Understanding Acoustic Emission Testing- Reading 1 Part B-A

charliechong

Understanding Acoustic Emission Testing- Reading 1 Part B-A

Understanding

Acoustic Emission

Testing, AET- Reading 1

My Pre-exam ASNT Self Study Notes

3rd September 2015

Charlie Chong/ Fion Zhang


E&P Applications

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Concrete Offshore structure

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Wind Energy Offshore structure

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Refinery Applications

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Refinery Applications

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http://wins-ndt.com/oil-chem/spherical-tanks/


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http://www.smt.sandvik.com/en/search/?q=stress+corrosion+cracking

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The Magical Book of Acoustic Emission

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Charlie Chong/ Fion Zhang


ASNT Certification Guide

NDT Level III / PdM Level III

AE - Acoustic Emission Testing

Length: 4 hours Questions: 135

1 Principles and Theory

• Characteristics of acoustic emission testing

• Materials and deformation

• Sources of acoustic emission

• Wave propagation

• Attenuation

• Kaiser and Felicity effects, and Felicity ratio

• Terminology (refer to acoustic emission glossary, ASTM 1316)

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2 Equipment and Materials

• Transducing processes

•Sensors

• Sensor attachments

• Sensor utilization

• Simulated acoustic emission sources

• Cables

• Signal conditioning

• Signal detection

• Signal processing

• Source location

• Advanced signal processing

Acoustic emission test systems

• Accessory materials

• Factors affecting test equipment

selection

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3 Techniques

• Equipment calibration and set up for

test

• Establishing loading procedures

• Precautions against noise

• Special test procedures

• Data displays

4 Interpretation and Evaluation

• Data interpretation

• Data evaluation

• Reports

5 Procedures

6 Safety and Health

7 Applications

• Laboratory studies (materialcharacterization)

• Structural applications

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Reference Catalog Number

NDT Handbook, Second Edition: Volume 5, Acoustic Emission Testing 130

Acoustic Emission: Techniques and Applications 752

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Fion Zhang at Shanghai

3rd September 2015

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http://meilishouxihu.blog.163.com/


Greek

Alphabet

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http://greekhouseoffonts.com/


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Video on - Leak Detection on Buried Water Piping using Acoustic

Emission


https://www.youtube.com/watch?v=9kq6JxIJDik

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Contents:

AE Codes and Standards

■ ASTM

■ ASME V

1. Reading 01- www.geocities.ws/raobpc/AET.html

2. Reading 02- Sidney Mindess University of British Columbia Chapter 16:

Acoustic Emission Methods

3. Reading 03- AET ndt-ed.org

4. Reading 04- Terms & Definitions ASTM E1316

5. Reading 05- Q&A 25 items

6. Reading 06- High Strength Steel- TWIP Steel

7. Reading 07- AET- optimum solution for leakage detection of water pipeline

8. Others reading.

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ASME V Article Numbers:

Gen Article 1

RT Article 2

Nil Article 3

UT Article 4 for welds

UT Article 5 for materials

PT Article 6

MT Article 7

ET Article 8

Visual Article 9

LT Article 10

AE Article 11 (FRP)

AE Article 12 (Metallic)

AE Article 13 (Continuous)

Qualif. Article 14

ACFM Article 15

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ASTM Standards

E569 - 07

Standard Practice for Acoustic Emission Monitoring of Structures

During Controlled Stimulation

E650 – 97 (2007)

Standard Guide for Mounting Piezoelectric Acoustic Emission Sensors

E749 - 07

Standard Practice for Acoustic Emission Monitoring During

Continuous Welding

E750 - 04

Standard Practice for Characterizing Acoustic Emission

Instrumentation

E751 - 07

Standard Practice for Acoustic Emission Monitoring During Resistance

Spot-Welding

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ASTM Standards

E976 - 05

Standard Guide for Determining the Reproducibility of Acoustic

Emission Sensor Response

E1067 - 07

Standard Practice for Acoustic Emission Examination of Fiberglass

Reinforced Plastic Resin (FRP) Tanks/Vessels

E1106 - 07

Standard Test Method for Primary Calibration of Acoustic Emission

Sensors

E1118 - 05

Standard Practice for Acoustic Emission Examination of Reinforced

Thermosetting Resin Pipe (RTRP)

E1139 - 07

Standard Practice for Continuous Monitoring of Acoustic Emission

from Metal Pressure Boundaries

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ASTM Standards

E1211 - 07

Standard Practice for Leak Detection and Location Using Surface-

Mounted Acoustic Emission Sensors

E1419 - 09

Standard Practice for Examination of Seamless, Gas-Filled, Pressure

Vessels Using Acoustic Emission

E1495 - 02

(2007)

Standard Guide for Acousto-Ultrasonic Assessment of Composites,

Laminates, and Bonded Joints

E1736 - 05

Standard Practice for Acousto-Ultrasonic Assessment of Filament-

Wound Pressure Vessels

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ASTM Standards

E1781 - 08

Standard Practice for Secondary Calibration of Acoustic Emission

Sensors

E1888 /E1888M – 07

Standard Practice for Acoustic Emission Examination of Pressurized

Containers Made of Fiberglass Reinforced Plastic with Balsa Wood

Cores

E1930 – 07

Standard Practice for Examination of Liquid-Filled Atmospheric and

Low-Pressure Metal Storage Tanks Using Acoustic Emission

E1932 - 07

Standard Guide for Acoustic Emission Examination of Small Parts

E2075 – 05

Standard Practice for Verifying the Consistency of AE-Sensor

Response Using an Acrylic Rod

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ASTM Standards

E2076 - 05

Standard Test Method for Examination of Fiberglass Reinforced Plastic

Fan Blades Using Acoustic Emission

E2191 - 08

Standard Practice for Examination of Gas-Filled Filament-Wound

Composite Pressure Vessels Using Acoustic Emission

E2374 - 04

Standard Guide for Acoustic Emission System Performance

Verification

E2478 - 06a

Standard Practice for Determining Damage-Based Design Stress for

Fiberglass Reinforced Plastic (FRP) Materials Using Acoustic

Emission

E2598 - 07

Standard Practice for Acoustic Emission Examination of Cast Iron

Yankee and Steam Heated Paper Dryers

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Typical AET Signal

https://dspace.lib.cranfield.ac.uk/bitstream/1826/2196/1/Acoustic%20Emission%20Waveform%20Changes%202006.pdf

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Typical AET Signal

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Study Note 1:

AET

http://www.geocities.ws/raobpc/AET.html

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http://www.geocities.ws/raobpc/AET.html


What is AE

Acoustic emission is the technical term for the noise emitted by materials and

structures when they are subjected to stress. Types of stresses can be (1)

mechanical, (2) thermal or (3) chemical. This emission is caused by the rapid

release of energy within a material due to events such as crack initiation and

growth, crack opening and closure, dislocation movement, twinning, and

phase transformation in monolithic materials and fiber breakage and fibermatrix

debonding in composites.

The subsequent extension occurring under an applied stress generates

transient elastic waves which propagate through the solid to the surface

where they can be detected by one or more sensors. The sensor is a

transducer that converts the mechanical wave into an electrical signal

(piezoelectric) . In this way information about the existence and location

(triangulation by multi-transducers) of possible sources is obtained. Acoustic

emission may be described as the "sound" emanating from regions of

localized deformation within a material.

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http://www.geocities.ws/raobpc/AET.html


Until about 1973, acoustic emission technology was primarily employed in the

non-destructive testing of such structures as pipelines, heat exchangers,

storage tanks, pressure vessels, and coolant circuits of nuclear reactor plants.

However, this technique was soon applied to the detection of defects in

rotating equipment bearings.

Applications:

Static subjects

Dynamic subjects

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Charlie Chong/ Fion Zhang

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http://www.geocities.ws/raobpc/AET.html


Acoustic Emission

Acoustic Emission (AE) refers to generation of transient elastic waves 瞬 间 弹

性 波 during rapid release of energy from localized sources within a material.

The source of these emissions in metals is closely associated with the

dislocation movement accompanying plastic deformation and with the

initiation and extension of cracks in a structure under stress. 应 力 作 用 下 , 结

构 中 的 裂 纹 萌 生 / 扩 展 ( 塑 性 变 形 ) 造 成 的 位 错 运 动 . 这 位 错 运 动 会 引 发 瞬 间 的 弹

性 波 .

Other sources of AE are: melting, phase transformation, thermal stresses,

cool down cracking and stress build up, twinning, fiber breakage and fibermatrix

debonding in composites.

其 他 会 引 起 瞬 间 的 弹 性 波 的 因 素 :

熔 化 , 相 变 , 热 应 力 冷 却 裂 纹 和 应 力 建 立 , 孪 晶 , 在 复 合 材 料 中 的 纤 维 断 裂 和 纤

维 - 基 体 界 面 脱 粘

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http://www.geocities.ws/raobpc/AET.html


AE Technique

The AE technique (AET) is based on the detection and conversion of high

frequency elastic waves emanating from the source to electrical signals. This

is accomplished by directly coupling piezoelectric transducers on the surface

of the structure under test and loading the structure. The output of the

piezoelectric sensors (during stimulus) is amplified through a low-noise

preamplifier, filtered to remove any extraneous noise and further processed

by suitable electronics. AET can non-destructively predict early failure of

structures. Further, a whole structure can be monitored from a few locations

and while the structure is in operation. AET is widely used in industries for

detection of faults or leakage in pressure vessels, tanks, and piping systems

and also for on-line monitoring welding and corrosion.

The difference between AET and other non-destructive testing (NDT)

techniques is that AET detects activities inside materials, while other

techniques attempt to examine the internal structures of materials by sending

and receiving some form of energy.

Charlie Chong/ Fion Zhang

http://www.geocities.ws/raobpc/AET.html


Types of AET

Acoustic emissions are broadly classified into two major types namely;

• continuous type (associated with lattice dislocation)

• burst type. (twinning, micro yielding, development of crack)

The waveform of continuous type AE signal is similar to Gaussian random

noise, but the amplitude varies with acoustic emission activity. In metals and

alloys, this form of emission is considered to be associated with the motion of

dislocations. Burst type emissions are short duration pulses and are

associated with discrete release of high amplitude strain energy. In metals,

the burst type emissions are generated by twinning, micro yielding,

development of cracks.

• Continuos type (Gaussian random noise) → Motion of dislocation,

• Burst type (discrete high amplitude strain energy) → twinning, micro

yielding, development of cracks

Charlie Chong/ Fion Zhang

http://www.geocities.ws/raobpc/AET.html


What is Normal (Gaussian) distribution

In probability theory, the normal (or Gaussian) distribution is a very common

continuous probability distribution. Normal distributions are important in

statistics and are often used in the natural and social sciences to represent

real-valued random variables whose distributions are not known.[1][2]

The normal distribution is remarkably useful because of the central limit

theorem. In its most general form, under mild conditions, it states that

averages of random variables independently drawn from independent

distributions are normally distributed. Physical quantities that are expected to

be the sum of many independent processes (such as measurement errors)

often have distributions that are nearly normal.[3] Moreover, many results and

methods (such as propagation of uncertainty and least squares parameter

fitting) can be derived analytically in explicit form when the relevant variables

are normally distributed.

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https://en.wikipedia.org/wiki/Normal_distribution


The normal distribution is sometimes informally called the bell curve.

However, many other distributions are bell-shaped (such as Cauchy's,

Student's, and logistic). The terms Gaussian function and Gaussian bell curve

are also ambiguous because they sometimes refer to multiples of the normal

distribution that cannot be directly interpreted in terms of probabilities.

The probability density of the normal distribution is:

Hereμ is the mean or expectation of the distribution (and also its median and

mode). The parameter σ is its standard deviation with its variance then σ 2 . A

random variable with a Gaussian distribution is said to be normally distributed

and is called a normal deviate.

If μ = 0 and σ = 1, the distribution is called the standard normal distribution

or the unit normal distribution denoted by N(0,1) and a random variable with

that distribution is a standard normal deviate.

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https://en.wikipedia.org/wiki/Normal_distribution


Probability density function for the normal distribution

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https://en.wikipedia.org/wiki/Normal_distribution


Cumulative distribution function of an acoustic emission

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https://en.wikipedia.org/wiki/Normal_distribution


Cumulative distribution function of an acoustic emission

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https://en.wikipedia.org/wiki/Normal_distribution


Discussion

Subject: What is the difference between an Gaussian random noise and an

engineering acoustic emission?

Answer: The waveform of continuous type AE signal is similar to Gaussian

random noise, but the amplitude varies with acoustic emission activity.

Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Normal_distribution


Crystal Twinning

Crystal twinning occurs when two separate crystals share some of the same

crystal lattice points in a symmetrical manner. The result is an intergrowth of

two separate crystals in a variety of specific configurations. A twin boundary

or composition surface separates the two crystals. Crystallographers classify

twinned crystals by a number of twin laws. These twin laws are specific to the

crystal system. The type of twinning can be a diagnostic tool in mineral

identification.

Twinning can often be a problem in X-ray crystallography, as a twinned

crystal does not produce a simple diffraction pattern.

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https://en.wikipedia.org/wiki/Crystal_twinning


Twin boundaries occur when two crystals of the same type intergrow, so that

only a slight misorientation exists between them. It is a highly symmetrical

interface, often with one crystal the mirror image of the other; also, atoms are

shared by the two crystals at regular intervals. This is also a much lowerenergy

interface than the grain boundaries that form when crystals of arbitrary

orientation grow together.

Twin boundaries are partly responsible for shock hardening and for many of

the changes that occur in cold work of metals with limited slip systems or at

very low temperatures. They also occur due to martensitic transformations:

the motion of twin boundaries is responsible for the pseudoelastic and shapememory

behavior of nitinol, and their presence is partly responsible for the

hardness due to quenching of steel. In certain types of high strength steels,

very fine deformation twins act as primary obstacles against dislocation

motion. These steels are referred to as 'TWIP' steels, where TWIP stands for

TWinning Induced Plasticity

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https://en.wikipedia.org/wiki/Crystal_twinning


What is Crystal Twinning

Crystal twinning occurs when two separate crystals share some of the same

crystal lattice points in a symmetrical manner.

Crystal-A

Crystal-B

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https://en.wikipedia.org/wiki/Crystal_twinning


Crystal Twinning

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https://en.wikipedia.org/wiki/Crystal_twinning


Crystal Twinning

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https://en.wikipedia.org/wiki/Crystal_twinning


Fivefold twinning in a gold nano-particle (electron microscope image).

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https://en.wikipedia.org/wiki/Crystal_twinning


Crystal Twinning- Diagram of twinned crystals of Albite. On the more perfect

cleavage, which is parallel to the basal plane (P), is a system of fine striations,

parallel to the second cleavage (M).

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https://en.wikipedia.org/wiki/Crystal_twinning


Crystal Twinning- Martensitic Formation

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https://en.wikipedia.org/wiki/Crystal_twinning


AET Set-up

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http://www.geocities.ws/raobpc/AET.html


Continuous type- Gaussian random noise

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http://www.geocities.ws/raobpc/AET.html


Continuous type

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http://www.geocities.ws/raobpc/AET.html


Discrete Burst Type

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http://www.geocities.ws/raobpc/AET.html


Discussion

Subject: explains on the weak damages signal w.r.t the severe damage in

term of the recorded peak signal.

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http://www.geocities.ws/raobpc/AET.html


Discrete Burst Type (Kaiser effect)

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Kaiser Effect

Plastic deformation is the primary source of AE in loaded metallic structures.

An important feature affecting the AE during deformation of a material is

‘Kaiser Effect’, which states that additional AE occurs only when the stress

level exceeds previous stress level. A similar effect for composites is termed

as 'Falicity effect'. (?)

Comments:

Kaiser effect- when the load is released and later applied, AE will not be

emitted until the previous maximum is reaches.

Falicity effect- an effect that deviate from Kaiser effect

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Kaiser Effect- which states that additional AE occurs only when the stress

level exceeds previous stress level. A similar effect for composites is termed

as 'Falicity effect'. (?)

http://www.ndt.net/ndtaz/content.php?id=476

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Felicity effect is an effect in acoustic emission that reduces Kaiser effect

at high loads of material. Under Felicity effect the acoustic emission resumes

before the previous maximum load was reached

Felicity effect

Kaiser effect

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https://en.wikipedia.org/wiki/Felicity_effect


Basic AE history plot showing

Kaiser effect (BCB), Felicity effect

(DEF), and emission during hold

(GH) 2

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Activity of AE Sources in Structural Loading

AE signals generated under different loading patterns can provide valuable

information concerning the structural integrity of a material. Load levels that

have been previously exerted on a material do not produce AE activity. In

other words, discontinuities created in a material do not expand or move until

that former stress is exceeded. This phenomenon, known as the Kaiser Effect,

can be seen in the load versus AE plot to the right. As the object is loaded,

acoustic emission events accumulate (segment AB). When the load is

removed and reapplied (segment BCB), AE events do not occur again until

the load at point B is exceeded. As the load exerted on the material is

increased again (BD), AE’s are generated and stop when the load is removed.

However, at point F, the applied load is high enough to cause significant

emissions even though the previous maximum load (D) was not reached.

This phenomenon is known as the Felicity Effect. This effect can be

quantified using the Felicity Ratio, which is the load where considerable AE

resumes, divided by the maximum applied load (F/D).

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Kaiser Effect- The phenomenon,

known as the Kaiser Effect, can be

seen in the load versus AE plot to

the right. As the object is loaded,

acoustic emission events

accumulate (segment AB). When

the load is removed and reapplied

(segment BCB), AE events do not

occur again until the load at point B

is exceeded

Charlie Chong/ Fion Zhang


Felicity Effect –

the applied load is high enough to

cause significant emissions even

though the previous maximum load

(D) was not reached. This

phenomenon is known as the

Felicity Effect.

(F)

(D)

Felicity Ratio= F/D

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AE Parameters

Various parameters used in AET include: AE burst, threshold, ring down

count, cumulative counts, event duration, peak amplitude, rise time, energy

and RMS voltage etc. Typical AE system consists of signal detection,

amplification & enhancement, data acquisition, processing and analysis units.

Charlie Chong/ Fion Zhang

http://www.geocities.ws/raobpc/AET.html


AE Parameters

Various parameters used in AET include:

• AE burst, threshold,

• ring down count,

• cumulative counts,

• event duration,

• peak amplitude,

• rise time, energy and

• RMS voltage etc.

Typical AE system consists of signal detection, amplification & enhancement,

data acquisition, processing and analysis units.

Charlie Chong/ Fion Zhang

http://www.geocities.ws/raobpc/AET.html


Sensors / Source Location Identification

The most commonly used sensors are resonance type piezoelectric

transducers with proper couplants. In some applications where sensors

cannot be fixed directly, waveguides are used. Sensors are calibrated for

frequency response and sensitivity before any application. The AE technique

captures the parameters and correlates with the defect formation and failures.

When more than one sensors is used,

• AE source can be located based by measuring the signal’s arrival time to

each sensor. By comparing the signal’s arrival time at different sensors,

the source location can be calculated through triangulation 三 角 测 量 and

other methods.

• AE sources are usually classified based on activity 活 动 力 and intensity 强

度 . A source is considered to be active if its event count continues to

increase with stimulus.

• A source is considered to be critically active if the rate of change of its

count or emission rate consistently increases with increasing stimulation

变 化 率 随 着 刺 激 增 加 不 断 提 高 .

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AET Advantages

AE testing is a powerful aid to materials testing and the study of deformation,

fatigue crack growth, fracture, oxidation and corrosion. It gives an immediate

indication of the response and behaviour of a material under stress, intimately

connected with strength, damage and failure. A major advantage of AE

testing is that it does not require access to the whole examination area. In

large structures / vessels permanent sensors can be mounted for periodic

inspection for leak detection and structural integrity monitoring.

Typical advantages of AE technique include:

1. high sensitivity,

2. early and rapid detection of defects, leaks, cracks etc.,

3. on-line monitoring,

4. location of defective regions,

5. minimization of plant downtime for inspection,

6. no need for scanning the whole structural surface and

7. minor disturbance of insulation.

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AET Limitations

On the negative side;

• AET requires stimulus. (process stimulus or externally test stimulus?)

• AE technique can only (1) qualitatively estimate the damage and predict (2)

how long the components will last. So,

• other NDT methods are still needed for thorough examinations and for

obtaining quantitative information.

• Plant environments are usually very noisy and the AE signals are usually

very weak. This situation calls for incorporation of signal discrimination and

noise reduction methods. In this regard, (1) signal processing and (2)

frequency domain analysis are expected to improve the situation.

