Free-edge delamination in advanced composite laminates

Abstract

The free-edge delamination in composite laminates results directly form the interlaminar stresses at the ply-interfaces and exposoed free-edge, and it reduces overall structural performance of laminates. In this study, various laminates with different fiber orientations and stacking sequences are analyzed and tested to investigate the modes of edge delamination, to develop a simple stacking method for the suppression of free-edge delamination and to understand the failure mechanism in thick laminates under compression-compression fatigue. Delamination tests on various laminates show that some laminates delaminate only by the effect of mode II interlaminar shear stress and the stress component also plays an important role on free-edge delamination even though it is zero at the free-edge. More attention should be paid to the effect of mode II interlaminar stress to predict free-edge delaminations adequately. Each mode of delamination shows differences on the delamination fracture surfaces. The PRS stacking (the disposition of plies to be symmetric with respect to the values of effective Poission``s ratio in one half of the laminate) and the PRS sublaminate approach (the repeated stacking of Poisson``s ratio symmetric sublaminates) are found to be very efficient stacking methods to suppress free-edge delamination in thin and thick laminates, respectively. Those methods do not require any computations and provide much safer stacking sequences. The delamination cracks in composite laminates under compression-compression fatigue load initiate due to the predominant effect of mode III interlaminar shear stress. The delamination cracks propagate, and then local buckling occurs at the delaminated plies. The delamination near the top and bottom surfaces is easier to grow than the delamination near the laminate mid-plane because of the constraint effect of adjacent plies on local buckling.