| In this dissertation, emphasis is placed on understanding the formability of sheet metal for different forming models (drawing, stretching and bending) and forming parameters (material properties, lubrication, tool geometry and press speed). In the study of deep drawing, redrawing and punch stretching, the Limit Draw Ratio, an important property of formability of sheet metal, is studied in detail. Influences of plastic anisotropy, contact friction, geometry and thickness variation on the Limit Draw Ratio are considered through a general two-dimension axisymmetrical shell of revolution theory incorporated into a general non-quadratic anisotropic yield criterion.; In the present investigation, a new model based on M-K method, using Barlat and Lian's non-quadratic anisotropic yield criterion, is developed by introducing a strain gradient term in the constitutive equation to consider the bending effect on the localized necking in anisotropic sheets. The effects of strain gradient, punch curvature and sheet thickness on the Forming Limit Diagram, besides the effects of strain and strain rate hardening, plastic anisotropy and deformation heating, are then considered for the first time. The fact that the forming limit increases as the punch curvature increases is predictable by the present model. Furthermore, the effect of contact friction on the localized necking is also studied based on the strip friction test and the strain distribution during strain localization is obtained. The findings of the effects of strain gradient, punch curvature, sheet thickness and contact friction on the localized necking (forming limit) are very helpful to understand the formability of sheet metal in practical applications.; In addition, a continuum model in finite deformation plasticity (both rate-dependent and rate-independent) with nonlinear combined isotropic/kinematic hardening is developed by considering the coupling effects of dislocation hardening, recovery softening and material rotation. Different stress responses in tension, compression and torsion (directional softening) are simulated and the complicated axial effects induced in torsion are predicted and they are in good agreement with available experimental data. All parameters in the constitutive equation developed here can be uniquely determined by the simple tests: tension, compression and torsion. (Abstract shortened with permission of author.)... |
