12 - Investigation into fatigue strength of natural/synthetic fiber-based composite materials

Abstract

This work presents a review of work done on synthetic and natural fiber composites in terms of their fatigue strength. Composite materials are generally regarded as having better fatigue strength than metals. This is because in composite materials subjected to fatigue, although initial damage may appear early, its propagation may be arrested by the internal structure of the composite, unlike metals in which a crack may rapidly grow to final fracture. In composites subjected to fatigue loading, damage takes the form of numerous microcracks predominantly in the matrix material or at the fiber–matrix interface. The damage is sustained and spreads throughout the bulk material. Accordingly, typical S–N curves in a composite, especially for a high-stiffness carbon fiber composite, are much flatter than those for a metal. Synthetic fiber composites have been extensively studied in terms of their fatigue properties and there are comprehensive data on these composites. Natural fiber composites, on the other hand, have been studied only in the past two decades and there are still insufficient data on their fatigue properties. However, some concrete results have begun to be inferred from these studies. The poorer tensile properties of natural fibers vis-à-vis synthetic fibers are also reflected in their poorer fatigue strength compared with synthetic fiber composites. Improved fiber–matrix bonding resulting from the surface treatment of natural fibers also improves their fatigue performance. Despite having poorer absolute fatigue strength, natural fiber composites exhibit less fatigue sensitivity compared with glass fiber-reinforced polymers, which can be correlated to their lower stiffness degradation during fatigue.