Electromagnetic testing emt-mft chapter 9b

Electromagnetic Testing 

MFLT/ ECT/ Microwave/RFT 

Chapter 9B – 漏磁检测 

More Reading on Magnetic Field Leakage Testing MFLT 

1st Feb 2015 

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Self Study Notes 

Charlie Chong/ Fion Zhang

Refinery & Appurtenances 


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http://en.wikipedia.org/wiki/CANDU_reactor















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NDT Level III Examinations 

Basic and Method Exams 

ASNT NDT Level III certification candidates are required to pass both the NDT Basic and a 

method examination in order to receive the ASNT NDT Level III certificate. 

Exam Specifications 

The table below lists the number of questions and time allowed for each exam. Clicking on an 

exam will take you to an abbreviated topical outline and reference page for that exam. For the full 

topical outlines and complete list of references, see the topical outlines listed in the American 

National Standard ANSI/ASNT CP-105, Standard Topical Outlines for Qualification of 

Nondestructive Testing Personnel. 

MFL 

Magnetic Flux Leakage Testing 

90 Questions Time: 2 hrs Certification: NDT only 


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Chapter 9B: 

Magnetic Field Testing 


ASM Metal Handbook Vol.17 Nondestructive evaluation and Quality control

PART 1. Magnetic Field Testing 

1.0 Introduction 

MAGNETIC FIELD TESTING includes some of the older and more widely 

used methods for the nondestructive evaluation of materials. Historically, 

such methods have been in use for more than 50 years in the examination of 

magnetic materials for defects such as cracks, voids, or inclusions of foreign 

material. More recently, magnetic methods for assessing other material 

properties, such as grain size, texture, or hardness, have received increasing 

attention. 

Because of this diversion of applications, it is natural to divide the field of 

magnetic materials testing into two parts, one directed toward defect 

detection and characterization and the other aimed at material properties 

measurements. This article is primarily concerned with the first class of 

applications, namely, the detection, classification, and sizing of material flaws. 

However, an attempt has also been made to provide at least an introductory 

description of materials characterization principles, along with a few examples 

of applications. This is supplemented by references to other review articles. 



Keywords: 

Field of magnetic materials testing into two parts, 

• one directed toward defect detection and characterization and 

• the other aimed at material properties measurements. 



All magnetic methods of flaw detection rely in some way on the detection and 

measurement of the magnetic flux leakage field near the surface of the 

material, which is caused by the presence of the flaw. For this reason, 

magnetic testing techniques are often described as flux leakage field or 

magnetic perturbation methods. The magnetic particle inspection method is 

one such flux leakage method that derives its name from the particular 

method used to detect the leakage field. Because the magnetic particle 

method is described in the article "Magnetic Particle Inspection" in this 

Volume, the techniques discussed in this article will be limited to other forms 

of leakage field measurement. 

Keywords: 

flux leakage field FLF or magnetic perturbation 扰动 methods. 


Although it is conceivable that leakage field fluctuations associated with 

metallurgical microstructure might be used in the analysis of material 

properties, the characterization methods now in use rely on bulk 

measurements of the hysteretic properties of material magnetization or of 

some related phenomenon, such as Barkhausen noise. The principles and 

applications of magnetic characterization presented in this article are not 

intended to be exhaustive, but rather to serve as illustrations of this type of 

magnetic testing. 

The principles and techniques of leakage field testing and magnetic 

characterization are described in the two sections that follow. These sections 

will discuss concepts and methods that are essential to an understanding of 

the applications described in later sections. The examples of applications 

presented in the third section will provide a brief overview of the variety of 

inspection methods that fall under the general heading of magnetic testing. 

Keywords: 

■ Barkhausen noise 


Barkhausen Noise 

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. 


http://en.wikipedia.org/wiki/Barkhausen_effect

Barkhausen noise: Magnetization (J) or flux density (B) curve as a function 

of magnetic field intensity (H) in ferromagnetic material. The inset shows 

Barkhausen jumps. 



