Abstract
Due to their high strength characteristics and excellent corrosion resistance behavior operating at different temperature ranges, titanium and nickel alloys have been gaining rapid popularity as part of weld overlay and metal spraying technologies. This has been raising the demand for a reliable and practical nondestructive evaluation technique to assess metallic coating thicknesses over nonmagnetic metals. Because of its frequency-dependent penetration depth, eddy current technology lends itself to assess such nonmagnetic layered structures. Nevertheless, impedance forward measurement techniques are sensitive to both coating/substrate conductivities and lift-off deviations from those used over the calibration blocks. This affects the accuracy and limits the capabilities of existing impedance-based inversion models in estimating coating thicknesses. This study takes the first step towards adopting AECC spectroscopy as an alternative forward measurement technique, which offers a reduced sensitivity to both deviations in a broad range of inspection frequencies. More importantly, a new AECC-based inversion algorithm is developed to estimate metallic coating thicknesses over nonmagnetic metals. This work is validated following the plane-wave approximation and COMSOL simulations over different coating thicknesses relevant to the industry.
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