  Theories of toughness of materials depend on an understanding of the
characteristic instabilities of the crack tip, and their possible interactions.
In this paper we examine the effect of dislocation emission on subsequent
cleavage of a crack and on further dislocation emission. The work is an
extension of the previously published Lattice Greens Function methodology. We
have developed a Cavity Greens Function describing a blunt crack and used it to
study the effect of crack blunting under a range of different force laws. As
the crack is blunted, we find a small but noticeable increase in the crack
loading needed to propagate the crack. This effect may be of importance in
materials where a dislocation source near the crack tip in a brittle material
causes the crack to absorb anti-shielding dislocations, and thus cause a
blunting of the crack. It is obviously also relevant to cracks in more ductile
materials where the crack itself may emit dislocations.
