Ultrasonic Measurement of Stress Using a Rotating EMAT

Abstract

The acoustic birefringence method is primarily used to measure differences of principal stresses in regions where principal stresses coincide with material symmetry axes. To determine the differences of principal stresses also requires knowledge of the unstressed birefringence at the measurement locations. Consequently, variability in texture can introduce errors in stress determination. In contrast, the ultrasonic measurement of shear stress is independent of texture. The shear stress causes a rotation φ of the pure-mode polarization directions of the SH waves used in our experiments. We constructed a motorized electromagnetic-acoustic transducer (EMAT) and used it to measure phase as the EMAT is rotated. We developed an algorithm to determine φ and the birefringence B. We measured the shear stress along parallel scanlines in a residual-stress specimen with known stress state. We calculated the shear stress gradient and used it in the stress-equilibrium equation to determine the normal stress acting along the scanline direction. The other plane stress component was determined from an acoustoelastic relation between B and the difference of normal stresses. Good agreement was obtained with theoretical stress values.