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.
An analytical calculation is presented for the response of an air-cored eddy-current probe to a surface-breaking metal fatigue crack in a flat plate, modelled as a region of zero conductivity, infinitely long and uniformly deep, in a homogeneous, conducting half-space. The problem is formulated in terms of the magnetic scalar potential, subject to surface impedance boundary conditions, with a line source to represent the crack. Solutions are found using spatial Fourier transforms. The theory is valid for both ferromagnetic and non-magnetic metals, providing that the electromagnetic penetration depth is small compared with the crack depth. The signal is expressed in terms of two integrals which, in general, must be evaluated numerically. Specific computations have been performed for a circular, single-coil probe.
In this paper we study the problem of electromagnetic fields around a conductor bar of rectangular cross section. A thin-skin model is established for calculating such fields. It is revealed that the behaviour of fields near the edges of the bar is dependent on a material property and frequency-related dimensionless parameter m. The current distribution on the surface of the conductor tends to be uniform if m is small and edge-concentrated if m is large. Experiments for testing the theory are carried out. The results of these experiments are also presented, which are found to be in good agreement with the theory in general.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.