ND T Abstracts

metals by visuaiising their magnetic leakage fields using magnetic particles. Some of the British Standards prescribe a minimum level of magnetic field strength H, some a level of flux density B and some a combination of both. The purpose of this present work was to place MPI on a rigourous scientific basis. The relationships between the size and shape of a defect, the magnetic properties of the steel, the magnetizing conditions and the leakage field were modelled and compared favourably with experiment. The model demonstrates that is actually the level of magnetization achieved on the faces of a defect rather than B or H which determines the leakage field strength. The interaction between a leakage field and magnetic particles was also modelled by considering the magnetic forces on an ideaiised spherical magnetic particle. This type of analysis shows how the sensitivity of MPI depends strongly on the permeability of the steel. Detailed analyses of some of the more common magnetization methods are presented as a function of permeability and simple method of measuring the field within a piece of steel was developed.

40703 Atherton, D.L.; Szpunar, J.A.; Szpunar, B. Magnetizat ion and stress effects in steel Nondestructive Characterisation of Materials 2, Montreal (Canada), 21- 23 Jul. 1986, pp. 585-594. Edited by J.F. Bussiere, J.P. Monchalin, C.O. Ruud, R.E. Green Jr. Plenum Press, 1987

Stress has long been known to be one of the major factors affecting the magnetic inspection techniques for steel. Stress effects are studied by two complementary approaches, experimental studies of magnetic flux leakage signals using experimental studies of magnetic flux leakage signals using laboratory test rigs to reproduce the behaviour of magnetic inspection tools used for pipelines and studies of the magnetic behaviour of representative steel specimens under tensile and compressive strains. It has been shown that the first magnetomechanical effect is that the initial application of stress to a magnetized ferromagnet causes a shift in magnetization towards the anhysteretic or equilibrium magnetization. Points on the anhysteretic magnetization curve are obtained, rather similarly to demagnetization, by applying AC of steadily diminishing amplitude but superimposed on a DC bias field.

40343 Crescenzo, E. Inspection of ferromagnetic materials by flux leakage (In French) Qualite. Revue Practique de Control lndustriel, No. 154, pp. 50-53 (1988)

40168 Jiles, D.C.; Utrata, D. Strain dependence of the magnetic propert ies of AISI 4130 and 4140 alloy steels Review of Progress in Quantitative Nondestructive Evaluation, Williamsburg, Virginia (United States), 22-26 Jun. 1987. Vol. 7B, pp. 1455-1462. Edited by D.D. Thompson and D.E. Chimenti, Plenum Press, 1988.

The magnetic properties of ferromagnetic materials are very sensitive to the mechanical treatment and condition of the material. Therefore magnetic methods for non destructive evaluation of the mechanical properties of steels have received increasing attention in recent years. Techniques ranging from magnetic Barkhausen measurements, magnetoacoustic emission, residual magnetization, field rotation, hysteresis and flux leakage have all been developed with varying degrees of success. These methods are usually aimed at solving one of two essential problems: to measure intrinsic properties of materials such as microstructure, elastic strain, plastic deformation or creep; or to measure extrinsic properties such as the existence of cracks, voids or corrosion.

39504 Shannon, R.W.E.; Braithwaite, J.C.; Morgan, L.L. Flux-leakage vehicles pass tests for pipeline inspection Oil and Gas Journal, Vol. 86, No. 32, pp. 47-59 (8 Aug. 1988)

Magnetic flux leakage equipment has been developed which can accurately locate and describe metal-loss defects and determines pipeline conditions before defects become critical. Detection and sizing of a range of stress corrosion cracks has been performed successfully in operating pipelines.

39146 Brudar, B. Magnet ic leakage fields calculated by the method of finite differences Proceedings of the 4th European Conference on Non-Destructive Testing, London (United Kingdom), 13-17 Sep. 1987. Vol. 1, pp. 665-674. Pergamon Press, 3173 pp. (1988)

Magnetic leakage fields around a surface crack in a ferromagnetic material have been measured and calculated on basis of simplified assumptions. In this article the detailed description of the necessary

suppositions at specified mesh points in the numerical procedure (FD) is given, taking into account the dependence of the relative permeability on the magnetic field strength.

