Abstract
This paper discusses the mechanisms governing the fracture toughness of polypropylene reinforced by either mica or glass flakes. The toughness of these compos ites is found to be controlled by the competition between shear yielding and voiding in the matrix. Shear yielding occurs around filler particles, because of debonding at the filler/matrix interface. The filler onentation does not affect the crack growth resistance, but there is a unique relationship between the latter and the average distance between par ticles, regardless of the nature of the filler. It has also been found that the glass-rubber relaxation of the matrix polypropylene is the governing parameter for the time dependence of the dynamic fracture toughness of these composites.