Review of magnetic methods for nondestructive evaluation (Part 2)

Review of magnetic methods for nondestructive evaluation ( Part 2) D.C. Jiles This paper gives a guideline to the literature of magnetic methods for nondestructive evaluation of materials for flaws and defects. This is a sequel to an earlier paper which described magnetic methods for nondestructive evaluation of stress, plastic deformation and microstructure. The present paper discusses magnetic particle inspection, magnetic flux leakage, leakage field calculations and eddy current inspection, including the remote field electromagnetic inspection method. An extensive survey of recent publications in the field is given.

Keywords:magnetic methods, flaws, defects, magnetic particle inspection, magnetic flux leakage, leakage field calculations, eddy current inspection, remote field electromagnetic inspection

The subject of this review is nondestructive evaluation of defects or flaws in ferromagnetic steels. This paper is a sequel to the review of magnetic methods for nondestructive evaluation of intrinsic, ie non-flaw-related, properties of ferromagnetic materials tl]. As in the earlier paper, the objective of the review is to provide a broad survey of the existing literature, giving a comprehensive summary of earlier work but without going into fine details, which have been covered in most cases by more specialized reviews of the particular techniques. The subject of flaw detection in materials using magnetic methods has a long history, going back as far as Saxbyt2] in the last century. Systematic development of testing techniques based on perturbations of the magnetic flux in iron and steel due to the presence of defects did not begin, however, until after the chance discovery of Hoke t3] that iron filings accumulated close to defects in hard steels while in the process of being ground. The technique of magnetic particle inspection, which was based on this discovery, was then developed by deForest t4] and Donne ts]. Later, as the subject of flaw detection became more quantitative, additional methods were developed in which the leakage field in .the vicinity of the flaw was measured with a magnetometer. Once the field strengths of the leakage fields were being measured on a routine basis, it became desirable to relate these to flaw size and shape, and therefore there arose the need for modelling the leakage fields from different crack geometries.

to deForest to develop the method further for practical use. DeForest's work involved devising methods of generating a magnetic field of sufficient strength in any direction in a specimen. This he solved by proposing that electrical contact electrodes (known as 'prods') with heavy-duty cables be used to pass large currents through test specimens in desired directions. Furthermore, he realized the need to use magnetic powders with uniform properties such as particle shape, size and saturation magnetization in order to obtain more reliable and reproducible results. DeForest and Donne formed the Magnaflux Corporation lrJ to exploit the MPI method in 1934. This company remains one of the principal suppliers of equipment for MPI. The MPI method is very simple in principle. It depends on the leakage of magnetic flux at the surface of a ferromagnetic material in the vicinity of surface-breaking or near-surface flaws (Figure 1). There are two main methods for generating a magnetic field in a material: the 'yoke' method and the 'prod' method. A magnetic yoke wound with a field coil can be used, as shown in Flux lines are perturbed near flaw

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Magnetic particle accumulation

Magnetic particle inspection (MPI) The technique of magnetic particle inspection was the first magnetic NDE method in widespread use. It was discovered accidentally by Hoke in 1918, but it was left

flux Fig. 1 Schematic diagram of leakage flux in the vicinity of a surface-breaking flaw with accumulation of magnetic particles

0308-9126/90/020083-10 © 1990 Butterworth & Co (Publishers) Ltd NDT International Volume 23 Number 2 April 1990

83

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Figure 2. Alternatively, a high electric current can be passed through the component using contact electrodes ('prods'). The current generates a circulating magnetic field as given by the right-hand rule (corkscrew rule). In both cases the best indication is given when the magnetic field is perpendicular to the largest dimension of the flaw or crack. It is a very reliable method, when used correctly, for finding these surface or near-surface flaws and gives a direct indication of the location and length of the flaw. There is little or no limitation on the size or shape of component being tested, although more care is needed in the application of the method to complex geometries. Nevertheless, the method does have some distinct limitations. It can of course only be used on ferromagnetic materials, and in addition the magnetic field must lie perpendicular to the direction of the flaw to give the best indication. Flaws can easily be overlooked by misorientation of the field. Finally, although the length of the flaw is easily found, the depth is difficult to ascertain. Although some indications of flaw depth based on the amount of powder accumulated at the crack or flaw have been suggested, these are not entirely satisfactory, Various enhancements have been made to the original 'dry powder method'. These include the use of waterborne suspensions or magnetic inks, known as the 'wet method', and also fluorescent powders which often give a clearer indication of small flaws when viewed under ultraviolet light. Another method that has found use in detection of flaws in structural components is the 'replica' or magnetographic method in which a magnetic tape is placed over the area to be inspected t71. The tape is magnetized by the strong surface field and is then removed and inspected for magnetic anomalies. The magnetic tape, which records an imprint of the flux leakage from the surfaces of components, can be inspected afterwards using magnetometers such as Hall probes or fluxgates. The advantage of this method over the magnetic particle inspection technique is that the magnetograph (usually a magnetic tape) can be read using a magnetometer to obtain a quantitative measure of flux leakage from regions where it would be difficult to use a magnetometer, e9 inside pipelines or for underwater applications.

