Abstract
The effect of molding conditions on the resistance to Mode II interlaminar crack propagation under monotonic and cyclic loading of unidirectional continuous glass-fiber composites with a polypropylene (PP) matrix was studied. The distribution of the fibers and of the crystalline and amorphous components of the matrix phase, the melting temperature and the amount of crystallinity are related to the molding conditions employed and to the resulting flexural strength and modulus, apparent interlaminar shear strength and Young’s modulus. Mode II crack propagation, either cyclic or monotonic, is strongly affected by the fiber-matrix interface and matrix morphology. The distribution of the soft amorphous PP phase in the semi-crystalline PP matrix appears to be the controlling parameter determining the fracture and fatigue resistance of the composite. The fractographic features clearly show the role that this phase plays during crack propagation. A relationship between the shear cusp size measured on the fatigue surfaces and the cyclic strain energy release rate is proposed.