Barkhausen noise: Domain wall motion with a Barkhausen jump 


http://upload.wikimedia.org/wikipedia/commons/7/79/Barkhausensprung.gif

Barkhausen noise 

A coil of wire wound on the ferromagnetic material can demonstrate the sudden, 

discontinuous jumps in magnetization. The sudden transitions in the magnetization of 

the material produce current pulses in the coil. These can be amplified to produce a 

series of clicks in a loudspeaker. This sounds as crackle, complete with skewed 

pulses which sounds like candy being unwrapped, Rice crispier, or a pine log fire. 

Hence the name Barkhausen noise. Similar effects can be observed by applying only 

mechanical stresses (e.g. bending) to the material placed in the detecting coil. 

These magnetization jumps are interpreted as discrete changes in the size or rotation 

of ferromagnetic domains. Some microscopic clusters of atomic spins aligned with the 

external magnetizing field increase in size by a sudden reversal of neighbouring spins; 

and, especially as the magnetizing field becomes relatively strong, other whole 

domains suddenly turn into the direction of the external field. Simultaneously, due to 

exchange interactions the spins tend to align themselves with their neighbours. The 

tension between the various pulls creates avalanching, where a group of neighbouring 

domains will flip in quick succession to align with the external field. So the material 

magnetizes neither gradually nor all at once, but in fits and starts. 



Practical use 

A set-up for non-destructive testing of ferromagnetic materials: green – magnetising 

yoke, red – inductive sensor, grey – sample under test. The amount of Barkhausen 

noise for a given material is linked with the amount of impurities, crystal dislocations, 

etc. and can be a good indication of mechanical properties of such a material. 

Therefore, the Barkhausen noise can be used as a method of non-destructive 

evaluation of the degradation of mechanical properties in magnetic materials 

subjected to cyclic mechanical stresses (e.g. in pipeline transport) or high-energy 

particles (e.g. nuclear reactor) or materials such as high-strength steels which may be 

subjected to damage from grinding. Schematic diagram of a simple non-destructive 

set-up for such a purpose is shown on the right. 

Barkhausen noise can also indicate physical damage in a thin film structure due to 

various nanofabrication processes such as reactive ion etching or using an ion milling 

machine 



A set-up for non-destructive testing of ferromagnetic materials: 

• green – magnetizing yoke, 

• red – inductive sensor, 

• grey – sample under test. 



Magnetic Dipole 


Magnetic Dipole 


2.0 Principles of Magnetic Leakage Field Testing MLFT 

2.1 Origin of Defect Leakage Fields. 

The origin of the flaw leakage field is illustrated in Fig. 1. Figure 1(a) shows a 

uniformly magnetized rod, which consists of a large number of elementary 

magnets aligned with the direction of magnetization. Inside the material, each 

magnetic pole is exactly compensated by the presence of an adjacent pole of 

opposite polarity, and the net result is that interior poles do not contribute to 

the magnetic field outside the material. At the surfaces, however, magnetic 

poles are uncompensated and therefore produce a magnetic field in the 

region surrounding the specimen. This is illustrated in Fig. 1(a) by flux lines 

connecting uncompensated elementary poles. 


If a slot is cut in the rod, as illustrated in Fig. 1(b), the poles on the surface of 

this slot are now also uncompensated and therefore produce a localized 

magnetic field near the slot. This additional magnetic field, which is 

represented by the extra flux lines in Fig. 1(b), is the leakage field associated 

with the slot. Figure 1, although adequate for a qualitative understanding of 

the origin of leakage fields, does not provide an exact quantitative description. 

The difficulty is the assumption that the magnetization remains uniform when 

the flaw is introduced. In general, this does not happen, because the 

presence of the flaw changes the magnetic field in the vicinity of the flaw, and 

this in turn leads to a change in magnetization near the flaw. With regard to 

Fig. 1, this means that the strengths and orientations of the elementary 

dipoles (magnets) actually vary from point to point in the vicinity of the flaw, 

and this variation also contributes to the flaw leakage field. The end result is 

that the accurate description of a flaw leakage field poses a difficult 

mathematical problem that usually requires a special-purpose computer code 

for its solution. 


Fig. 1 Origin of defect leakage fields. (a) Magnetic flux lines of a magnet 

without a defect. (b) Magnetic flux lines of a magnet with a surface defect. 