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A Few Typical Applications

• Detection and location of leak paths in end-shield of reactors (frequency

analysis)

• Identification of leaking pressure tube in reactors

• Condition monitoring of 17 m Horton sphere during hydro testing (24

sensors)

• On-line monitoring of welding process and fuel end-cap welds

• Monitoring stress corrosion cracking, fatigue crack growth

• Studying plastic deformation behaviour and fracture of SS304, SS316,

Inconel, PE-16 etc

• Monitoring of oxidation process and spalling behaviour of metals and

alloys

Charlie Chong/ Fion Zhang

http://www.geocities.ws/raobpc/AET.html


Acoustic Emission Testing applications are most suitable

for:

1. Aboveground Storage Tank Screening for Corrosion & Leaks

2. Pressure Containment Vessels (Columns, Bullets, Cat Crackers)

3. Horton Spheres & legs

4. Fiberglass Reinforced Plastic Tanks and Piping

5. Offshore Platform Monitoring

6. Nuclear components inspection

7. Tube Trailers

8. Railroad tank cars

9. Bridge Critical Members monitoring

10. Pre- & Post-Stressed Concrete Beams

11. Reactor Piping

12. High Energy Seam Welded Hot Reheat Piping Systems in Power Plants.

13. On-Stream Monitoring

14. Remote Long Term Monitoring

http://www.techcorr.com/services/Inspection-and-Testing/Acoustic-Emission-Testing.cfm

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Acoustic Emission Testing Advantages

Compared to conventional inspection methods the advantages of the

Acoustic Emission Testing technique are:

• Tank bottom Testing without removal of product.

• Inspection of Insulated Piping & Vessels

• Real time monitoring during cool-down & start-ups

• Real Time Monitoring Saves Money

• Real Time Monitoring Improves Safety

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Tank AET

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End of Reading 1

Charlie Chong/ Fion Zhang


Study Note 2:

Acoustic Emission Method

Sidney Mindess

University of British Columbia

Chapter 16: Acoustic Emission Methods

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16

Acoustic Emission

Methods

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Dam

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http://www.boomsbeat.com/articles/116/20140118/tianzi-mountains-china.htm


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Content:

16.1 Introduction

16.2 Historical Background

16.3 Theoretical Considerations

16.4 Evaluation of Acoustic Emission Signals

16.5 Instrumentation and Test Procedures

16.6 Parameters Affecting Acoustic Emissions from Concrete

The Kaiser Effect · Effect of Loading Devices · Signal

Attenuation · Specimen Geometry · Type of aggregate ·Concrete Strength

16.7 Laboratory Studies of Acoustic Emission

Fracture Mechanics Studies · Type of Cracks · Fracture Process

Zone (Crack Source) Location · Strength vs. Acoustic Emission

Relationships · Drying Shrinkage · Fiber Reinforced Cements

and Concretes · High Alumina Cement · Thermal Cracking ·

Bond in Reinforced Concrete · Corrosion of Reinforcing Steel

in Concrete

16.8 Field Studies of Acoustic Emission

16.9 Conclusions

Charlie Chong/ Fion Zhang


Foreword:

Acoustic emission refers to the sounds, both audible and sub-audible

(ultrasonic?, subsonic?) , that are generated when a material undergoes

irreversible changes, such as those due to cracking.

Acoustic emissions (AE) from concrete have been studied for the past 30

years, and can provide useful information on concrete properties. This review

deals with the parameters affecting acoustic emissions from concrete,

including discussions of the Kaiser effect, specimen geometry, and concrete

properties. There follows an extensive discussion of the use of AE to monitor

cracking in concrete, whether due to:

(1) externally applied loads,

(2) drying shrinkage, or

(3) thermal stresses.

AE studies on reinforced concrete are also described. While AE is very useful

laboratory technique for the study of concrete properties, its use in the field

remains problematic.

Charlie Chong/ Fion Zhang


16.1 Introduction

It is common experience that the failure of a concrete specimen under load is

accompanied by a considerable amount of audible noise. In certain

circumstances, some audible noise is generated even before ultimate failure

occurs. With very simple equipment- a microphone placed against the

specimen, an amplifier, and an oscillograph — subaudible sounds can be

detected at stress levels of perhaps 50% of the ultimate strength; with the

sophisticated equipment available today, sound can be detected at much

lower loads, in some cases below 10% of the ultimate strength. These sounds,

both audible and subaudible, are referred to as acoustic emission. In general,

acoustic emissions are defined as “the class of phenomena whereby transient

转 瞬 即 逝 的 elastic waves are generated by the rapid release of energy from

localized sources within a material.” These waves propagate through the

material, and their arrival at the surfaces can be detected by piezoelectric

transducers.

Keywords: Audible & Sub-audible sounds

Charlie Chong/ Fion Zhang


Acoustic emissions, which occur in most materials, are caused by irreversible

changes, such as

(1) dislocation movement,

(2) twinning,

(3) phase transformations,

(4) crack initiation, and propagation,

(5) debonding between continuous and dispersed phases in composite

materials, and so on.

In concrete, since the first three of these mechanisms do not occur, acoustic

emission is due primarily to:

1. Cracking processes

2. Slip between concrete and steel reinforcement

3. Fracture or debonding of fibers in fiber-reinforced concrete

Charlie Chong/ Fion Zhang


16.2 Historical Background

The initial published studies of acoustic emission phenomena, in the early

1940s, dealt with the problem of predicting rockbursts in mines; this technique

is still very widely used in the field of rock mechanics, in both field and

laboratory studies.

The first significant investigation of acoustic emission from metals (steel, zinc,

aluminum, copper, and lead) was carried out by Kaiser. Among many other

things, he observed what has since become known as the Kaiser effect: “the

absence of detectable acoustic emission at a fixed sensitivity level, until

previously applied stress levels are exceeded.”

While this effect is not present in all materials, it is a very important

observation, and it will be referred to again later in this review. The first study

of acoustic emission from concrete specimens under stress appears to have

been carried out by Rüsch, who noted that during cycles of loading and

unloading below about 70 to 85% of the ultimate failure load, acoustic

emissions were produced only when the previous maximum load was

reached (the Kaiser effect).

Charlie Chong/ Fion Zhang


At about the same time, but independently, L’Hermite also measured acoustic

emission from concrete, finding that a sharp increase in acoustic emission

(magnitude or event count?) coincided with the point at which Poisson’s ratio

also began to increase (i.e., at the onset of significant matrix cracking in the

concrete).

Charlie Chong/ Fion Zhang


Poisson's ratio, named after Siméon Poisson, is the negative ratio of

transverse to axial strain. When a material is compressed in one direction, it

usually tends to expand in the other two directions perpendicular to the

direction of compression. This phenomenon is called the Poisson effect.

Poisson's ratio ѵ (nu) is a measure of this effect. The Poisson ratio is the

fraction (or percent) of expansion divided by the fraction (or percent) of

compression, for small values of these changes.

Conversely, if the material is stretched rather than compressed, it usually

tends to contract in the directions transverse to the direction of stretching.

This is a common observation when a rubber band is stretched, when it

becomes noticeably thinner. Again, the Poisson ratio will be the ratio of

relative contraction to relative expansion, and will have the same value as

above. In certain rare cases, a material will actually shrink in the transverse

direction when compressed (or expand when stretched) which will yield a

negative value of the Poisson ratio.

Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Poisson%27s_ratio


Figure 1: A cube with sides of length L of an isotropic linearly elastic material

subject to tension along the x axis, with a Poisson's ratio of 0.5. The green

cube is unstrained, the red is expanded in the x direction by ∆L due to tension,

and contracted in the y and z directions by ∆L'.

Poisson Ratio = ∆L‘/ ∆L

Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Poisson%27s_ratio


In 1965, however, Robinson used more sensitive equipment to show that

acoustic emission occurred at much lower load levels than had been reported

earlier, and hence, could be used to monitor earlier microcracking (such as

that involved in the growth of bond cracks in the interfacial region between

cement and aggregate).

In 1970, Wells built a still more sensitive apparatus, with which he could

monitor acoustic emissions in the frequency range from about 2 to 20 kHz.

However, he was unable to obtain truly reproducible records for the various

specimen types that he tested, probably due to the difficulties in eliminating

external noise from the testing machine. Also in 1970, Green reported a much

more extensive series of tests, recording acoustic emission frequencies up to

100 kHz. Green was the first to show clearly that acoustic emissions from

concrete are related to failure processes within the material; using source

location techniques, he was also able to determine the locations of defects. It

was this work that indicated that acoustic emissions could be used as an

early warning of failure. Green also noted the Kaiser effect, which suggested

to him that acoustic emission techniques could be used to indicate the

previous maximum stress to which the concrete had been subjected. As we

will see below, however, a true Kaiser effect appears not to exist for concrete.

Charlie Chong/ Fion Zhang


Green also noted the Kaiser effect, which suggested to him that acoustic

emission techniques could be used to indicate the previous maximum stress

to which the concrete had been subjected. As we will see below, however, a

true Kaiser effect appears not to exist for concrete.

Charlie Chong/ Fion Zhang


Nevertheless, even after this pioneering work, progress in applying acoustic

emission techniques remains slow. An extensive review by Diederichs et al.

(et al means: and others), covers the literature on acoustic emissions from

concrete up to 1983. However, as late as 1976, Malhotra noted that there was

little published data in this area, and that “acoustic emission methods are in

their infancy.” Even in January, 1988, a thorough computer-aided search of

the literature found only some 90 papers dealing with acoustic emissions from

concrete over about the previous 10 years; while this is almost certainly not a

complete list, it does indicate that there is much work to be carried out before

acoustic emission monitoring becomes a common technique for testing

concrete. Indeed, there are still no standard test methods which have even

been suggested for this purpose.

Charlie Chong/ Fion Zhang


16.3 Theoretical Considerations

When an acoustic emission event occurs at a source with the material, due to

(1) inelastic deformation or (2) to cracking, the stress waves travel directly

from the source to the receiver as body waves. Surface waves may then arise

from mode conversion. When the stress waves arrive at the receiver, the

transducer responds to the surface motions that occur.

It should be noted that the signal captured by the recording device may be

affected by:




the nature of the stress pulse generated by the source,

the geometry of the test specimen, and

the characteristics of the receiver,

making it difficult to interpret the recorded waveforms.

Charlie Chong/ Fion Zhang


Two basic types of acoustic emission signals can be generated (Figure 16.1):

• Continuous emission is “a qualitative description of the sustained signal

level produced by rapidly occurring acoustic emission events.” These are

generated by events such as plastic deformations in metals, which occur

in a reasonably continuous manner.

• Burst emission is “a qualitative description of the discrete signal related to

an individual emission event occurring within the material,” such as that

which may occur during crack growth or fracture in brittle materials.

These burst signals are characteristic of the acoustic emission events

resulting from the loading of cementitious materials.

Charlie Chong/ Fion Zhang


FIGURE 16.1 The two basic types of acoustic emission signals. (A) Continuous

emission. (B) Burst emission.

Charlie Chong/ Fion Zhang


16.4 Evaluation of Acoustic Emission Signals

A typical acoustic emission signal from concrete is shown in Figure 16.2.12

However, when such acoustic events are examined in much greater detail, as

shown in Figure 16.3, the complexity of the signal becomes even more

apparent; the scatter in noise, shown in Figure 16.3, makes it difficult to

determine exactly the time of arrival of the signal; this means that very

sophisticated equipment must be used to get the most information out of the

acoustic emission signals. In addition, to obtain reasonable sensitivity, the

acoustic emission signals must be amplified. In concrete, typically, system

gains in the range of 80 to 100 decibels (dB) are used.

Comments:

20log (I/I o ) = 80, (I/I o ) = 10000

20log(I/I o ) = 100, (I/Io) = 100000

Charlie Chong/ Fion Zhang


FIGURE 16.2 A typical acoustic emission signal from concrete. (From

Berthelot, J.M. et al., private communication, 1987. With permission.)

Charlie Chong/ Fion Zhang


FIGURE 16.3 Typical view of an acoustic emission event as displayed in an

oscilloscope screen. (Adapted from Maji, A. and Shah, S.P., Exp. Mech., 26,

1, 1988, p. 27.)

Charlie Chong/ Fion Zhang


FIGURE 16.3 Typical view of an acoustic emission event as displayed in an

oscilloscope screen. (Adapted from Maji, A. and Shah, S.P., Exp. Mech., 26,

1, 1988, p. 27.)

Charlie Chong/ Fion Zhang


There are a number of different ways in which acoustic emission signals may

be evaluated.

Acoustic Emission Counting (ring-down counting)

This is the simplest way in which an acoustic emission event may be

characterized. It is “the number of times the acoustic emission signal exceeds

a preset threshold during any selected portion of a test,” and is illustrated in

Figure 16.4. A monitoring system may record:

FIGURE 16.4 The principle of acoustic emission counting (ring-down counting).

Charlie Chong/ Fion Zhang


1. The total number of counts (e.g., 13 counts in Figure 16.4). Since the

shape of a burst emission is generally a damped sinusoid, pulses of higher

amplitude will generate more counts.

2. The count rate. This is the number of counts per unit of time; it is

particularly useful when very large numbers of counts are recorded.

3. The mean pulse amplitude. This may be determined by using a root-mean

square meter, and is an indication of the amount of energy being

dissipated.

Clearly, the information obtained using this method of analysis depends upon

both the gain and the threshold setting. Ring-down counting is affected

greatly by the characteristics of the transducer, and the geometry of the test

specimen (which may cause internal reflections) and may not be indicative of

the nature of the acoustic emission event. In addition, there is no obvious way

of determining the amount of energy released by a single event, or the total

number of separate acoustic events giving rise to the counts.

Charlie Chong/ Fion Zhang


■ Event counting — Circuitry is available which counts each acoustic

emission event only once, by recognizing the end of each burst emission in

terms of a predetermined length of time since the last count (i.e., since the

most recent crossing of the threshold). In Figure 16.4, for instance, the

number of events is three. This method records the number of events, which

may be very important, but provides no information about the amplitudes

involved.

Charlie Chong/ Fion Zhang

since the most recent

crossing of the threshold


■ Rise time — This is the interval between the time of first occurrence of

signals above the level of the background noise and the time at which the

maximum amplitude is reached. This may assist in determining the type of

damage mechanism.

Charlie Chong/ Fion Zhang


■ Signal duration — This is the duration of a single acoustic emission event;

this too may be related to the type of damage mechanism.

■ Amplitude distribution — This provides the distribution of peak

amplitudes. This may assist in identifying the sources of the emission events

that are occurring.

■ Frequency analysis — This refers to the frequency spectrum of individual

acoustic emission events. This technique, generally requiring a fast Fourier

transformation analysis of the acoustic emission waves, may help

discriminate between different types of events. Unfortunately, a frequency

analysis may sometimes simply be a function of the response of the

transducer, and thus reveal little of the true nature of the pulse.

Charlie Chong/ Fion Zhang


Energy analysis — This is an indication of the energy released by an

acoustic emission event; it may be measured in a number of ways, depending

on the equipment, but it is essentially the area under the amplitude vs. time

curve (Figure 16.4) for each burst. Alternatively, the area under the envelope

of the amplitude vs. time curve may be measured for each burst.

Charlie Chong/ Fion Zhang


Defect location — By using a number of transducers to monitor acoustic

emission events, and determining the time differences between the detection

of each event at different transducer positions, the location of the acoustic

emission event may be determined by using triangulation techniques. Work

by Maji and Shah, for instance, has indicated that this technique may be

accurate to within about 5 mm.

Analysis of the wave-form— Most recently, it has been suggested that an

elaborate signals processing technique (deconvolution - 反 褶 积 ) applied to the

wave-form of an acoustic emission event can provide information regarding

the volume, orientation, and type of microcrack. Ideally, since all of these

methods of data analysis provide different information, one would wish to

measure them all. However, this is neither necessary nor economically

feasible. In the discussion that follows, it will become clear that the more

elaborate methods of analysis are useful in fundamental laboratory

investigations, but may be inappropriate for practical applications.

Charlie Chong/ Fion Zhang


FIGURE 16.5 The main elements of a modern acoustic emission detection

system.

Charlie Chong/ Fion Zhang


The Fourier transform- (Deconvolution- 反 褶 积 of Frequency)

The Fourier transform decomposes a function of time (a signal) into the

frequencies that make it up, similarly to how a musical chord can be

expressed as the amplitude (or loudness) of its constituent notes. The Fourier

transform of a function of time itself is a complex-valued function of frequency,

whose absolute value represents the amount of that frequency present in the

original function, and whose complex argument is the phase offset of the

basic sinusoid in that frequency. The Fourier transform is called the frequency

domain representation of the original signal. The term Fourier transform refers

to both the frequency domain representation and the mathematical operation

that associates the frequency domain representation to a function of time.

The Fourier transform is not limited to functions of time, but in order to have a

unified language, the domain of the original function is commonly referred to

as the time domain. For many functions of practical interest one can define an

operation that reverses this: the inverse Fourier transformation, also called

Fourier synthesis, of a frequency domain representation combines the

contributions of all the different frequencies to recover the original function of

time.

Charlie Chong/ Fion Zhang


Fourier-Transform (FT)

The Fourier theorem states that any waveform can be duplicated by the

superposition of a series of sine and cosine waves. As an example, the

following Fourier expansion of sine waves provides an approximation of a

square wave. The three curves in the plot show the first one term (black line),

four terms (blue line), and sixteen terms (red line) in the Fourier expansion.

As more terms are added the superposition of sine waves better matches a

square wave.

Charlie Chong/ Fion Zhang

http://www.tissuegroup.chem.vt.edu/chem-ed/data/fourier.html


Fourier-Transform (FT) of Frequency

To understand any complicated signal, one of the first step is to generate the Fourier

transform of that signal. Fourier transform is a mathematical function that decomposes

a time varying signal, as shown in figure to the right, into several sinusoidal waves.

These sinusoidal waves will have different frequency, amplitude and phases but when

you add them all together, the original waveform is magically recreated. The

fundamental idea here is complexity reduction by splitting a waveform into

manageable chunks. For reasons that initially baffled me, the powers there be chose

sinusoidal waves as this manageable chunk.

Charlie Chong/ Fion Zhang

https://ranabasheer.wordpress.com/2014/03/16/why-do-we-use-fourier-transform/


Signal Evaluation: Analysis of the wave-form

Charlie Chong/ Fion Zhang

http://sirius.mtm.kuleuven.be/Research/NDT/AcousticEmissions/index.html


Signal Evaluation: Acoustic Emission Counting (ring-down counting)

Ring-down count= 13

Charlie Chong/ Fion Zhang


Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

Raise time

mV/μs

Signal duration μs

Event counts = 3 in unit time

Charlie Chong/ Fion Zhang


Signal Evaluation: Amplitude Distribution- Triangulation to locate source

Charlie Chong/ Fion Zhang

http://iopscience.iop.org/0964-1726/21/3/035009;jsessionid=DE0B79359A6ADDA1365CAC54ABA381A2.c2


Signal Evaluation: Amplitude Distribution- Triangulation to locate source

Charlie Chong/ Fion Zhang

http://iopscience.iop.org/0964-1726/21/3/035009;jsessionid=DE0B79359A6ADDA1365CAC54ABA381A2.c2


Signal Evaluation: Frequency analysis

Charlie Chong/ Fion Zhang


Signal Evaluation:

Energy analysis- it is essentially the area under the amplitude vs. time curve

Note: all areas under curves or only areas above threshold.

Charlie Chong/ Fion Zhang


Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

ring-down counting

Charlie Chong/ Fion Zhang


Signal Evaluation: Raise Time/ Event Counts/ Signal Duration

Charlie Chong/ Fion Zhang


16.5 Instrumentation and Test Procedures

Instrumentation (and, where necessary, the associated computer software) is

available, from a number of different manufacturers, to carry out all of the

methods of signal analysis described above. It might be added that advances

in instrumentation have outpaced our understanding of the nature of the

elastic waves resulting from microcracking in concrete. The main elements of

a modern acoustic emission detection system are shown schematically in

Figure 16.5.

Charlie Chong/ Fion Zhang


FIGURE 16.5 The main elements of a modern acoustic emission detection

system.

Charlie Chong/ Fion Zhang


FIGURE 16.5 The main elements of a modern acoustic emission detection

system.

Raw Display?

Selective Display?

Charlie Chong/ Fion Zhang


A brief description of the most important parts of this system is as follows:

1. Transducers: Piezoelectric transducers (generally made of lead zirconate

titanate, PZT) are used to convert the surface displacements into electric

signals. The voltage output from the transducers is directly proportional to

the strain in the PZT, which depends in turn on the amplitude of the

surface waves. Since these transducers are high impedance devices, they

yield relatively low signals, typically less than 100μV. There are basically

two types of transducers. (a) Wide-band transducers are sensitive to

acoustic events with frequency responses covering a wide range, often

several hundred kHz. (b) Narrow-band transducers are restricted to a

much narrower range of frequencies, using bandpass filters. Of course, the

transducers must be properly coupled to the specimen, often using some

form of silicone grease as the coupling medium.

