Deformation micromechanisms in polymeric matrices and carbon fibre reinforced composites

Abstract

The objective of this study is to identify the deformation micromechanisms operative in carbon fibre reinforced polymer composites and in the base polymers that act as the matrix material. The aim is to understand the reasons for the low delamination toughness encountered in the composites, and add to the knowledge required to attempt a rational synthesis of tougher composites. The focus is on micromechanisms of deformation and fracture in both matrix and composite, and in correlating these to the macroscopic fracture toughness. Fracture toughness tests, fractography and in situ crack observations in the scanning electron microscope were the main tools employed to achieve these objectives. The primary fracture micromechanism identified in both matrix and composite has been termed microrupture, and a qualitative model describing this mechanism has been proposed. Other micromechanisms observed in the matrices were: ductile tearing induced by the cavitation of elastomeric particles, microshear band formation at the free surface, fibrillation leading to the formation of craze-like structures in the toughened epoxies, and yielding in diffuse shear bands which was very prominent in the thermoplastic Lexan and also evident in the modified epoxies. Of these the last is completely absent in the composite due to constraint of the fibres. The others are all manifest in the composite but to a much lesser extent than in the matrix. The most significant reason for the lack of toughness in the composite as identified by this investigation, is the ease of matrix cracking perpendicular to the fibre direction due to crack tunnelling along the fibre-matrix interface. Rational synthesis of tough composites would require that the interfacial strength and toughness be better than the cohesive strength and toughness of the matrix, so that effective load transfer from the fibres to the matrix can occur at the delamination crack tip.