Quasi-static bending fatigue of carbon cord-rubber composites used in timing belts

Abstract: Cord-rubber composites such as timing belts are subjected to coupled tension and bending under typical service conditions. Due to its increased modulus, carbon cords are replacing the traditional glass cords as the reinforcing materials in timing belt products. The bending fatigue behaviour of carbon cord reinforced hydrogenated butadiene rubber (CC-HNBR) composites is of increasing interest for both understanding their failure mechanism and supporting new industrial product development. In this work, a simple experimental set-up that replicated in a simplified way the real-pulley situation encountered in a timing belt operation was developed, in order to investigate the effects of the applied tension, bending curvature, frequency and R ratio on the bending fatigue life of carbon cord reinforced hydrogenated nitrile butadiene rubber (CC-HNBR) composites. Furthermore, numerical investigation of the stress distribution within the CC-HNBR composite, under both uniaxial tension and coupled tension and bending loading, were carried out using finite element analysis (FEA). Cord-dominated fracture was observed close to the point where the specimen just left the pulley using a thermal imaging camera at high stress levels. This location is due to the combined effects of bending and maximum tension at this site. There was a reduction in the bending fatigue life as a result of a higher level of bending strain introduced by a smaller diameter pulley. Frequency had negligible effects on the bending fatigue life within testing regimes probably resulting from the rubber only generating limited heat build-up even at the highest test frequencies. Higher R ratios led to a longer bending fatigue life potentially because of strain induced crystallisation of the HNBR matrix at the tip of any cracks that are generated. This study provides a basic investigation into the bending fatigue behaviour of CC-HNBR composites under coupled tension and bending loading conditions, shedding some light on failure characteristics of CC-HNBR composites under interaction of bending and tension deformations.