Intralaminar crack growth rates of a glass fibre multiaxial laminate subjected to variable amplitude loading

The literature shows that variable amplitude loading can be much more severe than constant amplitude loading in a fatigue damage context. However, the underlying physical explanations for this are still unknown. Therefore, this paper investigates if the local energy release rate at each intralaminar crack tip in a tension/tension fatigue loaded multidirectional laminate can be used to determine the crack growth rate of these cracks subjected to variable amplitude loading. The local energy release rate is investigated through a developed finite element model in which any number of cracks can be included to account for their interaction. The position of the cracks is determined based on previous experimental work. The experimental results from the previous work are compared to the numerical results in this work. At similar crack density levels the energy release rates increase in the variable amplitude tests compared to constant amplitude tests, but not enough to significantly impact the crack growth rate. It is concluded that less than 5% of the increased crack density rate and crack growth rate caused by variable amplitude block loading is due to increases in local energy release rate. The primary reason is more likely found on a microstructural level.