Theoretical Characterization and Comparison of Resonant-Probe Microwave Eddy-Current Testing with Conventional Low-Frequency Eddy-Current Methods

During the past two years small ferromagnetic resonators (FMR) operating at frequencies above 1000 MHz have been shown to offer promise as a new type of eddy-current probe for surface-breaking cracks in metals. Strong signals have been obtained from both electrical discharge machining (EDM) notches and actual fatigue cracks, and the FMR probes have been shown to have lift-off discrimination capability. Since the skin depth at these microwave frequencies is of the order of microns, the surface currents must penetrate the crack and, in order to detect its presence, the flaw detection mechanism must be necessarily somewhat different in detail than for conventional low-frequency eddy-current methods, where the flaw has dimensions typically of the order of the skin depth. A general theory, based on the Lorentz reciprocity relation and applicable to all types of detection systems, is reviewed and used as a vehicle for comparing microwave with low-frequency eddy-current techniques. In the microwave case, the FMR resonator is emphasized but other probe geometries are also considered.