Abstract
The work described in this paper arises from a program for the detection and measurement of surface cracks in metals carried out at University College London. The instrument which was developed for the purpose, the Crack Microgauge, employs the acpd (alternating current potential difference) method. An alternating electric current at a frequency of 6 kHz is applied to the specimen, and the instrument measures the voltage between the probe terminals which are applied to the surface of the specimen. By examining the variation of the voltage readings with position on the surface and, in particular, the jump in readings obtained when the probe crosses the crack, the crack can be detected and features of its geometry deduced. The correlation between instrument readings and information about the crack geometry must be made by use of a theoretical model of the electromagnetic field produced in the crack neighborhood. The authors have been principally concerned in the study of this mathematical problem. In this paper we have attempted to bring together in summary form the most significant results arising from the studies on several different projects.
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