Keywords:

• Since these transducers are high impedance devices, they yield relatively

low signals, typically less than 100μV.

• Wide band & Narrow Band

Charlie Chong/ Fion Zhang


Discussion

Subject:

A brief description of the most important parts of this system is as follows:

1. Transducers: Piezoelectric transducers (generally made of lead zirconate titanate, PZT) are used to convert the surface displacements into electric

signals. The voltage output from the transducers is directly proportional to the strain in the PZT, which depends in turn on the amplitude of the surface

waves. Since these transducers are high impedance devices, they yield relatively low signals, typically less than 100μV. There are basically two types

of transducers. (a) Wide-band transducers are sensitive to acoustic events with frequency responses covering a wide range, often several hundred

kHz. (b) Narrow-band transducers are restricted to a much narrower range of

frequencies, using bandpass filters. Of course, the transducers must be properly coupled to the specimen, often

using some form of silicone grease as the coupling medium.

Keywords:

• Since these transducers are high impedance devices, they yield relatively low signals, typically less than 100μV.

• Wide band & Narrow Band

Question:

Band pass (selective, High, Low?) as part of transducer constructions? Or

post transducer electronic?

Charlie Chong/ Fion Zhang


PZT:- If the p.d or the stress is changing the resulting effect also changes. Therefore if

an alternating potential difference with a frequency equal to the resonant frequency of

the crystal is applied across it the crystal will oscillate. A number of crystalline

materials show this effect – examples of these are quartz, barium titanate, lithium

sulphate, lead metaniobate, lead zirconate titanate (PZT) and polyvinylidine difluoride.

Piezoelectric transducers can act as both as a transmitter and a detector of vibrations.

However there are certain conditions. The crystal must stop vibrating as soon as the

alternating potential difference is switched off so that they can detect the reflected

pulse. For this reason a piece of damping material with an acoustic impedance the

same as that of the crystal is mounted at the back of the crystal. (See Figure 2).The

transducer is made with a crystal that has a thickness of one half of the

wavelength of the ultrasound, resonating at its fundamental frequency. A layer of

gel is needed between the transducer and the body to get good acoustic coupling (see

acoustic impedance).

Charlie Chong/ Fion Zhang

http://www.schoolphysics.co.uk/age16-19/Medical%20physics/text/Piezoelectric_transducer/index.html


The transducer is made with a crystal that has a thickness of one half of the

wavelength of the ultrasound, resonating at its fundamental frequency.

Example: Frequency= 519Hz, Wavelength λ = Speed/ frequency =

5890/519=11.35mm. The thickness of the transducer= 5.7mm approx.

s= 5890m/s

Charlie Chong/ Fion Zhang

http://www.olympus-ims.com/en/ndt-tutorials/thickness-gage/appendices-velocities/


AET

Transducer

In 0.1KHz~2.0KHz

Charlie Chong/ Fion Zhang


UT Transducers 2.0~5.0 MHz (≠ AET Transducer)

Charlie Chong/ Fion Zhang


2. Preamplifier: Because of the low voltage output (≤100μV) , the leads from

the transducer to the preamplifier must be as short as possible; often, the

preamplifier is integrated within the transducer itself. Typically, the gain in the

preamplifier is in the range 40 to 60 dB (x100, x1000). (Note: The decibel

scale measures only relative amplitudes. Using this scale:

where V is the output amplitude and Vi is the input amplitude. That is, a gain

of 40 dB will increase the input amplitude by a factor of 100; a gain of 60 dB

will increase the input amplitude by a factor of 1000, and so on.)

Charlie Chong/ Fion Zhang


3. Passband filters: are used to suppress the acoustic emission signals that

lie outside of the frequency range of interest.

(high pass, low pass, selective pass)

4. The main amplifier: further amplifies the signals, typically with a gain of

an additional 20 to 60 dB.

5. The threshold discriminator: is used to set the threshold voltage above

which signals are counted (or analyze) .

The remainder of the electronic equipment depends upon the way in which

the acoustic emission data are to be recorded, analyzed, and displayed.

Acoustic emission testing may be carried out in the laboratory or in the field.

Basically, one or more acoustic emission transducers are attached to the

specimen. The specimen is then loaded slowly, and the resulting acoustic

emissions are recorded.

Charlie Chong/ Fion Zhang


There are generally two (or more) categories of tests:

1. To use the acoustic emission signals to learn something about the internal

structure of the material, and how structural changes (i.e., damage) occur

during the process of loading. In this case, the specimens are generally

loaded to failure.

2. To establish whether the material or the structure meet certain design or

fabrication criteria. In this case, the load is increased only to some

predetermined level (“proof ” loading). The amount and nature of the

acoustic emissions may be used to establish the integrity of the specimen

or structure, and may also sometimes be used to predict the service life.

(i.e., hydrostatic testing)

3. Inservice monitoring where the loadings are the service loading? (e.g.,

monitoring of crack growth in a inservice coke drum)

4. Other?

Charlie Chong/ Fion Zhang


16.6 Parameters Affecting Acoustic Emissions from Concrete

16.6.1 The Kaiser Effect

The earliest acoustic emission studies of concrete, such as the work of Rüsch,

indicated that a true Kaiser effect (see above) exists for concrete; that is,

acoustic emissions were found not to occur in concrete that had been unloaded

until the previously applied maximum stress had been exceeded on

reloading. This was true, however, only for stress levels below about 75 to 85%

of the ultimate strength of the material; for higher stresses, acoustic emissions

began again at stresses somewhat lower than the previous maximum stress.

Subsequently, a number of other investigators have also concluded that

concrete exhibits a Kaiser effect, at least for stresses below the peak stress of

the material. (felicity effect)

Keypoints:

For concrete This was true, however, only for stress levels below about 75 to

85% of the ultimate strength of the material

Charlie Chong/ Fion Zhang


Spooner and Dougill confirmed that this effect did not occur beyond the peak

of the stress-strain curve (i.e., in the descending portion of the stress-strain

curve), where acoustic emissions occurred again before the previous

maximum strain was reached. It has also been suggested that a form of the

Kaiser effect occurs as well for cyclic thermal stresses in concrete, and for

drying and wetting cycles. On the other hand, Nielsen and Griffin have

reported that the Kaiser effect is only a very temporary effect in concrete; with

only a few hours of rest between loading cycles, acoustic emissions are again

recorded during reloading to the previous maximum stress. They therefore

concluded “that the Kaiser effect is not a reliable indicator of the loading

history for plain concrete.” Thus, it is unlikely that the Kaiser effect could be

used in practice to determine the previous maximum stress that a structural

member has been subjected to.

Comments:

The continual curing of concrete matrix repair the previous loading induced

effects (microcracks, disbonding etc.) and return the concrete back to almost

preloading condition.

Charlie Chong/ Fion Zhang


Kaiser Effect- Concrete

For concrete This

was true, however,

only for stress

levels below about

75 to 85% of the

ultimate strength

of the material

that this effect did not

occur beyond the

peak of the stressstrain

curve (i.e., in

the descending

portion of the stressstrain

curve), where

acoustic emissions

occurred again

before the previous

maximum strain was

reached.

Charlie Chong/ Fion Zhang


Spooner and Dougill conclusion on Kaiser Effect- Concrete:

They therefore concluded “that the Kaiser effect is not a reliable indicator of

the loading history for plain concrete.”

Charlie Chong/ Fion Zhang


16.6.2 Effect of Loading Devices

As is well known, the end restraint of a compression specimen of concrete

due to the friction between the ends of the specimen and the loading platens

can have a considerable effect on the apparent strength of the concrete.

These differences are also reflected in the acoustic emissions measured

when different types of loading devices are used. For instance, in

compression testing with stiff steel platens, most of the acoustic emission

appears at stresses beyond about half of the ultimate stress; with more

flexible platens, such as brush platens, significant acoustic emission appears

at about 20% of the ultimate stress. This undoubtedly reflects the different

crack patterns that develop with different types of platens, but it nonetheless

makes inter-laboratory comparisons, and indeed even studies on different

specimen geometries within the same laboratory, very difficult.

Charlie Chong/ Fion Zhang


16.6.3 Signal Attenuation

The elastic stress waves that are generated by cracking attenuate as they

propagate through the concrete. Thus, large acoustic emission events that

take place in the concrete far from a pick-up transducer may not exceed the

threshold excitation voltage due to this attenuation, while much smaller

events may be recorded if they occur close to the transducer. Very little

information is available on acoustic emission attenuation rates in concrete. It

has been shown that more mature cements show an increasing capacity to

transmit acoustic emissions. Related to this, Mindess has suggested that the

total counts to failure for concrete specimens in compression are much higher

for older specimens, which may also be explained by the better transmission

through older concretes.

Charlie Chong/ Fion Zhang


As a practical matter, the maximum distance between piezoelectric

transducers, or between the transducers and the source of the acoustic

emission event, should not be very large. Berthelot and Robert required an

array of transducers arranged in a 40-cm square mesh to locate acoustic

emission events reasonably accurately. They found that for ordinary concrete,

with a fifth transducer placed in the center of the 40 x 40-cm square mesh,

only about 40% of the events detected by the central transducer were also

detected by the four transducers at the corners; with high strength concrete,

this proportion increased to 60 to 70%. Rossi also found that a 40-cm square

mesh was needed for a proper determination of acoustic emission events.

Although more distant events can, of course, be recorded, there is no way of

knowing how many events are “lost” due to attenuation. This is an area that

requires much more study.


16.6.4 Specimen Geometry

It has been shown that smaller specimens appear to give rise to greater

levels of acoustic emission than do larger ones. The reasons for this are not

clear, although the observation may be related to the attenuation effect

described above. After an acoustic emission event occurs, the stress waves

not only travel from the source to the sensor, but also undergo (1) reflection,

(2) diffraction, and (3) mode conversions within the material. The basic

problem of wave propagation within a bounded solid certainly requires further

study, but there have apparently been no comparative tests on different

specimen geometries.

Charlie Chong/ Fion Zhang


16.6.5 Type of Aggregate

It is not certain whether the mineralogy of the aggregate has any effect on

acoustic emission. It has been reported that concretes with a smaller

maximum aggregate size produce a greater number of acoustic emission

counts than those with a larger aggregate size; however, the total energy

released by the finer aggregate concrete is reduced. This is attributed to the

observation that concretes made with smaller aggregates start to crack at

lower stresses; in concretes with larger aggregate particles, on the other hand,

individual acoustic events emit higher energies. For concretes made with

lightweight aggregates, the total number of counts is also greater than for

normal weight concrete, perhaps because of cracking occurring in the

aggregates themselves.

Charlie Chong/ Fion Zhang


16.6.6 Concrete Strength

It has been shown that the total number of counts to the maximum load is

greater for higher strength concretes. However, as was mentioned earlier, for

similar strength levels the total counts to failure appears to be much higher for

older concretes.

Charlie Chong/ Fion Zhang


16.7 Laboratory Studies of Acoustic Emission

By far the greatest number of acoustic emission studies of concrete have

been carried out in the laboratory, and have been largely “theoretical” in

nature:

1. To determine whether acoustic emission analysis could be applied to

cementitious systems

2. To learn something about crack propagation in concrete

Charlie Chong/ Fion Zhang


16.7.1 Fracture Mechanics Studies

A number of studies have shown that acoustic emission can be related to

crack growth or fracture mechanics parameters in cements, mortars, and

concretes. Evans et al. showed that acoustic emission could be correlated

with crack velocity in mortars. Morita and Kato and Nadeau, Bennett, and

Mindess were able to relate total acoustic emission counts to Kc (the fracture

toughness). In addition, Lenain and Bunsell found that the number of

emissions could be related to the sixth power of the stress intensity factor, K.

(K 6 ?) Izumi et al. showed that acoustic emissions could also be related to the

strain energy release rate, G. In all cases, however, these correlations are

purely empirical; no one has yet developed a fundamental relationship

between acoustic emission events and fracture parameters, and it is unlikely

that such a relationship exists.

Charlie Chong/ Fion Zhang


16.7.1 Fracture Mechanics Studies

A number of studies have shown that acoustic emission can be related to crack growth or fracture mechanics parameters in

cements, mortars, and concretes. Evans et al. showed that acoustic emission could be correlated with crack velocity in mortars.

Morita and Kato and Nadeau, Bennett, and Mindess were able to relate total acoustic emission counts to Kc (the fracture

toughness). In addition, Lenain and Bunsell found that the number of emissions could be related to the sixth power of the stress

intensity factor, K. (K 6 ?) Izumi et al. showed that acoustic emissions could also be related to the strain energy release rate, G. In

all cases, however, these correlations are purely empirical; no one has yet developed a fundamental relationship between

acoustic emission events and fracture parameters, and it is unlikely that such a relationship exists.

Charlie Chong/ Fion Zhang


16.7.2 Type of Cracks

A number of attempts have been made to relate acoustic events of different

frequencies, or of different energies, to different types of cracking in concrete.

For instance, Saeki et al., by looking at the energy levels of the acoustic

emissions at different levels of loading, concluded that the first stage of

cracking, due to the development of bond cracks between the cement paste

and the aggregate, emitted high energy signals; the second stage, which they

termed “crack arrest,” emitted low energy signals; the final stage, in which

cracks extended through the mortar, was again associated with high energy

acoustic events. Similarly, Tanigawa and Kobayashi used acoustic energies

to distinguish the onset of “the proportional limit, the initiation stress and the

critical stress.” On the other hand, Tanigawa et al. tried to relate the fracture

type (pore closure, tensile cracking, and shear slip) to the power spectra and

frequency components of the acoustic events. The difficulty with these and

similar approaches is that they tried to relate differences in the recorded

acoustic events to preconceived notions 先 入 为 主 的 观 念 of the nature of

cracking in concrete; direct cause and effect relationships were never

observed.

Charlie Chong/ Fion Zhang


16.7.3 Fracture Process Zone (Crack Source) Location

Perhaps the greatest current interest in acoustic emission analysis is its use

in locating fracture processes, and in monitoring the damage that concrete

undergoes as cracks progress. Okada et al. showed that the location of crack

sources obtained from differences in the arrival times of acoustic emissions

was in good agreement with the observed fracture surface. At about the same

time, Chhuy et al. and Lenain and Bunsell were able to determine the length

of the damaged zone ahead of the tip of a propagating crack using onedimensional

acoustic emission location techniques. In subsequent work,

Chhuy et al., using more elaborate equipment and analytical techniques,

were able to determine damage both before the initiation of a visible crack

and after subsequent crack extension. Berthelot and Robert and Rossi used

acoustic emission to monitor concrete damage as well.

Charlie Chong/ Fion Zhang


They found that, while the number of acoustic events showed the progression

of damage both ahead and behind the crack front, this technique alone could

not provide a quantitative description of the cracking. However, using more

elaborate techniques, including amplitude analysis and measurements of

signal duration, Berthelot and Robert concluded that “acoustic emission

testing is practically the only technique which can provide a quantitative

description of the progression in real time of concrete damage within test

specimens.” Later, much more sophisticated signals processing techniques

were applied to acoustic emission analysis.

In 1981, Michaels et al.15 and Niwa et al. developed deconvolution

techniques 反 褶 积 技 术 to analyze acoustic waveforms, in order to provide a

stress-time history of the source of an acoustic event. Similar deconvolution

techniques were subsequently used by Maji and Shah to determine the

volume, orientation and type of microcrack, as well as the source of the

acoustic events. Such sophisticated techniques have the potential eventually

to be used to provide a detailed picture of the fracture processes occurring

within concrete specimens.

Charlie Chong/ Fion Zhang


16.7.4 Strength vs. Acoustic Emission Relationships

Since concrete quality is most frequently characterized by its strength, many

studies have been directed towards determining a relationship between

acoustic emission activity and strength. For instance, Tanigawa and

Kobayashi concluded that “the compressive strength of concrete can be

approximately estimated by the accumulated AE counts at relatively low

stress level.” Indeed, they suggested that acoustic emission techniques might

provide a useful nondestructive test method for concrete strength. Earlier,

Fertis had concluded that acoustic emissions could be used to determine not

only strength, but also static and dynamic material behavior. Rebic, too, found

that there is a relationship between the “critical” load at which the concrete

begins to be damaged, which can be determined from acoustic emission

measurements, and the ultimate strength; thus, acoustic emission analysis

might be used as a predictor of concrete strength.

Charlie Chong/ Fion Zhang


Sadowska-Boczar et al. tried to quantify the strength vs. acoustic emission

relationship using the equation:

Where:

Fr is the rupture strength,

Fp is the stress corresponding to the first acoustic emission signal, and

a and b are constants for a given material and loading conditions.

Using this linear relationship, which they found to fit their data reasonably well,

they suggested that the observation of acoustic emissions at low stresses

would permit an estimation of strength, as well as providing some

characterization of porosity and critical flaw size.

Charlie Chong/ Fion Zhang


Unfortunately, the routine use of

acoustic emissions as an

estimator of strength seems to be

an unlikely prospect, in large part

because of the scatter in the data,

as has been noted by Fertis. As an

example of the scatter in data.

Figure 16.6 indicates the variability

in the strength vs. total acoustic

emission counts relationship; the

within-batch variability is even

more severe, as shown in Figure

16.7.23

Charlie Chong/ Fion Zhang

FIGURE 16.6 Logarithm of total acoustic emission counts vs.

compressive strength of concrete cubes. (From Mindess, S., Int.

J. Cem. Comp. Lightweight Concr., 4, 173, 1982. With

permission.)


FIGURE 16.7 Within-batch variability of total acoustic emission counts vs. applied compressive

stress on concretecubes. (From Mindess, S., Int. J. Cem. Comp. Lightweight Concr., 4, 173,

1982. With permission.)

Charlie Chong/ Fion Zhang


16.7.5 Drying Shrinkage

Acoustic emission has been used to try to monitor shrinkage in cement

pastes and mortars. Nadeau et al. found that, in hardened pastes, the

acoustic emission resulted from cracking due to the unequal shrinkage of the

hydration products. Mortar gave less acoustic emission than hardened paste,

suggesting that the fracture processes at the sand/cement paste interface are

not an important source of acoustic emission. Jeong et al. also suggested that,

in autoclaved aerated concrete, the acoustic emissions during drying could be

related to microcracking. Again, however, it is unlikely that acoustic emission

measurements will be able to be used as a means of predicting the shrinkage

as a function of time.

Charlie Chong/ Fion Zhang


16.7.6 Fiber Reinforced Cements and Concretes

A number of acoustic emission studies have been carried out on fiber

reinforced cements and concretes. Lenain and Bunsell, in a study of asbestos

cement, found that acoustic emissions resulted both from cracking of the

matrix and fiber pullout.

They noted that the Kaiser effect was not found for this type of fiberreinforced

composite, since on unloading of a specimen the partially pulled

out fibers were damaged, and particles of cement attached to them were

crushed, giving rise to acoustic emissions on unloading. Because these

damaged fibers were then less able to resist crack growth, on subsequent

reloading cracks started to propagate at lower stress levels than on the

previous cycle, thus, giving off acoustic emissions below the previously

achieved maximum load.

Akers and Garrett also studied asbestos cement; they found that acoustic

emission monitoring could be used to detect the onset and development of

prefailure cracking.

Charlie Chong/ Fion Zhang


However, they concluded that “there is no basis whatsoever for using

amplitude discrimination in acoustic emission monitoring for distinguishing

between the various failure modes which occur in this material.” On the other

hand, Faninger et al. argued that in fiber-reinforced concrete the amplitude

pattern of the acoustic emission signals did make it possible to distinguish

whether fracture had occurred in the fibers or between them. Similarly, Jeong

et al. stated that acoustic emission frequency analysis could distinguish

between different micro-fracture mechanisms in fiber-reinforced autoclaved

aerated concrete.

Charlie Chong/ Fion Zhang


Fiber Reinforced Cements and Concretes

Charlie Chong/ Fion Zhang


16.7.7 High Alumina Cement

In concretes made with high alumina (calcium aluminate) cement, the

conversion from CAH 10 * to C 3 AH 6 * on prolonged aging can lead to a large

increase in porosity and therefore a large decrease in strength. There has

thus been considerable interest in finding a nondestructive technique to

monitor high alumina cement concrete (HAC) members. Parkinson and

Peters concluded that the conversion process itself is not a source of acoustic

emission activity, since no acoustic emissions were generated during the

accelerated conversion of pastes at the critical w/c ratio of 0.35. However, at

the high w/c ratio of 0.65, conversion was accompanied by a high level of

acoustic emission activity, due to the fracture processes taking place during

conversion, associated perhaps with the liberation of excess water. Arrington

and Evans suggested that the structural integrity of HAC could be evaluated

from the shape of the acoustic emission vs. load plot, the emissions recorded

while the specimens were held under a constant load, and the decay of

emission activity with time.

