Coupled effects of texture, hardening and non-proportionality of strain on ductility in ferritic steel

Non-proportionality of straining, initial texture and hardening have been investigated in polycrystal BCC steel using a crystal plasticity finite element framework. Two extreme forms of hardening are investigated; namely, isotropic latent hardening and anisotropic self-hardening. Dislocation density evolutions on all independent slip systems have been calculated in order to investigate the establishment of dislocation distributions and their dependence on non-proportionality, hardening and initial texture. Also, the limit strains under both hardening rules have been predicted along the non-proportional strain paths to identical final uniaxial and biaxial states. Further, a measure of non-proportionality has been introduced which enables differentiation of the effects of initial texture. The results for two textures (initially random and textured polycrystal) are discussed and the coupled effects of texture, hardening and non-proportionality on forming limit investigated. Examination of dislocation distributions for an initially random texture subjected to non-proportional straining shows that differences in structures develop which are more pronounced under uniaxial as opposed to biaxial straining. Non-proportionality in strain path is shown to have a significant effect on strains to localization, leading both to increases and decreases in ductility depending upon level of non-proportionality. The basis for this behavior is the path dependence of the development of dislocation distributions. There is the potential, therefore, for the selection of pre-forming texture and non-proportionality path in order to maximize forming limit strains. Crown