Crystal plasticity modeling of ultrasonic softening effect considering anisotropy in the softening of slip systems

In this study, a novel approach to modeling the ultrasonic softening effect during metal plasticity is developed, where the slip systems experience differential softening depending on their orientation relative to the ultrasonic direction. The directional softening model was implemented within a Visco-Plastic Self-Consistent (VPSC) model, where the material and ultrasonic softening parameters are calibrated based on micro-tensile test data of pure copper with tension along the rolling direction of sheet. The model is then validated against the stress-strain response of the samples with tension along transverse direction. The VPSC modeling results provide new insights into ultrasonic softening, particularly that the stress reduction is not homogeneous in the whole aggregate. The degree of softening shows a strong dependence on grain orientation. A higher level of stress reduction is observed in plastically hard, high Taylor factor grains. A decrease in Taylor factor is predicted, especially in the grain subset with higher stress reduction, which agrees with experimental data. Although a traditional isotropic softening model is also capable in predicting the ultrasonically softened stress-strain response for different texture inputs, this decrease in Taylor factor cannot be captured.