Application of the double torsion specimen to the study of fracture in fibre reinforced plastics

Abstract

The increasing use of fibre reinforced polymers in structural components often requires an accurate assessment of the strength of the component. The strength of composite materials is usually based on the strength of an individual lamina. This is then combined in a manner depending on the orientation of the plies within the laminate. The actual failure process is often ignored in this type of analysis. Composite failure is the result of damage accumulation from a number of failure modes, in particular, fibre failure, matrix failure and failure of the interface between the fibres and matrix. Measurement of the interfacial strength requires specialised testing techniques in order to obtain accurate characterization of the interfacial failure processes. This research uses a double torsion specimen reinforced with fibres in a number of configurations. The testing techniques developed allow the interaction of a matrix crack with fibres, resulting in the failure of the interface. Finite element analysis has been used to gain an insight into the deformation mechanisms. A compliance change analysis has been developed so that the load in the fibres can be calculated. Results from the finite element analysis confirm the analytical procedures and show that, for the fibre/resin combination tested, the interface has a lower fracture toughness than the matrix material. The interaction between the fibres and matrix shows that the mechanism of fibre bridging inhibits the propagation of matrix cracks. This produces an apparent increase in the toughness of the composite system. To confirm the failure processes occurring, the technique of acoustic emission has been used to monitor the development of the specimen failure. In line with other workers, it is shown that matrix failure produces low amplitude events and interfacial failure produces mid amplitude events. Fibre failure did not occur to any significant degree. This thesis shows how the contribution from the presence of an interface affects the fracture of composite materials and how, via the reinforced double torsion specimen, this contribution can be measured and interpreted.