A model for the crush of polymer matrix composite tubes.

Abstract

Understanding the crush behavior of structures manufactured from polymer matrix composites is important to the automotive industry because of the potential use of these materials in areas of the vehicle where crash energy management must be considered. Because of the many variables involved in the fabrication of composites, it is desirable to develop a modelling technique to allow the prediction of crush performance based on a limited number of material properties. The role of mode I interlaminar fracture in the crush of composites was studied to determine if a model could be developed to predict its contribution to the crushing process. Laminated strips, with a midplane delamination at the end, were compressed against a crush plate to propagate the delamination crack down the length of the strip. Energy models were developed to account for strain, friction, and mode I fracture energies. These models were then evaluated to predict the force required to propagate the crack as a function of crack length. The force and crack length at which the process reached a steady state condition were also predicted. The model was modified to account for mode II interlaminar fracture and in-plane fracture that occur in composite tubes undergoing stable crush. Crush force as a function of crack depth was predicted for round tubes. The model predictions for the laminated strips were in excellent agreement with experimental results both during the early stages of crack propagation and during steady state crack growth. Crushed tubes from previous studies were examined. Based on assumed properties for the tube materials and an observed crack depth, the model provided a reasonable prediction of crush force. The modelling technique used in this study, in combination with finite element analysis appears to be a promising approach for prediction of crush performance of composite structures.