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.
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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