3. Eddy Currents
Ferromagnetic probes resonating at microwave frequencies have previously been shown to function as sensitive detectors of surface breaking flaws in metals. A swept frequency mode of operation was used, with the resonance line of the probe displayed on an oscilloscope and the presence of a flaw indicated by a shift of the resonance line. This type of presentation lacks good discrimination between lift-off and and flaw signals and also reduces the probe scanning speed because the entire resonance line must be swept at each measurement point on the test specimen. Our new system operates under cw conditions, with a network analyser type of display giving the probe input impedance in polar coordinates. This provides liftoff discrimination and also enhances sensitivity. Experimental curves and their theoretical interpretations will be presented, and a portable prototype instrument will be described.
Auld, B. A.; Ezekiel, A.; Pettibone, D.; and Winslow, D. K., "Surface Flaw Detection with Ferromagnetic Resonance Probes" (1980). Proceedings of the DARPA/AFML Review of Progress in Quantitative NDE, July 1978–September 1979. 11.