certainly not reproducible and are likely to be unreliable for other reasons such as the lack of control over the relative orientation of field and flaw. Therefore it is standard procedure to apply a controlled magnetic field to the specimen, either through a magnetic yoke or by the prod magnetization method. Recently much of the research effort in MPI has been directed towards establishing standard procedures and conditions for applying the method. Gregory et al tSJ have shown that for complex geometries the magnetic particle method is less successful and has sometimes failed to reveal structural faults in aerospace components because of less magnetization in some regions of those components. Optimum conditions for the application of MPI to inspection of welds were described by Massa 191. Once again the critical factor was to determine adequate levels of magnetization in order to reveal the presence of defects. No simple solution was found in that case, but tables of values of critical field strength H for various geometries were given. Some standard procedures have been recommended in the UK t1°1, where a field strength of at least 30 Oe (2400 A m -x) was suggested for using MPI on steels. However, opinions seem to differ over the necessity of the recommendation. Work on MPI applications to pressure vessels and pipelines by Raine et al tl q has shown that the recommendations of BS 6072 do not seem to be generally applicable. Their work indicated that field strengths lower than 30 Oe were quite adequate for satisfactory MPI indications. Edwards and Palmer have investigated procedures for applying the method to tubular slSecimens threaded on a current-carrying conductort12~ (Figure 3) and for a cylindrical bar using prod magnetizationtl 3]. It was shown that care is needed to generate sufficient field strength for adequate magnetization of the specimen in these cases. Often it is most convenient to use an electromagnet to get an adequate magnetic field in the testpiece, as shown in Figure 2. It is sometimes assumed that the optimum magnetizing condition corresponds to the maximum permeability; however, this was not found to be true in the work of Oehl and Swartzendrubert141. They found that for cylindrical defects with square cross-sections the ratio of leakage field to applied field reached a maximum at an applied field of between 10 and 30 Oe in steels, depending on the lift-off. This was well removed from the maximum

Applications of the M P I method Although it is possible to apply the MPI technique solely on the remanent magnetization of a specimen and its accompanying field, the results obtained in this way are

84

Fig. 3 Magnetizing a hollow tube specimen using the threaded tube method. A conductor carrying a current / (A) is threaded through the specimen

NDT International April 1990

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Forster tT] has described the use of the magnetographic method for the detection of flaws on the inside surfaces of tubes and in pipe welds. A recent enhancement of the fluorescent MPI method has been reported by Chen t15], who used automated scanning of steel components for crack indications followed by digital image-processing techniques to enhance the results. The automation of the inspection method has the advantage of eliminating subjective evaluation of results and is therefore desirable. The 'wet method' in which a magnetic colloid is used has many similarities with the Bitter pattern technique for magnetic domain observations. Because of the finer particles used in the wet method, it has some advantages in spatial resolution over the dry powder method and therefore can be successfully used for detection of smaller flaws. The next stage of the evolution of MPI should be the development of expert systems, since even with automated measurement there are some disadvantages compared with a human operator. For example, an experienced inspector will know where flaws are most likely to occur and therefore can concentrate his efforts on that area. The transfer of this expertise to computer systems would be advantageous. In conclusion, therefore, MPI is a well established technique and most of the problems associated with it appear to have been solved. The remaining research effort is directed towards optimizing conditions for its use. A review of magnetic particle inspection was given by Bezer/16'17] some years ago. More recent reviews of the subject have been given by Bezert~s] and Dunn tt9], who have provided introductory surveys of the field. The standard reference work on MPI is Principles of Magnetic Particle Testing by Betz/2°], which contains much information on the subject, including the history of its development, the underlying principles, methods of generating fields in the materials and descriptions of the dry powder methods, the wet (or magnetic ink) method and the fluorescent powder method.

