Metastable austenite driven work-hardening behaviour in a TRIP-assisted dual phase steel

The mechanically-induced transformation behaviour of the metastable austenite phase in a high-strength industrial TRIP-assisted Dual Phase steel was monitored in situ using high-energy synchrotron diffraction under uniaxial loading. This allowed direct quantification of the impact of the transformation of the metastable austenite phase (16 vol %), embedded in a ferrite-bainite-martensite matrix, on the work hardening behaviour of this steel. Our results show that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding. We provide experimental evidence which demonstrates for the first time that the austenite transformation increases the work-hardening contribution, σw thereby supporting a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding and continues to offset the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Further we show conclusively that martensite yielding does not occur until the completion of the mechanically induced transformation of austenite. Plastic deformation of martensite is immediately followed by local plastic instability leading to necking and ultimate failure of this material

Viscoplastic regularization of local damage models: revisited

Local damage models are known to produce pathological mesh dependent results. Regularization techniques are therefore mandatory if local damage models are used for academic research or industrial applications. The viscoplastic framework can be used for regularization of local damage models. Despite of the easy implementation of viscoplasticity, thismethod of regularization did not gain much popularity in comparison to the non-local or gradient damage models. This work is an effort to further explore viscoplastic regularization for quasi-static problems. The focus of this work is on ductile materials. Two different types of strain rate hardening\ud models i.e. the Power law (with a multiplicative strain rate part) and the simplified Bergström van Liempt (with an additive strain rate part) models are used in this study. The modified Lemaitre’s anisotropic damage model with a strain\ud rate dependency was used in this study. It was found that the primary viscoplastic length scale is a function of the hardening and softening (damage) parameters and does not depend upon the prescribed strain rate whereas the secondary length scale is a function of the strain rate. As damage grows, the effective regularization length gradually decreases.When the effective regularization length gets shorter than the element length numerical results become mesh dependent again. This loss of objectivity can not be solved but the effect can be minimized by selecting a very fine mesh or by prescribing high deformation velocities