Grain size effect on L_cr elastic wave for surface stress measurement of carbon steel

Abstract

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.

Keywords:

• Grain size
• correction of stress measurement
• ultrasonic wave
• elastic plastic deformation