Experimental fatigue crack growth behavior and predictions under multiaxial variable amplitude service loading histories

This study presents experimental crack growth data for naturally initiated fatigue cracks in tubular specimens of 2024-T3 aluminum alloy subjected to both uniaxial and multiaxial variable amplitude flight loading spectra. Experimental crack growth behavior is compared to predictions based on two state-of-the-art analysis codes: UniGrow and FASTRAN. UniGrow is based on the idea that residual stress distributions surrounding the crack tip are responsible for causing load sequence effects in variable amplitude crack growth, while FASTRAN attributes these effects to varying degrees of plasticity induced closure in the crack wake. For variable amplitude fatigue tests performed under pure axial nominal loading conditions, both UniGrow and FASTRAN analyses were found to produce conservative growth life predictions despite good agreement with constant amplitude crack growth data. For variable amplitude torsion and combined axial-torsion crack growth analyses, however, the conservatism in growth life predictions was found to reduce. This is attributed to multiaxial nominal stress state effects, such as T-stress and mixed-mode crack growth, which are not accounted for in either UniGrow or FASTRAN, but were observed in constant amplitude crack growth tests to increase experimental crack growth rates. Additionally, by comparing experimental crack growth lives between tests performed using full and edited versions of the same loading history, it was found that a 94% reduction in loading history length resulted in differences in experimental crack growth life of less than a factor of 5. Since cracks in this study were initiated naturally, the effect of initial crack geometry assumptions on crack growth predictions was also investigated.