An experimental investigation into fatigue damage development in open and bolted hole specimens

An extensive experimental program has been carried out to investigate and understand the sequence of damage development throughout the life of open-hole and bolted composite laminates under quasi-static loading and tension-tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/-45₂/0₂]_S defined as ply scaled and [45/90/-45/0]_2S defined as sub-laminate scaled, were used. Specimens were cycled at 5Hz with various amplitudes to 1×10⁶ cycles unless failure occurred prior to this limit. A number of tests from the open-hole study were interrupted at various points as the stiffness dropped and analysed for damage using X-ray CT. The ply level open-hole case showed a similar overall failure mode of delamination in quasi-static and fatigue loading. The sub-laminate case showed a failure mode change from quasi-static fibre dominated pull-out to delamination in fatigue. For the ply-level bolted case, the fatigue life increased, with a failure mode change from delamination in open-hole fatigue tests, to pull-out. It is shown in this paper how the role of delamination is critical in the case of fatigue loading and how this interacts with bolt clamp-up forces, even when delamination does not dominate in static tests.