A method was proposed of simultaneous identification of a semi-elliptical crack on the back surface and the plate thickness using the direct-current potential difference method of multiple-probe type. The geometry of the crack was designated by six parameters : the two-dimensional location of the crack center on the back surface, the length, the depth, and the surface and inward angles of the crack. In addition to these six geometrical parameters, the plate thickness was evaluated. The evaluation was carried out based on the distribution of electric potential differences between two adjacent probes in the current direction within a square area on the front surface. In this study, the distribution of potential difference was obtained by the finite element method for twenty-seven different crack geometries. The results showed that the proposed method successfully evaluates the geometry of the crack on the back surface and the plate thickness. The method will be applicable to the non-destructive evaluation of a crack initiated on the inner surface of pipes with wall thinning which is often the case in various aged plants.

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