Source: Ref 1 


2.2 Experimental Techniques. 

One of the first considerations in the experimental application of magnetic 

leakage field methods is the generation of a suitable magnetic field within the 

material. In some ferromagnetic materials, the residual field (the field that 

remains after removal of an external magnetizing field) is often adequate for 

surface flaw detection. In practice, however, residual magnetization is rarely 

used because use of an applied magnetizing field ensures that the material is 

in a desired magnetic state (which should be known and well characterized) 

and because applied fields provide more flexibility (that is, one can produce a 

high or low flux density in the specimen as desired. Experience has shown 

that control of the strength and direction of the magnetization can be useful in 

improving flaw detectability and in discriminating among different types of 

flaws. In general, the magnitude of the magnetization should be chosen to 

maximize the flaw leakage field with respect to other field sources that might 

interfere with flaw detection; the optimum magnetization is usually difficult to 

determine in advance of a test and is often approached by trial-and-error 

experimentation. The direction of the field should be perpendicular to the 

largest flaw dimension to maximize the effect of the flaw on the leakage field. 


Keywords: 

The direction of the field should be perpendicular to the largest flaw 

dimension to maximize the effect of the flaw on the leakage field. 


It is possible to generate a magnetic field in a specimen either directly or 

indirectly. In direct magnetization, current is passed directly through the part. 

With the indirect approach, magnetization is induced by placing the part in a 

magnetic field that is generated by an adjacent current conductor or 

permanent magnet. This can be done, for example, by threading a conductor 

through a hollow part such as a tube or by passing an electric current through 

a cable wound around the part. Methods of magnetizing a part both directly 

and indirectly are illustrated schematically in Fig. 2. 


Fig. 2 Methods of magnetization. (1) Head-shot method. (b) Magnetization 

with prods. (c) Magnetization with a central conductor. (d) Longitudinal 

magnetization. (e) Yoke magnetization 


The flaw leakage field can be detected with one of several types of magnetic 

field sensors. Aside from the use of magnetic particles, the sensors most 

often used are the inductive coil and the Hall effect device. The inductive coil 

sensor is based on Faraday's law of induction, which states that the voltage 

induced in the coil is proportional to the number of turns in the coil multiplied 

by the time rate of change of the flux Ф threading the coil. It follows that 

detection of a magnetostatic field requires that the coil be in motion so that 

the flux through the coil changes with time. The principle is illustrated in Fig. 3, 

in which the coil is oriented so as to sense the change in flux parallel to the 

surface of the specimen. If the direction of coil motion is taken as x, then the 

induced electromotive force, E, in volts is given by: 

for Ф = B∙A 


where N is the number of turns in the coil, A is its cross-sectional area, and B 

is the flux density, in Gauss, parallel to the surface of the part. Thus, the 

voltage induced in the coil is proportional to the gradient of the flux density 

along the direction of coil motion multiplied by the coil velocity. Figure 4 

shows the flux density typical of the leakage field from a slot, along with the 

corresponding signal from a search coil oriented as in Fig. 3. 


Fig. 3 Flux leakage measurement using a search coil. Source: 


Fig. 4 Leakage flux and search coil signal as a function of position. 


Unlike the inductive coil, which provides a measure of the flux gradient, a Hall 

effect sensor directly measures the component of the flux itself in the direction 

perpendicular to the sensitive area of the device. Because the response of a 

Hall effect sensor does not depend on the motion of the probe, it can be 

scanned over the surface to be inspected at any rate that is mechanically 

convenient. In this respect, the Hall device has an advantage over the coil 

sensor because there is no need to maintain a constant scanning speed 

during the inspection. On the other hand, Hall effect sensors are more difficult 

to fabricate, are somewhat delicate compared to inductive coil sensors, and 

require more complex electronics. Other magnetic field sensors that are used 

less often in leakage field applications include the flux gate magnetometer, 

magnetoresistive sensors, magnetic resonance sensors, and magnetographic 

sensors, in which the magnetic field at the surface of a part is registered on a 

magnetic tape pressed onto the surface. 

Keywords: 

Hall device has an advantage over the coil sensor because there is no need 

to maintain a constant scanning speed during the inspection. 


