Effects of the presence of compression in transverse cyclic loading on fibre–matrix debonding in unidirectional composite plies

Fatigue of composite materials is of great concern in load-carrying structures. The first type of damage to appear is generally transverse cracks in off-axis plies. These cracks form when fibre–matrix debonds coalesce. The underlying mechanism is hence fatigue growth of debonds at the fibre–matrix interfaces. In the present study, debond growth has been characterized under tensile and compressive cyclic loading of single glass fibres embedded in polymer matrix. The debond length was determined by in situ microscopy with transmitted polarized light showing the more damaging effect of tension–compression cyclic loading than tension–tension cyclic loading. A boundary element model has been developed and interfacial fracture mechanics concepts applied over the numerical results aiming to give an explanation of this experimental fact. These results may be used to formulate a fatigue growth law at a local microscopic level, at a stage prior to the formation of any visible damage, i.e. transverse cracks. Ideas of how to develop this methodology further are also discussed.