Fatigue crack growth - a modeling perspective

Modeling of fatigue crack growth processes require calculations of crack tip stress fields, including the influence of premature crack contact and crack opening displacements. Two problems that have been of recent interest to the senior author will be described in this paper; contact of crack surfaces with asperities, and crack growth behavior under the case of crystallographic slip at crack tips. The asperity micro-contact problem was solved using a statistical description of asperity heights, asperity densities, and material flow properties. The results showed a nonlinear relationship between crack opening stress levels and the asperity dimensions, with asperity density playing a modifying role. The present analysis represented a considerable improvement over the earlier geometric models of roughness. In a related topic, crystallographic slip has been known to play an important role in microscopic fatigue crack growth. For microscopic cracks, finite elements are used to make predictions of crack opening stress levels in the case of double slip at crack tips. As the orientations of the two slip directions, with respect to the crack growth direction, were varied, the results revealed that the crack opening stress levels were in the range zero to 0.4 of the maximum stress. This range partially accounts for the variability in the crack growth rates observed for crystallographic cracks.