Modelling the compressive response behaviour of monolithic and sandwich composite structures

Failure in compression of fiber composite laminates with an open hole is by the initiation and growth of a microbuckle from the edge of the hole. The geometric inhomogeneity induces fiber rotation under increasing applied load; deformation localizes within a band and a microbuckle is initiated. The microbuckle then propagates in a stable manner for 2–3 mm and the component fails at a higher load than the initiation load. This process in carbon-epoxy and carbon-PEEK laminates was compared to an equivalent crack containing cohesive stresses. It is a crack bridging analysis that predicts the size of the buckled region as a function of the applied load, with the local stress supported by the buckled fibers decreasing linearly with the closing displacement of the microbuckle. The model is able to predict successfully the effects of hole size and lay-up upon the compressive strength and has been incorporated into a user-friendly computer program. Tests had been performed to obtain both the laminate un-notched strength and the compressive energy release rate associated with fiber microbuckling, which are required as the model's input. From a design point of view, it is desirable to predict these laminate properties from the mechanical properties of the fibers and the matrix and from the lay-up geometry.