Creep-fatigue interaction and hygrothermal aging effect on the fatigue behavior of carbon/glass hybrid fiber filament-wound tubes

Carbon/glass hybrid fiber reinforced polymer (C/GFRP) tubes, which offer both high performance and cost-effectiveness, are often subjected to the synergistic effects of fatigue and creep during their service life as transportation carriers, which reduces the safety of the structure. This study investigates the tension–tension fatigue behavior of C/GFRP tubes under constant stress ratio at different stress levels. The influence of a hygrothermal environment on fatigue failure modes, fatigue life, and stiffness degradation was examined via laboratory accelerated aging (150 days of immersion in distilled water at 60 °C). The creep displacement evolution was investigated by experimental and analytical means. Finally, a modified fatigue stiffness degradation model accounting for creep effects was proposed based on the creep growth curve. During fatigue loading, the primary load-bearing responsibility gradually shifts from the resin to the fibers as the resin deforms. This transition alters the material’s viscoelastic behavior, evolving from resin-dominated viscoelasticity toward fiber-dominated elasticity. Consequently, the total energy dissipated per loading cycle significantly decreases. Hygrothermal aging alters the failure mode, causing irregular serrated matrix fractures due to interface degradation, and significantly reduces fatigue life. After 150 days of accelerated aging, the fatigue life retention rates of the C/GFRP tubes at stress levels of 0.50, 0.45, 0.40, and 0.38 were 16.3 %, 61.6 %, 57.1 %, and 45.8 %, respectively. Creep effects lead to increased stiffness during fatigue in tubes. The modified stiffness degradation model effectively characterizes the actual stiffness evolution of C/GFRP tubes during fatigue process by separating the cyclic creep.