Issue 37

N.R. Gates et alii, Frattura ed Integrità Strutturale, 37 (2016) 166-172; DOI: 10.3221/IGF-ESIS.37.23 170 study, the cause for these shifts in prediction accuracy can be narrowed down to two likely sources: multiaxial stress state effects, such as the presence of T-stress, and/or differences in loading history profile. Figure 3 : Variable amplitude crack growth life predictions based on (a) FASTRAN and (b) UniGrow analyses. Figure 4 : Variable amplitude crack growth curves for lowest loading level (a) axial, (b) torsion, and (c) combined axial-torsion loading. Concerning loading history profile, the axial channel of the variable amplitude service loading history, as shown in Fig. 1(a), is composed of smaller cycles with significant tensile mean stress mixed with occasional larger cycles at a much smaller R ratio. The shear channel, on the other hand, is composed of small amplitude cycles, ranging between approximately zero to minimum conditions, mixed with occasional larger amplitude cycles of an approximate zero to maximum range. Given these differences, significantly more crack closure would be expected for mode I crack growth under pure torsion loading due to the smaller tensile mean stress values. This agrees with the more conservative FASTRAN life predictions for the axial loading history, as compared to the shear history. Residual stresses, on the other hand, are most significantly affected by the maximum stress values in a loading history. Therefore, the effect of the (a) (b) (a) (b) (c)