2.3 Analysis of Leakage Field Data. 

In most applications of the leakage field method, there is a need not only to 

detect the presence of a flaw but also to estimate its severity. This leads to 

the problem of flaw characterization, that is, the determination of flaw 

dimensions from an analysis of leakage field data. The most widely used 

method of flaw characterization is based on the assumptions that the leakage 

field signal amplitude is proportional to the size of the flaw (which usually 

means its depth into the material) and that the signal amplitude can therefore 

be taken as a direct measure of flaw severity. In situations where all flaws 

have approximately the same shape and where calibration experiments show 

that the signal amplitude is indeed proportional to the size parameter of 

concern, this empirical method of sizing works quite well. 

Keywords: 

The most widely used method of flaw characterization is based on the 

assumptions that the leakage field signal amplitude is proportional to the size 

of the flaw (which usually means its depth into the material) () 


There are, however, many situations of interest where flaw shapes vary 

considerably and where signal amplitude is not uniquely related to flaw depth, 

as is the case for corrosion pits in steel tubing. In addition, different types of 

flaws, such as cracks and pits, can occur in the same part, in which case it 

becomes necessary to determine the flaw types present as well as their 

severity. 

In such cases, a more careful analysis of the relationship between signal and 

flaw characteristics is required if serious errors in flaw characterization are to 

be avoided. One of the earliest attempts to use a theoretical model in the 

analysis of leakage field data was based on the analytic solution for the field 

perturbed by a spherical inclusion. Two conclusions were drawn from this 

analysis. First, when one measures the leakage flux component normal to the 

surface of the part, the center of the flaw is located below the scan plane at a 

distance equal to the peak-to-peak separation distance in the flaw signal (Fig. 

5), and second, the peak-to-peak signal amplitude is proportional to the flaw 

volume. A number of experimental tests of these sizing rules have confirmed 

the predicted relationships for nonmagnetic inclusions in steel parts 


Keywords: 

Two conclusions were drawn from this analysis.: 

• First, when one measures the leakage flux component normal to the 

surface of the part, the center of the flaw is located below the scan plane 

at a distance equal to the peak-to-peak separation distance in the flaw 

signal (Fig. 5), and 

• Second, the peak-to-peak signal amplitude is proportional to the flaw 

volume. A number of experimental tests of these sizing rules have 

confirmed the predicted relationships for nonmagnetic inclusions in steel 

parts 

peak-to-peak signal amplitude is 

proportional to the flaw volume. 

Lateral 

dimension 


Fig. 5 Dependence of magnetic signal peak separation (a) on the depth of a 

spherical inclusion (b) 


Further theoretical and experimental data for spheroidal inclusions and 

surface pits have shown, however, that the simple characterization rules for 

spherical inclusions do not apply when the flaw shape differs significantly 

from the ideal sphere. In such cases, the signal amplitude depends on the 

lateral extent of the flaw and on its volume, and characterization on the basis 

of leakage field analysis becomes much more complicated. 

Finally, there has been at least one attempt to apply finite-element 

calculations of flaw leakage fields to the development of characterization 

rules for a more general class of flaws. Hwang and Lord performed most of 

their computations for simple flaw shapes, such as rectangular and triangular 

slots and inclusions, and from the results devised a set of rules for estimating 

the depth, width, and angle of inclination of a flaw with respect to the surface 

of the part. One of their applications to a flaw of complex shape is shown in 

Fig. 6. 


Fig. 6 Characterization of a ferrite-tail type of defect. The dashed line shows 

the flaw configuration estimated from the leakage field data. 


The promising results obtained from the finite-element work of Hwang and 

Lord, as well as the analytically based work on spheroidal flaws, suggest that 

the estimation of flaw size and shape from leakage field data is feasible. 

Another numerical method potentially applicable to flux leakage problems is 

the boundary integral method, which may prove useful in flaw 

characterization. Unfortunately, much more work must be done on both the 

theoretical basis and on experimental testing before it will be possible to 

analyze experimental leakage field data with confidence in terms of flaw 

characteristics. 


