Issue 21

D. Croccolo et alii, Frattura ed Integrità Strutturale, 21 (2012) 13-20 ; DOI: 10.3221/IGF-ESIS.21.02 17 diagrams refer to a remote stress of RS=357.5 ± 292.5 (black curves) and to a remote stress of RS=233.75 ± 191.25 (gray curves) which correspond to a high and low fatigue remote stress respectively. The diagrams of Fig. 6 point out a significant discrepancy among the three actual stress ranges, especially when they are calculated close to the hole; the OH specimen exhibits a stress range of about 1,800MPa, the I06 exhibits a stress range of about 1,050MPa and I2 exhibits a stress range of about 800MPa when RS=357.5 ± 292.5MPa (black curves of Fig. 6). If the remote stress decreases the discrepancies among actual ranges change: the OH range remains greater than I06 and I2 ones, but the I06 and I2 ranges tend to become equal. Indeed when RS=233.75 ± 191.25MPa (gray curves of Fig. 6) the OH specimen exhibits a stress range of about 1,200MPa whereas the I06 and the I2 specimens exhibit the same stress range of about 400MPa. This is the reason why the fatigue strength of I06 and I2 specimens tends to be the same when the remote stress decreases. This occurrence is well indicated in Fig. 7 where the stress ranges are plotted as a function of the specific interference for different remote stresses: in the diagram of Fig. 6b the stress range at I06 and at I2 is, again, the same so that the fatigue strength should be similar; this event is also confirmed by the S-N diagram of Fig. 4 in which at 425MPa of maximum remote stress the number of cycles reached by I06 and by I2 specimens is quite the same. The diagrams of Fig. 7 are also useful to indicate the minimum interference level sufficient to improve the fatigue strength for a stated remote stress; for instance, in the case of 357.5 ± 292.5MPa of remote stress an interference level of 0.6% is high enough for increasing the fatigue strength, whereas for 357.5 ± 292.5MPa of remote stress an interference level of about 1% is suggested. Figure 5 : FEA Model.