Issue 37

N.R. Gates et alii, Frattura ed Integrità Strutturale, 37 (2016) 166-172; DOI: 10.3221/IGF-ESIS.37.23 171 loading history profile is not expected to have as large of an effect on residual stress distributions so long as the frequency and magnitude of maximum stress cycles are similar, which is the case in this study. As such, the fact that growth life predictions are more consistent between axial and torsion loading histories for UniGrow analyses, as compared to FASTRAN analyses, is not surprising. In addition to load history profile, even for constant amplitude loading conditions, mode I crack growth rates for nominal torsion loading of the notched tubular specimens were considerably higher than those for axial nominal loading at the same applied SIF range. This was attributed to the presence of compressive tangential stress (T-stress) at the crack tip, resulting in an increased plastic zone size and crack driving force under multiaxial nominal stress states. Because UniGrow and FASTRAN are both meant to model crack growth under uniaxial nominal loading conditions, neither program accounts for multiaxial stress state effects, such as T-stress, on mode I crack growth rates. Therefore, for a given SIF range, both models will predict lower crack growth rates than what would be expected in experiments for torsion and combined axial-torsion nominal loadings. As a result, the overall reduction in conservatism for the pure torsion variable amplitude growth life predictions is to be expected. Finally, crack growth analyses were performed for the variable amplitude combined axial-torsion (AT) loading histories. Given the tension dominated nature of the variable amplitude loading history applied in this study, crack growth behavior for the combined loading tests is expected to be similar to that observed for the axial only tests. By studying the results, this is found to be generally true. Figs. 3 and 4(c) show that crack growth life predictions for both axial only and combined loading tests tend to be conservative based on both FASTRAN and UniGrow analyses. Additionally, the degree of conservatism is found to increase with decreasing loading levels in both cases. While crack growth prediction trends are qualitatively similar for the axial only and combined loading conditions, growth life predictions are generally found to be less conservative for combined loading tests. Similar to the growth life predictions for the pure torsion tests, some of this difference is likely due to the crack growth models’ inability to account for increased growth rates due to the effect of T-stress on mode I crack growth. However, there are also additional factors that can contribute to this discrepancy which are only brought about under combined loading situations. Fig. 1(b) shows that the variable amplitude service loading history investigated in this study contains a number of significant non-proportional loading events. When non-proportionally varying stresses are present in a crack growth analysis, it becomes especially difficult to calculate crack driving forces. Because the principal stress directions are not constant under non-proportional loading conditions, a growing crack is continuously subjected to varying degrees of mixed-mode loading, regardless of its orientation. Additionally, the tendency of a mode I crack to grow under the influence of maximum principal stress can cause increased crack meandering and crack face roughness as cracks try to align with the changing principal stress direction. While there are many factors, in addition to plasticity induced closure and residual stress effects, which have the potential to influence crack growth behavior under variable amplitude combined loading conditions, some of these effects act to increase crack growth rates (e.g. T-stress and mixed-mode loading), while others tend to hinder crack growth (e.g. crack path meandering). For the loading conditions and specimen geometry of interest in the current study, the combined effect of all of these mechanisms appears to result in less conservative crack growth predictions for combined loading histories than for axial only histories. This agrees with the higher crack growth rates observed at higher SIF ranges for both in- phase and 90° out-of-phase constant amplitude combined loading conditions, when compared to those for axial loading. S UMMARY AND CONCLUSIONS n general, variable amplitude crack growth predictions based on both FASTRAN and UniGrow analyses were found to be conservative, regardless of the initial crack geometry assumption, for both axial and combined axial-torsion loading conditions. The accuracy of crack growth predictions for pure torsion loading conditions, however, was found to vary depending on the crack growth model, although all predictions were less conservative than in the case of axial and combined loadings. This is despite the fact that, for both programs, the majority of experimental crack growth lives under constant amplitude axial loading conditions were predicted within a factor of ±3 and generally found to fall between predictions based on the TT and CCH crack geometry assumptions. Additionally, comparisons with constant amplitude crack growth data show that the shift in conservatism between the different nominal loading conditions can likely be attributed to multiaxial stress state effects on mode I crack growth, such as the presence of T-stress and the potential for mixed-mode crack growth conditions. These effects are not accounted for in either crack growth model investigated. I