Grain size effect on L-cr elastic wave for surface stress measurement of carbon steel

摘要

Based on critical refraction longitudinal wave (L-cr wave) acoustoelastic theory, correction method for grain size effect on surface stress measurement was discussed in this paper. Two fixed distance L-cr wave transducers were used to collect L-cr wave, and difference in time of flight between L-cr waves was calculated with cross-correlation coefficient function, at last relationship of L-cr wave acoustoelastic coefficient and grain size was obtained. Results show that as grain size increases, propagation velocity of L-cr wave decreases, one cycle is optimal step length for calculating difference in time of flight between L-cr wave. When stress value is within stress turning point, relationship of difference in time of flight between L-cr wave and stress is basically consistent with L-cr wave acoustoelastic theory, while there is a deviation and it is higher gradually as stress increasing. Inhomogeneous elastic plastic deformation because of inhomogeneous microstructure and average value of surface stress in a fixed distance measured with L-cr wave were considered as the two main reasons for above results. As grain size increasing, L-cr wave acoustoelastic coefficient decreases in the form of power function, then correction method for grain size effect on surface stress measurement was proposed. Finally, theoretical discussion was verified by fracture morphology observation.