An Analytical Investigation of Plasticity Induced Closure Involving Short Cracks

Plasticity induced crack closure and its effect on the fatigue growth of short cracks is investigated numerically. A plane stress finite-element analysis is used to simulate a nonpropagating crack of length 65 μm in a single-edge-cracked specimen under two stress levels and two values of stress ratio R. A comparison of results is then made with a propagating crack of an initial length of 25 μm, which is allowed to grow to a final length of 65 μm. R values of 0.1 and −1.0 with stress levels of 60 and 90% of the yield stress for the material are considered. The plastic zone size for each case studied is approximately equal to the initial crack length, which provides conditions characteristic of short cracks.

It is observed that the displacement profile behind the crack tip is unrealistic unless cumulative history is considered. Crack closure develops as the crack propagates because of formation of a wake of residual deformation behind the crack tip. Closure develops rapidly for fully reversed loading but requires longer propagation distances to develop under R = 0.1 loading. Closure load/maximum load appears to eventually achieve a steady state value, which decreases with increasing stress or decreasing R. Numerically determined load-displacement data demonstrate that closure, as determined from deviations from linearity in such a plot, is easier to determine using displacements measured near the crack tip than from far-field displacements.