Fatigue and fracture of short fibre composites exposed to extreme temperatures

Fibre-reinforced composites have been used for more than 50 years and are still being evolving in terms of material integrity, manufacturing process and its performance under adverse conditions. The advent of graphite fibres from polyacrylonitrile organic polymer has resulted in a high performance material, namely carbon based composites, performing better in every respect than glass fibre-reinforced plastic (GFRP). However, glass fibres are still in high demand for wide applications, where the cost takes precedence over performance. Owing to its quasi-isotropic properties, randomly orientated short fibre reinforced composites, particularly chopped strand mat (CSM) and sheet moulding compound, are playing a critical role in boat building industry and automotive industry, respectively. As structural performance of composite material is being improved, GFRPs are expected to replace metals in more harsh applications, in which high cyclic loadings and elevated temperatures are applied. Furthermore, heat deflection temperature of common thermosetting resin is in the range from 65ºC to 85ºC under applied stress of 1.8MPa. The thermal effects on short-fibre thermosetting composites have not been flourishingly investigated. Fatigue prediction of mechanical structure is not only critical at the design stage, but is much more critical for the maintenance strategy. The fatigue, fracture and durability of GFRP-CSM are complex issues because of so many variables contributing to thermal and mechanical damages. Despite a number of approaches to modeling fatigue damage of GFRP using phenomenological methodologies based on the strength and stiffness degradation, or physical modelling based on micro-mechanics, their performance under adverse thermo-mechanical loading has not been fully understood to benefit the end users.