Residual Stress measurements in Metal Objects using Four Coils

Author:  Jack A. Soules
Source:  US Patent 5,898,302
Doc ID:  1999002
Year of Publication:  1999
Abstract:  
A device for measuring residual stress in ferromagnetic and non-ferromagnetic metal objects. The device having four electrically identical induction coils which form a four terminal alternating current bridge circuit. There is a fine wire shield formed of fingers for shielding the coils from stray capacitance. The bridge having four coil terminals. Two diagonally opposite coil terminals are connected to a variable frequency constant voltage generator. The other two diagonally opposite coil terminals are connected to a low noise broad band preamplifier. The preamplifier amplifies any unbalance in the bridge. There is a double pole double throw switch connected to two coil terminals which are diagonally opposite each other. Connected to the preamplifier is an amplifier. The amplifier is connected to a phase detector which is connected to a computer. The phase detector detects in phase and quadrature component signals. The computer has software for determining changes in differential resistivity as a function of frequency and for converting resistivity differences into residual stress using an algorithm.