| Weight reduction has long been identified as a key priority for improving automotive fuel economy, and many studies often suggest substituting lightweight materials for typical steel applications. However, replacing steel in the structure and body of an automobile with lighter materials such as aluminum, magnesium, plastics, composites, etc., can be costly and is not simple or straightforward. Aluminum sheet, in particular, has lower formability at room temperature than typical sheet steel.; Numerical analysis is critically important to understanding the complex deformation mechanics that occur during sheet forming processes. Finite element analysis (FEA) and simulations are used in automotive design and formability processes to predict deformation behavior accurately during stamping operations. Although available commercial FEA codes offer a library of material models applicable to a variety of applications, they often do not offer highly specialized material models developed for a specific material and process. Also, very few available material models are capable of handling complex forming simulations that incorporate the temperature-dependence of materials, especially for anisotropic materials such as aluminum sheets.; The goal of this research was to develop a temperature-dependent anisotropic material model for use in a coupled thermo-mechanical finite element analysis of the forming of aluminum sheets. The anisotropic properties of the aluminum alloy sheet AA3003-H111 were characterized for a range of temperatures 25°C-260°C (77°F-500°F) and for different strain rates. After performing standard ASTM uniaxial tensile tests, the anisotropy coefficients for two accurate material models: Barlat YLD96 and Barlat YLD2000-2d, both in the plane stress condition, were calculated for several elevated temperatures.; The developed temperature-dependent anisotropic material models (YLD96 and YLD2000-2d) were then implemented in the commercial FEM code LS-Dyna as a user material subroutine (UMAT) using the cutting-plane algorithm proposed by Simo et al. (1985) for the integration of a general class of elastoplastic constitutive models. Exact implementation of the yield functions and their derivatives are presented. The temperature-dependent material models were then used to numerically simulate the thermo-coupled finite element model for pure stretch conditions in order to compare the accuracy of the UMAT's ability to predict both forming behavior and failure locations with experimental results of the pure stretch forming process for AA3003-H111 under several elevated temperatures.; The favorable comparison found between the numerical and experimental data shows that a promising future exists for the development of more accurate temperature-dependent yield functions to apply to thermo-hydroforming process. |
