A high frequency eddy-current, non-destructive testing apparatus with automatic probe positioning suitable for scanning applications

Abstract

By considering the equivalent circuit of an inductive probe in the vicinity of a metal object it is shown that at high frequencies the reactance reflected into the probe circuit is independent of the resistivity of the metal object. This reflected reactance can be used to control the degree of coupling between the probe and the object with the result that the reflected resistance is determined solely by the resistivity of the metal. A prototype instrument is described which makes use of these principles. When the instrument is placed near a metal object the test probe is automatically adjusted to be at the correct distance from the object. The instrument can be scanned over a surface with an irregular topography without making mechanical contact, and the information from the instrument will distinguish between changes in topography, variations in resistivity, and mechanical defects such as cracks.