PART 2. Principles of Magnetic Characterization of Materials 

1.0 Metallurgical and Magnetic Properties. 

The use of magnetic measurements to monitor the metallurgical properties 

of ferromagnetic materials is based on the fact that variables such as 

crystallographic phase, chemical composition, and microstructure, which 

determine the physical properties of materials, also affect their magnetic 

characteristics. Some parameters, such as grain size and orientation, 

dislocation density, and the existence of precipitates, are closely related to 

measurable characteristics of magnetic hysteresis, that is, to the behavior of 

the flux density, B, induced in a material as a function of the magnetic field 

strength, H. This relationship can be understood in principle from the physical 

theory of magnetic domains. Magnetization in a particular direction increases 

as the domains aligned in that direction grow at the expense of domains 

aligned in other directions. Factors that impede domain growth also impede 

dislocation motion; hence the connection, at a very fundamental level, 

between magnetic and mechanical properties. 


Keywords: 

Metallurgical properties: 

• Crystallographic phase, 

• Chemical composition, 

• Grain size and orientation, 

• Dislocation density, and 

• Existence of precipitates, 

are closely related to measurable characteristics of magnetic hysteresis, that 

is, to the behavior of the flux density, B, induced in a material as a function of 

the magnetic field strength, H. 


Hysteresis Curves 


Other magnetic properties, such as the saturation magnetization, which is the 

maximum value B can achieve, or the Curie temperature at which there is a 

transition to a nonmagnetic state, are less dependent on microstructure, but 

are sensitive to such factors as crystal structure and chemical composition. 

Interest in the magnetic characterization of materials, principally steels, 

derives from many such relationships between measurable magnetic 

parameters and metallurgical properties. These relationships are, however, 

quite complicated in general, and it is often difficult to determine how or if a 

particular measurement or combination of measurements can be used to 

determine a property of interest. 


Nevertheless, the prospect of nondestructive monitoring and quality control is 

an attractive one, and for this reason research on magnetic materials 

characterization continues to be an active field. It is not the purpose of this 

article to explore such magnetic methods in depth, but simply to point out that 

it is an active branch of nondestructive magnetic testing. The more 

fundamental aspects of the relationship between magnetism and metallurgy 

are discussed in Ref 26 and 28. Engineering considerations are reviewed in 

Ref 24. The proceedings of various symposia also contain several papers 

that provide a good overview of the current status of magnetic materials 

characterization. 


Curie temperature 

In physics and materials science, the Curie temperature (Tc), or Curie point, is the temperature 

where a material's permanent magnetism changes to induced magnetism. The force of 

magnetism is determined by magnetic moments. The Curie temperature is the critical point 

where a material's intrinsic magnetic moments change direction. Magnetic moments are 

permanent dipole moments within the atom which originate from electrons' angular momentum 

and spin. Materials have different structures of intrinsic magnetic moments that depend on 

temperature. At a material's Curie Temperature those intrinsic magnetic moments change 

direction. 

Permanent magnetism is caused by the alignment of magnetic moments and induced magnetism 

is created when disordered magnetic moments are forced to align in an applied magnetic field. 

For example, the ordered magnetic moments (ferromagnetic, figure 1) change and become 

disordered (paramagnetic, figure 2) at the Curie Temperature. Higher temperatures make 

magnets weaker as spontaneous magnetism only occurs below the Curie Temperature. Magnetic 

susceptibility only occurs above the Curie Temperature and can be calculated from the Curie- 

Weiss Law which is derived from Curie's Law. In analogy to ferromagnetic and paramagnetic 

materials, the Curie temperature can also be used to describe the temperature where a 

material's spontaneous electric polarisation changes to induced electric polarisation or the 

reverse upon reduction of the temperature below the Curie temperature. 


http://en.wikipedia.org/wiki/Curie_temperature

Curie temperature 

Below T c 

Ferromagnetic 

Ferrimagnetic 

Antiferromagnetic 

Above T c 

↔ Paramagnetic 





Ferromagnetism The magnetic moments in a ferromagnetic material. The 

moments are ordered and of the same magnitude in the absence of an 

applied magnetic field. 



Paramagnetism The magnetic moments in a paramagnetic material. The 

moments are disordered in the absence of an applied magnetic field and 

ordered in the presence of an applied magnetic field. 



Ferrimagnetism The magnetic moments in a ferrimagnetic material. The 

moments are aligned oppositely and have different magnitudes due to being 

made up of two different ions. This is in the absence of an applied magnetic 

field. 



