Multi-Scale Modelling of Texture Evolution and Surface Roughening of BCC Metals During Sheet Forming
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This thesis examines the qualitative and quantitative variation in local plastic deformation and surface roughening due to crystallographic texture in body-centered cubic materials, specifically interstitial-free steel sheet and molybdenum foil and sheet. Complex forming operations currently used in industrial manufacturing lead to high material failure rates, due in part to the severity of the applied strain path. A multi-scale model was developed to examine the contribution of mesoscopic and local microscopic behaviour to the macroscopic constitutive response of bcc metals during deformation. The model integrated a dislocation-based hardening scheme and a Taylor-based crystal-plasticity formulation into the subroutine of an explicit dynamic FEM code, LS-DYNA. Numerical analyses using this model were able to predict not only correct grain rotation during deformation, but variations in plastic anisotropy due to initial crystallographic orientation. Simulations of molybdenum foil under uniaxial tension supported the existence of bending due to local variations in plastic anisotropy, confirmed with good quantitative agreement by experimental measurements of surface roughening. A series of two-stage strain-path tests were performed, revealing a prestrain-dependent softening of both the steel and molybdenum samples when an orthogonal secondary strain path is applied. Numerical analyses of these tests overestimate macroscopic hardening during complex loading, due in part to the dynamic nature of the FEM code used.