39137 Marshall, J. Evaluating the effectiveness of Hall.effect probes for detecting fatigue cracks in DtaC steel Nondestructive Testing - Australia, Vol. 25, No. 2, pp. 35-38 (Mar./Apr. 1988)

An experimental programme is described to introduce in situ Hall effect transducers into the wing/body joints of Australian airforce aircraft to maintain inspection standards while reducing down-time and cost. Extensive trials have shown several areas for improvement.

38845 Kalwa, E.; Piekarski, K. Qualitative and quanti tat ive determination of densely occurring defects in steel ropes by magnet ic testing method Materials Evaluation, Vol. 46, No. 6, pp. 767-770 (May 1988)

Frequently occurring defects such as series of broken wires, corrosion patches, or abrasions significantly weaken steel ropes, but they are the most difficult to determine quantitatively. The Hail-effect sensor was used to measure the tangential component of the flux leakage to test the instrument's ability to estimate these kinds of defects. Specially designed models were used to simulate the presence of groups of broken wires and corrosion patches in the rope. It was found that the signal from this sensor clearly indicates the presence of even a high density of defects and can be used for their quantitative evaluation.

38535 Kalwa, E.; Piekarski, K. Determination of flaws located at different depth levels in the cross- section of steel rope NDT International, Vol. 21, No. 2, pp. 77-82 (Apr. 1988)

Signals received from the magnetic testing of steel rope vary depending on the location of the defect in the rope's cross-section. Measurements of the radial and tangential components of the leakage flux performed with a single Hall-effect probe have shown significant variations of the distribution of these components around the rope. The Hall-effect sensor, built in such a way that it measures the average value of the tangential component, indicates stronger and narrower signals from defects positioned closer to the rope's surface. This effect can be used to determine the depth of the flaw's location, which is especially useful in the testing of locked-coil ropes.

38533 Atherton, D.L. Finite element calculations and computer measurements of magnetic flux leakage patterns for pits British Journal of Nondestructive Testing, Vol. 30, No. 3, pp. 159-162 (May 1988)

Two dimensional finite element calculations for anomalous leakage fluxes, generated by far side grooves in pipes inspected, using magnetic leakage flux anomaiy detectors, are compared with precision, computer controlled, experimental measurements of three dimensional synthetic corrosion pits. The experimental measurements are presented as contour maps and surface plots. There is a strong linear correlation between peak anomalous radial flux densities calculated using two dimensional finite element techniques and those measured experimentally for the corresponding three dimensional anomalies.

38252 Atherton, D.L.; Welbourn, C. A rotat ing-drum rig for testing pipeline anomaly detectors under simulated line pressure Materials Evaluation, Vol. 46, No. 1, pp. 101-107 (Jan. 1988)

A rig for testing magnetic flux leakage detectors for pipeline monitoring applications is described. It uses short sections of line pipe supported and driven by two pairs of truck wheels. Test detectors are pressed against the outside of the pipe. Detector design, including the use of neodymium-iron- boron magnets and finite-element flux leakage caiculations, is described. Experimental results are presented and signal processing and noise are discussed. A hydraulically pressurized sealed pipe has been used to investigate the effects of line-pressure stress on detector performance.

38251 Atherton, D L.; Cheng, C.W. A large Hall probe read head sensor for magnetic flux inspection of pipelines Canadian Society for Nondestructive Testing, Vol. 8, No. 6, pp. 44, 46-50 (Nov./Dec. 1987)

A magnetic flux leakage sensor using a flux concentn~tor in the form of a read head with a Hall probe in the back air gap for use in pipeline inspection tools is described. Unlike the more usual inductive sense coils the read head produces a velocity independent signal. It also increases the scan width

40 NDT&E International Volume 26 Number 1 1993