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The alternating current (AC) potential difference technique for measuring the growth of regular and irregular surface cracks is described. This technique is based on injecting high frequency alternating current into the metal specimen and measuring the change in voltage on the surface produced by the presence of a crack. The high frequency current tends to flow in a thin layer of the metal surface; therefore, low currents are required to produce measurable voltages on the specimen surface. Although AC techniques are increasingly employed for the measurement of surface cracks, one of the difficulties with the approach is the problem of interpreting the measured data in terms of crack shape and size. The objective of this paper is to present an inversion algorithm that can be used to determine the shape and size of surface cracks from measurements of the surfaces' voltage. This inversion algorithm is based on a model of the electromagnetic field problem, and the algorithm enables the voltage data obtained from measurements in the crack region to be interpreted directly in terms of the crack shape and size. Examples of the application of the inversion algorithm to the interpretation of voltage measurements obtained from a single semielliptical and two semielliptical intersecting surface cracks are described.
nondestructive evaluation, surface cracks, alternating current, potential difference, alternating current potential difference (ACPD), inversion
Senior research engineer, Southwest Research Institute, San Antonio, TX