Fatigue-induced dual stiffness behavior of filled styrene–butadiene rubber

Payne and Mullins effects are widely observed in reinforced rubber materials. The mechanisms by which these two effects work are not fully understood. Several models have been proposed, including molecular slippage model, bond rupture model, and filler rupture model. In this study, two different compounds of styrene–butadiene rubber were prepared using carbon black and silica as reinforcement fillers, respectively, and subjected to cyclic fatigue process. Tensile, creep, and relaxation tests were performed on fatigued samples to assess the residual stress–strain behavior by comparing with the results from similar tests using pristine (no fatigue) samples. When the tensile stiffness behavior of fatigued specimens was evaluated, we noted that the stiffness versus strain behavior which exhibited a monotonic decreasing–increasing behavior with the pristine specimens changed to what we call “dual-stiffness” condition, where the specimens went through a first (low) turning point as with the pristine samples, but then dropped off of a peak to go through a second softening stage, similar to the first softening stage of the pristine material. We believe that such spiking (dual) stiffness behavior characterized by a “Peak” point represents a combination of both Payne and the Mullins effects active during fatigue loading. We conclude that molecular slippage and bond rupture are the main factors affecting the physical properties of carbon black-filled compounds, while breakage and recombination of the filler are the key mechanisms affecting the silica-filled compounds during the fatigue process.