Transition-behaviours in fatigue-driven delamination of GFRP laminates following step changes in block amplitude loading

Fatigue damage accumulation in laminated fiber reinforced polymer composites is highly sensitive to interactions of load events occurring in variable amplitude loading. This work aims to experimentally characterise fatigue-driven delamination growth due to step changes in block amplitude loading. Double cantilever beam glass/epoxy specimens are subjected to two-level block loading and constant amplitude cyclic loading under mode I crack opening. Crack propagation is monitored using a recently developed highly accurate digital image-based method for automated tracking of delamination fronts in translucent materials (Bak and Lindgaard, 2020). The results prove a significant difference in the crack growth rate following from a step change in block amplitude loading in comparison to the crack growth rate under constant amplitude cyclic loading at the same value of the strain energy release rate. Special emphasis is placed on the transition crack growth phenomenon, which is strongly influenced by the cyclic load history. Any of the load amplitude transitions under investigation, cause a characteristic and non-negligible transition-behaviour, which is currently ignored in state-of-the-art prediction models for fatigue-driven delamination growth. Supplementary tests are conducted to discuss the role of bridging fibres in the crack wake during load amplitude transitions.