Effect of Strain on the Elastic Anisotropy and Ultrasonic Wave Velocities in Low-Carbon Steel

Abstract

The results of experimental studies of the effect of plastic deformation on the velocities of transverse, longitudinal and head waves in hot-rolled sheet steel 20 are presented. Sheet-type specimens cut from the sheet along and across the rolling direction were tested in stages for uniaxial tension with a residual strain step of 4%.The tensile diagrams obtained for the specimens have the yield plateau, which is typical for low-carbon steel. After each stage, the specimens were unloaded and structurally sensitive ultrasonic measurements were carried out using shear, longitudinal and head wave transducers.It is found that the velocities of shear and longitudinal waves vary slightly until the moment of loss in local stability and the velocity of head waves decreases monotonically and in fact linearly with plastic deformation. The change in the anisotropy of the material elastic properties during plastic deformation is also investigated: monotonic dependences of the acoustic anisotropy parameter (birefringence of shear waves) and the relative difference in the velocities of longitudinal and head waves on the residual plastic strain are obtained. The presented ultrasonic investigations provide information about changes in the material structure due to plastic deformation: texture evolution and microdamage accumulation. Correlations between the material damage parameter and the elastic anisotropy parameters are obtained, which can be used for ultrasonic monitoring the state of plastically deformed steel. Additional optical studies of microstructure evolution on the surface have shown that during plastic deformation, grain rotations and localization of plastic deformation (Luders–Chernov bands) occur, as evidenced by the defocusing of individual microzones of the image, and persistent slip bands are formed in individual grains.