Antiferromagnetism The magnetic moments in an antiferromagnetic 

material. The moments are aligned oppositely and have the same 

magnitudes. This is in the absence of an applied magnetic field. 



2.0 Experimental Techniques. 

A typical setup for measuring the B-H characteristic of a rod specimen is 

shown in Fig. 7. The essential elements are an electromagnet for generating 

the magnetizing field, a coil wound around the specimen for measuring the 

time rate of change of the magnetic flux, B, in the material, and a magnetic 

field sensor, in this case a Hall effect probe, for measuring the magnetic field 

strength, H, parallel to the surface of the part. The signal generator provides a 

low-frequency magnetizing field, typically of the order of a few Hertz, and the 

output of the flux measuring coil is integrated over time to give the flux density 

in the material. In the arrangement shown in Fig. 7, an additional feature is 

the provision for applying a tensile load to the specimen for studies of the 

effects of stress on the hysteresis data. When using a rod specimen such as 

this, it is important that the length-to-diameter ratio of the specimen be large 

so as to minimize the effects of stray fields from the ends of the rod on the 

measurements of B and H. 


Fig. 7 Experimental arrangement for hysteresis loop measurements. 


Another magnetic method that uses a similar arrangement is the 

measurement of Barkhausen noise. As the magnetic field strength, H, is 

varied at a very slow rate, discontinuous jumps in the magnetization of the 

material can be observed during certain portions of the hysteresis cycle. 

These jumps are associated with the sudden growth of a series of magnetic 

domains that have been temporarily stopped from further growth by such 

obstacles as grain boundaries, precipitates, or dislocations. Barkhausen 

noise is therefore dependent on microstructure and can be used 

independently of hysteresis measurements, or in conjunction with such 

measurements, as another method of magnetic testing. The experimental 

arrangement differs from that shown in Fig. 7 in that a single sensor coil, 

oriented to measure the flux normal to the surface of the specimen, is used 

instead of the Hall probe and the flux winding. 


The review articles and conference proceedings cited above contain 

additional detail on experimental technique and a wealth of information on the 

interpretation of hysteresis and Barkhausen data. However, it should be noted 

that test methods and data interpretation are often very specific to a particular 

class of alloy, and techniques that seem to work well for one type of material 

may be totally inappropriate for another. The analysis of magnetic 

characterization data is still largely empirical in nature, and controlled testing 

of a candidate technique with the specific alloy system of interest is advisable. 


PART 3. Application 

1.0 Flaw Detection by the Flux Leakage Method. 

Perhaps the most prevalent use of the flux leakage method is the 

inspection of ferromagnetic tubular goods, such as gas pipelines, down hole 

casing, and a variety of other forms of steel piping. In applications in the 

petroleum industry, the technique is highly developed, but details on 

inspection devices and methods of data analysis are, for the most part, 

considered proprietary by the companies that provide inspection services. 

Still, the techniques currently in use have certain features in common, and 

these are exemplified by the typical system described below. The device 

shown in Fig. 8 is an inspection tool for large-diameter pipelines. 

Magnetization is provided by a large electromagnet fitted with wire brushes to 

direct magnetic flux from the electromagnet into the pipe wall. To avoid 

spurious signals from hard spots in the material, the magnetization circuit is 

designed for maximum flux density in the pipe wall in an attempt to 

magnetically saturate the material. 


Leakage field sensors are mounted between the pole pieces of the magnet in 

a circle around the axis of the device to provide, as nearly as possible, full 

coverage of the pipe wall. In most such tools, the sensors are the inductive 

coil type, oriented to measure the axial component of the leakage field 

gradient. Data are usually recorded on magnetic tape as the system is 

propelled down a section of pipe. After the inspection, the recorded signals 

are compared with those from calibration standards in an attempt to interpret 

flaw indications in terms of flaw type and size. 


Fig. 8 Typical gas pipeline inspection pig. The tool consists of a drive unit, an 

instrumentation unit, and a center section with an electromagnetic and flux 

leakage sensors. 

