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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alloy at R=+0.1 show perfect matching of fracture surfaces, Fig. 11a at P min with a zig-zag crack profile with no observable oxide layer. While Fig. 11b the single crystal alloy of same composition, shows some dry oxide debris in the Mode-II-part (on (111) slip plane) due to oxide induced abrasion at R= -1.0 but not in the Mode I part of the deflected crack. But both micrographs in Fig. 11 show good matching of fracture surfaces at P min , even though there is some oxide debris in the Mode II section of the crack path at R= -1.0. This crack surface oxide is a dry oxide observed after the experiment. During the experiment it was a viscous oxide that can impart some sliding to push out the oxide that dried.

Fig. 12 Crack path profiles in planar slip AL-4.5Zn-1.25Mg alloys in vacuum Petite (2008) at P min for (a) R = 0.1 and (b) R = -1. Schematic Fig. 9b show the gaps between the unmatched fracture surfaces at that allow for the environment to enter to the crack tip and induce environmentally assisted fatigue damage. Available examples of  K th data in vacuum suggest that RICC contributions is not significant enough to reduce  K th with R in planar slip alloys as well as in non-planar slip alloys. The factor of environmental access dominates the roughness effect affecting near threshold region for fatigue damage. Vacuum results question the importance of PICC, OICC, and RICC concepts on FCG behavior at threshold, if there are no other deformation processes like creep, void growth, or cleavage at high R contributing when K max → K IC . We suggest that for modelling and analysis direct use of applied  K and K max at a given environment is sufficient to use than any corrected version of  K to  K eff . Conclusions The following conclusions are drawn from our analysis that addresses issues related to PICC, OICC and RICC on FCG behavior at the near threshold region in both lab air and in vacuum environments. 1. Supported by analytical and experimental data in vacuum, one can deduce that PICC, OICC and RICC effect on FCG are overestimated in lab air and in aqueous environments. 2. The PICC and RICC models do not include the environmental component and do not seem to validate the data in vacuum results. 3. Analysis of corrosion processes at room temperature and transition toward the end-product phases in the wake of the crack result in a negligible reduction of  K applied with R by PICC due to deformable viscous nature of the oxide layer. 4. The OICC mechanism is possible at high temperatures where dry oxide may form behind the crack-tip to the size relevant to the cyclic crack tip opening displacement (CTOD). In summary, the experimentally observed R-ratio effects on FCG and threshold behavior at room temperature in humid air are less linked to crack closure behind the crack tip but to the access of the environment to the crack tip region that