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There has been a long history of the use of two electromagnetic techniques to measure surface-breaking cracks in metals. Both the alternating current potential drop (ACPD) technique and the eddy current technique have given good agreement with experimental results, even though the theoretical models on which their interpretations are based use contrasting assumptions for the boundary condition on the metal surface. The model for the ACPD technique assumes that the magnetic scalar potential satisfies the 2D Laplace equation, while eddy current modeling assumes an approximation of Born type in which the surface field is unperturbed by the presence of the crack. This paper considers a general model matching the thin-skin electromagnetic field around a surface-breaking crack to that in the free space above and shows that the two contrasting boundary conditions are extremes of a more general one. The Laplace approximation is valid for high permeability materials such as mild steel, while the Born approximation is appropriate for materials of low permeability and high conductivity such as aluminum. Experimental investigations of the magnetic fields near semielliptical cracks in mild steel and aluminum show quantitative agreement with the theory.

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This paper describes the principles of the a.c. field measurement technique, a non-contacting electromagnetic method of crack detection and sizing in metals. The development of the technique is covered, followed by examples of some of the many different applications the technique has been used for, including multi-sensor arrays for rapid manual inspection or for deployment by robotic manipulator in hazardous environments. Finally, recent work on replacing the normal multi-turn coil magnetic field sensors with newly available GMR sensors is discussed.

The stress-magnetism effect can be used as a novel and convenient potential NDE method, called the magnetic memory method. However, whether and how this can be used as a quantitative measurement are still subjects that are as yet rarely studied by researchers. In this paper, circle tensile stress within the elastic region was applied to a ferromagnetic sample under geomagnetic field. The relationships between the surface leakage magnetic field and the tensile stress are presented. The stress-magnetic field equivalent model is derived and discussed and reconciled with the experimental results. A new model of stress-permeability that represents the relationship of the material permeability versus stress is proposed, which reconciles with the experimental results. It confirms that quantitative measurement and estimation of the integrity of ferromagnetic items by magnetic memory testing is feasible.

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Reliable and cost-effective inspection of rail tracks is of paramount importance to ensure the safety of rolling stock operations. In this paper alternating current field measurement (ACFM) sensors are used to carry out experiments on artificially induced rail surface defects at various speeds using testing configurations that simulate actual inspection conditions found in the field. From the obtained results it can be clearly seen that the ACFM sensors can detect the artificially induced rail surface defects even when relatively significant lift-off is involved, i.e. 5 mm and that the effect of increasing speed on the amplitude of the Bx signal, which is directly related to the depth of the crack, is negligible. However, clustered defects cannot be easily resolved and the overall amplitude is related to the spacing of the defects within a cluster. The order of clustered defects also significantly influences the maximum amplitude of the recorded Bx signal. The validity of the results obtained from the tests on artificially induced defects was verified by conducting further ACFM tests on a rail sample removed from service that contained mild rolling contact fatigue cracks.