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 


Concrete Offshore structure 


Wind Energy Offshore structure 


Refinery Applications 


Refinery Applications 

Charlie Chong/ Fion Zhang 

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


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


The Magical Book of Acoustic Emission 



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) 


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 


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 


Reference Catalog Number 

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

Acoustic Emission: Techniques and Applications 752 


Fion Zhang at Shanghai 

3rd September 2015 


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

Greek 

Alphabet 



http://greekhouseoffonts.com/


Video on - Leak Detection on Buried Water Piping using Acoustic 

Emission 

■ 

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


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. 


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 


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 



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 



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 



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 



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 


Typical AET Signal 

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


Typical AET Signal 


Study Note 1: 

AET 

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


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. 



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 








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 



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. 



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 



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. 


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. 



Probability density function for the normal distribution 



Cumulative distribution function of an acoustic emission 



Cumulative distribution function of an acoustic emission 



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. 



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. 


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 



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 









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



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). 



Crystal Twinning- Martensitic Formation 



AET Set-up 



Continuous type- Gaussian random noise 



Continuous type 



Discrete Burst Type 



Discussion 

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

term of the recorded peak signal. 



Discrete Burst Type (Kaiser effect) 



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 



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 



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 


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

Basic AE history plot showing 

Kaiser effect (BCB), Felicity effect 

(DEF), and emission during hold 

(GH) 2 


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). 


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 


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 


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. 



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. 



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 . 


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. 



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. 



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 



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 



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 



Tank AET 



End of Reading 1 


Study Note 2: 

Acoustic Emission Method 

Sidney Mindess 

University of British Columbia 

Chapter 16: Acoustic Emission Methods 


16 


Methods 


Dam 


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


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 


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. 



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 


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 


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). 


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). 


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. 


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 


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. 


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. 


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. 


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. 


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. 


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

emission. (B) Burst emission. 


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 


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

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


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.) 


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.) 


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). 


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. 


■ 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. 


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. 


■ 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. 


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. 


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. 


FIGURE 16.5 The main elements of a modern acoustic emission detection 

system. 


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. 


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. 


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. 


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

Signal Evaluation: Analysis of the wave-form 


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

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

Ring-down count= 13 


Signal Evaluation: Raise Time/ Event Counts/ Signal Duration 

Raise time 

mV/μs 

Signal duration μs 

Event counts = 3 in unit time 


Signal Evaluation: Amplitude Distribution- Triangulation to locate source 


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

Signal Evaluation: Amplitude Distribution- Triangulation to locate source 


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

Signal Evaluation: Frequency analysis 


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. 



ring-down counting 




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. 



system. 



system. 

Raw Display? 

Selective Display? 


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 


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? 


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). 


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 


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

AET 

Transducer 

In 0.1KHz~2.0KHz 


UT Transducers 2.0~5.0 MHz (≠ AET Transducer) 


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.) 


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. 


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? 


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 


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. 


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. 


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.” 


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. 


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. 


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. 


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. 


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. 


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 


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. 


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. 


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. 


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. 


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. 


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. 


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. 


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 


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.) 


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. 


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. 


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. 


Fiber Reinforced Cements and Concretes 


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. 


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. 


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. 


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. 


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. 


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. 


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. 


Corrosion of Reinforcing Steel in Concrete 


Corrosion of Reinforcing Steel in Concrete 


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. 


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. 


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). 


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.” 


Concrete Structures 






Concrete Structures- The Troll A platform 








Concrete Structures- Draugen 




Study Note 3: 

Introduction to Acoustic Emission Testing 

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



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. 


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. 


Twinning 


AET 


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. 


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. 


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. 


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. 


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. 


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. 


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. 



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). 


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). 


Kaiser/Felicity effects 

Felicity effect 

Felicity ratio = F/D 

Kaiser effect 


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. 


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) 


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. 


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 


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? 


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. 


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. 


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. 


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. 


Most of the energy is directed 

in the 90º and 270º directions, 

perpendicular to the crack 

surfaces. 

90º 

270º 


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. 


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) 


Decay time 

Decay Time: 

highly damped, nonmetallic material → order of 100 microseconds (s -6 ) 

lightly damped metallic material → tens of milliseconds (s -3 ) 


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) 


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 ) 


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 


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. 


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. 


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. 


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. 


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 


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. 


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 


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 


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 


The major axis of the ellipse is perpendicular to the surface of the solid. 


Surface wave 


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 


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 


Surface wave - Following Contour 

Surface wave 


Surface wave – One wavelength deep 

λ 

λ 


Rayleigh Wave 


http://web.ics.purdue.edu/~braile/edumod/waves/Rwave_files/image001.gif

Rayleigh Wave 


Love Wave 


http://web.ics.purdue.edu/~braile/edumod/waves/Lwave_files/image001.gif

Love Wave 


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. 


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. 


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. 


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 


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 


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 


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. 


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 


Types of Wave 

New! 

• Plate wave- Love 

• Stoneley wave 

• Sezawa 


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 


Plate or Lamb waves are similar to surface waves except they can only be 

generated in materials a few wavelengths thick. 


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: 


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 


When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb 

waves, or generalized Rayleigh waves. 