Magnetic flux leakage (MFL) The magnetic flux leakage method is derived from the magnetic particle inspection method. Both depend on the perturbation of magnetic flux in a ferromagnetic material caused by surface or near-surface flaws. Whereas in the MPI method detection relies on the accumulation of magnetic powder, or sometimes the use of a magnetic recording tape [21'22], to indicate the presence of a defect, the flux leakage technique utilizes a magnetometer. The use of a magnetometer for detecting leakage fields was first suggested by Zuschlag t23]. The magnetometer allows a quantitative measurement of the leakage field in the vicinity of a flaw to be obtained. Furthermore, the field components in three directions, perpendicular and parallel to the flaw and normal to the surface, can be measured, although in practice only components parallel to the surface are usually measured. However, the method only began to gain wide acceptance after the design of a practical flux-leakage-measuring system by Hastings t24,2s].

NDT International April 1990

This was capable of detecting surface and subsurface flaws on the inner surface of steel tubes, a location that was quite unsuitable for the MPI method. The flux leakage technique has now been further developed so that it can be used not only for the detection of flaws but also for their characterization t26'271. The magnetometer probe is most often a Hall probe or induction coil. It is scanned across the surface of the component looking for anomalies in the flux density which indicate the location of a flaw, as in Figure 4. The leakage flux as a function of distance across a crack is shown in Figure 5, together with a typical search coil output as it is moved across the crack. A drawback of this method compared with the particle method is that while large areas of material can be tested quickly using MPI, the scanning of a magnetometer over the surface can be time-consuming. Therefore the flux leakage method has advantages in situations where the location of flaws is known or where the location can be predicted with a reasonable chance of success. Under those circumstances a careful magnetic inspection can be conducted over a confined area. Another situation when flux leakage magnetometry has advantages over the particle method is where the part to be tested is not easily accessible for a visual inspection. An example is the inside surfaces of long tubes and pipelines t2a]. Once again, under these circumstances, magnetic flux leakage magnetometry has been an important and highly successful technique for detection of both flaws and stresses t29]. Subsequent work on detection of flux anomalies in pipelines indicated that these could be related to stresses within the pipe wall t3°'aH. The method has been particularly useful for the inspection of defects in underground pipelines, as in the work of Atherton t321 and Khalileev and co-workers [33'34]. Magnetometer scanning surface J J of component ~/

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As with the MPI method, the flux leakage signal depends on the level of magnetization within the material. Pashagin et al t351 have also shown that it depends on the method of magnetization, such as magnetizing by current flow or via magnetizing coils. Introductory reviews of the flux leakage method t36,37J and also a more advanced review t3m have been given by Forster. These discuss both the experimental and theoretical foundations of the method. The demagnetizing fields Hd of defects were calculated for ellipsoidal flaws inside the material, and consequently the true internal field in the defect, Hi, was calculated from the measured applied field H~ using the equation H i = H~ - Hd

Once again this procedure is equivalent to assuming that the defect behaves like a simple magnetic dipole. Such an assumption is only valid as a first approximation. If the permeability of the material is #=~o., the permeability of the flaw is #haw and the demagnetizing factor, which depends on the shape of the flaw, is N d, then the internal field is Hi =

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One of the difficulties here is that of course in general the magnetic induction in a ferromagnet is not known as a function of field H and hence #=ro, is not known as a function of H. These reviews were completed with experimental results on the variation of leakage flux with lift-off position above the crack and also as a function of depth within the crack, as shown in Figure 6. These showed that as the lift-offdistance increased, the leakage field became insensitive to crack depth at lift-offdistances above 1 mm for crack depths of 2.5 mm with varying widths of 0.2-2 mm.

