“…Compared with the whole body affected in DCPD, in ACPD the current field concentrates in a narrower layer near the material surface, thus a lower excitation current is sufficient to supply easily measurable potentials [10,13,29,40,41,74,76,84,96,124,125,127,131,137,139], thereby minimising the risk of localised heating [13,96,139]. Hence, ACPD is able to achieve higher sensitivity than DCPD, especially for small crack lengths near the surface of materials [13,34,40,41,132,141] and the effect of specimen geometries is reduced [13]. Verpoest et al [132] have applied high frequency ACPD to detect and monitor surface cracks in un-notched steel rods and gave the conclusions that a 40 current frequency would allow detection of cracks in an area of 0.05% of the specimen cross-section, which was unachievable by the other NDT techniques.…”