A simple model to predict crack growth in bonded joints and laminates under variable-amplitude fatigue

It is now well established that adhesives and fibre-reinforced polymers (FRPs) can fail at far lower loads under fatigue loading than under quasi-static loading. Most of the work on the fatigue of these materials has been under constant amplitude (CA); however, recent work has shown that fatigue crack growth acceleration and unstable crack growth can be seen under conditions of variable-amplitude (VA) fatigue. In this paper, further evidence of this effect is given. This phenomenon is attributed to the influence of VA fatigue on the evolution of a damage zone ahead of the main crack front and results from the characterization of these damage zones are also presented. Finally, a simple fracture mechanics-based model for predicting crack growth under VA fatigue is proposed. This is shown to be entirely consistent with the experimental observations.