A Novel Microwave Ferromagnetic Resonance Probe for Eddy Current Detection of Surface Flaws in Metals

A new type of eddy current probe has been developed for detecting surface flaws in metals. By contrast with traditionel devices it operates at microwave frequencies and uses an yttrium iron garnet (YIG) ferromagnetic resonator. Such resonators experience a frequency shift when brought into proximity with a metal surface because of the perturbation of the microwave field external to the resonator. A crack in the metal disturbs the surface currents and produces an additional perturbation of the frequency. This paper reports the basic principles of operation and results of experiments performed on machined slots in aluminium. Fractional frequency shifts as large as 0.5 percent were observed for a slot with an opening 115 ¿m wide. An unloaded fatigue crack in a nonmagnetic stainless steel test sample was also easily detected, in accord with the simple theory, which predicts a response proportional to the slot opening.