Finite element modelling of the progressive crushing of braided composite tubes under axial impact Carla McGregor a , Reza Vaziri a, * , Xinran Xiao b,1 a Composites Group, Departments of Civil Engineering and Materials Engineering, The University of British Columbia, 6250 Applied Science Lane, Vancouver, B.C., Canada, V6T 1Z4 b General Motors Corporation, Research & Development, MC 480 106 710, 30500 Mound Road, Warren, MI 48090 9055, USA article info Article history: Received 2 December 2008 Received in revised form 9 September 2009 Accepted 21 September 2009 Available online 29 September 2009 Keywords: Braided composite tubes Progressive crushing Energy absorption Numerical simulation Damage mechanics abstract 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 dis- tinguishing 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. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The automotive industry is continually trying to reduce the weight and thus the emissions of vehicles while maintaining the same level of safety and crashworthiness. To this end, lightweight, high-performance composite materials are replacing conventional materials in many structural and non-structural components. The front side rail is one such component that is a candidate for redesign. This component is responsible for absorbing front impact energy in a controlled and reliable manner, bringing the passenger compartment to rest under acceptable levels of deceleration. Current metallic front rail component accomplish this through progressive plastic folding, whereas composite tubes absorb this impact energy through a combination of complex damage mech- anisms, including fibre breakage, matrix cracking, delamination and friction. The ensuing crush morphology can be generally categorized into one of three modes: (1) fragmentation mode – sequential damage of material at the micro-structural level, (2) splaying mode – formation of continuous fronds in combination with transverse tearing, and (3) progressive folding mode – in a manner similar to that observed in metal tubes. As shown in Fig. 1 , the braided tubes of interest in this study progressively crush in a splaying mode with the formation of continuous fronds that bend outward (with plug initiator) or both outward and inward (without plug initiator). Although the literature indicates that stable progressive crush- ing and higher levels of specific energy absorption (SEA) are attainable in composite tubes [1–6], two factors are delaying the widespread use of them in vehicles: (1) high material and manufacturing costs, and (2) lack of numerical models capable of accurately predicting their response. Improvements to low-cost manufacturing methods, such as resin transfer moulding, are making economical mass production more realistic. However, time- consuming testing and evaluation of prototypes is still necessary due to the proven difficulty associated with simulating the response of these quasi-brittle tubes under axial compressive loads. * Corresponding Author. Tel.: þ1 604 822 2800. E-mail address: reza.vaziri@ubc.ca (R. Vaziri). 1 Current address: Composite Vehicle Research Center, Department of Mechan- ical Engineering, Michigan State University, 2555 Engineering Building, East Lans- ing, MI 48824 Contents lists available at ScienceDirect International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng 0734-743X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2009.09.005 International Journal of Impact Engineering 37 (2010) 662–672