Progress in braided composite tube crush simulation

The high Specific Energy Absorption (SEA) of composite tubular structures makes them attractive candidates in energy absorbing structural applications such as front rails in vehicles. To incorporate primary composite components in vehicle structures requires numerical simulation tools that can predict the structural performance of the vehicle under various loading conditions including crashworthiness. In previous studies, axial crush simulations of braided composite tubes tended to generate global buckling, which are inconsistent with the steady crush behavior observed in experiments. It was found that the constitutive models based on the continuum damage mechanics (CDM) framework are inadequate to represent the unloading response of damaged composites. In axial crush experiments, braided composite tubes form multiple continuous crush fronds. Local unloading occurs when material moving out of the crush front becomes part of the crush frond. Improper representation of the material unloading response affects the computed total energy absorption of the structure. To address this issue, an analog model was developed to describe the unloading path of compressively damaged composites. This approach was implemented in CODAM, as a user defined composite CDM model for the explicit finite element code LS-DYNA®. The improved CODAM model results in a significantly improved prediction of the tube crush.