*Note that cement chemistry notation is being used: C= CaO; A= Al 2 O 3 ; H=

H 2 O.

Charlie Chong/ Fion Zhang


Perhaps the most extensive series of tests on HAC, carried out at the Fulmer

Research Institute in the U.K., was reported by Williams. Apart from

observing that the Kaiser effect existed up to the point at which the beams

cracked, some tentative suggestions were made for monitoring HAC beams

with acoustic emissions:

1. If, on loading a beam, no acoustic emission is noted, then the applied load

is still less than about 60% of the ultimate load; if acoustic emission occurs,

then this percentage of the ultimate load has been exceeded.

If, upon unloading such a beam, further acoustic emission activity is recorded,

then the beam is cracked. The amount of acoustic emission during this

unloading could indicate the degree to which the cracking load had been

exceeded.

Charlie Chong/ Fion Zhang


2. If a beam is under its service load, it would behave similarly on application

of a superimposed load. The presence or absence of acoustic emissions

during this further loading and unloading might indicate the condition of the

beam.

3. If a beam under service load showed no acoustic emission activity during

further loading, but did so at a later date when loaded to the same level, then

the strength must have decreased during that time interval.

As well, Williams noted similar behavior on testing of ordinary prestressed

concrete beams, and suggested that these techniques could be used to

evaluate any type of concrete structure, as long as acoustic emissions not

connected with beam damage could be eliminated.

Charlie Chong/ Fion Zhang


16.7.8 Thermal Cracking

Relatively little work has been carried out on acoustic activity when concrete

is subjected to high temperatures, such as those that may be encountered in

fires. However, Hinrichsmeyher et al. carried out tests up to temperatures of

900°C. They claimed that acoustic emission analysis during heating enabled

them to distinguish the different types of thermally induced cracking that

occurred. They noted a thermal Kaiser effect in the temperature range 300 to

600°C, which might help in determining the maximum temperature reached

in a previous heating cycle. The technique was even sensitive enough to

record the acoustic emissions from the quartz inversion at 573°C.

Charlie Chong/ Fion Zhang


16.7.9 Bond in Reinforced Concrete

A number of acoustic emission studies of debonding of reinforcing bars in

reinforced concrete have been carried out. Kobayashi et al. tested simulated

beam-column connections with a 90° hooked reinforcing bar subjected to

various cyclic loading histories. They found that the penetration of a surface

crack down to the level of the bar gave rise to only one or two acoustic events;

most acoustic emission signals were generated by the internal cracking

around the bar due to fracture at the lugs (ribs) of the bars. Acoustic emission

signals were able to indicate, with reasonable accuracy, the degree of

debonding. They suggested that acoustic emission techniques could be used

to determine the amount of bond deterioration in concrete structures during

proof testing, or due to overloads. In addition, several studies of bond

degradation at elevated temperatures have been carried out. Royles et al.

studied simple pullout specimens at temperatures up to 800°C.

Charlie Chong/ Fion Zhang


They found that acoustic emissions were associated with the adhesive failure

at the steel-concrete interface, followed by local crushing under the ribs of the

reinforcing bars. They suggested that acoustic emissions could be used to

identify the point of critical slip. In further work, Royles and Morley suggested

that acoustic emission techniques might be useful in estimating the quality of

the bond in reinforced concrete structures that had been subjected to fires.

Charlie Chong/ Fion Zhang


16.7.10 Corrosion of Reinforcing Steel in Concrete

The deterioration of concrete due to corrosion of the reinforcing steel is a

major problem, which is usually detected only after extreme cracking has

already taken place. Weng et al. found that measurable levels of acoustic

emission occurred even during the corrosion of unstressed reinforced

concrete. They suggested that, at least in the laboratory, acoustic emission

monitoring would assist in characterizing corrosion damage. In subsequent

work, Dunn et al. developed a relationship between the observed damage

and the resulting acoustic emissions. Damage could be detected in its early

stages, and by a combination of total counts and amplitude measurements,

the nature of the corrosion damage could be determined.

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Corrosion of Reinforcing Steel in Concrete

Charlie Chong/ Fion Zhang


Corrosion of Reinforcing Steel in Concrete

Charlie Chong/ Fion Zhang


16.8 Field Studies of Acoustic Emission

As shown in the previous section, acoustic emission analysis has been used

in the laboratory to study a wide range of problems. Unfortunately, its use in

the field has been severely limited; only a very few papers on field application

have appeared, and these are largely speculation on future possibilities. The

way in which acoustic emission data might be used to provide information

about the condition of a specimen or a structure has been described by

Cole; his analysis may be summarized as follows:

1. Is there any acoustic emission at a certain load level? If no, then no

damage is occurring under these conditions; if yes, then damage is

occurring.

2. Is acoustic emission continuing while the load is held constant at the

maximum load level? If no, no damage due to creep is occurring; if yes,

creep damage is occurring. Further, if the count rate is increasing, then

failure may occur fairly soon.

Charlie Chong/ Fion Zhang


3. Have high amplitude acoustic emissions events occurred? If no, individual

fracture events have been relatively minor; if yes, major fracture events

have occurred.

4. Does acoustic emission occur if the structure has been unloaded and is

then reloaded to the previous maximum load? If no, there is no damage or

crack propagation under low cycle fatigue; if yes, internal damage exists

and the damage sites continue to spread even under low loads.

5. Does the acoustic emission occur only from a particular area? If no, the

entire structure is being damaged; if yes, the damage is localized.

6. Is the acoustic emission in a local area very localized? if no, damage is

dispersed over a significant area; if yes, there is a highly localized stress

concentration causing the damage.

Charlie Chong/ Fion Zhang


16.9 Conclusions

From the discussion above, it appears that acoustic emission techniques may

be very useful in the laboratory to supplement other measurements of

concrete properties. However, their use in the field remains problematic.

Many of the earlier studies held out high hopes for acoustic emission

monitoring of structures. For instance, McCabe et al. suggested that, if a

structure was loaded, the absence of acoustic emissions would indicate that it

was safe under the existing load conditions; a low level of acoustic emissions

would indicate that the structure should be monitored carefully, while a high

level of acoustic emission could indicate that the structure was unsafe. But

this is hardly a satisfactory approach, since it does not provide any help with

quantitative analysis. In any event, even the sophisticated (and expensive)

equipment now available still provides uncertain results when applied to

structures, because of our lack of knowledge about the characteristics of

acoustic emissions due to different causes, and because of the possibility of

extraneous noise (vibration, loading devices, and so on).

Charlie Chong/ Fion Zhang


Another serious drawback is that acoustic emissions are only generated

when the loads on a structure are increased, and this poses considerable

practical problems. Thus, one must still conclude, with regret, that “acoustic

emission analysis has not yet been well developed as a technique for the

evaluation of phenomena taking place in concrete in structures.”

Charlie Chong/ Fion Zhang


Concrete Structures

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Concrete Structures

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Concrete Structures

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Concrete Structures- The Troll A platform

Charlie Chong/ Fion Zhang


Concrete Structures- The Troll A platform

Charlie Chong/ Fion Zhang


Concrete Structures- The Troll A platform

Charlie Chong/ Fion Zhang


Concrete Structures- The Troll A platform

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Concrete Structures- Draugen

Charlie Chong/ Fion Zhang


End of Reading 2

Charlie Chong/ Fion Zhang


Study Note 3:

Introduction to Acoustic Emission Testing

http://www.ndt-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_Intro.htm

Charlie Chong/ Fion Zhang


1.0 Introduction

Acoustic Emission (AE) refers to the generation of transient elastic waves

produced by a sudden redistribution of stress in a material. When a structure

is subjected to an external stimulus (change in pressure, load, or

temperature), localized sources trigger the release of energy, in the form of

stress waves, which propagate to the surface and are recorded by sensors.

With the right equipment and setup, motions on the order of picometers

(10 -12 m) can be identified. Sources of AE vary from natural events like:

1. earthquakes and rock bursts to

2. the initiation and growth of cracks,

3. slip and dislocation movements,

4. melting,

5. twinning, and

6. phase transformations

in metals. In composites, matrix cracking and fiber breakage and de-bonding

contribute to acoustic emissions.

Charlie Chong/ Fion Zhang


AE’s have also been measured and recorded in polymers, wood, and

concrete, among other materials. Detection and analysis of AE signals can

supply valuable information regarding the origin and importance of a

discontinuity in a material. Because of the versatility of Acoustic Emission

Testing (AET),

It has many industrial applications e.g.

1. assessing structural integrity,

2. detecting flaws,

3. testing for leaks, or

4. monitoring weld quality and

5. is used extensively as a research tool.

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Twinning

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AET

Charlie Chong/ Fion Zhang


Acoustic Emission is unlike most other nondestructive testing (NDT)

techniques in two regards. The first difference pertains to the origin of the

signal. Instead of supplying energy to the object under examination, AET

simply listens for the energy released by the object. AE tests are often

performed on structures while in operation, as this provides adequate loading

for propagating defects and triggering acoustic emissions.

The second difference is that AET deals with dynamic processes, or changes,

in a material. This is particularly meaningful because only active features (e.g.

crack growth) are highlighted. The ability to discern between developing and

stagnant defects is significant. However, it is possible for flaws to go

undetected altogether if the loading is not high enough to cause an acoustic

event.

Furthermore, AE testing usually provides an immediate indication relating to

the strength or risk of failure of a component. Other advantages of AET

include fast and complete volumetric inspection using multiple sensors,

permanent sensor mounting for process control, and no need to disassemble

and clean a specimen.

Charlie Chong/ Fion Zhang


Unfortunately, AE systems can only qualitatively gauge how much damage is

contained in a structure. In order to obtain quantitative results about size,

depth, and overall acceptability of a part, other NDT methods (often ultrasonic

testing) are necessary. Another drawback of AE stems 逆 from loud service

environments which contribute extraneous noise to the signals. For

successful applications, signal discrimination and noise reduction are crucial.

Charlie Chong/ Fion Zhang


2.0 A Brief History of AE Testing

Although acoustic emissions can be created in a controlled environment, they

can also occur naturally. Therefore, as a means of quality control, the origin of

AE is hard to pinpoint. As early as 6,500 BC, potters were known to listen for

audible sounds during the cooling of their ceramics, signifying structural

failure. In metal working, the term "tin cry" (audible emissions produced by the

mechanical twinning of pure tin during plastic deformation) was coined

around 3,700 BC by tin smelters in Asia Minor. The first documented

observations of AE appear to have been made in the 8th century by Arabian

alchemist Jabir ibn Hayyan. In a book, Hayyan wrote that Jupiter (tin) gives

off a ‘harsh sound’ when worked, while Mars (iron) ‘sounds much’ during

forging. Many texts in the late 19th century referred to the audible emissions

made by materials such as tin, iron, cadmium and zinc. One noteworthy

correlation between different metals and their acoustic emissions came from

Czochralski, who witnessed the relationship between tin and zinc cry and

twinning. Later, Albert Portevin and Francois Le Chatelier observed AE

emissions from a stressed Al-Cu-Mn (Aluminum-Copper-Manganese) alloy.

Charlie Chong/ Fion Zhang


The next 20 years brought further verification with the work of Robert

Anderson (tensile testing of an aluminum alloy beyond its yield point), Erich

Scheil (linked the formation of martensite in steel to audible noise), and

Friedrich Forster, who with Scheil related an audible noise to the formation of

martensite in high-nickel steel. Experimentation continued throughout the

mid-1900’s, culminating in the PhD thesis written by Joseph Kaiser entitled

"Results and Conclusions from Measurements of Sound in Metallic Materials

under Tensile Stress.” Soon after becoming aware of Kaiser’s efforts,

Bradford Schofield initiated the first research program in the United States to

look at the materials engineering applications of AE. Fittingly, Kaiser’s

research is generally recognized as the beginning of modern day acoustic

emission testing.

Charlie Chong/ Fion Zhang


3.0 Theory - AE Sources

As mentioned in the Introduction, acoustic emissions can result from the

initiation and growth of cracks, slip and dislocation movements, twinning, or

phase transformations in metals. In any case, AE’s originate with stress.

When a stress is exerted on a material, a strain is induced in the material as

well. Depending on the magnitude of the stress and the properties of the

material, an object may return to its original dimensions or be permanently

deformed after the stress is removed. These two conditions are known as

elastic and plastic deformation, respectively.

The most detectible acoustic emissions take place when a loaded material

undergoes plastic deformation or when a material is loaded at or near its yield

stress. On the microscopic level, as plastic deformation occurs, atomic planes

slip past each other through the movement of dislocations. These atomicscale

deformations release energy in the form of elastic waves which “can be

thought of as naturally generated “ultrasound” traveling through the object.

Charlie Chong/ Fion Zhang


Crack: When cracks exist in a metal, the stress levels present in front of the

crack tip can be several times higher than the surrounding area. Therefore,

AE activity will also be observed when the material ahead of the crack tip

undergoes plastic deformation (micro-yielding).

Fatigue Crack: Two sources of fatigue cracks also cause AE’s.

■ The first source is emissive particles (e.g. nonmetallic inclusions) at the

origin of the crack tip. Since these particles are less ductile than the

surrounding material, they tend to break more easily when the metal is

strained, resulting in an AE signal.

■ The second source is the propagation of the crack tip that occurs through

the movement of dislocations and small-scale cleavage produced by triaxial

stresses.

Charlie Chong/ Fion Zhang


The amount of energy released by an acoustic emission and the amplitude of

the waveform are related to the magnitude and velocity of the source event.

AE Amplitude: The amplitude of the emission is proportional (∝) to the

(a) velocity of crack propagation and the (b) amount of surface area created.

Large, discrete crack jumps will produce larger AE signals than cracks that

propagate slowly over the same distance.

Detection and conversion of these elastic waves to electrical signals is the

basis of AE testing. Analysis of these signals yield valuable information

regarding the origin and importance of a discontinuity in a material. As

discussed in the following section, specialized equipment is necessary to

detect the wave energy and decipher which signals are meaningful.

Charlie Chong/ Fion Zhang


Charlie Chong/ Fion Zhang

http://www.nature.com/nmat/journal/v10/n11/full/nmat3167.html


Activity of AE Sources in Structural Loading

AE signals generated under different loading patterns can provide valuable

information concerning the structural integrity of a material. Load levels that

have been previously exerted on a material do not produce AE activity. In

other words, discontinuities created in a material do not expand or move until

that former stress is exceeded. This phenomenon, known as the Kaiser Effect,

can be seen in the load versus AE plot to the right. As the object is loaded,

acoustic emission events accumulate (segment AB). When the load is

removed and reapplied (segment BCB), AE events do not occur again until

the load at point B is exceeded. As the load exerted on the material is

increased again (BD), AE’s are generated and stop when the load is removed.

However, at point F, the applied load is high enough to cause significant

emissions even though the previous maximum load (D) was not reached.

This phenomenon is known as the Felicity Effect. This effect can be

quantified using the Felicity Ratio, which is the load where considerable AE

resumes, divided by the maximum applied load (F/D).

Charlie Chong/ Fion Zhang


Kaiser Effect:

Load levels that have been previously exerted on a material do not produce

AE activity. This phenomenon, known as the Kaiser Effect

Felicity Effect:

The applied load is high enough to cause significant emissions even though

the previous maximum load was not reached. This phenomenon is known as

the Felicity Effect.

Felicity Ratio:

Felicity Ratio, which is the load where considerable AE resumes, divided by

the previous maximum applied load (F/D).

Charlie Chong/ Fion Zhang


Kaiser/Felicity effects

Felicity effect

Felicity ratio = F/D

Kaiser effect

Charlie Chong/ Fion Zhang


Knowledge of the Kaiser Effect and Felicity Effect can be used to determine if

major structural defects are present. This can be achieved by applying

constant loads (relative to the design loads exerted on the material) and

“listening” to see if emissions continue to occur while the load is held. As

shown in the figure, if AE signals continue to be detected during the holding

of these loads (GH), it is likely that substantial structural defects are present.

In addition, a material may contain critical defects if an identical load is

reapplied and AE signals continue to be detected. Another guideline

governing AE’s is the Dunegan corollary, which states that if acoustic

emissions are observed prior to a previous maximum load, some type of new

damage must have occurred. (Note: Time dependent processes like corrosion

and hydrogen embrittlement tend to render the Kaiser Effect useless)

Dict:

Corollary: something that results from something else.

Charlie Chong/ Fion Zhang


Dunegan corollary

states that if acoustic emissions are observed prior to a previous maximum

load, some type of new damage must have occurred. (Note: Time dependent

processes like corrosion and hydrogen embrittlement tend to render the

Kaiser Effect useless)

Charlie Chong/ Fion Zhang


Q. What is the Dunegan Corollary?

a. It states that if acoustic emissions are observed prior to a previous

maximum load, some type of new damage must have occurred.

b. When the applied load is high enough to cause significant emissions even

though the previous maximum load was not reached.

c. Gauging signal arrival times or differences in the spectral content of true

AE signals and background noise.

d. the number of times a signal crosses a preset threshold

Corollary: is a statement that follows readily from a previous statement.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUXJ

QZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Comments:

Emissions are observed prior to a previous maximum load;

• Felicity effect, (when the applied load is high enough)

• Dunegan corollary, (when the load is less than the preceding load)

Keywords:

• Kaiser effect,

• Felicity effect,

• Dunegan corollary

Charlie Chong/ Fion Zhang


Noise

The sensitivity of an acoustic emission system is often limited by the amount

of background noise nearby. Noise in AE testing refers to any undesirable

signals detected by the sensors. Examples of these signals include frictional

sources (e.g. loose bolts or movable connectors that shift when exposed to

wind loads) and impact sources (e.g. rain, flying objects or wind-driven dust)

in bridges. Sources of noise may also be present in applications where the

area being tested may be disturbed by mechanical vibrations (e.g. pumps).

To compensate for the effects of background noise, various procedures can

be implemented. Some possible approaches involve fabricating special

sensors with electronic gates for noise blocking, taking precautions to place

sensors as far away as possible from noise sources, and electronic filtering

(either using signal arrival times or differences in the spectral content of true

AE signals and background noise).

Comments:

■ Spectral filtering

■ Time of flight filtering

■ Placement

■ Sensor with electronic gate?


Pseudo Sources

In addition to the AE source mechanisms described above, pseudo source

mechanisms produce AE signals that are detected by AE equipment.

Examples include liquefaction and solidification, friction in rotating bearings,

solid-solid phase transformations, leaks, cavitation, and the realignment or

growth of magnetic domains (See Barkhausen Effect).

Comments:

Noise ≡ Pseudo Sources?

Charlie Chong/ Fion Zhang


Barkhausen Effect

The Barkhausen effect is a name given to the noise in the magnetic output of a ferromagnet when the

magnetizing force applied to it is changed. Discovered by German physicist Heinrich Barkhausen in 1919, it is

caused by rapid changes of size of magnetic domains (similarly magnetically oriented atoms in ferromagnetic

materials). Barkhausen's work in acoustics and magnetism led to the discovery, which provided evidence that

magnetization affects whole domains of a ferromagnetic material, rather than individual atoms alone. The

Barkhausen effect is a series of sudden changes in the size and orientation of ferromagnetic domains, or

microscopic clusters of aligned atomic magnets (spins), that occurs during a continuous process of

magnetization or demagnetization. The Barkhausen effect offered direct evidence for the existence of

ferromagnetic domains, which previously had been postulated theoretically. Heinrich Barkhausen discovered

that a slow, smooth increase of a magnetic field applied to a piece of ferromagnetic material, such as iron,

causes it to become magnetized, not continuously but in minute steps.

Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Barkhausen_effect


Wave Propagation

A primitive wave released at the AE source

is illustrated in the figure right. The

displacement waveform is a step-like

function corresponding to the permanent

change associated with the source process.

The analogous velocity and stress

waveforms are essentially pulse-like. The

width and height of the primitive pulse

depend on the dynamics of the source

process. Source processes such as

microscopic crack jumps and precipitate

fractures are usually completed in a fraction

of a microsecond or a few microseconds,

which explains why the pulse is short in

duration. The amplitude and energy of the

primitive pulse vary over an enormous range

from submicroscopic dislocation movements

to gross crack jumps.

Charlie Chong/ Fion Zhang


Primitive AE wave

released at a source. The

primitive wave is

essentially a stress pulse

corresponding to a

permanent displacement

of the material. The

ordinate quantities refer to

a point in the material.

Charlie Chong/ Fion Zhang


Waves radiates from the

source in all directions, often

having a strong directionality

depending on the nature of the

source process, as shown in

the second figure. Rapid

movement is necessary if a

sizeable amount of the elastic

energy liberated during

deformation is to appear as an

acoustic emission.

Angular dependence of acoustic emission radiated from a growing

microcrack. Most of the energy is directed in the 90 and 270 o directions,

perpendicular to the crack surfaces.

