New Method for Estimating the Depth of Small Surface-Breaking Cracks from Photo Inductive Data

Abstract

Since the depth of surface-breaking cracks often determines the remaining life of a part, the characterization of surface-breaking cracks is a key problem in nondestructive evaluation. Eddy currents have been used for this purpose [1]. However, traditional eddy current methods have poor success when the dimensions of the crack are small compared with the inner radius of the eddy current probe. This is as expected, since the spatial resolution of an eddy current probe is comparable with its inner radius. Moulder et al [2] and Nakagawa [3] have described a new eddy-current based method of nondestructive evaluation, known as the photoinductive method. It offers greatly enhanced spatial resolution by combining the eddy current method with a laser. Images made with the photoinductive method provide a map of the surface-breaking portion of the crack and directly measure its length along the surface. In essence, the photoinductive method allows one to map the square, E · E, of the electric field, E, on the surface of the metallic part. In this paper, we develop and present a new method for estimating the depth and the shape of surface-breaking cracks in metallic parts from photoinductive data.