Electronic Thesis and Dissertation Repository


Doctor of Philosophy


Mechanical and Materials Engineering


Dr. Jeff Wood


Polymer composite design in energy absorbing components requires a failure criterion that can predict the energy involved in its fracture under different modes of loading. Present mixed mode criteria are mainly empirical or semi-empirical, and are only suitable for a small range of composite types.

The purpose of this study was to develop a mechanistic failure criterion that is applicable to a wide range of polymer composites. An energy based mechanistic failure criterion is proposed to characterize the toughness of unidirectional (UD) and randomly oriented short fibre composites (random fibre composites).

In UD and random composites, the criterion predicts the energy absorbed in the material during fracture based only on the constituents and interfacial properties. In UD composites, the criterion accounts for the resin fracture energy, hackle formation, interfacial debonding and effects of the plastic zone size. In random fibre composite the criterion also includes the effect of fibre orientation and fibre pull-out energy. The pull-out energy was calculated with the help of a finite element model.

Several experiments were performed to determine the failure mechanisms that influence the energy absorbed in the fracture of the polymer composite materials. Mixed mode loading was applied to the composite specimens using a compact tension shear (CTS) fixture. The comparison of the criterion predictions and experimental data shows a very good match. The criterion is able to predict the critical strain energy release rate (CSERR) of the epoxy and UD composites within 7% error margin. In random fibre composites, the criterion is able to predict an upper and a lower bound for the value of CSERR that fits well with the experiment.