Sensor 

Pipe 

Pole 

Pole 

Coil 

Sensor 


In addition to systems for inspecting rotationally symmetric cylindrical parts, 

flux leakage inspection has been applied to very irregular components, such 

as helicopter rotor blade D-spars, gear teeth, and artillery projectiles. Several 

of these special-purpose applications have involved only laboratory 

investigations, but in some cases specialized instrumentation systems have 

been developed and fabricated for factory use. These systems are uniquely 

adapted to the particular application involved, and in most cases only one or 

at most several instrumentation systems have been built. Even in the case of 

laboratory investigations, special-purpose detection probe and magnetizing 

arrangements have been developed for specific applications. 


One such system for automated thread inspection on drill pipe and collars is 

described: The device consists of an electromagnet and an array of sensors 

mounted outside a nonmagnetic cone that threads onto the tool joint. The 

assembly is driven in a helical path along the threads by a motor/clutch 

assembly. To minimize the leakage flux signal variations caused by the 

threads, signals from the sensor array are compared differentially. The 

system is capable of operating in a high field strength mode for the detection 

of cracks and corrosion pits and also in a residual field mode for the detection 

of other forms of damage. At last report, the system was undergoing field 

tests and was found to offer advantages, in terms of ease of application and 

defect detection, over the magnetic particle technique normally used for 

thread inspection. 


The flux leakage method is also finding application in the inspection of ropes 

and cables made of strands of ferromagnetic material. One approach is to 

induce magnetization in the piece by means of an encircling coil energized by 

a direct current (dc). With this method, one measures the leakage field 

associated with broken strands using a Hall effect probe or an auxiliary 

sensor coil. A complementary method with alternating current (ac), which is 

actually an eddy current test rather than flux leakage, is to measure the ac 

impedance variations in an encircling coil caused by irregularities in the 

cross-sectional area of the specimen. Haynes and Underbakke describe 

practical field tests of an instrumentation system that utilizes both the ac and 

dc methods. They conclude that instrumentation capable of a combination of 

inspection techniques offers the best possibility of detecting both localized 

flaws and overall loss of cross section caused by generalized corrosion and 

wear. They also present detailed information on the practical characteristics 

of a commercially available device that makes use of both the ac and dc 

methods. 



The flux leakage method is also finding application in 

the inspection of ropes and cables

Another area in which the flux leakage method has been successfully 

implemented is the inspection of rolling-element antifriction bearings. A 

schematic illustration of the method as applied to an inner bearing race is 

shown in Fig. 9. In this application, the part is magnetized by an 

electromagnet, as indicated in Fig. 9(a). The race is then rotated by a spindle, 

and the surface is scanned with an induction coil sensor. Typically, the race is 

rotated at a surface speed of about 2.3 m/s (7.5 ft/s), and the active portion of 

the raceway is inspected by incrementally indexing the sensor across the 

raceway. Magnetizing fields are applied in the radial and circumferential 

orientations. It has been shown that radial field inspection works best for 

surface flaws, while circumferential field inspection shows greater sensitivity 

to subsurface flaws. Data have been collected on a large number of bearing 

races to establish the correlation between leakage field signals and inclusion 

depths and dimensions determined by metallurgical sectioning. 


Fig. 9 Flux leakage inspection of a bearing race. (a) Magnetization of inner 

race. (b) Perturbation in the magnetic flux at the surface of the inner race. (c) 

Probe scanning the surface 


Finally, the flux leakage method has also been adapted to the inspection of 

steel reinforcement in concrete beams. The basic function of the magnetic 

field disturbance (MFD) inspection equipment is to provide maps of the 

magnetic field across the bottom and sides of the beam. An electromagnet on 

an inspection cart, which is suspended on tracks below the beam, provides a 

magnetic field that induces magnetization in permeable structures in its 

vicinity, such as steel rebars, cables, and stirrups. An array of Hall effect 

sensors distributed across the bottom and sides of the beam measures the 

field produced by magnetized structures within the beam. If a flaw is present 

in one of these magnetized structures, it will produce a disturbance of the 

normal magnetic field pattern associated with the unflawed beam. Thus, the 

idea behind the MFD system is to search the surface of the beam for field 

anomalies that indicate the presence of flaws in reinforcing steel within the 

structure. 

