Modelling fatigue transition behaviour of FRP in mode I block loading

Abstract

According to experiments the fatigue damage behaviour of fibre reinforced polymers (FRP) experiences a load sequence effect. In this report the ability of the cohesive fatigue damage model of Dávila (2020) to simulate the behaviour after a load amplitude change is investigated. For this a double cantilever beam (DCB) with fictitious material properties is loaded in mode I. Two element layers with each its own cohesive law are used to take into account the different behaviour of the epoxy and the bridging fibres in the wake of the crack. The calculated crack growth rate is compared to the experiments done by Jensen et al. (2021), since the conditions of the simulations are most similar to that of their experiments.
When the base and bridge layer only differ in their quasi-static cohesive law no load sequence effect is seen, since the conditions at the start of the second load block are the same as for the constant amplitude test at the same crack length. By allowing fatigue damage to accumulate only in the base layer and the bridge layer to only experience quasi-static loading, a history effect is seen. However, the high-to-low transition does not give the same transition behaviour as seen in experiments. In addition, the results of the constant amplitude tests are unrealistic.
When the fatigue damage accumulation of the bridge layer is faster than that of the base layer, a transition behaviour comparable, although less profound, to that of experiments is seen. This difference in fatigue damage accumulation is achieved by adjusting the coefficients of the fatigue damage rate function to the values corresponding to another R-ratio. This result may indicate a possible underlying mechanism for the transition behaviour of FRP.