Investigation on Stress Relaxation Behavior of High-Strength Steel Sheets Based on Elasto-viscoplasticity

Stress relaxation is the phenomenon where stress of materials decreases under constant strain. In several previous studies, it was found that the stress relaxation makes uniform elongation larger, showing a possibility that this phenomenon can be utilized to increase the forming limit in combination with the flexible slide motion of a servo press. However, the stress relaxation phenomenon has not yet been sufficiently clarified. Authors previously investigated the stress relaxation behavior by applying several models where stress relaxation was described as an elasto-viscoplasticity behavior. However, a unified and quantitative description of strain rate sensitivity of flow stress and stress relaxation has not been sufficiently studied. In this study, we investigated the influence of strain, strain rate and relaxation time on stress relaxation phenomena of high strength steel sheets. Strain rate sensitivity of flow stress was modelled with m-power law. Stress relaxation behavior was also successfully approximated by a model derived from the m-power law with the parameters obtained by strain rate sensitivity tests, which suggests that both the strain rate sensitivity and the stress relaxation were based on a unified elasto-viscoplasticity. The mechanisms of stress relaxation was also discussed through numerical analyses.