Failure of Unidirectional Fiber Reinforced Composites: A Case Study in Strength of Materials

The traditional approach of strength of materials applied to failure of unidirectional fiber reinforced composites is examined in view of the failure mechanisms observed. While the elastic limit of isotropic materials such as metals is given by the measured yield strength and reflects the manifestation of the underlying crystalline slip processes that is not the case for unidirectional composites that are orthotropic and have different independent mechanisms operative under axial tension, axial compression, transverse tension, transverse compression, and in-plane shear. The paper discusses why the generalization of yield theories of isotropic metals to textured metals of orthotropic symmetry cannot be used for describing failure of unidirectional composites. Furthermore, the failure criteria proposed in the literature to describe interactions under combined loading are scrutinized to clarify their inadequacy to treat those interactions. The use of tractions on an assumed failure plane to formulate failure criteria is also discussed as without basis in the failure mechanisms observed. Finally, a way forward for physical modeling of failure in unidirectional composites is proposed.