Correlation of defect depth with diffusion time of eddy currents for the defects in conducting materials using transient eddy current NDE

Detailed experimental studies have been carried out to determine the diffusion time, tm, at which the eddy current due to the defect is maximum for the defects located at different depths below the top surface of a stack of aluminum plates. These subsurface defects were detected using a transient eddy current non-destructive evaluation system based on a fluxgate sensor with differential excitation coil in the form of a double 'D'. The diffusion time, tm, directly provides information about the depth of the defect and therefore has been evaluated for defects located at different depths from 2 to 20 mm below the top surface of the plate. The defect depth was varied in steps of 2 mm, while the overall total thickness of the stack of plates was kept constant at 21.5 mm. Each defect represented a localized loss of conducting volume, which was 70 mm in length, 1 mm in width and 1.5 mm in height. The experimental results show that the square root of the diffusion time tm is proportional to the depth of defect. In this system, the full sensitivity of the fluxgate sensor (0.5–1 nT) has been realized in a relatively noisy laboratory environment using an intelligent data acquisition system with built-in software.