Charlie Chong/ Fion Zhang


Most of the energy is directed

in the 90º and 270º directions,

perpendicular to the crack

surfaces.

90º

270º

Charlie Chong/ Fion Zhang


Angular dependence of acoustic emission radiated from a growing

microcrack. Most of the energy is directed in the 90 and 270 o directions,

perpendicular to the crack surfaces.


As these primitive waves travel through a material, their form is changed

considerably. Elastic wave source and elastic wave motion theories are being

investigated to determine the complicated relationship between the AE

source pulse and the corresponding movement at the detection site. The

ultimate goal of studies of the interaction between elastic waves and material

structure is to accurately develop a description of the source event from the

output signal of a distant sensor.

However, most materials-oriented researchers and NDT inspectors are not

concerned with the intricate knowledge of each source event. Instead, they

are primarily interested in the broader, statistical aspects of AE. Because of

this, they prefer to use narrow band (resonant) sensors which detect only a

small portion of the broadband of frequencies emitted by an AE. These

sensors are capable of measuring hundreds of signals each second, in

contrast to the more expensive high-fidelity sensors used in source function

analysis. More information on sensors will be discussed later in the

Equipment section.

Charlie Chong/ Fion Zhang


The signal that is detected by a sensor is a combination of many parts of the

waveform initially emitted. Acoustic emission source motion is completed in a

few millionths of a second. As the AE leaves the source, the waveform travels

in a spherically spreading pattern and is reflected off the boundaries of the

object. Signals that are in phase with each other as they reach the sensor

produce constructive interference which usually results in the highest peak of

the waveform being detected. The typical time interval from when an AE wave

reflects around the test piece (repeatedly exciting the sensor) until it decays,

ranges from the order of 100 microseconds in a highly damped, nonmetallic

material to tens of milliseconds in a lightly damped metallic material.

Decay Time:

highly damped (intrinsic) , nonmetallic material → order of 100 microseconds

(10 -6 s)

lightly damped metallic material → tens of milliseconds (10 -3 s)

Charlie Chong/ Fion Zhang


Decay time

Decay Time:

highly damped, nonmetallic material → order of 100 microseconds (s -6 )

lightly damped metallic material → tens of milliseconds (s -3 )

Charlie Chong/ Fion Zhang


highly damped, nonmetallic

material ~.0001 s

lightly damped metallic

material, ~.001 s.

Decay time

Decay Time:

highly damped, nonmetallic material → order of 100 microseconds (10 -6 s)

lightly damped metallic material → tens of milliseconds (10 -3 s)

Charlie Chong/ Fion Zhang


Attenuation

The intensity of an AE signal detected by a sensor is considerably lower than

the intensity that would have been observed in the close proximity of the

source. This is due to attenuation.

There are three main causes of attenuation,

(1) beginning with geometric spreading. As an AE spreads from its source in

a plate-like material, its amplitude decays by 30% every time it doubles its

distance from the source. In three-dimensional structures, the signal decays

on the order of 50%. This can be traced back to the simple conservation of

energy.

(2) Another cause of attenuation is material damping, as alluded 指 出 to in the

previous paragraph. While an AE wave passes through a material, its elastic

and kinetic energies are absorbed and converted into heat. (σ abs )

(3) The third cause of attenuation is wave scattering. Geometric

discontinuities (e.g. twin boundaries, nonmetallic inclusions, or grain

boundaries) and structural boundaries both reflect some of the wave energy

that was initially transmitted. (σ scat )

Charlie Chong/ Fion Zhang


Attenuation:

1. Spread (30% for 2D, 50% for 3D for each doubling of distance from

source),

2. Material damping, absorption.

3. Scattering (reflection & difrraction)

3

1

2

3

Charlie Chong/ Fion Zhang


Measurements of the effects of attenuation on an AE signal can be performed

with a simple apparatus known as a Hsu-Nielson Source. This consists of a

mechanical pencil with either 0.3 or 0.5 mm 2H lead that is passed through a

cone-shaped Teflon shoe designed to place the lead in contact with the

surface of a material at a 30 degree angle. When the pencil lead is pressed

and broken against the material, it creates a small, local deformation that is

relieved in the form of a stress wave, similar to the type of AE signal produced

by a crack. By using this method, simulated AE sources can be created at

various sites on a structure to determine the optimal position for the

placement of sensors and to ensure that all areas of interest are within the

detection range of the sensor or sensors.

Charlie Chong/ Fion Zhang


Teflon shoe

http://www.ndt.net/ndtaz/content.php?id=474


Wave Mode and Velocity

As mentioned earlier, using AE inspection in conjunction with other NDE

techniques can be an effective method in gauging the location and nature of

defects. Since source locations are determined by the time required for the

wave to travel through the material to a sensor, it is important that the velocity

of the propagating waves be accurately calculated. This is not an easy task

since wave propagation depends on the material in question and the wave

mode being detected. For many applications, Lamb waves are of primary

concern because they are able to give the best indication of wave

propagation from a source whose distance from the sensor is larger than the

thickness of the material. For additional information on Lamb waves, see the

wave mode page in the Ultrasonic Inspection section.

Charlie Chong/ Fion Zhang


Lamb waves in acoustic emission testing

Acoustic emission uses much lower frequencies than traditional ultrasonic

testing, and the sensor is typically expected to detect active flaws at distances

up to several meters. A large fraction of the structures customarily testing with

acoustic emission are fabricated from steel plate - tanks, pressure vessels,

pipes and so on. Lamb wave theory is therefore the prime theory for

explaining the signal forms and propagation velocities that are observed

when conducting acoustic emission testing. Substantial improvements in the

accuracy of AE source location (a major techniques of AE testing) can be

achieved through good understanding and skillful utilization of the Lamb wave

body of knowledge.

Charlie Chong/ Fion Zhang


Ultrasonic and acoustic emission testing contrasted

An arbitrary mechanical excitation applied to a plate will generate a

multiplicity of Lamb waves carrying energy across a range of frequencies.

Such is the case for the acoustic emission wave.

In acoustic emission testing, the challenge is to recognize the multiple Lamb

wave components in the received waveform and to interpret them in terms of

source motion.

This contrasts with the situation in ultrasonic testing, where the first challenge

is to generate a single, well-controlled Lamb wave mode at a single frequency.

But even in ultrasonic testing, mode conversion takes place when the

generated Lamb wave interacts with flaws, so the interpretation of reflected

signals compounded from multiple modes becomes a means of flaw

characterization.

Plate or Lamb waves are similar to surface waves except they can only be

generated in materials a few wavelengths thick.

Charlie Chong/ Fion Zhang


2.2.5 Rayleigh Characteristics

Rayleigh waves are a type of surface wave that travel near the surface of

solids. Rayleigh waves include both longitudinal and transverse motions that

decrease exponentially in amplitude as distance from the surface increases.

There is a phase difference between these component motions. In isotropic

solids these waves cause the surface particles to move in ellipses in planes

normal to the surface and parallel to the direction of propagation – the major

axis of the ellipse is vertical. At the surface and at shallow depths this motion

is retrograde 逆 行 , that is the in-plane motion of a particle is counterclockwise

when the wave travels from left to right.

http://en.wikipedia.org/wiki/Rayleigh_wave

Charlie Chong/ Fion Zhang


Rayleigh waves are a type of surface acoustic wave that travel on solids.

They can be produced in materials in many ways, such as by a localized

impact or by piezo-electric transduction, and are frequently used in nondestructive

testing for detecting defects. They are part of the seismic waves

that are produced on the Earth by earthquakes. When guided in layers they

are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized

Rayleigh waves.

Charlie Chong/ Fion Zhang


Q29: The longitudinal wave incident angle which results in formation of a

Rayleigh wave is called:

A. Normal incidence

B. The first critical angle

C. The second critical angle

D. Any angle above the first critical angle

Charlie Chong/ Fion Zhang


Surface (or Rayleigh) waves travel the surface of a relatively thick solid

material penetrating to a depth of one wavelength.

Surface waves combine both (1) a longitudinal and (2) transverse motion to

create an elliptic orbit motion as shown in the image and animation below.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/rayleigh.swf

Charlie Chong/ Fion Zhang


The major axis of the ellipse is perpendicular to the surface of the solid. As

the depth of an individual atom from the surface increases the width of its

elliptical motion decreases. Surface waves are generated when a

longitudinal wave intersects a surface near the second critical angle and

they travel at a velocity between .87 and .95 of a shear wave. Rayleigh

waves are useful because they are very sensitive to surface defects (and

other surface features) and they follow the surface around curves.

Because of this, Rayleigh waves can be used to inspect areas that other

waves might have difficulty reaching.

Wave velocity:

• Longitudinal wave velocity =1v,

• The velocity of shear waves through a material is approximately half that

of the longitudinal waves, (≈0.5v)

• Surface waves are generated when a longitudinal wave intersects a

surface near the second critical angle and they travel at a velocity

between .87 and .95 of a shear wave. ≈(0.87~0.95)x0.5v

Charlie Chong/ Fion Zhang


The major axis of the ellipse is perpendicular to the surface of the solid.

Charlie Chong/ Fion Zhang


Surface wave

Charlie Chong/ Fion Zhang


Surface wave or Rayleigh wave are formed when shear waves refract to 90.

The whip-like particle vibration of the shear wave is converted into elliptical

motion by the particle changing direction at the interface with the surface. The

wave are not often used in industrial NDT although they do have some

application in aerospace industry. Their mode of propagation is elliptical along

the surface of material, penetrating to a depth of one wavelength. They will

follow the contour of the surface and they travel at approximately 90% of the

velocity of the shear waves.

Depth of penetration of

about one wavelength

Direction of wave propagation

Charlie Chong/ Fion Zhang


Surface wave has the ability to follow surface contour, until it meet a sharp

change i.e. a surface crack/seam/lap. However the surface waves could be

easily completely absorbed by excess couplant of simply touching the part

ahead of the waves.

Transducer

Wedge

Surface discontinuity

Specimen

Charlie Chong/ Fion Zhang


Surface wave - Following Contour

Surface wave

Charlie Chong/ Fion Zhang


Surface wave – One wavelength deep

λ

λ

Charlie Chong/ Fion Zhang


Rayleigh Wave

Charlie Chong/ Fion Zhang

http://web.ics.purdue.edu/~braile/edumod/waves/Rwave_files/image001.gif


Rayleigh Wave

Charlie Chong/ Fion Zhang


Love Wave

Charlie Chong/ Fion Zhang

http://web.ics.purdue.edu/~braile/edumod/waves/Lwave_files/image001.gif


Love Wave

Charlie Chong/ Fion Zhang


Surface (Rayleigh) waves are not as common as the longitudinal and shear

waves, but are used to great advantage in a limited number of applications

that require an ability of the wave to follow the contours of irregularly shaped

surfaces such as jet engine blades and vanes.

Rayleigh waves extend from the surface to a depth of about one wavelength

into the material and thus are only sensitive to surface or very near-surface

flaws.

They are very sensitive to surface conditions including the presence of

residual coupling compounds as well as finger damping.

Rayleigh waves are usually generated by mode conversion using angle beam

search units designed to produce shear waves just beyond the second critical

angle.

Charlie Chong/ Fion Zhang


Other Reading: Rayleigh Waves

Surface waves (Rayleigh waves) are another type of ultrasonic wave used in

the inspection of materials. These waves travel along the flat or curved

surface of relatively thick solid parts. For the propagation of waves of this type,

the waves must be traveling along an interface bounded on one side by the

strong elastic forces of a solid and on the other side by the practically

negligible elastic forces between gas molecules. Surface waves leak energy

into liquid couplants and do not exist for any significant distance along the

surface of a solid immersed in a liquid, unless the liquid covers the solid

surface only as a very thin film. Surface waves are subject to attenuation in a

given material, as are longitudinal or transverse waves. They have a velocity

approximately 90% of the transverse wave velocity in the same material. The

region within which these waves propagate with effective energy is not much

thicker than about one wavelength beneath the surface of the metal.

Charlie Chong/ Fion Zhang


At this depth, wave energy is about 4% of the wave energy at the surface,

and the amplitude of oscillation decreases sharply to a negligible value at

greater depths. Surface waves follow contoured surfaces. For example,

surface waves traveling on the top surface of a metal block are reflected from

a sharp edge, but if the edge is rounded off, the waves continue down the

side face and are reflected at the lower edge, returning to the sending point.

Surface waves will travel completely around a cube if all edges of the cube

are rounded off. Surface waves can be used to inspect parts that have

complex contours.

Charlie Chong/ Fion Zhang


Q110: What kind of wave mode travel at a velocity slightly below the shear

wave and their modes of propagation are both longitudinal and transverse

with respect to the surface?

a) Rayleigh wave

b) Transverse wave

c) L-wave

d) Longitudinal wave

Charlie Chong/ Fion Zhang


Q: Which of the following modes of vibration exhibits the shortest wavelength

at a given frequency and in a given material?

A. longitudinal wave

B. compression wave

C. shear wave

D. surface wave

Charlie Chong/ Fion Zhang


Q192: Surface waves are reduced to an energy level of approcimately 1/25 of

the original power at a depth of ?

A. 25mm

B. 102mm

C. 1 wavelength

D. 4 wavelength

Charlie Chong/ Fion Zhang


2.2.6 Lamb Wave:

Lamb waves propagate in solid plates. They are elastic waves whose

particle motion lies in the plane that contains the direction of wave

propagation and the plate normal (the direction perpendicular to the plate). In

1917, the english mathematician horace lamb published his classic analysis

and description of acoustic waves of this type. Their properties turned out to

be quite complex. An infinite medium supports just two wave modes traveling

at unique velocities; but plates support two infinite sets of lamb wave modes,

whose velocities depend on the relationship between wavelength and plate

thickness.

Charlie Chong/ Fion Zhang


Since the 1990s, the understanding and utilization of lamb waves has

advanced greatly, thanks to the rapid increase in the availability of computing

power. Lamb's theoretical formulations have found substantial practical

application, especially in the field of nondestructive testing.

The term rayleigh–lamb waves embraces the rayleigh wave, a type of wave

that propagates along a single surface. Both rayleigh and lamb waves are

constrained by the elastic properties of the surface(s) that guide them.

http://en.wikipedia.org/wiki/Lamb_wave

http://pediaview.com/openpedia/Lamb_waves

Charlie Chong/ Fion Zhang


Types of Wave

New!

• Plate wave- Love

• Stoneley wave

• Sezawa

Charlie Chong/ Fion Zhang


Plate or Lamb waves are the most commonly used plate waves in

NDT. Lamb waves are complex vibrational waves that propagate parallel to

the test surface throughout the thickness of the material. Propagation of Lamb

waves depends on the density and the elastic material properties of a

component. They are also influenced a great deal by the test frequency and

material thickness. Lamb waves are generated at an incident angle in which

the parallel component of the velocity of the wave in the source is equal to the

velocity of the wave in the test material. Lamb waves will travel several

meters in steel and so are useful to scan plate, wire, and tubes.

Lamb wave influenced by: (Dispersive Wave)





Density

Elastic material properties

Frequencies

Material thickness

Charlie Chong/ Fion Zhang


Plate or Lamb waves are similar to surface waves except they can only be

generated in materials a few wavelengths thick.

Charlie Chong/ Fion Zhang

http://www.ndt.net/ndtaz/files/lamb_a.gif


Plate wave or Lamb wave are formed by the introduction of surface wave

into a thin material. They are a combination of (1) compression and surface or

(2) shear and surface waves causing the plate material to flex by totally

saturating the material. The two types of plate waves:

Charlie Chong/ Fion Zhang


With Lamb waves, a number of modes of particle vibration are possible, but

the two most common are symmetrical and asymmetrical. The complex

motion of the particles is similar to the elliptical orbits for surface

waves. Symmetrical Lamb waves move in a symmetrical fashion about the

median plane of the plate. This is sometimes called the extensional mode

because the wave is “stretching and compressing” the plate in the wave

motion direction. Wave motion in the symmetrical mode is most efficiently

produced when the exciting force is parallel to the plate. The asymmetrical

Lamb wave mode is often called the “flexural mode” because a large portion

of the motion moves in a normal direction to the plate, and a little motion

occurs in the direction parallel to the plate. In this mode, the body of the plate

bends as the two surfaces move in the same direction.

The generation of waves using both piezoelectric transducers and

electromagnetic acoustic transducers (EMATs) are discussed in later sections.

Keywords:

Symmetrical = extensional mode

Asymmetrical = flexural mode

Charlie Chong/ Fion Zhang


When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb

waves, or generalized Rayleigh waves.

Lamb waves – 2 modes

Charlie Chong/ Fion Zhang


Symmetrical = extensional mode

Asymmetrical = flexural mode

Charlie Chong/ Fion Zhang


Symmetrical = extensional mode

Asymmetrical = flexural mode

Charlie Chong/ Fion Zhang


Symmetrical = extensional mode

Charlie Chong/ Fion Zhang


Other Reading: Lamb Wave

Lamb waves, also known as plate waves, are another type of ultrasonic wave

used in the nondestructive inspection of materials. Lamb waves are

propagated in plates (made of composites or metals) only a few wavelengths

thick. A Lamb wave consists of a complex vibration that occurs throughout the

thickness of the material. The propagation characteristics of Lamb waves

depend on the density, elastic properties, and structure of the material as well

as the thickness of the test piece and the frequency. Their behavior in general

resembles that observed in the transmission of electromagnetic waves

through waveguides.

There are two basic forms of Lamb waves:

• Symmetrical, or dilatational

• Asymmetrical, or bending

Charlie Chong/ Fion Zhang


The form is determined by whether the particle motion is symmetrical or

asymmetrical with respect to the neutral axis of the test piece. Each form is

further subdivided into several modes having different velocities, which can

be controlled by the angle at which the waves enter the test piece.

Theoretically, there are an infinite number of specific velocities at which Lamb

waves can travel in a given material. Within a given plate, the specific

velocities for Lamb waves are complex functions of plate thickness and

frequency.

In symmetrical (dilatational) Lamb waves, there is a compressional

(longitudinal) particle displacement along the neutral axis of the plate and an

elliptical particle displacement on each surface (Fig. 4a). In asymmetrical

(bending) Lamb waves, there is a shear (transverse) particle displacement

along the neutral axis of the plate and an elliptical particle displacement on

each surface (Fig. 4b). The ratio of the major to minor axes of the ellipse is a

function of the material in which the wave is being propagated.

Charlie Chong/ Fion Zhang


Fig. 4 Diagram of the basic patterns of (a) symmetrical (dilatational) and (b)

asymmetrical (bending) Lamb waves. The wavelength, , is the distance

corresponding to one complete cycle.

Charlie Chong/ Fion Zhang


Q1: The wave mode that has multiple or varying wave velocities is:

A. Longitudinal waves

B. Shear waves

C. Transverse waves

D. Lamb waves

Charlie Chong/ Fion Zhang


2.2.7 Dispersive Wave:

Wave modes such as those found in Lamb wave have a velocity of

propagation dependent upon the operating frequency, sample thickness and

elastic moduli. They are dispersive (velocity change with frequency) in that

pulses transmitted in these mode tend to become stretched or dispersed.

Charlie Chong/ Fion Zhang


Dispersion refers to the fact that in a real medium such as water, air, or glass,

a wave traveling through that medium will have a velocity that depends upon

its frequency. Dispersion occurs for any form of wave, acoustic,

electromagnetic, electronic, even quantum mechanical. Dispersion is

responsible for a prism being able to resolve light into colors and defines the

maximum frequency of broadband pulses one can send down an optical fiber

or through a copper wire. Dispersion affects wave and swell forecasts at

sea and influences the design of sound equipment. Dispersion is a physical

property of the medium and can combine with other properties to yield very

strange results. For example, in the propagation of light in an optical fiber, the

glass introduces dispersion and separates the wavelengths of light according

to frequency, however if the light is intense enough, it can interact with the

electrons in the material changing its refractive index. The combination of

dispersion and index change can cancel each other leading to a wave that

can propagate indefinitely maintaining a constant shape. Such a wave has

been termed a soliton.

Charlie Chong/ Fion Zhang

http://www.rpi.edu/dept/chem-eng/WWW/faculty/plawsky/Comsol%20Modules/DispersiveWave/DispersiveWave.html


Discussion

Subject: Wave Mode and Velocity

As mentioned earlier, using AE inspection in conjunction with other NDE techniques can be an effective method in gauging the location and nature of defects. Since source locations are

determined by the time required for the wave to travel through the material to a sensor, it is important that the velocity of the propagating waves be accurately calculated. This is not an easy task

since wave propagation depends on the material in question and the wave mode being detected. For many applications, Lamb waves

are of primary concern because they are able to give the best indication of

wave propagation from a source whose distance from the sensor is larger

than the thickness of the material.

