Detection of the interface between two metals by eddy current thermography

Abstract

The capability of eddy current thermography in detecting the interface between two (different) metals is investigated numerically. The simulations concern the detection of: (a) the boundary between two halves of a plate, each half made of a different material, and (b) the position and shape of a small region, consisting of a different material than the surrounding one. A simple observation of numerical snapshots (isotherms) gives only a vague indication about the interface; however, the numerical experiment results are improved considerably by data processing techniques, like image subtraction, and the depiction of the norm of the spatial derivatives of: (i) the instantaneous temperature and (ii) the mean value of temperature over a time period. The most effective technique proved to be the latter one, which determines the interface quite clearly, even in the case where the electrical and thermal properties of the two materials are similar. The sharpness of the interface depends on how dissimilar the two materials are, on the number of gradations in the temperature field (a function of the sensitivity of the infrared camera as well as the maximum increase in temperature within the work-piece), on the image update rate and on the time period over which the mean value of temperature is calculated. It depends also on the temperature distribution, which is affected by the exciting frequency. Higher exciting frequencies generate higher heating rates, but this does not ensure sharper images for the interface than those obtained by lower frequencies.