A strain rate-dependent distortional hardening model for nonlinear strain paths

Abstract

In this paper, a strain rate-dependent distortional hardening model is firstly proposed to describe strain rate-dependent material behaviors under linear and nonlinear strain paths changes in $0\le {\theta }_{pathchange}\le {180}^{\circ }$. The proposed model is formulated based on the simplified strain rate-independent distortional hardening model (Choi and Yoon, 2023). Any yield function could be used for the strain rate-dependent isotropic and anisotropic yielding. For the linear strain path, the strain rate-dependent isotropic hardening behavior could be explained by two state variables representing rate-dependent yielding and convergence rate of flow stress under monotonically increasing loading condition, respectively. For the nonlinear strain paths, the strain rate-dependent material behaviors such as Bauschinger effect, yield surface contraction, permanent softening, and nonlinear transient behavior could be described by modifying the evolution equations of the simplified strain rate-independent distortional hardening model with a logarithmic term of strain rate. For the verification purpose, it was used the strain-rate dependent tension-compression experiments of TRIP980 and TWIP980 (Joo et al., 2019). In addition, a high speed U-draw bending test was conducted with original and pre-strained specimens. The springback prediction in high speed U-draw bending test was performed by using strain rate-independent isotropic, strain rate-dependent isotropic-kinematic and distortional hardening models. It is identified that the proposed model showed the most accurate prediction for the pre-strained specimen where the possible bilinear and trilinear path change in $0\le {\theta }_{pathchange}\le {180}^{\circ }$ is observed while it showed the same accuracy for the original specimen where main strain path change occur in forward-reverse manner (${\theta }_{pathchange}={180}^{\circ }$).