QUASI-STATIC BENDING FATIGUE OF CARBON CORD–RUBBER COMPOSITES USED IN TIMING BELTS

Abstract

ABSTRACT Cord–rubber composites such as timing belts are subjected to coupled tension and bending under typical service conditions. Due to their increased modulus, carbon cords are replacing traditional glass cords as reinforcing materials in timing belt products. The bending fatigue behavior of carbon cord–reinforced hydrogenated butadiene rubber (CC-HNBR) composites is of increasing interest for both understanding their failure mechanism and supporting the development of new industrial products. In this work, a simple experimental setup that replicated in a simplified way the real-pulley situation encountered in a timing belt operation was developed to investigate the effects of applied tension, bending curvature, frequency, and R ratio on the bending fatigue life of CC-HNBR composites. Furthermore, a numerical investigation of the stress distribution within the CC-HNBR composite, under both uniaxial tension and coupled tension and bending loading, was carried out using finite element analysis. Cord-dominated fracture was observed close to the point at which 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 regimens probably resulting from the rubber generating only limited heat buildup even at the highest test frequencies. Higher R ratios led to a longer bending fatigue life, potentially due to the strain-induced crystallization of the HNBR matrix at the tip of any generated cracks. This study provides a basic investigation into the bending fatigue behavior of CC-HNBR composites under coupled tension and bending loading conditions, shedding some light on the failure characteristics of CC-HNBR composites under the interaction of bending and tension deformations.