High-Frequency Hall Coefficient Spectroscopy for Nondestructive Characterization of Shot-Peened IN718

The authors have recently developed a new technique for nondestructive Hall coefficient measurement based on inductive sensing of the Hall-Corbin current produced by the injection of high-frequency alternating current into the component under test. In this study, the feasibility of characterizing the combined influence of near-surface residual stress and cold work based on Hall impedance spectroscopy was investigated in shot-peened fully hardened IN718 coupons. First, a simple analytical approximation is proposed that allows the direct prediction of the measured frequency-dependent Hall impedance from the depth-dependent Hall coefficient and conductivity profiles. This approximation is based on the simplistic approximation originally developed for eddy current conductivity depth profiling that has been modified for Hall coefficient measurements. According to this approximation, Hall coefficient measurements exhibit roughly half the penetration depth of eddy current conductivity measurements taken at the same frequency. Typical depth profiles of the Hall coefficient and electric conductivity were estimated from residual stress and cold work depth profiles obtained by destructive X-ray diffraction (XRD) measurements. The corresponding Hall coefficient and electric conductivity depth profiles were determined using gauge factors obtained from experiments previously conducted to study the influence of applied stress and cold work on the Hall coefficient and electric conductivity in fully hardened IN718. These Hall coefficient and conductivity depth profiles were then used to predict the Hall impedance spectra using COMSOL FE simulations and the analytical approximation. The results of finite element simulations were in reasonable agreement with the analytical approximation and validate the predicted lower penetration depth of Hall current compared to the conduction current. Finally, the feasibility of Hall impedance measurements was demonstrated by experiments conducted between 100 kHz and 30 MHz on fully hardened IN718 coupons surface treated to three different shot-peening levels of 4A, 8A, and 12A Almen intensity.