Analysis and Characterization of Damage Using a Generalized Composite Material Model Suitable for Impact Problems

The need for accurate material models to simulate the deformation, damage, and failure of polymer matrix composites under impact conditions is becoming more and more critical, because these materials are gaining increased usage in the aerospace and automotive communities. Although there are several composite material models currently available within commercial transient dynamic finite-element codes, several features have been identified as lacking in these models, which could substantially enhance the predictive capability of composite impact simulations. One specific desired feature includes incorporating both plasticity and damage within the material model. Another desired feature relates to using experimentally based tabulated input to define the evolution of yield stresses, plastic strains, and damage parameters as opposed to specifying discrete input properties (such as modulus and strength) and using analytical functions to track the response of the material. To begin to address these needs, an orthotropic, macroscopic constitutive model incorporating both plasticity and damage is being developed for implementation within a commonly used commercial transient dynamic finite-element code. The plasticity model is based on extending the Tsai-Wu composite failure model into a strain hardening-based orthotropic plasticity model with a nonassociative flow rule. The evolution of the yield surface is determined based on tabulated stress-strain curves in the various normal and shear directions, and is tracked by using the effective plastic strain. To compute the evolution of damage, a strain-equivalent semicoupled formulation is used, in which a load in one direction results in a stiffness reduction in multiple material coordinate directions. A detailed analysis is carried out to ensure that the strain-equivalence assumption is appropriate for the derived plasticity and damage formulations that are used in the current model. Procedures to develop the appropriate input curves for the damage model are presented and the process required to develop an appropriate characterization test matrix is discussed.