Transition Between Stage I and Stage II Modes of Fatigue Crack Growth

Abstract

Fatigue crack propagation in metals may exhibit two consecutive modes: an initial crystallographic shear mode (Stage I) followed by a noncrystallographic tensile mode (Stage II). The mode of cracking is assumed to be governed by the character of slip in the vicinity of the crack tip, i.e., Stage I is the result of co-planar reversible slip, while Stage II is associated with homogeneous, ``wavy'' slip. The slip character is, in turn, assumed to depend upon the likelihood of screw dislocations to cross slip out of the primary slip planes, which requires recombination of the Shockley partials. Thus the transition from Stage I to Stage II is postulated to occur when the separation of the partials, influenced by the crack-tip stress field and the material's stacking-fault energy, is reduced to the order of the Burger's vector. A continuous dislocation distribution representing the crack and plastic zones is used to analyze the transition from Stage I to Stage II. In the model, the closest screw dislocation in the plastic zone adjacent to the crack tip is dissociated into discrete partial dislocations, while the dislocations representing the crack and remaining plastic zone are presented as a continuous distribution of dislocations. Although it was found that the separation of partials is very sensitive to the ratio of the applied stress to the yield stress and the stacking-fault energy, the separation is not significantly reduced as the crack length increases. It is therefore concluded that strong obstacles to dislocation movement or dislocation entanglements would be required ahead of the crack to effect recombination of the partials. This result implies that a transition in the mode of crack growth should not be observed in single crystals of metals which display planar slip characteristics.