Finite Element Analysis of Evaluation of Crack on Back Surface by Direct-Current Potential Difference Method with Two Pairs of Current-Supply Probes

In the conventional direct-current potential difference method (DC-PDM), uniform current is supplied by a pair of probes set far from a crack and the change in the potential difference by the crack is measured by another pair of probes near the crack. The identification of the crack is done based on the measurement result. But when the crack exists on the back surface, the change in the potential difference is not large and the identification is usually difficult. In this study, it was assumed that current was supplied by a pair of remote probes and a pair of adjacent probes to a plate with a crack on the back surface. The distribution of electric potential and the potential difference between a pair of potential measurement probes on the surface were analyzed by the finite element method. When the adjacent current-supply probes were set in between the potential measurement probes, larger normalized potential difference was obtained than the case where only the remote current was supplied. Furthermore, it was found that the increase in the potential difference by the adjacent current depends on the distance between adjacent current-supply probes and the crack depth. Based on this characteristic, a method of crack depth evaluation was proposed.