Prediction of forming limit strains under strain-path changes: Application of an anisotropic model based on texture and dislocation structure

Strain-path changes strongly influence the forming limit:strains of sheet metals. The value of the limit strains is greatly affected by material-related effects such as initial anisotropy, transient hardening, Bauschinger effect and cross hardening. A model which can describe these mechanical behaviours has been developed on the physical basis of texture and dislocation structure , and applied in conjunction with the Marciniak-Kuczynski analysis of the forming limit strains. The results are represented in forming limit diagrams (FLDs) in which the forming limit strains are indicated. The calculation successfully predicts some of the experimental tendencies which cannot be reproduced by conventional phenomenological models. Furthermore, the model has been used to discuss the effects of texture and dislocation structure on the FLDs. Especially, it is suggested that transient hardening caused by the latent part of the persistent dislocation structure significantly reduces the forming limit strain for a strain-path change from equi-biaxial stretching to uniaxial tension. (C) 1998 Elsevier Science Ltd. All rights reserved.status: publishe

High Strain Rate Properties of High Strength Steel Sheets

High strain rate properties of sheet steel largely depend on its strength level and strengthening mechanisms. The strain rate sensitivity of flow stress has a close relation with the hardness of a softer phase, which indicates superior strain rate sensitivity of multi-phase steels such as DP and TRIP steels. Simple-shear tests have revealed an increase of the flow stress with strain even for ultra high strength steel at large strains. Based on these experimental evidences and physical understandings, an appropriate constitutive model for crash simulation is proposed and discussed