Thin sheet metals and ultrathin metal foils produced by industrial rolling processes are textured polycrystalline materials and their mechanical behaviors may depend strongly on the orientation of applied loading. Consideration of such plastic anisotropy in advanced modeling of these materials is of the paramount importance in designing optimal manufacturing processes for automotive and other applications using finite element methods. This research addresses several critical issues in anisotropic plasticity modeling and its applications in analyzing micro channel forming of ultrathin stainless-steel foils. An experimental study has first been carried out on the accuracy and sensitivity of measuring the plastic strain ratios of an aluminum alloy AA6111-T4 thin sheet under uniaxial tension by digital image correlation. The plastic strain ratios are found to be virtually constant at the axial strain of 2$\%$ and beyond. Besides large measurement uncertainties at small strains, non-homogeneous deformation and out-of-plane translation of test coupon are found to be the main causes of their observed variations. With the aid of extensive numerical optimization calculations, a theoretical analysis has then been conducted to evaluate two formulations of convex fourth-order stress functions. Gotoh's yield function is found to be more capable and better suited for parameter identification while fourth-order Yld2000 function may be used to convexify a calibrated but non-convex Gotoh's yield function with reduced plastic anisotropy. Polycrystalline plasticity modeling has subsequently been considered to account for the discrete nature of individual grains in ultrathin foils. A quadratic crystal plasticity model of FCC single crystals with orthotropic plastic anisotropy has been successfully calibrated using finite element polycrystal plasticity modeling and uniaxial tension test data. Both macroscopic and polycrystalline plasticity models have been successfully incorporated in finite element analyses of micro channel forming of 304L stainless steel thin foils. Plastic anisotropy, grain heterogeneity distribution, strain hardening, and contact friction are all shown to affect the springback of the formed micro-channels and polycrystalline finite element simulations provide an improved prediction of experimentally measured springback.
Dr. Wei Tong
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Sheng, Jie, "Anisotropic Plasticity Modeling of Thin Sheets and Its Application to Micro Channel Forming of Steel Foils" (2021). Mechanical Engineering Research Theses and Dissertations. 38.