Lamb waves – 2 modes 










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 


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. 


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. 


Q1: The wave mode that has multiple or varying wave velocities is: 

A. Longitudinal waves 

B. Shear waves 

C. Transverse waves 

D. Lamb waves 


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. 


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. 


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. 


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. 


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. 


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. 


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” 


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 


Equipment- Probe 


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 


FIGURE 16.5 The main elements of a modern acoustic emission detection system. 


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. 


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 


AET 


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) 


AET Signals 


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. 


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 


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. 


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. 


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). 


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 


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 


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. 


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 


■ 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. 


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. 


■ 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. 


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. 


■ 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. 


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. 


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. 


Hysterisis Loop- magnetization or demagnetization. 

Barkhausen noise 

generated if the magnetic 

field was induced on the 

areas with discontinuiies 

(throughout the whole loop) 


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. 


■ 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. 


AET Application 


■ 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. 


■ 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. 


■ 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. 



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. 




Study Note 4: 

ASTM E1316 Term & Definitions 


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. 


E1316-05

• 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 ). 


E1316-05

• 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. 


E1316-05

• 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 . 


E1316-05

• 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. 


E1316-05

• 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. 


E1316-05

• 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) 


E1316-05

• 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. 


E1316-05

• 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. 


E1316-05

• 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). 


E1316-05

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. 


E1316-05

AE signal amplitude measured as a ratio of 

1μV in dB AE 

E1316-05 


• 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). 


E1316-05

• 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. 


E1316-05

• 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. 


E1316-05

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. 


E1316-05

FIG. 1 Burst Emission on a Continuous Emission Background. (Sweep Rate- 

5 ms/cm.) 


E1316-05

FIG. 1 Burst Emission on a Continuous Emission Background. (Sweep Rate- 

5 ms/cm.) 

Burst Emission 

Continuous Emission Background 


E1316-05

FIG. 2 Continuous Emission. (Sweep Rate- 5 ms/cm.) 


E1316-05

FIG. 3 Continuous Emission. (Sweep Rate- 0.1 ms/cm.) 


E1316-05

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. 


E1316-05

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) 


E1316-05

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. 


E1316-05

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. 


E1316-05

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+ 


E1316-05

Linear, Planar, 3D 

Linear 

3D 

Planar 


E1316-05

3D 


E1316-05

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. 


E1316-05

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. 


E1316-05

Methods of Location 

• Hit method 

• Differential time method 

• Continuous method 

- signal attenuation-based source location 

- correlation-based source location 


E1316-05

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. 


E1316-05

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. 


E1316-05

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. 


E1316-05

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. 


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. 


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.) 


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. 


E1316-05



Study Note 5: 

Q&A 


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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. 


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. 




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. 




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. 




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. 




Q11. What is the Dunegan Corollary? 

A. It states that if acoustic emissions are observed prior to a previous 


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 


D. the number of times a signal crosses a preset threshold. (count, n) 




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 


C. It states that if acoustic emissions are observed prior to a previous 


D. the number of times a signal crosses a preset threshold. 




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 


C. It states that if acoustic emissions are observed prior to a previous 



Q14. Examples of electronic filtering: 

A. Gauging signal arrival times or differences in the spectral content of true 


B. using in-line amplifiers. 

C. using flat response amplifiers. 

D. an electronic filter. 




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. 




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. 







Q19. The term ""rise time"" refers to: 

A. the time interval between the first threshold crossing and the signal 

peak. 


C. the measure of the area under the envelope of the rectified linear voltage 

time signal from the transducer. 


Q20. The term ""duration"" refers to: 

A. is the time difference between the first and last threshold crossings. 



D. low frequency and high frequency. 


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 




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 






Study Note 6: 

High Strength Steel- TWIP Steel 

(Twinning as source of Acoustic Emission) 



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. 



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). 



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. 



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. 



Figure 1: Ductility-strength relationship of mild and high strength steels 



Figure 1: Ductility-strength relationship of mild and high strength steels (M) 



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. 



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. 



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. 



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. 


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). 



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. 



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 



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. 



Dotted red line: more representing the higher tensile strength of TWIP Steel? 

TWIP Steel ? 



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. 



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. 



Figure 3: The diagram shows the very high stresses that TRIP/TWIP steels 

(red) can resist, compared to conventional deep drawing steels (blue). 



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. 



Table 1: Effect of alloying elements on properties of high manganese steels 



■ 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. 



■ 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. 





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. 


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. 



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). 



Brescia, Italy 



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. 



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. 



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. 



■ 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. 



■ 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 



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. 



Fig 2: Cross-correlation function plot. 



Fig 3: Coherence function plot. 



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. (?) 



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. 



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. 



■ 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. 



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. 



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. 



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. 





■ ωσμ∙Ωπ∆ ∇ º≠δ≤>ηθφФρ|β≠Ɛ∠ ʋ λαρτ√ ≠≥ѵФε ≠≥ѵФdsssa 


More Reading 



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 


Peach – 我爱桃子 


Good Luck 


Good Luck 


Charlie https://www.yumpu.com/en/browse/user/charliechong 

Chong/ Fion Zhang

Understanding Acoustic Emission Testing- Reading 1 Part B-A

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