Fatigue behaviour of pseudo-ductile thin ply hybrid composites

The fatigue behavior of unidirectional interlayer hybrid composites consisting of thin ply TC35 carbon/epoxy and S-glass/epoxy prepregs has been studied by looking at the fatigue threshold and delamination growth rate before and after the stress level at which fragmentation of the carbon ply initiates. For the pristine specimens, without any fragmentation, there is no stiffness reduction up until 74,000 cycles when fatigued at 90% of the fragmentation initiation stress level. For the specimens initially overloaded beyond the fragmentation stress level, they can withstand several thousand cycles at high amplitude loads with only a gradual increase in the damage. In these specimens, the stiffness reduction rate depends on the maximum fatigue stress level. At 90% of the fragmentation initiation stress level, the stiffness reduction rate is faster compared to 80 %. This is due to the higher cyclic strain energy release rate at the 90% compared with the 80%. A correlation between the stiffness reduction rate and the damage growth rate over the course of cyclic loading is presented, highlighting the effect of different maximum fatigue loading levels.