The effects of stress ratio and temperature on fatigue behavior of an epoxy adhesive

Epoxy adhesives are widely used in outdoor structural applications for bonding, where they are often subjected to cyclic loading patterns at different environmental temperatures, nevertheless, their synergistic effect on fatigue behavior has not yet been fully experimentally investigated. This study examined the tension–tension fatigue behavior of a structural epoxy at stress ratios (R) of 0.1, 0.5 and 0.9 and temperatures of 20 °C, 40 °C and 55 °C, where the onset glass transition temperature (T_g-onset) was 49 °C. At each temperature, increasing R extended fatigue life, particularly at 20 °C. This effect was slightly reduced at 40 °C, particularly at R = 0.9, because creep was accelerated at high temperature. At R = 0.5 and R = 0.9, fatigue life decreased with increasing temperature, while at R = 0.1, little change was observed when temperature increased from 20 °C to 40 °C. At 55 °C, exceeding T_g-onset, fatigue life significantly decreased across all stress ratios due to thermal softening. Stiffness degradation was predominantly attributed to damage initiation resulted from cyclic loading and thermal softening at elevated temperature, while creep effect was very limited, as observed at R = 0.9. The measured total energy dissipation for cyclic loading and creep exhibited that the contribution of creep increased with temperature. Finally, the constant life diagrams were formulated using a linear model by connecting the constant life points at R = 0.1 and R = 0.9, which showed good agreement at R = 0.5.