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Applications of the flux leakage method Stumm [391has described several devices based on the flux leakage technique which can be used for testing of ferromagnetic tubes for flaws. One of the problems identified with these instruments was the difference in defect signal between an external (near-side) flaw and an internal (far-side) flaw. The signal from an internal flaw was much smaller than the signal from a comparable external flaw or a much larger internal flaw. One device, called the 'rotomar, enabled the tube to be passed through a rotating magnetizing yoke which generated a circumferential magnetic field for detecting longitudinal defects in the tube wall. The leakage flux was measured using an array of Hall probes. Tube diameters of 20-450 mm could be inspected. A second instrument, the 'tubomar, rotated the tube while maintaining the detection system stationary. The detection and field generation systems were identical to those used in the rotomat system. The method of magnetization, however, was dependent upon the size of the pipe. For large pipes the pipe was threaded with a central conductor which then carried the current required to obtain the optimum field strength in the pipe wall. For smaller pipes the magnetic yoke method was employed.

NDT International April 1990

A third, somewhat different system described was the 'discomat', which was used primarily for weld inspection. The detection system containing five Hall probes was rotated on a disc at typically 50 rev s- t and the tube was magnetized transversely to the direction of the weld using a magnetizing yoke. Owston t4°] has reported on measurement of leakage flux from fatigue cracks and artificial flaws such as saw slots in mild steel. These he attempted to interpret in terms of a simple dipole model of the type described by Zatsepin and Shcherbinin. The magnetic leakage field parallel to the surface and perpendicular to the cracks was measured as a function of lift-off (ie distance from the field detector to the surface of the test material). Results indicated that the leakage field increased linearly with slot depths of up to 0.2 in ( ~ 5 ram), while the derivative d2H/dz 2, where H is the magnetic field and z is the distance from the centre of the slot measured in the surface of the specimen, was found to be proportional to 1/l 4, where I is the lift-off distance. Barton et alf4~1 investigated the flux leakage method for testing of bearings, which revealed that magnetic signatures associated with pits, voids and inclusions could be identified in the leakage fields of bearing races. Forster has also written a number of papers on the subject of flux leakage at a fairly elementary level. The testing of round billets for cracks using leakage flux probes has been reported c42]. This included descriptions of both the 'tubomat' and the 'rotomat' devices. The locations of defects identified by these automated flux leakage detectors are automatically marked for ease of identification with a paint spray. These systems were capable of inspecting 12 m long billets of diameter 90-230 mm in 16 s. The most extensive review of the subject of magnetic flux leakage as an NDE tool presently available is 'NDE applications of magnetic leakage field methods' by Beissner et al t¢31. This must be considered to be the standard reference on magnetic flux leakage at this time. It contains a history of the development of the method, discusses the underlying theory, including the analysis of leakage field results and defect characterization, and finally describes a number of applications.

Leakage field calculations Once quantitative measurements of leakage fields became a routine technique, it was natural to want to interpret the signals in terms of flaw size and shape. Therefore leakage field calculations for specific flaw shapes began to be made, so that the theoretical profiles could be compared with experimental observations, with the objective of characterizing flaws from their leakage fields. Two of the most significant papers in the early development of this subject were by Shcherbinin and Zatsepin v~4'45], which were based on approximating surface defects by linear magnetic dipoles and calculating the dipole magnetic field. In this way, expressions were obtained for the normal and tangential components of the magnetic leakage field due to a defect. Zatsepin and Shcherbinin realized that the exact calculation of leakage fields arising from real defects presented an extremely complicated problem, which was intractable

NDT International April 1990

given the numerical methods and state of computer technology at that time. They therefore looked for a relatively simple problem, the leakage field of flaws which could be approximated as point, line or strip dipoles, and calculated the fields using analytical expressions. The results were then compared with observations of leakage fields due to defects, as shown in Figure 7. Notice that the horizontal component of field across the flaw leads to a unipolar response, while the normal component leads to a bipolar response. However, these early papers did not relate the leakage field to the field in the material, and therefore there were limitations on its applicability to actual measured leakage fields. Subsequent papers on leakage field calculations by Shcherbinin and Pashagin t46- 4s] were based on the same model. Experimental measurements were made on specimens of carbon steel which had artificial flaws machined in the surfaces. These defects were typically h = 0.2-3.0 mm deep, 2b = 1.0 mm wide and 21 = 1.030.0 mm long. The magnetic fields were generated using the magnetic yoke method, with a maximum magnetic field of 12 kA m-1. Measurements of the leakage field were made using a fluxgate (or 'ferroprobe') magnetometer. Field components tangential to the surface of the specimen normal to the flaw, Hx, and normal to the surface of the specimen, Hy, were measured. It was found, for example, that the relationship between the field Hy and the magnetizing field He remained almost linear, irrespective of the length of flaw, l, although the actual ratio Hr/H o increased as the flaw length increased, and in fact appeared to reach a saturation level as l --, ~ . The tangential field Hx perpendicular to the flaw was also measured. For a given flaw it was found to decay with displacement z from the centre of the flaw. Hx was also found to increase with flaw length, although this variation was dependent upon the magnetizing He. The calculations of the leakage fields by Shcherbinin and co-workers were

