On the shielding effect of a multiligament zone of a crack in WC-Co

A crack tip shielding analysis of the fracture of cobalt reinforced tungsten carbide hardmetals is developed. It is shown that a multiligament zone formed by ductile binder ligaments behind the crack tip strongly affects the cleavage process in the WC matrix. This is due to a decrease in both the crack tip opening displacement and the plastic zone size ahead of a blunting crack tip. Consequently, normal stresses build up and, as a result, cleavage of the matrix is facilitated even under a low local stress intensity factor perceived by the crack tip. The cooperation of the cleavage of the matrix and the tearing of binder ligaments by dimple rupture results in a meandering crack path with a surprisingly high ligament area fraction. This ligament area fraction is about 15% higher than the volume fraction of the cobalt binder. It is suggested that the facilitated cleavage process promotes an advance of the crack front and maximizes the reduction of the potential energy of a body. The partition between carbide and binder phase related fracture is predicted. (C) 1997 Acta Metallurgica Inc.