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. 4 FCG data for (a) AA7075-T7351 alloy (wavy slip), and (b) a single crystal PWA-1480 alloy, (planar slip) tested in high vacuum are revealing small variation with R-ratio. Short cracks tested in laboratory air often observed to propagate considerably faster than their long crack counterparts under the same applied effective stress intensity range, ΔK. This phenomenon is commonly explained using the PICC argument, which suggests that short cracks have less closure than long cracks due to an undeveloped crack wake. Figure 5a shows the FCG behavior of small cracks (SC) and long cracks (LC) for three different alloys, AA7075-T651, Ti-6Al-4V, and EN460, tested under vacuum (~10 -5 Pa), as reported by Petit and his group, Petite (1998) and Petite et al. (2000). The experimental data for SC and LC at R=0.1 are well within the experimental scatter, as observed in Fig. 5a. It should be noted that these three alloys have significant differences in terms of microstructures, yield strengths, elastic moduli, and slip modes. These differences may contribute to the observed scatter due to variations in local grain deformation at the vicinity of the crack tip or may affect the measurement resolutions. Recently, Yoshinaka et al. (2016) observed a similar trend in Ti-6Al-4V alloy (Fig. 5b) in an ultra-high vacuum (UHV of 10 -7 Pa) between small and long cracks FCG behavior. Experimental data for FCG rates below 10 10 m/cycle often exhibit a slightly larger scatter compared to higher FCG rates. These results demonstrate again that in UHV both small and long cracks behave very similarly in terms of  K applied following the same scatter.