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87

analogous to the fringing field calculations for idealized magnetic recording heads by Karlquist [49j. Significant progress in the calculation of leakage fields was made subsequently by Hwang and Lord ES°J using finite element methods. This was the first attempt to use numerical methods to find exact solutions to the field caused by defects. It represented a real breakthrough in the development of the subject because it enabled the leakage field to be calculated from the existing field and permeability in the bulk of the material, and it represents a landmark in the calculation of leakage fields. Some results of their calculations are shown in Figure 8. The earlier work of Zatsepin and Shcherbinin, while commendable, was not easily adaptable to the range of shapes of defects encountered in practice. The leakage field profiles obtained by Hwang and Lord for the case of a simple rectangular slot were in excellent agreement with observation. This paper was therefore instrumental in demonstrating how the finite element technique could be used for modelling fields of defects. It was clear from the work that the finite element method was sufficiently flexible that its successful application in the case of a simple slot defect indicated its likely successful application in the case of more complex defects an extension that the Zatsepin-Shcherbinin model was not capable of.

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This was followed by a series of papers of which the most significant were by Lord and Hwang ts 1]and Lord et al I521. Lord and Hwang t51J extended the application of the finite element method to a variety of more complicated flaw shapes. It was deduced from this that the finite element method provided the possibility of defect characterization from the leakage field profile. It was found, for example, that the peak-to-peak value of the leakage flux BN increased with increasing flaw depth, while the separation between the peaks depended on flaw width. These results were in agreement with experimental observation and indicated the potential benefits of finite element techniques for interpreting leakage flux measurements with the objective of characterizing the defects. Lord et a1152] have remarked that the cornerstone of an approach to the application of magnetic flux leakage techniques to NDT is the development of an adequate mathematical model for magnetic field/defect interactions. It might also be added that modelling of the (B, H) or hysteresis characteristics in ferromagnetic materials adds another dimension to this problem, since even if the internal magnetic field is known there are a range of possible values of the flux density inside the material. These different possible values of B will certainly affect the leakage flux for a given type of defect with a given

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88

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NDT International April 1990

magnetizing field Ho inside the material. Lord et al concluded that the complex defect geometries, together with the non-linear magnetization characteristics of ferromagnetic steels, make closed-form or analytical solutions of magnetic field/defect interactions virtually impossible. However, they were able to show a number of examples of successful application of finite element modelling and to conclude optimistically about the potential for defect characterization for nondestructive testing. Lordt531 has given a review of the application of the finite element numerical method for calculation of leakage fields arising from magnetic field/defect interactions. In this he has indicated that much progress has been made in theoretical modelling of leakage field signatures using numerical methods, but that there are many complex problems still to be solved. Perhaps the most significant of these is taking into account the hysteresis in (B, H) characteristics of ferromagnetic materials before applying the finite element or other numerical calculation of the leakage field. The work of Lord has firmly established the use of numerical methods, such as finite element calculations, as the technique with the most promise for characterization of defects from leakage flux measurements. This has led to much interest in the area of numerical modelling of leakage fields, eg the recent work of Atherton and co.workerstS4- sr] and Bruda~ 57]. However, it was realized that three-dimensional finite element calculations are ultimately desirable for more precise characterization of real defects. An analytic solution for the leakage field of surfacebreaking cracks has recently been presented by Edwards and Palmer tsal in which the crack is approximated as a semi-elliptic slot in the material. The advantages of an analytic expression are that the fields due to defects can be rapidly calculated, and in addition the equations can be differentiated to find the forces on magnetic particles. From these results Edwards and Palmer calculate the magnetic field strength H needed to detect 1 #m and 10 /tm wide slots for a range of slot depths and permeabilities. They conclude that fields in the range 102-103 A m -1 are required for crack detection, and these are in agreement with those used in practice, eg BS 6072tl°] which recommends a magnetic induction of 0.72 T, corresponding to a field of 5700 A m - 1 in a steel of relative permeability 100. Reviews of the subject of magnetic leakage field calculations and the interpretation of experimental measurements have been given by Dobmann tsg] and Holler and Dobmann tr°]. Dobmann tsg] has discussed the problems of both detection and sizing of defects and has attempted to relate these to theoretical model predictions. Theoretical work on flux leakage has lagged considerably behind experimental development. Dobmann and Holler tr°] have reviewed the theoretical modelling of the effects of various flaws on the leakage flux, and Forster t61] has attempted to correlate observed magnetic flux leakage measurements with expectations based on finite element model calculations. His results showed serious discrepancies. This has made it difficult to characterize flaws quantitatively in some cases, although qualitatively the explanations are relatively simple and have been very successful.

