PSI - Issue 52

D. Kujawski et al. / Procedia Structural Integrity 52 (2024) 293–308 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 5 Long and short cracks FCG data in vacuum for (a) Ti64, AA7075, and E460 alloys, Petite (1998) and Petite et al. (2000), and (b) for Ti 6Al-4V alloy, Yoshinaka et al. (2016). It should be noted that the FCG behavior of both small and long cracks, as shown in Figs. 5a and 5b, is plotted in terms of the applied  K without any reference or modification to crack closure. As small cracks have limited crack wake and are not affected by crack closure, this implies that long crack behavior should also not be affected by any type of closure. Therefore, the similitude concept associated with  K applicability is preserved for both small and long cracks. However, it should be noted that these interesting data were limited only to a single R =0.1. The lack of an effect of R-ratios on the threshold  K value (  K TH ) for many alloys tested in vacuum, as shown in Fig. 6, suggests that crack closure may not play a significant role in determining  K TH in these conditions. This is noteworthy as crack closure is often considered to be a key factor in determining  K TH , and the absence of closure in vacuum conditions implies that other factors may be at play. It should be noted that the alloys shown in Fig. 6, Vasudevan et al. (2005) exhibit a wide range of properties, including variations in microstructure, slip modes, chemical compositions, and heat treatments. This suggests that the lack of an R-ratio effect on  K TH may be a general phenomenon that is not strongly influenced by the specific properties of a given alloy.