Fatigue crack growth behavior in the presence of notches and multiaxial nominal stress states

The primary focus of this study was to evaluate the effects of notches and multiaxial nominal stress states on mode I crack growth behavior under constant amplitude loading conditions. As a result, a variety of fatigue tests were performed using notched tubular and plate specimens made from 2024-T3 aluminum alloy. Because crack growth data were recorded for naturally initiated fatigue cracks, which can develop in different locations at the notch root, notch effects on crack growth were studied with respect to different initial crack geometry assumptions. By using an effective SIF model to account for stress concentration and boundary condition effects on crack growth from notches, crack growth rate correlations were found to improve over those based on long crack assumptions. Additionally, tubular specimen tests performed using different nominal loading paths allowed for the evaluation of multiaxial stress state effects, including those from non-proportional loading, on the resulting mode I crack growth behavior. Crack growth rates for specimens subjected to multiaxial nominal stress states were observed to be higher than those for uniaxial loading conditions at the same nominal mode I SIF range. While T-stress corrections were able to account for the higher growth rates in some cases, they were unable to collapse all of the tubular specimen crack growth data into a single narrow band. Furthermore, the presence of mixed-mode crack growth effects was determined to be a likely cause for curvilinear crack paths and higher crack growth rates observed for longer cracks growing under 90° out-of-phase non-proportional loading conditions.