NDT International April 1990

Eddy current inspection Eddy current methods for nondestructive evaluation are not strictly magnetic methods as such. That is, they do not depend for their validity on any inherent magnetic properties of the material under test. In fact, they can be applied to any conducting material. Furthermore, the literature for eddy current techniques is more extensive than that of all the other magnetic methods combined and so it would not be possible to give a comprehensive review of the subject in this paper. Nevertheless, in the interests of completeness, eddy currents do deserve mention since they are used on magnetic materials. The eddy current inspection method depends on the change in inductance of a search coil in the vicinity of a conducting test specimen caused by the generation of electrical currents in the test specimen when it is subjected to a time-varying magnetic field (Figure 9). The response is usually monitored in the form of a complex impedance plane map. Eddy currents can be used for the detection of cracks and other defects, because the defects interrupt the flow of the eddy currents generated in the material. This results in a different complex impedance of the eddy current pick-up coil when it is positioned over the flaw compared with when it is positioned over an undamaged region of the material. Eddy currents can also be used for checking thickness of coatings, determining permeability and conductivity, evaluating changes in heat treatment and microstructure, and estimating tensile strength, chemical composition and ductility. However, whereas the interpretation of results in non-magnetic materials is relatively straightforward, in ferromagnetic materials it is more difficult because the eddy current response depends on permeability. In ferromagnetic materials such as steels the permeability varies in a complicated way with the generating field. Much of the early work on development of eddy current techniques was due to Forster t62-651. A survey of the field until 1970 was given by Libby t66] and more recently by Lord [67].

Applications of eddy current inspection Application of eddy current techniques to magnetic materials by Vroman tra] has shown that the results are Time -varying magnetic field H(t) Time - dependent current I(t)

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89

easier to interpret if the material is subjected to a saturating DC field at the same time as the AC field which generates the eddy currents. This ensures that the prior state of magnetization of the material no longer affects the observed results, while also preventing large changes in permeability. Cecco and Bax [69] saturated the magnetization within their steam generator tubing while scanning for defects. In this way they reported that they obtained improved sensitivity over earlier attempts to use eddy currents for ferromagnetic tubes, which had been hampered by inherent 'magnetic noise'. Deeds and Dodd [7°] have reported on detection of defects such as cracks, wall thinning or holes in steam generator tubing using eddy currents. Dodd and Simpson t711 developed a low-frequency eddy current system for accurate determination of small changes in permeability of weakly magnetic materials, which was used to evaluate the fabrication of austenitic stainless steels. This was able to determine the amount of cold working in components and to determine the amount of 6-ferrite in welds, which reduces the incidence of hot cracking in the welds. Detection of the depth of case hardening in steels by eddy current inspection has been reported by Kuznetsov and Skripova t~2]. Clark and Junker t~31 have established the feasibility of an eddy current NDT approach for the detection of temper embrittlement in low-alloy steels, which was known to be difficult to detect by other methods. Eddy currents have also been used for mapping localized changes in residual stress using automated scanning equipment by Clark and Taszarek t~41. However, they concluded that the method was unable to discriminate between tensile and compressive residual stresses - only the absolute magnitude of stress could be ascertained. Lord and Palanisamy t75] have also used eddy currents for inspection of steam generator tubing and modelled the results using finite element techniques. T h e r e m o t e field electromagnetic technique

