Non-destructive surface and subsurface characterization of the machined parts by using fiber optic Eddy current sensor

Abstract

This paper introduces a fiber-optic eddy current sensor (FECS) to enable non-destructive surface and subsurface characterization of the subtractive or additive manufactured metal parts. The surface and subsurface characteristics of the parts are either globally or locally altered by the manufacturing process conditions. Those characteristics can be obtained by nondestructively scanning the machined parts. The proposed FECS consists of a fiber optic sensor for displacement measurement and an eddy current sensor for the subsurface metallurgical property characterization. FECS was designed and devised by winding a copper coil around the probe of the fiber optic sensor. The eddy current is affected by the electrical conductivity and magnetic permeability of the parts. Therefore, the eddy current sensor was used for measuring the magnetic permeability for non-destructive material characterization, and the fiber optic sensor was applied to profile the geometry of the part. The polynomial fitting error of FECS was estimated at 4.77 % for the fiber optic sensor and 0.29 % for the eddy current sensor, respectively. To test the performance of FECS, machined parts under a series of process conditions were fabricated, and those parts were scanned by FECS. As a result, the permeability measured by the FECS showed good agreement with that obtained by a permeability meter for the baseline comparison. Also, the residual stress of those parts measured by X-ray diffraction showed a similar trend to the results obtained by the FECS.