Fatigue life and failure mechanisms of cord-reinforced rubber composites: Influence of fiber orientation, layer architecture, and matrix thickness

This study investigates the fatigue life of cord-reinforced rubber composites subjected to fully reversed three-point bending. Although nominally a bending test, the high displacement amplitudes and low specimen stiffness lead to a tension-dominated loading state due to geometric nonlinearity. The research focuses on the influence of fiber orientation (0/90° vs. ± 45°) and the number of reinforcement layers on fatigue durability. It was demonstrated that traditional parameters, such as strain amplitude or maximum strain, are insufficient to describe fatigue life under non-zero-mean-strain conditions. Instead, the Smith-Watson-Topper (SWT) and Walker models were successfully employed to unify the results. The findings reveal that ± 45° configurations exhibit superior fatigue life compared to 0/90° layups under strain-controlled conditions, primarily due to fiber reorientation and lower internal stress levels. Analysis using the Walker model showed that single-layer composites are more sensitive to maximum strain (γ = 0.3), whereas double-layer systems are dominated by strain amplitude (γ = 0.7) due to interlaminar shear and internal friction. Furthermore, a critical relationship between matrix thickness and reinforcement ratio was identified: increasing rubber thickness improves durability in single-layer systems but deteriorates it in double-layer composites due to weakened crack-bridging mechanisms. SEM analysis confirmed these findings, highlighting the role of fiber pull-out and interlaminar degradation. Finally, the study highlights a lack of correlation between static tensile strength and fatigue performance, underscoring the need to account for viscoelastic effects in the design of cord-rubber structures.