PSI - Issue 66

Nur Mohamed Dhansay et al. / Procedia Structural Integrity 66 (2024) 87–101 Author name / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 6. Crack profile in the ZX, XZ and XY orientations for the AF/SR conditions. Fracture surfaces are shown at (a) 90x and (b) 1000x magnification. Crack profile images are shown in (c). Crack propagation is downwards. 4. Discussion When observing a Δ K th versus K max graph, such as in Fig. 5(b), the behavioural difference between extrinsic and intrinsic is more visually pronounced. For R < 0.6, a decrease in Δ K th occurs with no significant change in K max i.e., a somewhat vertical slope. Whereas for R > 0.6, K max now observes changes with no significant change in Δ K th i.e., a somewhat horizontal slope. Similarly, Boyce and Ritchie (Boyce and Ritchie, (2001)) observed this type of behaviour at R ~ 0.55 and refer to extrinsic and intrinsic regions as global “closure-affected” and global “closure-free”, respectively. The investigation by Newman et al (Newman et al., (2003)), found that “closure-affected” regions were largely caused by asperities near the crack tip i.e. RICC. Other, near-threshold FCGR investigations, where varying levels of R-ratio were applied, show similar behaviour in the “closure-affected” and “closure-free” regions. It is likely that the near-threshold behaviour in this investigation, observed for R < 0.6, is due to RICC. 4.1. Extrinsic influence of microstructural morphology It was previously argued that one of the reasons for the observed anisotropic behaviour in fatigue are based on crack closure extrinsic influences. A detailed discussion is given in previous work by the authors (Becker et al., (2020)) on the anisotropic observations being due to morphological texture as opposed to crystallographic texture during the “closure-affected” regime. This will only be summarised here.