Creep-fatigue interaction of viscoelastic GFRP composite laminates under fatigue loading

Fiber-reinforced polymer composites are being used for various structural applications in civil construction, aerospace, automotive, and energy industries due to their numerous advantageous mechanical properties. Most of these structures are exposed to different loading types that degrade their properties during their operational lifetime. Among the different types of loadings, fatigue and creep are the main mechanisms that cause material failure. Therefore, FRP composites' behavior under creep and fatigue loading should be evaluated for their long-term application. Since engineering structures are subjected to complex loading patterns, it is expected that both creep and fatigue could simultaneously contribute to material degradation during their life. Also, due to the viscoelastic nature of FRP composites, their time-dependent behavior may interact with fatigue degradation to accelerate their deterioration. The interaction of creep and fatigue takes place not only in complex loading patterns but also in constant amplitude fatigue loading with non-zero mean stress in which material undergoes time-dependent deformations. This thesis aims to investigate the fatigue behavior of GFRP composites considering both time- and cycle-dependent behavior as well as their interaction. For this purpose, a robust experimental and analysis methodology are introduced to assess GFRP composites' long-term behavior considering their viscoelastic behavior. An extended experimental work were performed for fatigue testing of [±45]2s, [90]8 and [0]8 GFRP laminates to obtain the evolution of time- and cycle-dependent properties as well as their interaction with each other at the laminate level. An experimental and analysis methodology are introduced to investigate the effect of fatigue damage on the viscoelastic properties of [±45]2s and [90]8 laminates by conducting DMA creep testing on the fatigue-damaged specimens using TTSP. The developed creep-fatigue interaction model is used to analyze the creep-fatigue interaction response of GFRP composites' laminates under fatigue loading. Moreover, the thermodynamics-based creep-fatigue analysis is proposed, in which the contribution of creep and fatigue behavior can be integrated to analyze the failure of GFRP laminates under fatigue loading.