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A comprehensive plasticity and fracture model was built for metal sheets with application to metal sheet forming and vehicle crash simulations. The combined Bai-Wierzbicki (BW [1]) and CPB06ex2 [2] (or Yld2000-2D [3]) anisotropic plasticity model was further extended to consider elevated temperature effects in additional to the effect of multiaxial stress states. A fully modularized framework was established to combine isotropic, kinematic, and cross hardening behaviors under non-linear loading conditions. The all strain based modified Mohr-Coulomb (eMMC) fracture model was used to consider material anisotropy and nonlinear strain path. The model has been implemented into Abaqus/Explicit as a user material subroutine (VUMAT). Test results on advanced high strength steels, aluminum alloy sheets and magnesium alloy sheets are used to validate the modeling and testing methodologies. Very good correlation was observed between experimental and simulation results.

Two analytical tools for failure predictions in free-expansion tube hydroforming, namely “Process Window Diagram” (PWD) and forming limit curve (FLC), are discussed in this paper. The PWD represents the incipient failure conditions of buckling, wrinkling and bursting of free-expansion tube hydroforming processes in the plane of process parameters, e.g. internal pressure versus axial compression. The PWD is a useful tool for design engineers to quickly assess part producibility and process design for tube hydroforming. An attempt is also made to draw the differences between FLCs for sheet and tube so that the appropriate FLC could be used to estimate the bursting or fracture limits in free-expansion tube hydroforming processes.

LS-DYNA3D has been widely used to perform computer simulation of sheet metal forming. In the material library of LS-DYNA3D there are a number of user defined material models. In order to take full advantage of the material subroutines, it is important for the users to be able to display user defined history variables in the post processing and to establish user-defined failure criterion. In this report, the development of a damage coupled plastic model is firstly described. The damage model is then programmed in a user defined material subroutine. This is followed by performing finite element simulation of sheet metal forming with the LS-DYNA3D that has incorporated the damage coupled plastic model. The way to display the user defined history variables and how to deal with the failure criterion during the postprocessing of ETA/DYNAFORM are described. History variable distributions at several time steps are displayed and discussed in this paper.

Applying an equilibrium approach to bifurcation the buckling of elastoplastic plates is investigated. The conditions for the onset of buckling are established for simply supported plates under biaxial loading conditions. The theory is also applied to study the elastoplastic buckling phenomena of the Yoshida Buckling Test (YBT). It demonstrated that sheet dimensions used in the original YBT specimen design does not take full advantage of the maximum value of the induced compressive stress. The peak value of the induced compressive stress, which is - 14% of the tension stress, occurs at a width to length ratio of the sheet of b/a = 0.914. While the same theoretical dimension ratio of a standard YBT specimen at b/a = 0.4 results in an average induced compressive stress to be about -6% of the tension stress. The sensitivity of the buckling load predictions to mechanical properties of material is examined for all possible combinations of ratios of applied stress resultants.