Influence of ply-angle on fracture in antisymmetric interfaces of CFRP laminates

Laminated composites are prone to fracture at layer interfaces. Such damage impairs their structural response and engenders important constraints in design. In this work, the influence of ply orientation on crack growth resistance was studied for three different antisymmetric interfaces and compared to a unidirectional reference one, using double cantilever beam specimens with equivalent stiffness, loaded under mode I conditions. Fracture toughness at initiation was found interface-independent. In all angle-ply specimens, distinct slow and fast phases of crack propagation were observed. Crack increments due to fast growth, were characterized using experimental energy release rates and verified from fracture surface analysis. The slow propagation phases were accompanied by large scale bridging involving intra-ply growth in the adjacent plies, with toughness increasing inversely with the angle. Mechanistic investigations suggest a consistent fracture pattern in terms of the interface angle. For each interface angle, a single traction-separation relation, obtained from the experimental energy release rate and crack opening displacements, was sufficient to model two consecutive slow propagation phases. These relations were used in 2D cohesive element models to predict very well the loading history.