PSI - Issue 66

A.R. Pelton et al. / Procedia Structural Integrity 66 (2024) 265–281 Pelton/ Structural Integrity Procedia 00 (2025) 000–000

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The effects of crystallographic directions in the Nitinol sheet were apparent in this investigation. At the macroscale, the TD had slower crack growth during both high ∆ K and low ∆ K cycling, had a greater fatigue threshold (K th ), and exhibited a more jagged crack path. At the microscale, the TD had a smaller strain magnitude in the vicinity of the crack trip and exhibited a higher crack opening threshold (K open ) than the TD. The authors stated that since fatigue cracking originates from plastic deformation, the reduced accumulation of plastic strain in the TD likely contributed to an intrinsic toughening mechanism that reduced its crack growth rates compared to the RD. The greater fatigue thresholds of 45 and TD indicate that their textures imparted a toughening mechanism that acted at low stress intensities. The authors concluded that this was a result of intrinsic toughening, since toughening at low stress intensities generally arises from intrinsic toughening mechanisms (LePage, Shaw et al. 2021). 3. Small vs Large Cracks Many of the investigations have focused on several key fracture-mechanics parameters associated with the onset of subcritical and critical cracking of so-called large cracks in superelastic Nitinol. As discussed above, these studies have provided insight on possible initiation sites (for example inclusions), effects of crystallography ( i.e ., drawing and rolling directions) as well as stress intensity threshold ( ∆ K th ) values that could be used for device design and safety efficacy. There is a growing body of recent work that has begun to investigate the possible effects of small cracks in superelastic Nitinol. Robertson and Ritchie (Robertson and Ritchie 2008) suggested a linear extrapolation of the crack-growth data on a log-log fatigue-crack growth-rate curve in order to determine the approximate small crack ( ∆ K th , small crack ) (Paris and Erdogan 1963). This extrapolation concept is shown schematically in Figure 11 (left). Robertson and Ritchie speculated that whereas the large-crack behavior is due to crack closure effects from multiple effects ( e.g ., interference or wedging of oxidation debris or fracture surface asperities inside the crack flanks), small cracks are not subjected to these same closure effects. Consequently, ∆ K th , small crack are thought to be some 40–60% lower than for large cracks based on this extrapolation (Robertson and Ritchie 2008, Robertson, Pelton et al. 2012). Three recent papers have expounded on the differences of small crack vs large crack growths in superelastic Nitinol. Specifically, Malito, et al . (Malito, Haghgouyan et al. 2024) investigated a modified method to combine aspects of total life fatigue and damage-tolerant fatigue to investigate small crack growth thresholds first formulated by McCarver and Ritchie in René 95 alloy (McCarver and Ritchie 1982, Suresh and Ritchie 1984). The investigation was performed on superelastic Nitinol wires (A f ~ 19  C) with focused-ion beam starter cracks under tension-tension conditions of 6% prestrain, 3% mean strain, and R = 0.75 at room temperature to a run out duration of 10,000,000 cycles (Malito, Haghgouyan et al. 2024). Their hybrid S-N and fracture mechanics-based fatigue testing program provided experimental small crack growth threshold for superelastic Nitinol obtained from physically small cracks with sizes comparable to native inclusions. Table 1 summarizes the calculated and experimental large and small crack stress intensity thresholds from Robertson and Ritchie (Robertson and Ritchie 2008), and Malito et al. (Malito, Haghgouyan et al. 2024). Brambilla, et al. implemented a fracture mechanics-based assessment to predict the fatigue durability of surrogate samples tested at different mean and alternate strains (Brambilla, Berti et al. 2024). As a motivation for this study, the authors noted that microstructurally short cracks show significantly faster crack growth rates and lower fatigue thresholds than large cracks and therefore possibly more significant for Nitinol medical devices (Robertson, Pelton et al. 2012). The investigators used symmetric double-edge notched samples that were obtained through electrical discharge machining from superelastic Nitinol sheet with a thickness of 250 µm for this fatigue crack growth study. These investigators selected the use of the cyclic J-integral as a crack driving force in order to account for strain energy dissipation during phase transformation as described in a recent publication (Haghgouyan, Young et al. 2021). Optical DIC was used to monitor cracks in the notched sheet at R values of 0.1 and 0.7; the crack growth rates were obtained from these DIC values. Two-dimensional finite element analyses in plane stress conditions were carried out to calculate the Δ J values during the fatigue crack growth tests.