Bridging the Gap: Uncovering the Secrets of Fibre Bridging with a Multiscale Approach to Fatigue Damage in Composite Materials

A primary cause of failure in composite structures is delaminations in the form of cracks between fibre-reinforced polymer (FRP) layers of the structures. For delaminations, a significantly increased damage development is observed under varying loads compared to current model predictions. These models are typically based on experiments with constant amplitude cyclic loading, as the mechanisms governing damage under variable amplitude spectrum loading are poorly understood. On the microscale, delaminations in FRP experience fibre bridging, where fibres bridge the gap between crack faces and provide a shielding effect at the delamination front. This effect will be characterised experimentally by microscale measurements of the individual bridging fibres under fatigue loading and measurements of the mesoscale traction profile using inverse methods. Additionally, the fatigue damage development of individual fibres will be monitored under different load amplitudes to characterise the effects of load interactions on the damage state of the delaminations. Using the experimental knowledge generated, a physicsbased micro-mesoscale model will be synthesised to simulate fatigue damage development of a mesoscale crack under variable amplitude spectrum loading. The model will utilise a discrete representation of bridging fibres to account for the microscale damage development of the bridging fibres. An elevation of the model to allow macro-scale simulations will be achieved by using the physics-based micro-mesoscale model to fit a computationally efficient surrogate model.