Fiber reinforced composites are widely used in aerospace, automotive and other industries due to their high stiffness & strength-to-weight ratio, corrosion resistant and energy absorption ability. The use of composites in primary energy absorbing components in vehicles, however, is very limited. The lack of robust and accurate models for the prediction of crashworthiness performance of composite structures is a critical factor. The capability of crash simulations is often examined with axial impact of tubes, a benchmark problem. Among the large amount of published works on modeling composite tubes under axial impact, very few were able to predict both the failure morphologies and force-displacement responses of a crushing tube, especially for the ones without a plug initiator at the crash front. One of the problems is related to the limitations of material constitutive models. The existing composite material models used in crash simulations generally contain a few non-measurable parameters while ignoring the irreversible strains which can be important to the prediction of energy absorption. The other problem is related to the robustness of the finite element (FE) models. Traditional elements tend to have instability issues under in-plane compression.In order to solve these problems, this study proposes a new crash model which composes of an Enhanced Continuum Damage Mechanics (ECDM) model and a Shell-Beam (SB) element modeling method. The ECDM model consists of a pre-failure sub-model, based on the modified Ladevèze model, and a post-failure sub-model. It considers both the matrix plasticity and irreversible strains due to damage. The SB element method enhances the shell element with real out-of-plane properties by introducing beam elements in the out-of-plane direction. As a result, the SB element is stable under in-plane compression while retaining the efficiency of the shells. The ECDM has been implemented in LS-DYNA as a user-defined material model.To evaluate the predictive capability of this new crash model, simulations are carried out for both quasi-static tests and dynamic tube crash tests of 2D triaxial braided composites (2D3A). For dynamic tube crash tests, two different groups of tests are done on composite tubes, with and without a plug initiator. The results show that the ECDM model is able to simulate the force and energy responses as well as the failure morphologies more accurately than a widely used CDM model available in LS-DYNA. On the other hand, the SB element method allows one to capture most of the damage modes of a composite tube under axial crash without the numerical difficulties experienced by using traditional elements, like negative element volume, infinite small timesteps, and instability. Both ECDM model and SB element method are also robust enough to be used on different geometries.To have a better understanding of ECDM model and shell-beam method, a sensitivity study is carried out at the end of this study for key model parameters, such as the tiebreak contact strength, friction coefficient, element deletion strain, etc.
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