Abstract
Fatigue crack propagation in most engineering materials has an initial stage of crack propagation, commonly designated Stage 1. Stage 1 cracks initiate and propagate from slip lines on the crystallographic planes of both polycrystal and single crystal alloys. A Stage 1 crack is usually thought to propagate solely by local shear stresses. A computer model of a crack and its plastic zones is developed. The positions of a series of dislocations are calculated in order that their combined stresses satisfy equilibrium equations and crack boundary conditions. The range of the crack tip displacement per loading cycle, which appears to be the best model for incremental crack growth, is independent of the mean stress value for mean stress greater than one-half the maximum stress and the stress range small. For all mean stress levels a minimum stress range is predicted by this program below which the cyclic tip displacement is zero.
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