Fatigue and Fracture Behavior of a Steel Cord/Rubber Composite

Abstract

The objective of this study was to investigate the fatigue and fracture behavior of steel cord/rubber composites used in tire belts under constant cyclic strain loading. The material was a specially made tire belt layer in the form of rubber sheets reinforced with unidirectional cords consisting of two pairs of twisted steel wires. Failure mechanisms, damage development, and fatigue life were determined for single belt layers with different cord orientations. Tests were conducted at cord angles of 22°, 72°, and 90° degrees with a cyclic strain amplitude of 8.3% at a frequency of 10 Hz. Five different stages of damage development were observed: microcrack initiation, microcrack multiplication, macrocrack formation, slow macrocrack propagation, and fast macrocrack propagation leading to final failure. In the case of the 22° cord specimens, where the in-plane shear component was dominant, damage development consisted of microcrack initiation at the cord/rubber interface, the formation of more microcracks and macrocracks, and finally the formation of a major fatal macrocrack along the cord direction. In the case of 90° cord specimens, dominated by transverse tension, initial microcracks occurred within the cord, they propagated across the thickness of the specimen, and finally a major macrocrack propagated across the entire width of the specimen. The final crack propagated in part along the cord/rubber interface and in part within the cord. In the case of the 72° cord specimens, where both in-plane shear and transverse tension are critical, the initial microcracks occurred within the cord and the final macrocrack along the interface. For the same cyclic strain amplitude, the 90° specimens had the shortest fatigue life, and the 72° specimens had the longest. Additional tests were conducted at different strain amplitudes. The normalized modulus decreases slowly and nearly linearly with normalized fatigue lifetime up to a certain value of the latter, approximately 80% of the normalized logarithmic lifetime, and then it drops sharply. Cyclic strain amplitude also affects the failure mechanisms. High amplitudes produce localized damage, whereas low amplitudes produce dispersed damage. A residual life model was proposed based on stiffness degradation.