Prediction of eddy current probe sensitivity for the sizing of case depth in ferrous components

The state-of-the-art eddy current techniques employ encircling coils for the nondestructive measurement of case depth in bearing components. The encircling coil technique measures bulk case depth properties in a test specimen. This approach lacks the necessary spatial resolution to detect local variations in case depth. A better approach to image case depth would be to scan a rotating test specimen with an eddy current probe, thus achieving the desired spatial resolution and 100 percent inspection. In this paper, the feasibility of employing two types of eddy current probes (absolute coil shielded pencil probe, and an absolute coil U-core probe) to measure case depth in induction hardened bearing components is theoretically studied using axisymmetric and two-dimensional finite element analysis techniques. The range of optimum probe excitation frequencies for higher detection sensitivity has been theoretically selected for the U-core probe.