Load history effects in fibre-reinforced polymer composite materials

Today, it is widely accepted that fatigue is one of the most common failure mechanisms in structural components, and pure static failure is rarely observed. Engineering structures are subjected to complex mechanical fatigue loading histories, which cause damage that eventually leads to functional and structural deterioration. One reason for this complexity is that the cyclic loading is accompanied by time-dependent deformations including creep and recovery. Fiber-reinforced polymer (FRP) composite materials are susceptible to time-dependent deformation, due to the viscoelastic nature of their matrix. Therefore, fatigue-creep-recovery interactions can play a role in the overall deformation behavior of FRP composite materials. The limited experimental investigations of FRP composite materials showed considerable effects of time-dependent phenomena on the fatigue behavior. However, no systematic analysis to clearly determine the effect of creep and recovery on developed damage mechanisms of FRP composites has yet been conducted. The aim of this research is to understand the fatigue behavior of glass fiber-reinforced composite (GFRP) materials and establish a reliable methodology to analytically/numerically model the fatigue behavior of FRP composite materials. The fatigue behavior of (±45)2S angle-ply glass/epoxy laminated composite at different stress levels has been investigated in this thesis. It was observed that cyclic-dependent parameters (cyclic creep, fatigue stiffness, and hysteresis loop area) as well as damage development were a function of applied stress levels. In tension-tension continuous fatigue, by decreasing the stress ratio, the damage became more severe, fatigue stiffness exhibited greater changes, the hysteresis loop area became larger and a higher and more uneven self-generated temperature was recorded. The effect of loading interruption on fatigue behavior was studied by subjecting the specimens to tension-tension constant amplitude fatigue loading interrupted at ¿max by creep intervals. At high stress levels, the fatigue life was improved when hold time was short as a result of the fiber realignment caused by the creep strain, while longer hold time caused considerable creep damage and therefore premature fatigue failure of the specimens. The cyclic loading was also interrupted at zero stress level. The stiffness degradation rates and hysteresis loop area decreased during the cyclic loading phase, while specimen recovery occurred after each stress removal. It was observed that specimens loaded under interrupted fatigue exhibited longer fatigue lives than those continuously loaded at high stress levels as a result of partial fatigue stiffness restoration and crack blunting. It was concluded that fatigue design allowables based on continuous fatigue could be conservative. A model based on the theory of viscoelasticity was developed to simulate specimen recovery in viscoelastic materials and predict their cyclic-dependent mechanical properties. The developed model predicted the cyclic-dependent parameters well - the fatigue stiffness, hysteresis loop area, cyclic creep, storage and loss moduli as well as tan(¿).