inspection

The remote field electromagnetic inspection technique is based on the diffusion of electromagnetic energy through a pipe wall. This can be used for the detection of corrosion or other forms of wall thinning in pipes and oil well castings. The method was developed in the 1950s and early 1960s, but it is only in recent years that it has suddenly received great attention as an NDE technique. One of the earliest descriptions of the method was by Schmidd TM, who used it for in situ detection of external casing corrosion in down-hole inspection in oil wells. This was an important breakthrough because there were no other techniques available at that time for detection of external corrosion. The advantages of the method over conventional eddy current and ultrasonics inspection, which can also be used for measuring wall thickness, include full 360 ° scanning of the pipe, rapid logging speeds and its insensitivity to dirt or scale on the pipe. Schmidt t771has reviewed the remote field electromagnetic inspection technique. He has shown that the development of improved spatial resolution of these instruments has allowed isolated pits and cracks in the pipe wall to be resolved, a result not possible with alternative techniques under the conditions encountered in down-hole inspection.

90

The method has been used for detection and classification of stress corrosion cracking. The advantage of the remote field technique is that it detects only a flux which has penetrated and been transmitted through the pipe wall. Even though both the transmitter and receiver coils are located within the pipe, there is very little direct coupling between them. Therefore the detected signal contains information about the pipe wall thickness. The variation of the detected signal with distance between exciter and receiver coils is shown in Figure 10. The technique is equally sensitive to both interior and exterior metal loss, and the observed phase lag was found to be linear with wall thickness, which is a very useful result. Finally, the problems that are normally encountered with lift-off in eddy current inspection are less evident in the remote field technique. This method has been investigated recently by Atherton and Sullivan tTs] for inspecting zirconium-2.5 % niobium nuclear reactor pressure tubes, which are non-magnetic, for the presence of defects. In this they operated the detection coil at a distance of one pipe diameter from the transmitter coil, as shown in Figure 11. They have also indicated that their results are consistent with Schmidt's original suggestion that the Signals are caused by a diffusion process through the wall close to the transmitter coil, followed by propagation along the outside of the tube, and then diffusion back through the pipe wall. Subsequently, finite element calculations were applied to model the propagation of the electromagnetic signals in the remote field eddy current technique/79]. The results of these calculations confirmed the mode of propagation originally suggested. A review of the history of eddy current inspection of steel tubing has been given by Black [8°], while Dorofeev roll has surveyed the use of eddy currents for evaluation of quality of steel components. Two elementary reviews of eddy current methods have appeared in the last few years, by Franklin ts2~ and by McMaster m3].

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[K_......I~ ~ Id e . IC~'

Direct coupling zone ,

0

I I

9o

Remote field zone ,

II P

,

I 5

,

I 4

,

[ 5

,

I 6

,

I 7

,

I '0 8

Pipe diameter from exciter coil Fig. 1 0 V a r i a t i o n o f phase and amplitude of detected signal as a f u n c t i o n of distance between exciter and receiver coils within the pipeline. ( A f t e r S c h m i d t [77])

NDT International April 1990

Pipe well

Energy flow path

Pick-up coil

15 16 17 18

/ Drive rod

Transmitter coil

19

Wheel support

Fig. 1 1 Arrangement for the remote field electromagnetic inspection of pipes as described by Atherton and Sullivan [78]

Conclusions Three major classes of technique for detection and characterization of flaws and defects in ferromagnetic materials have been discussed. These are magnetic particle inspection, magnetic flux leakage and eddy current or electromagnetic inspection. The relative advantages and disadvantages of each technique have been discussed, and a wide survey of existing literature has been given. The objective of this review has been to provide an introduction to the various magnetic methods for nondestructive evaluation, and in conjunction with the preceding pape rtl] it gives a comprehensive review of the field. It is not possible in a paper such as this to discuss each method in detail, nor is it even desirable. The references given here do provide access to the specialized literature of each method and further details can be obtained from these.

20 21 22 23 24 25 26 27

28 29

Acknowledgement

30

This work was sponsored by the Center for NDE at Iowa State University and was performed at the Ames Laboratory. Ames Laboratory is operated for the US Department of Energy by Iowa State University under Contract W-7405-ENG-82.

31

References

33

1 2 3 4 5 6 7 8 9 10 11 12 13 14

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Author The author is in the Center for Nondestructive Evaluation and the Department of Material Science and Engineering, Iowa State University, Ames, Iowa 50011, USA.

Paper received 19 May 1 989

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NDT International April 1990