Volume 5, Issue 8 (September 2008)
Elastic-Plastic Finite-Element Analyses of Compression Precracking and Its Influence on Subsequent Fatigue-Crack Growth
Compression precracking has been used in a renewed effort to generate fatigue-crack growth rates in the threshold and near-threshold regimes, as an alternative to the traditional load-reduction procedure. But concerns have been expressed on the influence of tensile residual stresses resulting from compressive yielding at the crack-starter notch. This paper uses elastic-plastic, finite-element analyses to study the influence of the tensile residual stresses induced by compression precracking on simulated crack growth under constant-amplitude loading. High-fidelity finite-element models (60 000 to 160 000 DOF) using two-dimensional plane-stress analyses were used to model plastic yielding during compressive loading and simulated fatigue-crack growth through the tensile residual-stress field. The finite-element code was ZIP2D and the course mesh had an element size of 2 μm. A refined mesh had an element size four times smaller (0.5 μm) than the course mesh. Fatigue-crack growth and crack-closure effects were simulated over a wide range in stress ratios (R=0 to 0.8) and load levels, after compression precracking. The crack-tip-opening displacement (CTOD) concept was used to judge the extent of the influence on the tensile residual stresses and the stabilization of the crack-opening loads. It was found that the tensile residual stress field decayed as the crack grew under cyclic loading. Once the crack had reached one compressive plastic-zone size, the tensile residual-stress field had dissipated. However, the stabilization of the crack-opening loads and merging of the CTOD values with and without compression precracking was found to be about 1.5 to 2 times the compressive plastic-zone size at R=0 loading. The effects dissipated at smaller distances from the notch tip for the higher stress ratios. The present results have validated the crack-extension criterion that had been proposed for use in the compressive precracking threshold test method, beyond which, cracks are growing under constant-amplitude (steady-state) behavior.