Rate-dependent isotropic-kinematic hardening model in tension-compression of TRIP and TWIP steel sheets

This paper presents a rate-dependent isotropic-kinematic hardening model in tension-compression of TRIP and TWIP steel sheets. The isotropic-kinematic hardening model is widely utilized to describe the Bauschinger effect, transient behavior and permanent softening under reverse loading which are indispensable for numerical simulation of springback in sheet metal forming. The isotropic-kinematic hardening model, however, has not yet been suggested for the strain rate effect higher than several tens per second although the high strain rate prevails in practical automotive sheet metal forming. This paper proposes a rate-dependent model based on tension-compression tests of TRIP980 and TWIP980 steel sheets at various strain rates ranging from 0.001 s(-1) to 100 s(-1). A proposed rate-dependent model is extended from the rate-independent Chaboche type model based on single-surface plasticity. Among three Chaboche type models, the Zang's model is selected as the basic rate independent model considering both a small change of the work-hardening rate in monotonic loading and a constant stress offset of permanent softening in reverse loading. With the basic rate-independent model, the material parameters are acquired at each strain rate to check their dependency on the strain rate and then formulated as linear or exponential functions of the logarithmic scale of the strain rate. Consequently, the present rate-dependent model is proposed with incorporation of the basic rate-independent model and the rate-dependent functions for the material parameters. (C) 2017 Elsevier Ltd. All rights reserved.