Question: from the additional reading, “Lamb waves, also known as plate

waves, are another type of ultrasonic wave used in the nondestructive

inspection of materials. Lamb waves are propagated in plates (made of

composites or metals) only a few wavelengths thick”. Discuss on this

statement.

Charlie Chong/ Fion Zhang


4.0 Equipment

Acoustic emission testing can be performed in the field with portable

instruments or in a stationary laboratory setting. Typically, systems contain a

sensor, preamplifier, filter, and amplifier, along with measurement, display,

and storage equipment (e.g. oscilloscopes, voltmeters, and personal

computers). Acoustic emission sensors respond to dynamic motion that is

caused by an AE event. This is achieved through transducers which convert

mechanical movement into an electrical voltage signal. The transducer

element in an AE sensor is almost always a piezoelectric crystal, which is

commonly made from a ceramic such as Lead Zirconate Titanate (PZT).

Transducers are selected based on operating frequency, sensitivity and

environmental characteristics, and are grouped into two classes:

(1) resonant and

(2) broadband.

The majority of AE equipment is responsive to movement in its typical

operating frequency range of 30 kHz to 1 MHz. For materials with high

attenuation (e.g. plastic composites), lower frequencies may be used to better

distinguish AE signals. The opposite holds true as well.

Charlie Chong/ Fion Zhang


Key Points:

• Two classes: resonant and broadband.

• The majority of AE equipment is responsive to movement in its typical

operating frequency range of 30 kHz to 1 MHz.

• For materials with high attenuation (e.g. plastic composites), lower

frequencies may be used to better distinguish AE signals. The opposite

holds true as well.

Charlie Chong/ Fion Zhang


The majority of AE equipment is responsive to movement in its typical

operating frequency range of

30 kHz to 1 MHz.

For materials with high attenuation (e.g. plastic composites), lower

frequencies may be used to better distinguish AE signals. The opposite holds

true as well.

Charlie Chong/ Fion Zhang


Q. The most common range of acoustic emission testing is?

A. 100-300KHz

B. 10-15KHz

C. 500-750KHz

D. 1-5mHz

What is the standard answer? (more reading) 2015/09/04, best guess “A”

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvU

XJQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Equipment- Probes

Case

Damping

materials

Wear plate

Electrode

Piezoelectric element

Couplants

Specimen

Charlie Chong/ Fion Zhang


Equipment- Probe

Charlie Chong/ Fion Zhang


Ideally, the AE signal that reaches the mainframe will be free of background

noise and electromagnetic interference. Unfortunately, this is not realistic.

However, sensors and preamplifiers are designed to help eliminate unwanted

signals. First, the preamplifier boosts the voltage to provide gain and cable

drive capability. To minimize interference, a preamplifier is placed close to the

transducer; in fact, many transducers today are equipped with integrated

preamplifiers. Next, the signal is relayed to a bandpass filter for elimination of

low frequencies (common to background noise) and high frequencies.

Following completion of this process, the signal travels to the acoustic system

mainframe and eventually to a computer or similar device for analysis and

storage. Depending on noise conditions, further filtering or amplification at the

mainframe may still be necessary.


Schematic Diagram of a Basic Four-channel Acoustic Emission Testing

System

Charlie Chong/ Fion Zhang


FIGURE 16.5 The main elements of a modern acoustic emission detection system.

Charlie Chong/ Fion Zhang


After passing the AE system mainframe, the signal comes to a

detection/measurement circuit as shown in the figure directly above. Note that

multiple-measurement circuits can be used in multiple sensor/channel

systems for source location purposes (to be described later). At the

measurement circuitry, the shape of the conditioned signal is compared with a

threshold voltage value that has been programmed by the operator. Signals

are either continuous (analogous to Gaussian, random noise with amplitudes

varying according to the magnitude of the AE events) or burst-type. Each time

the threshold voltage is exceeded, the measurement circuit releases a digital

pulse. The first pulse is used to signify the beginning of a hit. (A hit is used to

describe the AE event that is detected by a particular sensor. One AE event

can cause a system with numerous channels to record multiple hits.) Pulses

will continue to be generated while the signal exceeds the threshold voltage.

Once this process has stopped for a predetermined amount of time, the hit is

finished (as far as the circuitry is concerned). The data from the hit is then

read into a microcomputer and the measurement circuit is reset.

Charlie Chong/ Fion Zhang


Hit Driven AE Systems and Measurement of Signal Features

Although several AE system designs are available (combining various options,

sensitivity, and cost), most AE systems use a hit-driven architecture. The hitdriven

design is able to efficiently measure all detected signals and record

digital descriptions for each individual feature (detailed later in this section).

During periods of inactivity, the system lies dormant. Once a new signal is

detected, the system records the hit or hits, and the data is logged for present

and/or future display.

Also common to most AE systems is the ability to perform routine tasks that

are valuable for AE inspection. These tasks include quantitative signal

measurements with corresponding time and/or load readings, discrimination

between real and false signals (noise), and the collection of statistical

information about the parameters of each signal.


AET

Charlie Chong/ Fion Zhang


AET

Charlie Chong/ Fion Zhang


6.0 AE Signal Features

With the equipment configured and setup complete, AE testing may begin.

The sensor is coupled to the test surface and held in place with tape or

adhesive. An operator then monitors the signals which are excited by the

induced stresses in the object. When a useful transient, or burst signal is

correctly obtained, parameters like amplitude, counts, measured area under

the rectified signal envelope (MARSE), duration, and rise time can be

gathered. Each of the AE signal feature shown in the image is described

below.

Abbreviation:

measured area under the rectified signal envelope (MARSE)

Charlie Chong/ Fion Zhang


AET Signals

Charlie Chong/ Fion Zhang


Amplitude, A, is the greatest measured voltage in a waveform and is

measured in decibels (dB). This is an important parameter in acoustic

emission inspection because it determines the detectability of the signal.

Signals with amplitudes below the operator-defined, minimum threshold will

not be recorded.

Rise time, R, is the time interval between the first threshold crossing and the

signal peak. This parameter is related to the propagation of the wave between

the source of the acoustic emission event and the sensor. Therefore, rise time

is used for qualification of signals and as a criterion for noise filter.

Duration, D, is the time difference between the first and last threshold

crossings. Duration can be used to identify different types of sources and to

filter out noise. Like counts (N), this parameter relies upon the magnitude of

the signal and the acoustics of the material.

Charlie Chong/ Fion Zhang


MARSE, E, sometimes referred to as energy counts, is the measure of the

area under the envelope of the rectified linear voltage time signal from the

transducer. This can be thought of as the relative signal amplitude and is

useful because the energy of the emission can be determined. MARSE is

also sensitive to the duration and amplitude of the signal, but does not use

counts or user defined thresholds and operating frequencies. MARSE is

regularly used in the measurements of acoustic emissions.

Counts, N, refers to the number of pulses emitted by the measurement

circuitry if the signal amplitude is greater than the threshold. Depending on

the magnitude of the AE event and the characteristics of the material, one hit

may produce one or many counts. While this is a relatively simple parameter

to collect, it usually needs to be combined with amplitude and/or duration

measurements to provide quality information about the shape of a signal

Charlie Chong/ Fion Zhang


7.0 Data Display

Software-based AE systems are able to generate graphical displays for

analysis of the signals recorded during AE inspection. These displays provide

valuable information about the detected events and can be classified into four

categories:





location,

activity,

intensity, and

data quality (crossplots).

Location displays identify the origin of the detected AE events. These can be

graphed by X coordinates, X-Y coordinates, or by channel for linear

computed-source location, planar computed-source location, and zone

location techniques.

Charlie Chong/ Fion Zhang


Examples of each graph are shown to the right.

Activity displays show AE activity as a function of time

on an X-Y plot (figure below left).

Each bar on the graphs represents a specified amount

of time. For example, a one-hour test could be divided

into 100 time increments. All activity measured within

a given 36 second interval would be displayed in a

given histogram bar. Either axis may be displayed

logarithmically in the event of high AE activity or long

testing periods. In addition to showing measured

activity over a single time period, cumulative activity

displays (figure below right) can be created to show the

total amount of activity detected during a test. This

display is valuable for measuring the total emission

quantity and the average rate of emission.

Charlie Chong/ Fion Zhang


Intensity displays are used to give statistical

information concerning the magnitude of the

detected signals. As can be seen in the

amplitude distribution graph to the near right,

the number of hits is plotted at each

amplitude increment (expressed in dB’s)

beyond the user-defined threshold. These

graphs can be used to determine whether a

few large signals or many small ones created

the detected AE signal energy. In addition, if

the Y-axis is plotted logarithmically, the

shape of the amplitude distribution can be

interpreted to determine the activity of a crack

(e.g. a linear distribution indicates growth).

Charlie Chong/ Fion Zhang


The fourth category of AE displays, crossplots, is

used for evaluating the quality of the data

collected. Counts versus amplitude, duration

versus amplitude, and counts versus duration are

frequently used crossplots. As shown in the final

figure, each hit is marked as a single point,

indicating the correlation between the two signal

features. The recognized signals from AE events

typically form a diagonal band since larger signals

usually generate higher counts. Because noise

signals caused by electromagnetic interference do

not have as many threshold-crossing pulses as

typical AE source events, the hits are located

below the main band. Conversely, signals caused

by friction or leaks have more threshold-crossing

pulses than typical AE source events and are

subsequently located above the main band. In the

case of ambiguous data, expertise is necessary in

separating desirable

Charlie Chong/ Fion Zhang


Amplitude/counts

Signal Analysis

The recognized signals from AE events typically form a

diagonal band since larger signals usually generate higher

counts. Because noise signals caused by electromagnetic

interference do not have as many threshold-crossing pulses

as typical AE source events,

Conversely, signals caused by

friction or leaks have more

threshold-crossing pulses than

typical AE source events and are

subsequently located above the

main band.

Because noise signals caused by

electromagnetic interference do not have as

many threshold-crossing pulses as typical AE

source events, the hits are located below the

main band

Charlie Chong/ Fion Zhang


8.0 AE Source Location Techniques

Multi-Channel Source Location Techniques:

Locating the source of significant acoustic emissions is often the main goal of

an inspection. Although the magnitude of the damage may be unknown after

AE analysis, follow up testing at source locations can provide these answers.

As previously mentioned, many AE systems are capable of using multiple

sensors/channels during testing, allowing them to record a hit from a single

AE event. These AE systems can be used to determine the location of an

event source. As hits are recorded by each sensor/channel, the source can

be located by knowing the velocity of the wave in the material and the

difference in hit arrival times among the sensors, as measured by hardware

circuitry or computer software. By properly spacing the sensors in this manner,

it is possible to inspect an entire structure with relatively few sensors.

Charlie Chong/ Fion Zhang


Source location techniques assume that AE waves travel at a constant

velocity in a material. However, various effects may alter the expected

velocity of the AE waves (e.g. reflections and multiple wave modes) and can

affect the accuracy of this technique. Therefore, the geometric effects of the

structure being tested and the operating frequency of the AE system must be

considered when determining whether a particular source location technique

is feasible for a given test structure.

Keywords:

■ Reflections and multiple wave modes

■ Geometric effects

Charlie Chong/ Fion Zhang


■ Linear Location Technique

Several source location techniques have

been developed based on this method.

One of the commonly used computedsource

location techniques is the linear

location principle shown to the right.

Linear location is often used to evaluate

struts on truss bridges. When the

source is located at the midpoint, the

time of arrival difference for the wave at

the two sensors is zero. If the source is

closer to one of the sensors, a

difference in arrival times is measured.

To calculate the distance of the source location from the midpoint, the arrival

time is multiplied by the wave velocity. Whether the location lies to the right

or left of the midpoint is determined by which sensor first records the hit.

This is a linear relationship and applies to any event sources between the

sensors.

Charlie Chong/ Fion Zhang


Because the above scenario implicitly assumes that the source is on a line

passing through the two sensors, it is only valid for a linear problem. When

using AE to identify a source location in a planar material, three or more

sensors are used, and the optimal position of the source is between the

sensors. Two categories of source location analysis are used for this situation:

zonal location and point location.

Charlie Chong/ Fion Zhang


■ Zonal Location Technique

As the name implies, zonal location aims to trace the

waves to a specific zone or region around a sensor.

This method is used in anisotropic materials or in

other structures where sensors are spaced relatively

far apart or when high material attenuation affects the

quality of signals at multiple sensors. Zones can be

lengths, areas or volumes depending on the

dimensions of the array. A planar sensor array with

detection by one sensor is shown in the upper right

figure. The source can be assumed to be within the

region and less than halfway between sensors.

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When additional sensors are applied, (1) arrival times and (2) amplitudes help

pinpoint the source zone. The ordered pair in lower right figure represents the

two sensors detecting the signal in the zone and the order of signal arrival at

each sensor. When relating signal strength to peak amplitude, the largest

peak amplitude is assumed to come from the nearest sensor, second largest

from the next closest sensor and so forth.

Charlie Chong/ Fion Zhang


■ Point Location

In order for point location to be justified, signals must be detected in a

minimum number of sensors: (1) two for linear, (2) three for planar, (3) four for

volumetric. Accurate arrival times must also be available. Arrival times are

often found by using (a) peak amplitude or the (b) first threshold crossing. The

velocity of wave propagation and exact position of the sensors are necessary

criteria as well. Equations can then be derived using sensor array geometry

or more complex algebra to locate more specific points of interest.

Charlie Chong/ Fion Zhang


9.0 AE Barkhausen Techniques

The Barkhausen effect

The Barkhausen effect refers to the sudden

change in size of ferromagnetic domains

that occur during magnetization or

demagnetization. During magnetization,

favorably oriented domains develop at the

cost of less favorably oriented domains.

These two factors result in minute jumps of

magnetization when a ferromagnetic

sample (e.g. iron) is exposed to an

increasing magnetic field (see figure).

Domain wall motion itself is determined by

many factors like microstructure, grain

boundaries, inclusions, and stress and

strain. By the same token, the Barkhausen

effect is too a function of stress and strain.

Charlie Chong/ Fion Zhang


Barkhausen Noise

Barkhausen noise can be heard if a coil of wire is wrapped around the sample

undergoing magnetization. Abrupt movements in the magnetic field produce

spiking current pulses in the coil. When amplified, the clicks can be compared

to Rice Krispies or the crumbling a candy wrapper. The amount of

Barkhausen noise is influenced by material imperfections and dislocations

and is likewise dependent on the mechanical properties of a material.

Currently, materials exposed to high energy particles (nuclear reactors) or

cyclic mechanical stresses (pipelines) are available for nondestructive

evaluation using Barkhausen noise, one of the many branches of AE testing.

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Hysterisis Loop- magnetization or demagnetization.

Barkhausen noise

generated if the magnetic

field was induced on the

areas with discontinuiies

(throughout the whole loop)

Charlie Chong/ Fion Zhang


10. Applications

Acoustic emission is a very versatile, non-invasive way to gather information

about a material or structure. Acoustic Emission testing (AET) is be applied

to inspect and monitor pipelines, pressure vessels, storage tanks, bridges,

aircraft, and bucket trucks, and a variety of composite and ceramic

components. It is also used in process control applications such as

monitoring welding processes. A few examples of AET applications follow.

■ Weld Monitoring

During the welding process, temperature changes induce stresses between

the weld and the base metal. These stresses are often relieved by heat

treating the weld. However, in some cases tempering the weld is not possible

and minor cracking occurs. Amazingly, cracking can continue for up to 10

days after the weld has been completed. Using stainless steel welds with

known inclusions and accelerometers for detection purposes and background

noise monitoring, it was found by W. D. Jolly (1969) that low level signals and

more sizeable bursts were related to the growth of microfissures and larger

cracks respectively. ASTM E 749-96 is a standard practice of AE monitoring

of continuous welding.

Charlie Chong/ Fion Zhang


■ Bucket Truck (Cherry Pickers) Integrity Evaluation

Accidents, overloads and fatigue can all occur when operating bucket trucks

or other aerial equipment. If a mechanical or structural defect is ignored,

serious injury or fatality can result. In 1976, the Georgia Power Company

pioneered the aerial manlift device inspection. Testing by independent labs

and electrical utilities followed. Although originally intended to examine only

the boom sections, the method is now used for inspecting the pedestal, pins,

and various other components. Normally, the AE tests are second in a chain

of inspections which start with visual checks. If necessary, follow-up tests

take the form of magnetic particle, dye penetrant, or ultrasonic inspections.

Experienced personnel can perform five to ten tests per day, saving valuable

time and money along the way. ASTM F914 governs the procedures for

examining insulated aerial personnel devices.

Charlie Chong/ Fion Zhang


AET Application

Charlie Chong/ Fion Zhang


■ Gas Trailer Tubes

Acoustic emission testing on pressurized jumbo tube trailers was authorized

by the Department of Transportation in 1983. Instead of using hydrostatic

retesting, where tubes must be removed from service and disassembled, AET

allows for in situ testing. A 10% over-pressurization is performed at a normal

filling station with AE sensors attached to the tubes at each end. A

multichannel acoustic system is used to detection and mapped source

locations. Suspect locations are further evaluated using ultrasonic inspection,

and when defects are confirmed the tube is removed from use. AET can

detect subcritical flaws whereas hydrostatic testing cannot detect cracks until

they cause rupture of the tube. Because of the high stresses in the

circumferential direction of the tubes, tests are geared toward finding

longitudinal fatigue cracks.

Charlie Chong/ Fion Zhang


■ Bridges

Bridges contain many welds, joints and connections, and a combination of

load and environmental factors heavily influence damage mechanisms such

as fatigue cracking and metal thinning due to corrosion. Bridges receive a

visual inspection about every two years and when damage is detected, the

bridge is either shut down, its weight capacity is lowered, or it is singled out

for more frequent monitoring. Acoustic Emission is increasingly being used

for bridge monitoring applications because it can continuously gather data

and detect changes that may be due to damage without requiring lane

closures or bridge shutdown. In fact, traffic flow is commonly used to load or

stress the bridge for the AE testing.

Charlie Chong/ Fion Zhang


■ Aerospace Structures

Most aerospace structures consist of complex assemblies of components that

have been design to carry significant loads while being as light as

possible. This combination of requirements leads to many parts that can

tolerate only a minor amount of damage before failing. This fact makes

detection of damage extremely important but components are often packed

tightly together making access for inspections difficult. AET has found

applications in monitoring the health of aerospace structures because

sensors can be attached in easily accessed areas that are remotely located

from damage prone sites. AET has been used in laboratory structural tests,

as well as in flight test applications. NASA's Wing Leading Edge Impact

Detection System is partially based on AE technology. The image to the right

(above) shows a technician applying AE transducers on the inside of the

Space Shuttle Discovery wing structure. The impact detection system was

developed to alert NASA officials to events such as the sprayed-on-foam

insulation impact that damaged the Space Shuttle Columbia's wing leading

edge during launch and lead to its breakup on reentry to the Earth's

atmosphere.

Charlie Chong/ Fion Zhang


Charlie Chong/ Fion Zhang


Others

• Fiber-reinforced polymer-matrix composites, in particular glass-fiber

reinforced parts or structures (e.g. fan blades)

• Material research (e.g. investigation of material properties, breakdown

mechanisms, and damage behavior)

• Inspection and quality assurance, (e.g. wood drying processes, scratch

tests)

• Real-time leakage test and location within various components (small

valves, steam lines, tank bottoms)

• Detection and location of high-voltage partial discharges in transformers

• Railroad tank car and rocket motor testing

There are a number of standards and guidelines that describe AE testing and

application procedures as supplied by the American Society for Testing and

Materials (ASTM). Examples are ASTM E 1932 for the AE examination of

small parts and ASTM E1419-00 for the method of examining seamless,

gas-filled, pressure vessels.

Charlie Chong/ Fion Zhang


End of Reading 3

Charlie Chong/ Fion Zhang


Study Note 4:

ASTM E1316 Term & Definitions

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Section B: Acoustic Emission (E750, E1067, and E1118)

The boldface designations in parentheses indicate the standards from which

the terms in that section were derived.

The terms defined in Section B are the direct responsibility of Subcommittee

E07.04 on Acoustic Emission Method.

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• acoustic emission (AE)- the class of phenomena whereby transient elastic

waves are generated by the rapid release of energy from localized sources

within a material, or the transient waves so generated. Acoustic emission

is the recommended term for general use. Other terms that have been

used in AE literature include (1) stress wave emission, (2) microseismic

activity, and (3) emission or acoustic emission with other qualifying

modifiers.

Acoustic emission channel- see channel, acoustic emission.

• acoustic emission count (emission count) (N)- see count, acoustic

emission.

Acoustic emission count rate- see count rate, acoustic emission (emission

rate or count rate) (N ).

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• acoustic emission event- see event, acoustic emission.

• acoustic emission event energy- see energy, acoustic event.

Acoustic emission sensor- see sensor, acoustic emission.

