Fatigue damage mechanics of composite materials. I: Experimental measurement of damage and post-fatigue properties

A new approach for modelling the post-fatigue strength and stiffness of notched fibre composite laminates has been developed. It is based on the observation of notch tip damage which can be quantified by the extent of the individual failure processes, splitting in the 0° plies and delamination between the 0° ply and off-axis plies. The notch tip damage zone grows stably under tensile cyclic loading in a self-similar manner and the size and shape of this damage zone is dependent on laminate geometry and constituent properties of the fibre, matrix and interface. The post-fatigue strength and stiffness of the laminate can be related uniquely to the split length, which defines the extent of damage growth. In this first paper in a series of four, observation is made and measurements taken of the damage growth mechanisms that make up the damage zone in carbon fibre/epoxy laminates. Radiographs are used to characterise the notch tip damage zone and to establish a qualitative relationship between post-fatigue strength (or stiffness), cyclic stress, damage size and numbers of cycles