Finite element modelling of the progressive crushing of braided composite tubes under axial impact

Composite tubular structures are of interest as viable energy absorbing components in vehicular front rail structures to improve crashworthiness. Desirable tools in designing such structures are models capable of simulating damage growth in composite materials. Our model (CODAM for COmposite DAMage), which is a continuum damage mechanics based model for composite materials with physically based inputs, has shown promise in predicting damage evolution and failure in composites. In this study, the model is used to simulate the damage propagation, failure morphology and energy absorption in triaxially braided composite tubes under axial compression. The model parameters are based on results from standard and specialized material testing and a crack band scaling law is used to minimize mesh sensitivity (or lack of objectivity) of the numerical results. Axial crushing of two-ply and four-ply square tubes with and without the presence of an external plug initiator are simulated in LS-DYNA. Refinements over previous attempts by the authors include the addition of a pre-defined debris wedge, a distinguishing feature in tubes displaying a splaying mode of failure, and representation of delamination using a tiebreak contact interface that allows energy absorption through the un-tying process. It is shown that the model adequately predicts the failure characteristics and energy absorption of the crushing events. Using numerical simulations, the process of damage progression is investigated in detail and energy absorptions in different damage mechanisms are presented quantitatively.