• acoustic emission signal amplitude- see signal amplitude, acoustic

emission.

• acoustic emission signal (emission signal)- see signal, acoustic emission.

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• acoustic emission signature (signature)- see signature, acoustic emission.


• acoustic emission transducer- see sensor, acoustic emission.

Acoustic emission waveguide- see waveguide, acoustic emission.

• acousto-Ultrasonics (AU)- a nondestructive examination method that uses

induced stress waves to detect and assess diffuse defect states, damage

conditions, and variations of mechanical properties of a test structure. The

AU method combines aspects of acoustic emission (AE) signal analysis

with ultrasonic materials characterization techniques.

• adaptive location- source location by iterative 反 复 的 use of simulated

sources in combination with computed location.

• AE activity, n- the presence of acoustic emission during a test.

• AE amplitude- See dB AE .

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• AE rms, n- the rectified, time averaged AE signal, measured on a linear

scale and reported in volts.

• AE signal duration- the time between AE signal start and AE signal end.

• AE signal end- the recognized termination of an AE signal, usually defined

as the last crossing of the threshold by that signal.

• AE signal generator- a device which can repeatedly induce a specified

transient signal into an AE instrument.

• AE signal rise time- the time between AE signal start and the peak

amplitude of that AE signal.

• AE signal start- the beginning of an AE signal as recognized by the system

processor, usually defined by an amplitude excursion 远 足 / 旅 途 / 前 进

exceeding threshold.

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• array, n- a group of two or more AE sensors positioned on a structure for

the purposes of detecting and locating sources. The sources would

normally be within the array.

• arrival time interval (∆t ij )- see interval, arrival time.

• attenuation, n- the decrease in AE amplitude per unit distance, normally

expressed in dB per unit length.

• average signal level, n- the rectified, time averaged AE logarithmic signal,

measured on the AE amplitude logarithmic scale and reported in dB ae units

(where 0 dB ae refers to 1 μV at the preamplifier input).

• burst emission- see emission, burst.

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• channel, acoustic emission- an assembly of a sensor, preamplifier or

impedance matching transformer, filters secondary amplifier or other

instrumentation as needed, connecting cables, and detector or processor.

NOTE 2- A channel for examining fiberglass reinforced plastic (FRP) may

utilize more than one sensor with associated electronics. Channels may be

processed independently or in predetermined groups having similar sensitivity

and frequency characteristics.

0 dB= 0 = 20log (I/I o ), (I/I o ) = 1 (no attenuation)

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• continuous emission- see emission, continuous.

• count, acoustic emission (emission count) (N)- the number of times the

acoustic emission signal exceeds (crossing) a preset threshold during any

selected portion of a test.

• count, event (N e )- the number obtained by counting each discerned 分 清

acoustic emission event once.

• count rate, acoustic emission (emission rate or count rate) (N)- the time

rate at which emission counts occur. (N/s?)

• count, ring-down- see count, acoustic emission, the preferred term.

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• couplant- a material used at the structure-to-sensor interface to improve

the transmission of acoustic energy across the interface during acoustic

emission monitoring.

• cumulative (acoustic emission) amplitude distribution F(V)- see

distribution, amplitude, cumulative.

• cumulative (acoustic emission) threshold crossing distribution F t (V)- see

distribution, threshold crossing, cumulative.

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• dB AE - a logarithmic measure of acoustic emission signal amplitude,

referenced to 1 μV at the sensor, before amplification.

Signal peak amplitude dB AE

(dB AE ) = (dB 1μV at sensor ) = 20 log10(A 1 /A o ) (1)

where:

A o = 1 μV at the sensor (before amplification), and

A 1 = peak voltage of the measured acoustic emission signal (also before

amplification).

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Acoustic Emission Reference Scale:

dB AE Value

Voltage at Sensor

0 1μV

20 10 μV

40 100 μV

60 1 mV

80 10 mV

100 100 mV

DISCUSSION- In the case of sensors with integral preamplifiers, the A o

reference is before internal amplification.

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AE signal amplitude measured as a ratio of

1μV in dB AE

E1316-05

Charlie Chong/ Fion Zhang


• dead time- any interval during data acquisition when the instrument or

system is unable to accept new data for any reason. (E 750) 3

• differential (acoustic emission) amplitude distribution F(V)- see

distribution, differential (acoustic emission) amplitude f(V).

• differential (acoustic emission) threshold crossing distribution ft(V)- see

distribution, differential (acoustic emission) threshold crossing.

• distribution, amplitude, cumulative (acoustic emission) F(V)- the

number of acoustic emission events with signals that exceed an arbitrary

amplitude as a function of amplitude V.

• distribution, threshold crossing, cumulative (acoustic emission) Ft

(V)- the number of times the acoustic emission signal exceeds an arbitrary

threshold as a function of the threshold voltage (V).

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• distribution, differential (acoustic emission) amplitude f(V)- the

number of acoustic emission events with signal amplitudes between

amplitudes of V and V + ∆V as a function of the amplitude V. f(V) is the

absolute value of the derivative of the cumulative amplitude distribution

F(V).

• distribution, differential (acoustic emission) threshold crossing

f t (V)- The number of times the acoustic emission signal waveform has a

peak between thresholds V and V + ∆V as a function of the threshold V.

f t (V) is the absolute value of the derivative of the cumulative threshold

crossing distribution F t (V).

• distribution, logarithmic (acoustic emission) amplitude g(V)- the

number of acoustic emission events with signal amplitudes between V and

α V (where α is a constant multiplier) as a function of the amplitude. This

is a variant of the differential amplitude distribution, appropriate for

logarithmically windowed data.

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• dynamic range- the difference, in decibels, between the overload level

and the minimum signal level (usually fixed by one or more of the noise

levels, low-level distortion, interference, or resolution level) in a system or

sensor.

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effective velocity, n- velocity calculated on the basis of arrival times and

propagation distances determined by artificial AE generation; used for

computed location.

emission, burst- a qualitative description of the discrete signal related to an

individual emission event occurring within the material.

NOTE 3- Use of the term burst emission is recommended only for describing

the qualitative appearance of emission signals. Fig. 1 shows an oscilloscope

trace of burst emission signals on a background of continuous emission.

emission, continuous- a qualitative description of the sustained signal level

produced by rapidly occurring acoustic emission from structural sources,

leaks, or both.

NOTE 4- Use of the term continuous emission is recommended only for

describing the qualitative appearance of emission signals. Fig. 2 and Fig. 3

show oscilloscope traces of continuous emission signals at two different

sweep rates.

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FIG. 1 Burst Emission on a Continuous Emission Background. (Sweep Rate-

5 ms/cm.)

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FIG. 1 Burst Emission on a Continuous Emission Background. (Sweep Rate-

5 ms/cm.)

Burst Emission

Continuous Emission Background

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FIG. 2 Continuous Emission. (Sweep Rate- 5 ms/cm.)

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FIG. 3 Continuous Emission. (Sweep Rate- 0.1 ms/cm.)

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energy, acoustic emission event- the total elastic energy released by an

emission event.

energy, acoustic emission signal- the energy contained in a detected

acoustic emission burst signal, with units usually reported in joules and

values which can be expressed in logarithmic form (dB, decibels).

evaluation threshold- a threshold value used for analysis of the examination

data. Data may be recorded with a system examination threshold lower than

the evaluation threshold. For analysis purposes, dependence of measured

data on the system examination threshold must be taken into consideration.

event, acoustic emission (emission event)- a local material change giving

rise to acoustic emission.

event count (Ne)- see count, event.

event count rate (N˙ e)- see rate, event count.

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examination area- that portion of a structure being monitored with acoustic

emission.

examination region- that portion of the test article evaluated using acoustic

emission technology.

Felicity effect- the presence of acoustic emission, detectable at a fixed

predetermined sensitivity level at stress levels below those previously applied.

(E 1067)

Felicity ratio- the ratio of the stress at which the Felicity effect occurs to the

previously applied maximum stress. (E 1067, E 1118)

NOTE 5- The fixed sensitivity level will usually be the same as was

used for the previous loading or test. (E 1118)

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instrumentation dead time- see dead time, instrumentation.

first hit location- a zone location method defined by which a channel among

a group of channels first detects the signal. (the location of the channel?

Probe?)

floating threshold- any threshold with amplitude established by a time

average measure of the input signal. (E 750)

hit- the detection and measurement of an AE signal on a channel.

interval, arrival time (∆t ij )- the time interval between the detected arrivals of

an acoustic emission wave at the ith and jth sensors of a sensor array.

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Kaiser effect- the absence of detectable acoustic emission at a fixed

sensitivity level, until previously applied stress levels are exceeded.

location accuracy, n- a value determined by comparison of the actual

position of an AE source (or simulated AE source) to the computed location.

location, cluster, n- a location technique based upon a specified amount of

AE activity located within a specified length or area, for example: 5 events

within 12 linear inches or 12 square inches.

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location, computed, n- a source location method based on algorithmic

analysis of the difference in arrival times among sensors.

NOTE 6- Several approaches to computed location are used, including

linear location, planar location, three dimensional location, and adaptive

location.

a) linear location, n- one dimensional source location requiring two or more

channels.

b) planar location, n- two dimensional source location requiring three or more

channels.

c) 3D location, n- three dimensional source location requiring five or more

channels.

d) adaptive location, n- source location by iterative 反 复 的 / 叠 代 的 use of

simulated sources in combination with computed location.

2+,3+,5+

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Linear, Planar, 3D

Linear

3D

Planar

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3D

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location, continuous AE signal, n- a method of location

based on continuous AE signals, as opposed to hit or difference in arrival

time location methods.

NOTE 7- This type of location is commonly used in leak location due to the presence

of continuous emission. Some common types of continuous signal location methods

include signal attenuation and correlation analysis methods.

(a) signal attenuation-based source location, n- a source location method that relies

on the attenuation versus distance phenomenon of AE signals. By monitoring the AE

signal magnitudes of the continuous signal at various points along the object, the

source can be determined based on the highest magnitude or by interpolation or

extrapolation of multiple readings.

(b) correlation-based source location, n- a source location method that compares the

changing AE signal levels (usually waveform based amplitude analysis) at two or more

points surrounding the source and determines the time displacement of these signals.

The time displacement data can be used with conventional hit based location

techniques to arrive at a solution for the source site.

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NOTE 7- This type of location is commonly used in leak location due

to the presence of continuous emission. Some common types of continuous

signal location methods include signal attenuation and correlation

analysis methods.

(a)

(b)

signal attenuation-based source location, n- a source location method

that relies on the attenuation versus distance phenomenon of AE

signals. By monitoring the AE signal magnitudes of the continuous

signal at various points along the object, the source can be determined

based on the highest magnitude or by interpolation or extrapolation of

multiple readings.

correlation-based source location, n- a source location method that

compares the changing AE signal levels (usually waveform based

amplitude analysis) at two or more points surrounding the source and

determines the time displacement of these signals. The time

displacement data can be used with conventional hit based location

techniques to arrive at a solution for the source site.

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Methods of Location

• Hit method

• Differential time method

• Continuous method

- signal attenuation-based source location

- correlation-based source location

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location, source, n- any of several methods of evaluating AE data to

determine the position on the structure from which the AE originated. Several

approaches to source location are used, including zone location, computed

location, and continuous location.

location, zone, n- any of several techniques for determining the general

region of an acoustic emission source (for example, total AE counts, energy,

hits, and so forth).

NOTE 8- Several approaches to zone location are used, including

independent channel zone location, first hit zone location, and arrival

sequence zone location.

(a) independent channel zone location, n- a zone location technique that

compares the gross amount of activity from each channel.

(b) first-hit zone location, n- a zone location technique that compares only

activity from the channel first detecting the AE event.

(c) arrival sequence zone location, n- a zone location technique that

compares the order of arrival among sensors.

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logarithmic (acoustic emission) amplitude distribution g(V)- see distribution,

logarithmic (acoustic emission) amplitude.

overload recovery time- an interval of nonlinear operation of an instrument

caused by a signal with amplitude in excess of the instrument’s linear

operating range.

performance check, AE system- see verification, AE system.

pressure, design- pressure used in design to determine the required

minimum thickness and minimum mechanical properties.

processing capacity- the number of hits that can be processed at the

processing speed before the system must interrupt data collection to clear

buffers or otherwise prepare for accepting additional data.

processing speed- the sustained rate (hits/s), as a function of the parameter

set and number of active channels, at which AE signals can be continuously

processed by a system without interruption for data transport.

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ate, event count (N˙

e )- the time rate of the event count.

rearm delay time- see time, rearm delay.

ring-down count- see count, acoustic emission, the preferred term.

sensor, acoustic emission- a detection device, generally piezoelectric, that

transforms the particle motion produced by an elastic wave into an electrical

signal.

signal, acoustic emission (emission signal)- an electrical signal obtained

by detection of one or more acoustic emission events.

signal amplitude, acoustic emission- the peak voltage of the largest

excursion attained by the signal waveform from an emission event.

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signal overload level- that level above which operation ceases to be

satisfactory as a result of signal distortion, overheating, or damage.

signal overload point- the maximum input signal amplitude at which the ratio

of output to input is observed to remain within a prescribed linear operating

range.

signal strength- the measured area of the rectified AE signal with units

proportional to volt-sec. (?)

DISCUSSION- The proportionality constant is specified by the AE instrument

manufacturer.

signature, acoustic emission (signature)- a characteristic set of

reproducible attributes of acoustic emission signals associated with a specific

test article as observed with a particular instrumentation system under

specified test conditions.

Charlie Chong/ Fion Zhang

E1316-05


signature, acoustic emission (signature)- a characteristic set of

reproducible attributes of acoustic emission signals associated with a specific

test article as observed with a particular instrumentation system under

specified test conditions.

Charlie Chong/ Fion Zhang

E1316-05


stimulation- the application of a stimulus such as force, pressure, heat, and

so forth, to a test article to cause activation of acoustic emission sources.

system examination threshold- the electronic instrument threshold (see

evaluation threshold) which data will be detected.

transducers, acoustic emission- see sensor, acoustic emission.

verification, AE system (performance check, AE system)- the process of

testing an AE system to assure conformance to a specified level of

performance or measurement accuracy. (This is usually carried out prior to,

during and/or after an AE examination with the AE system connected to the

examination object, using a simulated or artificial acoustic emission source.)

Charlie Chong/ Fion Zhang

E1316-05


voltage threshold—a voltage level on an electronic comparator such that

signals with amplitudes larger than this level will be recognized. The voltage

threshold may be user adjustable, fixed, or automatic floating. (E 750)

waveguide, acoustic emission—a device that couples elastic energy from a

structure or other test object to a remotely mounted sensor during AE

monitoring. An example of an acoustic emission waveguide would be a solid

wire of rod that is coupled at one end to a monitored structure, and to a

sensor at the other end.

Charlie Chong/ Fion Zhang

E1316-05


End of Reading 4

Charlie Chong/ Fion Zhang


Study Note 5:

Q&A

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUXJ

QZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q1. The most common range of acoustic emission testing is?

A. 100-300KHz

B. 10-15KHz

C. 500-750KHz

D. 1-5mHz

Q2. Discontinuities that are readily detectable by acoustic emission testing

are:

A. all of the above.

B. leaks.

C. plastic deformation.

D. growing cracks.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q3. The total energy loss of a propagating wave is called:

A. attenuation.

B. dispersion.

C. diffraction.

D. scatter.

Q4. The Kaiser effect refers to:

A. the behavior where emission from a source will not occur until the

previous load is exceeded.

B. velocity changes due to temperature changes.

C. low amplitude emissions from aluminum structures.

D. none of the above.

Charlie Chong/ Fion Zhang

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JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q5. The felicity effect is useful in evaluating:

A. fiber-reinforced plastic components.

B. high alloy steel castings.

C. large structural steel members. (?)

D. ceramics.

Q6. The Kaiser effect is useful in distinguishing:

A. mechanical noise from growing discontinuities.

B. electrical noise from mechanical noise.

C. electrical noise from growing discontinuities.

D. electrical noise from continuous emissions.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q7. The terms ""counts"" refers to:

A. the number of times a signal crosses a preset threshold.

B. the number of events from a source.

C. the number of transducers required to perform a test.

D. none of the above.

Q8. The acoustic emission signal amplitude is related to:

A. the intensity of the source. (as well as source nearness to the

transducer?)

B. the preset threshold.

C. the band pass filters.

D. background noises.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q9. Threshold settings are determined by:

A. the background noise level.

B. the test duration.

C. the attenuation of the material.

D. the graininess of the material.

Q10. Background noise can be reduced by:

A. electronic filtering.

B. using flat response amplifiers.

C. using in-line amplifiers.

D. using heavier gauge coaxial cable.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q11. What is the Dunegan Corollary?

A. It states that if acoustic emissions are observed prior to a previous

maximum load, some type of new damage must have occurred.

B. When the applied load is high enough to cause significant emissions even

though the previous maximum load was not reached. (felicity effect)

C. Gauging signal arrival times or differences in the spectral content of true

AE signals and background noise.

D. the number of times a signal crosses a preset threshold. (count, n)

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q12. What is the Felicity Effect?

A. When the applied load is high enough to cause significant emissions

even though the previous maximum load was not reached.

B. Gauging signal arrival times or differences in the spectral content of true

AE signals and background noise.

C. It states that if acoustic emissions are observed prior to a previous

maximum load, some type of new damage must have occurred.

D. the number of times a signal crosses a preset threshold.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q13. The Felicity Ratio is:

A. The load where considerable AE resumes, divided by the maximum

applied load (F/D).

B. Gauging signal arrival times or differences in the spectral content of true

AE signals and background noise.

C. It states that if acoustic emissions are observed prior to a previous

maximum load, some type of new damage must have occurred.

D. the number of times a signal crosses a preset threshold.

Q14. Examples of electronic filtering:

A. Gauging signal arrival times or differences in the spectral content of true

AE signals and background noise.

B. using in-line amplifiers.

C. using flat response amplifiers.

D. an electronic filter.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q15. A Hsu-Nielson Source:

A. measures the effects of attenuation on an AE signal.

B. using in-line amplifiers.

C. using flat response amplifiers.

D. an electronic filter.

Q16. Two types of AE transducers are:

A. resonant and broadband.

B. barium and silica

C. active and passive.

D. low frequency and high frequency.

Charlie Chong/ Fion Zhang

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JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q17. The most common AE transducer element is made of:

A. lead zirconate titanate (PZT).

B. barium titanate

C. Quartz

D. barium sulfide.

Q18. The term ""MARSE"" refers to:

A. the measure of the area under the envelope of the rectified linear

voltage time signal from the transducer.

B. the number of events from a source.

C. the number of transducers required to perform a test.

D. the number of times a signal crosses a preset threshold.

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q19. The term ""rise time"" refers to:

A. the time interval between the first threshold crossing and the signal

peak.

B. the number of events from a source.

C. the measure of the area under the envelope of the rectified linear voltage

time signal from the transducer.

D. the number of times a signal crosses a preset threshold.

Q20. The term ""duration"" refers to:

A. is the time difference between the first and last threshold crossings.

B. the number of events from a source.

C. the number of transducers required to perform a test.

D. low frequency and high frequency.

Charlie Chong/ Fion Zhang


Q21. The term ""amplitude"" refers to:

A. is the greatest measured voltage in a waveform and is measured in

decibels (dB).

B. is the time difference between the first and last threshold crossings.

C. the measure of the area under the envelope of the rectified linear voltage

time signal from the transducer.

D. the time interval between the first threshold crossing and the signal peak.

Q22. AE displays provide valuable information about the detected events

and can be classified into four categories:

A. location, activity, intensity, and data quality (crossplots).

B. X,Y,Z, and L.A scan, B scan, C, scan, and Z scan.

C. Class 1, Class 2, Class 3, and Class 4

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


Q23. Four types of AE Source Location Techniques:

A. Multi-source location, Linear Location, Zonal location, and Point

Location.

B. A scan, B scan, C, scan, and Z scan.

C. location, activity, intensity, and data quality (crossplots).

D. Source location, Zonal location, Point Location, and Linear location.

Q24. The term ""Barkhausen Noise"" refers to:

A. the sudden change in size of ferromagnetic domains that occur

during magnetization or demagnetization.

B. low amplitude emissions from aluminum structures.

C. the behavior where emission from a source will not occur until the previous

load is exceeded.

D. velocity changes due to temperature changes

Charlie Chong/ Fion Zhang

http://www.gwhizmobile.com/mobile/CatalogDetail.php?tag=flash&key=0Aq7qOvnO3eKsdDBoVjZvUX

JQZjhKWlpKNjVERmt4aEE&action=view&title=ASNT%20Level%20III%20Basic%20AE%2FET


End of Reading 5

Charlie Chong/ Fion Zhang


Study Note 6:

High Strength Steel- TWIP Steel

(Twinning as source of Acoustic Emission)

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Multi Phase Twinning-Induced Plasticity (TWIP) Steel

(Korean Article )

The iron-manganese TWIP steels, which contain 17-20% of manganese,

derive their exceptional properties from a specific strengthening mechanism:

twinning.

