Constraint effects on energy absorption in unidirectional PMC tubes

An investigation was carried out to understand the effect of an external co nstraint on the failure modes and energy absorption capability of unidirect ional polymeric composite (PMC) tubes. The constraint was a steel ring that was tightened on the outside of the tube. This ring could slide during tub e crushing. The tightness of the ring was determined using a strain gage po sitioned on the ring. The effect of the constraint was shown to depend on t he initial position of the ring along the tube. The study was performed on fiberglass/polyester tubes and the results showed that the external constra int not only can improve the specific energy absorption of the tubes, but a lso can reduce significantly the undesired high initial peak loads. A nonli near finite element approach was used to model the fundamental failure mode s that dominate the energy absorption of the tubes with an external constra int. These modes including delamination and longitudinal crack propagation (splitting), were modeled using the finite element code ABAQUS. A user frie ndly FORTRAN subroutine was implemented to the code to incorporate the mate rial failure model for predicting the fiber/matrix failures that occur as c rushing is experienced by the tube. The predicted model results compared we ll with the experimental data for the various crushing stages of the polyes ter tubes.