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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lengthwise in crack direction for XZ and transversely elongated in the XY. Therefore, per a unit length/area of crack front, the XY orientation experiences more PBG (length of PBG) than the ZX and XZ (diameter/width of PBG). This produces varying levels of asperities for the crack front to interact with, which is orientation dependent, and thus resulting in the observed anisotropic RICC effects for the AF and SR conditions. The nature of the crack propagation influence occurs on two length scales i.e. local crack tip behaviour by α ’ laths and far crack tip by behaviour by PBG. Kumar et al (Kumar et al., (2018)) had also observed crack deflections along PBG boundaries and primary laths. The comparison between AF, SR and DA microstructural conditions highlights two distinct differences i.e., (i) α grain size, (ii) presence of β phase. The DA condition provides a larger, more coarse grain, which cause larger amounts of crack tortuosity, deflection and branching (Krüger et al., (2015)). These can be observed in Fig. 6(c) and Fig. 7(c). This results in larger Δ K th , for DA than AF and SR, in the global “closure-affected” region. Furthermore, the rough DA fracture surface is accompanied by faceted fracture, as a result of crystallographic cracking through α laths. It should be noted that facets are smooth, however, the crack deflections caused by it, results in the observed crack tortuosity and rough fracture surface. Even though the AF and SR conditions also exhibits faceted fracture, the larger grain size of the DA condition produced larger, more pronounced, facets and therefore larger crack tortuosity. In addition, this means that the smaller α s won’t contribute significantly to RICC as the larger α p does, as observed by Saxena and Radhakrishnan (Saxena and Radhakrishnan, (1998)). The presence of β phase introduces an inhomogeneity between the crystal structures i.e. α / β interfaces (Tan et al., (2015)). This may cause localised strain build-up at the interface, contributing to crack tortuosity. Moreover, the more ductile β phase contributes to RICC, not via faceting, but through the means of a ductile micro-tearing process (Saxena and Radhakrishnan, (1998)). Ductile micro-tearing features can be observed in Fig. 7(b). Between the AF, SR and DA conditions, α grain size and the presence of β -phase we consider a difference between the two conditions while the presence of PBGs in both are considered a similarity. However, we observed a difference regarding the PBG: (i) for the AF and SR conditions, the authors (Becker et al., (2020)) previously found that the PBG has an orientation specific crack closure role on crack behaviour, and currently (ii) The PBG in the DA condition’s fracture surface are not as pronounced as they are in the AF and SR conditions. It is likely that the observations of the DA condition’s crack and PBG interactions are due to an increase in ductility as well as observed grain boundary α . Furthermore, the DA condition’s PBG does not seem to contain specific roughness’s within an individual PBG as has been observed within the SR condition. Within the global “closure-affected” region (RICC), it is likely that the PBG does not play as a significant role in the DA condition as it does within the AF and SR conditions. 4.2. Intrinsic influence of microstructural morphology As previously mentioned, we consider a global “ closure-free ” regime to be when R ≥ 0.6, within this investigation. Generally, when using macroscopic techniques, closure mechanisms are not detectable. However, Davidson (Davidson, (1998)) conducted in-situ SEM fatigue tests and observed, at R > 0.8, that crack closure could be detected within 10 μ m of the crack tip. This means that RICC may still be of influence, particularly in the DA condition, but to a rather small extent. This is evident based on the behavioural changes between global “closure affected” and global “closure-free” as observed within Δ Kth versus Kmax graphs. After applying the DA heat treatment, the α ’ martensitic structure transforms into a larger α p grain with lamellar α s in a matrix of β ( α + β ) (Ter Haar and Becker, (2018)). The larger α grain size and presence of β phase increases the ductility of the bi-modal structure to greater than 15% e f i.e. above the requirements for surgical implants and aerospace applications (ASTM F136-13, (2012); Inagaki et al., (2014)). In addition, the secondary α + β structure provides a relatively high strength for the material given the high ductility achieved (Ter Haar and Becker, (2018)). Each phase i.e. α ’, α , and β have different levels of plastic flow capabilities and therefore different levels of energy dissipation. Based on the phase and grain size constituents, the DA condition will result in larger amounts of plastic flow and energy dissipation than the AF and SR conditions, resulting in larger intrinsic Δ K th . The increased ductility presents another crack shield mechanism which can be considered as an overlap between extrinsic and intrinsic mechanism i.e. crack tip blunting (Liu and Pons, (2018); Saxena and Radhakrishnan, (1998)). During the fatigue process, a crack opens, extends and closes. As the crack tip open and extends, the tip becomes relatively blunt and typically resharpens as the crack tip closes. However, in ductile material, the resharpening of a crack tip is not as pronounced as in brittle material i.e. the crack remains blunt. This means that crack tip blunting is more likely to occur