Keywords: 

magnetic field disturbance (MFD) 


A flaw, such as a broken wire in a cable or a fractured rebar, produces a 

distinctive magnetic field anomaly that depends on the size of the 

discontinuity and its distance from the sensor. Because the signal shape that 

results from such an anomaly is known, flaw detection is enhanced by 

searching magnetic field records for specific signal shapes, that is, those that 

are characteristic of discontinuities in magnetic materials. In the MFD system, 

this is accomplished by a computer program that compares signal shapes 

with typical flaw signal shapes. The program produces a correlation 

coefficient that serves as a measure of similarity of the observed signal shape 

to a typical flaw signal shape. Flaw detection is therefore not only enhanced 

by signal shape discrimination but also automated by computer processing of 

the magnetic field data. Laboratory tests have demonstrated the ability of the 

system to detect fracture in steel rebars and cables in a large pre-stressed 

concrete structure. Also planned are field tests of the equipment in the 

inspection of bridge decks for reinforcement corrosion damage. 


Steel rebars and cables in a large pre-stressed concrete structure. 


2.0 Nondestructive Characterization of Materials. 

Only two examples of magnetic methods for monitoring material 

properties are given because the examples chosen should suffice to illustrate 

the types of tests that might be employed. Measurements of magnetic 

characteristics can, however, provide a wealth of data, and various features 

of such data can yield information on different material properties. For 

example, it has been demonstrated that different features of magnetic 

hysteresis data can be interpreted in terms of heat treatment and 

microstructure, plastic deformation, residual stress, and mechanical hardness. 

An example of the effects of mechanical hardness on hysteresis data is 

shown in Fig. 10. These data were obtained in the absence of applied tensile 

stress with the experimental arrangement shown in Fig. 7. Specimens of 

different hardness were prepared by tempering at different temperatures. The 

grain size (ASTM No. 7) was the same for all four specimens used in these 

tests. Other data showed, however, that grain size has little effect on 

hysteretic behavior for the classes of alloys studied. 


Fig. 10 Effect of mechanical hardness on hysteresis loop data. (a) AISI 410 

stainless steel. (b) SAE 4340 steel. 


The main point illustrated in Fig. 10 is that the mechanically harder 

specimens of the same alloy are also harder to magnetize; that is, the flux 

density, B, obtained at a large value of H is smaller for mechanically harder 

specimens than for softer specimens. For one alloy, AISI 410 stainless steel, 

the hysteresis loop intersects the B = 0 axis at larger values of H for the 

harder specimen than for the softer specimen; that is, the coercive force is 

greater for the harder material. However, for the other material, SAE 4340 

steel, the coercive force does not change with hardness. This suggests that, 

for the two alloys considered here, the saturation flux density provides a more 

reliable measure of hardness than the coercive force. Mayos et al. used two 

quite different techniques to measure the depth of surface decarburization of 

steels. One method was a variation of a standard eddy current test, with the 

difference from standard practice being that eddy current probe response was 

measured in the presence of a low-frequency (~0.1 Hz) magnetic field. This 

arrangement provides a measure of incremental permeability, that is, the 

magnetic permeability corresponding to changes in the applied field about 

some quasistatic value. The second method employed was Barkhausen 

noise analysis. 


Keywords: 

• For one alloy, AISI 410 stainless steel, the hysteresis loop intersects the B 

= 0 axis at larger values of H for the harder specimen than for the softer 

specimen; that is, the coercive force is greater for the harder material. 

However, for the other material, SAE 4340 steel, the coercive force does 

not change with hardness. 

• This suggests that, for the two alloys considered here, the saturation flux 

density provides a more reliable measure of hardness than the coercive 

force. 


Depth of decarburization was analyzed by varying the frequency of the 

excitation field, thus changing the skin depth in the material. Experiments 

were performed with both artificial samples containing two layers of different 

carbon content and industrial samples in which carbon concentration varied 

smoothly with distance from the surface. It was shown that certain features of 

both Barkhausen noise and incremental permeability data can be correlated 

with depth of decarburization. The Barkhausen noise method showed a 

somewhat stronger sensitivity to depth, but was useful over a smaller range of 

depths than the incremental permeability method. It can be concluded that 

both methods are useful, with the optimum choice depending on accuracy 

requirements and the expected depth of decarburization. 


Offshore Structures 




VLCC 








Pipeline & Piping 


Pipeline & Piping 



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Electromagnetic testing emt-mft chapter 9b

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