Highly sensitive nondestructive evaluation of shallow surface cracks is realized through the distributions of d-c potential drop obtained by scanning the closely coupled four-point-probes sensor around the crack. A methodology is developed for evaluating the depth and length of a three-dimensional surface crack from the potential drop profiles measured across and along the crack, where the experimental result is compared with the corresponding prediction of finite element analysis. The highly sensitive characteristic of the measured profiles is also extended to the potential drop imaging for identifying the location of cracks in a clear pictorial form. It is verified that the method is a powerful tool for characterizing very small fatigue cracks (sub-millimeter depth) on the surface of metallic structures
