Measurement of shape of 3-dimensional surface crack using ICFPD technique

In order to evaluate the shape of a 3-D surface crack nondestructively, the ICFPD (Induced Current Focusing Potential Drop) technique was used. Using an ICFPD probe, the values of surface potential drops were measured around the surface crack introduced on a SUS 304 stainless steel plate and the size and shape of the crack were estimated based on the calibration curve plotted with the measurement results for 2-D cracks. The evaluation showed that by this technique the maximum depth of the 3-D crack could be exactly estimated but the depths near the end of the crack were underestimated. From the measurement of the distribution of the surface potential drop, it was shown that the flowing path of the induced current bypassed the end of the crack on the surface if the depth of the crack at the measured points was longer than the length to the end of crack. The difference between the true and the estimated crack depth was shown to be linearly proportional to the ratio of crack depth to surface crack length. An estimation based on the relation suggested that the shape of a 3-D crack could be estimated by the ICFPD technique.