Cyclic fatigue fracture of composites: What has testing revealed about the physics of the processes so far?

•Identifying open questions in fatigue fracture of fibre-reinforced polymer composites.•Discussing microscopic damage mechanisms and their effects on the macroscopic scale.•Discussing the physics of driving force and of fatigue damage resistance.•Identifying topics for further research in fatigue fracture of polymer composites. Where for metallic materials fatigue fracture testing has contributed significantly to understanding macroscopic and microscopic fatigue failure, the understanding of fatigue fracture in fibre reinforced polymers (FRP) still seems limited. It appears that the research on fatigue in FRPs raises more questions, rather than providing a framework for understanding the underlying mechanisms. Open questions include for example the correct formulation of the driving force and fatigue damage resistance, how to account for mixed mode loading and fibre bridging? But also the question to what extent averaging and homogenising over time- and length scales hinders the understanding in relation to the microscopic mechanisms? To contribute to the development of understanding, this paper discusses several of these open questions. The selected questions were discussed at a recent workshop on ‘Physics of fatigue damage growth’ at TU Delft with participants from several laboratories covering expertise on fatigue fracture testing of both metals and polymer composites. The discussion focuses on potential experimental and simulation approaches that may lead to a better understanding of the physics of the fatigue fracture process.