The iron-manganese TWIP steels, which contain 17-20% of manganese,

derive their exceptional properties from a specific strengthening mechanism:

twinning. The steels are fully austenitic and nonmagnetic, with no phase

transformation. The formation of mechanical twins during deformation

generates high strain hardening, preventing necking and thus maintaining a

very high strain capacity.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


The properties of different steels are determined by their crystal lattice

structures, that is the spatial arrangement of their atoms. Adding alloying

elements makes certain crystal structures more likely to form which allows the

properties of the steel to be fine-tuned. It is concluded from thermodynamic

calculations that a combination of manganese, silicon and aluminum would

probably be suitable for the development of the new lightweight construction

steel. These elements are lighter than iron and they force the crystal lattice

into certain structures: iron can switch between different crystal lattices, or

iron atoms can switch their positions and form different arrangements in the

crystal lattices.

There is, for example, an FCC.: face-centered cubic arrangement, known as

"austenite". In this case, the iron atoms sit on the corners of the crystal lattice

cube with an atom in the center of each face of the cube. Then there is the

BCC.: body-centered cubic layout. Again, the iron atoms are arranged on the

corners, but with another one in the cube's center. There is also a type in

which the iron atoms are distributed in a hexagonal arrangement. The bodycentered

cubic and the hexagonal forms are both traditionally referred to as

martensite. The crystal lattice changes, and with it, the character of the steel,

depending on the alloy element content (the alien atoms in the crystal lattice).

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Conventional high strength steels were manufactured by adding the alloying

elements such as Nb, Ti, V, and/or P in low carbon or IF (interstitial free)

steels. These steels can be manufactured under the relatively simple

processing conditions and have widely been applied for weight reduction.

However, as the demands for weight reduction are further increased, new

families of high strength steel have been developed. These new steels

grades include DP (dual phase), TRIP (TRansformation Induced Plasticity),

FB (ferrite-bainite), CP (complex phase) and TWIP (TWin Induced Plasticity)

steels.

The critical part of the steel manufacturing steels is to control the processing

parameters so that the microstructure and, hence, the strength-elongation

balance could be optimized. Various high added value products are

developed to satisfy increasing customer demands, as shown in Figure 1.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Keywords:

• DP (dual phase),

• TRIP (TRansformation Induced Plasticity),

• FB (ferrite-bainite),

• CP (complex phase) and

• TWIP (TWin Induced Plasticity) steels.

The critical part of the steel manufacturing steels is to control the processing

parameters so that the microstructure and, hence, the strength-elongation

balance could be optimized.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Figure 1: Ductility-strength relationship of mild and high strength steels

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Figure 1: Ductility-strength relationship of mild and high strength steels (M)

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Recently, new group of austenitic steels with 15-25 percent of manganese

contents and 3 percent of aluminum and silicon has been developed for

automotive use. This group is divided into transformation induced plasticity

steels (HMS-TRIP) and twinning induced plasticity steels (HMS-TWIP) due to

the characteristic phenomena occurring during plastic deformation inside the

grains.

At 700 MPa, the TRIP steels are also exceptionally strong. However, their

ductility is moderate, at approximately 35 percent. This characteristic – ductile

yet strong – is the result of changes in the crystal lattice. When forces act on

the steel, it changes from the face-centered cubic form, austenite to the body

centered cubic form, martensite. It is the collective shear of the crystal lattice

planes (the transformation) that makes traditional TRIP steel ductile.

Keywords: (improved formability?)

When forces act on the steel, it changes from the face-centered cubic form,

austenite to the body centered cubic form, martensite.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


However, with conventional TRIP steel, a certain amount of the austenite

portion is transformed to martensite – a rigid crystal structure that allows

hardly any stretching. In crash tests, this steel offers only about 5 percent

additional ductility.

With the increased share of manganese, silicon and aluminum atoms in the

iron crystal, the TRIP effect is twice as profound, thus providing double

additional ductility. The reason for twinning is that the alloy elements make

two martensitic transformations possible – first a change from austenite to

hexagonal martensite, and then from the hexagonal structure to the bodycentered

cubic martensite.

Keypoints: TWIP Hardening Mechanism?

The reason for twinning is that the alloy elements make 2 (two) Martensitic

transformations possible –

(1) first a change from austenite to hexagonal martensite, and then from the

hexagonal structure to the (2) tetragonal body-centered cubic martensite.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


with conventional TRIP steel, a certain amount of the austenite portion is

transformed to martensite – a rigid crystal structure that allows hardly any

stretching. In crash tests, this steel offers only about 5 percent additional

ductility.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Back to Basic: Martensite

Martensite is a body-centered tetragonal form of iron in which some carbon is

dissolved. Martensite forms during quenching, when the face centered cubic

lattice of austenite is distorted into the tetragonal body centered tetragonal

structure without the loss of its contained carbon atoms into cementite and

ferrite. Instead, the carbon is retained in the iron crystal structure, which is

stretched slightly so that it is no longer cubic. Martensite is more or less ferrite

supersaturated with carbon. Compare the grain size in the micrograph with

tempered martensite.

Charlie Chong/ Fion Zhang

http://www.threeplanes.net/martensite.html


Martensitic Transformation: Mysterious Properties Explained

The difference between austenite and martensite is, in some ways, quite

small: while the unit cell of austenite is a perfect cube, in the transformation to

martensite this cube is distorted so that it's slightly longer than before in one

dimension and shorter in the other two. The mathematical description of the

two structures is quite different, for reasons of symmetry, but the chemical

bonding remains very similar. Unlike cementite, which has bonding

reminiscent of ceramic materials, the hardness of martensite is difficult to

explain in chemical terms. The explanation hinges on the crystal's subtle

change in dimension, and the speed of the martensitic transformation.

Austenite is transformed to martensite on quenching at approximately the

speed of sound - too fast for the carbon atoms to come out of solution in the

crystal lattice. The resulting distortion of the unit cell results in countless

lattice dislocations in each crystal, which consists of millions of unit cells.

These dislocations make the crystal structure extremely resistant to shear

stress - which means, simply that it can't be easily dented and scratched.

Picture the difference between shearing a deck of cards (no dislocations,

perfect layers of atoms) and shearing a brick wall (even without the mortar).

Charlie Chong/ Fion Zhang

http://www.threeplanes.net/martensite.html


Keywords: Hexagonal & BCC Martensite

The reason for twinning is that the alloy elements make two martensitic

transformations possible – first a change from austenite to hexagonal

martensite, and then from the hexagonal structure to the body-centered cubic

martensite.

Charlie Chong/ Fion Zhang

http://www.threeplanes.net/martensite.html


The twinning causes a high value of the instantaneous hardening rate (n

value) as the microstructure becomes finer and finer. The resultant twin

boundaries act like grain boundaries and strengthen the steel. TWIP steels

combine extremely high strength with extremely high formability. The n value

increases to a value of 0.4 at an approximate engineering strain of 30% and

then remains constant until a total elongation around 50%. At the same time,

it hardens without breaking and it resists tensile pressures up to 1100 MPa

and it could be stretched to approximately 90 percent of its length without

breaking (Figure 2).

It is means in practice that when forces act on the steel, as in the deep draw

process, some of the austenite first transforms to the first martensite stage,

the hexagonal crystal form. When the steel is put under increasing stress, the

hexagonal lattice switches to the final, body-centered cubic form, similar to

conventional TRIP steel. This means that the steel retains a good part of its

ductility even after deep draw processing.

Austenite → ε martensite → γmartensite

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Figure 2: The stress-strain diagram clearly shows the differing characters of

TRIP and TWIP steel. TRIP steel can resist high stresses without deforming.

TWIP steel deforms with low stresses, but does not break until strain reaches

around 90 percent.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Dotted red line: more representing the higher tensile strength of TWIP Steel?

TWIP Steel ?

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Also, the TRIP steel is particularly useful for side impact protection. The

material deforms and absorbs the energy of the impact. It also becomes very

strong as it hardens, which prevents the side sections from collapsing too

much and protects vehicle occupants from injury.

However, the double TRIP effect does not explain why an alloy with 15-25

manganese content is particularly ductile. This is caused by small faults in the

crystal structure called "stacking faults". Stacking faults can be visualized as a

shift in the grid of atomic planes neatly arranged side by side and one on top

of the other. If an extra stack of two atomic planes is introduced into the lattice

from above, the regular stacking sequences are disturbed and therefore form

a stacking fault. This folding mechanism takes place on a mirror plane,

creating regularly mirrored sections of crystal. Experts refer to this as twinning,

which is what manifests itself externally as extreme ductility.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Typical mechanical property ranges of these different steels are indicated in

Figure 3. It is obvious that High Manganese Steels show extraordinary

strength-ductility relationships with a resist tensile stress up to 1100 MPa.

Conventional high-strength bodywork steels rupture at around 700 MPa or

even less.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Figure 3: The diagram shows the very high stresses that TRIP/TWIP steels

(red) can resist, compared to conventional deep drawing steels (blue).

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


High manganese steels composed of single austenite phase or multi phase

with high fraction of austenite phase can be alloyed with a large amount of

alloying elements. Effect of alloying element on properties of high manganese

steels is shown in Table 1.

■ C

As discussed above, carbon improves the stability of austenite and

strengthens the steels. It inhibits the formation of ε-martensite by increasing

the stacking fault energy.

■ Mn

Manganese stabilizes austenite. However if its content is less than 15%, α'-

martensite is formed, which aggravates the formability.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


Table 1: Effect of alloying elements on properties of high manganese steels

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


■ Mn

The γ => ε transformation temperatures decrease with increasing Mn content.

■ Si

Silicon improves strength by solid solution strengthening.

ilicon addition is effective for refining ε martensite plates and increasing

fracture strength, although it does not improve ductility.

■ Al

The high aluminum content in high manganese steels increases the stacking

fault energy of austenite. The formation of ε-martensite is suppressed by

aluminum addition. An aluminum addition is also very effective for improving

of low temperature toughness. Aluminum can segregate on the grain

boundaries during solidification, and produce a low melting point intermetalic

compound such as Fe 2 Al 5 having a melting point about 1170°C on the grain

boundaries, which cause a weakness in the casting structure.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


■ B, Ti, Zr

Adding small amounts of boron, titanium and zirconium into the high

manganese steels HMS alloyed with aluminum can improve the hot ductility

of the steels.

■ N

Nitrogen is an effective strengthening element in austenite e.g. adding

nitrogen to the Fe16.5Mn alloy decrease the martensite start temperature and

also reduces the volume fraction of ε-martensite.

TWIP steels have very good mechanical advantages for the improvement of

the automotive design, a very good crash resistance and they also reduce the

vehicle weight. This new class of steels is a good example of the

development of new materials for the benefit of the human being.

Charlie Chong/ Fion Zhang

http://cn.totalmateria.com/page.aspx?ID=CheckArticle&LN=KO&site=kts&NM=207


End of Reading 6

Charlie Chong/ Fion Zhang


Acoustic Emission Technique the

optimum solution for leakage detection

and location on water pipelines

Marco Fantozzi

ASM Brescia S.p.A., Via Lamarmora 230, 25124 Brescia, Italy.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


ABSTRACT

Leaks in water pipelines cause unnecessary waste of limited resources, thus

the necessity of leakage prevention and detection.

The experience of water distribution companies shows that the reduction of

leakage and the preservation of a low leakage level can be achieved with a

strategy that requires a loss analysis followed by leak detection and location

survey.

Effective techniques of leak detection by acoustic emission have been

developed and tested and this paper describes the experience and results

obtained with the application of these techniques in the last fifteen years in

several water systems including but not limited to those managed by ASM

BRESCIA S.p.A. in Italy.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


ASM introduction

Since 1908, ASM is the Municipal Services Board of Brescia, which is a town

of 200,000 inhabitants situated in the North of Italy. ASM, which is largely

owned by the Municipality of Brescia, is in charge of several services, the

main of which being: production and distribution of electricity, district heating,

street lighting and traffic lights, distribution of natural gas, collection, treatment

and distribution of drinking water, sewage treatment, urban transport, parking

management, telematic services, collection and disposal of urban solid waste

(including separate waste collection, landfill management and incineration of

the rest with combined production of energy and heat).

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


Brescia, Italy

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


Active approach to leak detection

The water systems managed by ASM, whose extension is around 2,200 km

are constructed mainly of ductile iron and cast iron pipes. Over 120 boreholes

and 30 spring sources supply the networks delivering to users a total of 47

million mc a year. Since 1988, ASM BRESCIA S.p.A. has been engaged in

an active program of leakage reduction.

Various methods of leakage monitoring and detection have been employed

by ASM. They include:

• District metering technique and step testing (using quadrina insertion flow

meters and data loggers)

• Leak detection and location using leak noise correlators

• Area surveys using acoustic loggers (Aqualogs)

• Analysis of the results by the Company's Maintenance Database

ASM's commitment to leakage reduction is demonstrated by the reduced

level of leakage achieved in many of the managed water networks.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


District metering technique

ASM decided to divide the network into a number of small zones called

districts that has proved by experience in different parts of the world, to be the

most efficient method of controlling leakage. Then, permanently closing the

boundary valves and installing flow meters on the few supplying mains can

continuously monitor the level of leakage. If an increase is registered in the

night consumption, a team is sent in to locate the leaks. In this way, leakage

is under permanent control, but intervention occurs only at the optimum

moment.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


Leak detection and location

The modern leak noise correlator is now the most effective and widely used

system for leak detection and location. For this reason the leak inspection on

water pipelines using the cross-correlation method were standardised in 1991

by a work group of the CNR (Italian National Research Council).

The code of practice highlights those elements necessary for carrying out the

leak detection survey in order to improve the quality and standardize the

activity. This document can be used by the Water Distribution Companies as

well as by Service Companies as a useful reference.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


■ Object and target

The method of testing requires the use of sensing devices placed on existing

pipelines fittings as well as conditioning, acquisition and signal analysis

instrumentation in order to detect and locate the leaks.

The method described applies to the control of underground supply and

distribution water pipelines of steel, ductile iron, cast iron, asbestos cement,

polyethylene and PVC. Cast iron, steel or asbestos cement pipe sections of a

maximum length of 250 meters can be controlled by using non-intrusive

sensing devices (accelerometers) and up to 600 meters by intrusive sensing

devices (hydrophones).

The maximum controllable length of plastic pipes such as PVC or (high and

low density) polyethylene is 50 metres only, when accelerometers are used,

and 120 meters when hydrophones are used.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


■ Method of testing (Linear)

The method of testing requires the use of non intrusive sensing devices

(accelerometers) or intrusive devices (hydrophones) placed on existing

pipeline fittings as well as conditioning, acquisition and signal analysis

instrumentation in order to detect and locate leaks.

The location of the leaking point in the pipe is obtained knowing: the distance

between the sensors that span the leak, the propagation velocity of the leak

sound in the pipeline and the time delay, measured by the cross-correlation

function (see figure 2), that the leak sound takes to reach the two sensors.

D = 2x + v∙∆T, x = ½•(D- v•∆T)

T 2 v

X= T 1 v

∆d = v(T1-T2) =v∙∆T

X= T 1 v

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


X = distance of the point of leak from the reference sensing device;

D = distance between the two sensing devices;

V = propagation wave speed;

∆t = time delay obtained from the peak position of the cross-correlation function.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


Fig 2: Cross-correlation function plot.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


Fig 3: Coherence function plot.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


The figure 1 shows how the position of the leaking point may be obtained and

the figure 2 shows an example of the cross-correlation function.

The diagram in figure 2 shows that the position of the leak, in relation to the

two sensing devices, is determined by detecting the maximum of the crosscorrelation

function related to the time delay of the signals. (?)

The coherence function shown in figure 3 allows establishing the reliability

rating of the measure carried out. It expresses the dependence of the signals,

detected at the two measurement points A and B, from a common leak noise

source. The Coherence is normally represented between zero and one,

therefore, the nearer the coherence is to one the closer is the link between

the two detected signals. (?)

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


On site inspection results

The results obtained over a sample of 4820 km of water distribution network

in different Italian cities that have been surveyed using the cross-correlation

technique in the last ten years are now outlined.

During the systematic survey concerning the above mentioned networks -

about half consisting of cast and ductile iron pipes and the other half of steel

pipes and asbestos cement pipes (only 33 km of plastic pipes have been

inspected) - a total of 3450 water leakages have been detected.

Out of the detected leaks, 3312 (96%) have been located exactly and have

undergone repair. Some of the remaining 174 leaks have been located during

the repair excavation at distances greater than 3-4 meters. The location

errors are essentially due to the uncertainty of the used distance between

sensors.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


Area surveys using acoustic loggers

In the last few years other acoustic techniques have been developed to

optimise water leakage management in identifying leakage areas prior to

directing leak detection operators to pinpoint the leak.

Thus have been developed systems for acoustic noise monitoring and

recording that can be permanently or time limited installed at hydrants, valves

or house connections. These "noise loggers" record typical noises in the

network during low consumption hours at night and identify areas of potential

leakage for further investigation. The ultimate advance consists in

transmission of leak presence from the noise loggers to a receiver module,

which may be hand carried or vehicle-mounted.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


■ Noise Logger

This logger is installed at fittings via a simple magnetic coupling, and is

battery powered with no maintenance requirement, and no problems for being

immersed in water.

The separation distance between loggers depend mainly on the pipe material,

with plastic pipes requiring closer spacing than metallic.

Each unit is intelligent and adapts itself to the environment. If no leak is

present, a radio signal is transmitted to indicate normal background

conditions. However, as soon as a leak is detected, the unit enters an alarm

state and transmits a radio signal to indicate a "leak condition". Signals are

received by a module that can be mounted in a patrolling vehicle, or can be

easily hand-held. This receiving module analyses and "homes in" on signals

to identify the location of units indicating a "leak condition", and thus the

approximate position of a likely leak.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


The reading of an area meter could easily include the monitoring of the

loggers within it, so that new leaks are localised at exactly the same time as

increases in the night flow are noticed.

This should mean a prescribed leakage level can be easily maintained,

because the detection time is greatly reduced.

This innovative technology offers the possibility of continuous, permanent

monitoring for leakage for the entire distribution system or just for those parts

that are known problem areas.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


THE FUTURE

The next step will probably be the automatic cross-correlation analysis

between permanently installed loggers.

The noise logger will be enabled to correlate a leak position with an adjacent

logger and transmit the exact position by interfacing through SCADA with a

GIS system. This process would enhance the leakage control process

significantly.

Comments:

SCADA (supervisory control and data acquisition) is a system operating with

coded signals over communication channels so as to provide control of

remote equipment .

A geographic information system (GIS) is a system designed to capture,

store, manipulate, analyze, manage, and present all types of spatial or

geographical data.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


CONCLUSIONS

In the last fifteen years the use of acoustic emission techniques has shown

that leaks can be accurately identified and localised much faster than with

any conventional method.

These experiences in leakage detection and location have proved that the

application of acoustic techniques gives Water industry the most effective

tools of conserving precious water resources.

In particular, the use of cross-correlation to detect and locate the leaks on

underground pipelines has gained larger and larger approval within the water

industry, because it offers a more accurate location of the leak, less

dependence from operator interpretation and it can be used in very noisy

conditions.

The obtainable benefits due to the application of the considered technique

are dependent on the care and manner in which it is applied and the results

are as good as the operators strictly observe the guideline.

With the application of the "noise loggers" which record typical noises in the

network during low consumption hours at night is now possible the permanent

acoustic monitoring of the distribution network. This new technology will help

to achieve further leakage reduction without increasing the costs for water

leak detection.

Charlie Chong/ Fion Zhang

http://www.ndt.net/article/wcndt00/papers/idn183/idn183.htm


End of Reading 7

Charlie Chong/ Fion Zhang


■ ωσμ∙Ωπ∆ ∇ º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λαρτ√ ≠≥ѵФε ≠≥ѵФdsssa

Charlie Chong/ Fion Zhang


More Reading

Charlie Chong/ Fion Zhang


Charlie Chong/ Fion Zhang


Others Reading

• http://www.globalspec.com/reference/63985/203279/Chapter-10-Acoustic-

Emission-Testing

• http://www.corrosionsource.com/(S(vf34kqncr0uklwzu0ioy5dz2))/FreeCont

ent/3/Combatting+Liquid+Metal+Attack+by+Mercury+in+Ethylene+and+Cr

yogenic+Gas+PlantsTask+1+-+Non-Destructive+Testing

• http://www.ndt.net/ndtaz/index.php?id=2

• https://www.ndeed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE

_Index.htm

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Peach – 我 爱 桃 子

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Good Luck

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Good Luck

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