Constraint effects on energy absorption in unidirectional polymeric composite 'PMC' tubes

The effect of using an external constraint on the crushing mechanisms and energy absorption of unidirectional polymer matrix composite tubes was studied in detail. The constraint is a steel ring that is tightened outside the tube and has the ability to slide along the tube as the crushing proceeds, supporting the splaying of the unidirectional fibers and improving the capability of the tubes to absorb energy in the crushing process. Both experimental and finite element approaches were used to evaluated and understand the role of major failure modes on the crushing performance of unconstrained and constrained tubes. Geometrical effects were studied by testing both circular and square tubes. The effect of matrix was evaluated by using polyester and vinyl ester matrix with fiberglass. The effect of initiator radius was studied by crushing the polymeric tubes with flat platens and initiator with 12 mm and 4 mm radii. Non-linear finite element technique was used to model the major failure modes including axial splitting and delamination for both quasi-static and dynamic loading conditions. Chang-Lessard and Chang-Chang material failure models were used in static and dynamic analysis, respectively. Analysis was carried out using ABAQUS and LS-DYNA non-linear codes. Experimental and computational results show that the external constraint can significantly improve the crushing performance of the polymeric tubes by improving the sustained crushing load level and reducing the undesired initial peak load. Also, the specific energy absorption was improved in all cases when an external constraint was used. Finite element models were able to predict the fundamental mechanisms that control the energy absorption capability of the tubes in the crushing process for both quasi-static and dynamic loading conditions.