DISLOCATION NUCLEATION FROM A CRACK-TIP - A FORMULATION BASED ON ANISOTROPIC ELASTICITY

The analysis of dislocation emission from a crack tip within the Peier ls framework [Rice (1992) J. Mech. Phys. Solids 40, 239-271; Rice et a l. (1992) Topics in Fracture and Fatigue, pp. 1-58; Sun et al. (1993) Mater. Sci. Engng A170, 67-85], heretofore developed for isotropic sol ids, is generalized to take into account elastic anisotropy. An incipi ent dislocation core, represented in terms of a critical configuration at the crack tip, is determined numerically (most simply in the shear -only version of the model, but also for a combined tension-shear vers ion that includes tension-shear coupling constrained by atomic modelin g). These solutions improve upon approximations based on an effective shear stress intensity. For fcc crystals and intermetallics, the nucle ation event analysed is that of a set of partial dislocations emitted sequentially. The anisotropic formulation accounts for corrections as large as 30% in the critical value of the stress intensity factor for atomic decohesion, or cleavage. The anisotropic critical crack extensi on force for dislocation emission may be greater or less than its isot ropic counterpart. For an embedded-atom-method (EAM) model of bcc alph a-Fe, the anisotropic values can be as large as 2.4 times the isotropi c ones in one crack orientation; in another crack orientation, the val ues are as much as 40% less than the isotropic analogs. For fcc struct ures (EAM nickel, aluminum and Ni3Al), the difference is within a +/- 10-25% range. For silicon, the isotropic formulation is good, with les s than a 14% difference from the anisotropic counterpart. The anisotro pic effects are found to increase with a standard ratio of elastic ani sotropy, and are important for predicting intrinsic ductile versus bri ttle response.