A simple method was proposed for determining the length and depth of a semi-elliptical surface crack embedded in a conductive body from several D.C. potential readings. This method was based on the comparison between measured potential values and calculated potential values obtained by boundary element electric field analyses. For each measured potential value a characteristic line could be calculated, which represented the possible combinations of crack shape parameters, i.e. the length and depth of the semi-elliptical surface crack. These crack shape parameters were estimated on a least squares basis from the intersections of the characteristic lines corresponding to several potential readings. A sensitivity matrix was introduced to predict errors involved in the estimation from errors in the potential measurements. The evaluation of this matrix enabled us to determine the optimum combination of locations of measuring probes for the estimation. This method was applied to the monitoring of the length and depth of surface cracks propagating under fatigue loadings. It was found that the present method was quite useful for the estimation of the length and depth of surface cracks. The sensitivity matrix was also found to be useful for evaluating the errors of the estimation.
