PSI - Issue 66

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

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Fig. 5: Backscattered scanning electron microscopy images from superelastic Nitinol wires that fractured after ~1000 cycles under conditions of 10% strain amplitude under rotary bending fatigue. The wire drawing direction is vertical. Note the vast number of cracks on the wire surface proximal to the fracture surface (top). Upon close inspection, each crack is associated with a non-metallic inclusion (Ti 4 Ni 2 O x ) (dark grey particles); similar images were observed with Nitinol that contained TiC inclusions. Both forms of the inclusions are ceramics and appear to form the initiation sites for fatigue cracks and that many cracks traverse across multiple inclusions. After (Pelton and Mitchell 2006, Pelton, Fino Decker et al. 2013). Two recent fatigue papers emphasize the effects of processing parameters on the strain-based total-life behavior of medical-grade Nitinol. In particular, these publications demonstrate the strong effects that ‘‘in situ’’ modification of the microstructure of Nitinol due to pre-strain and mean strain have on the fatigue properties of implanted medical devices (Pelton, Berg et al. 2022, Launey, Ong et al. 2023). As shown in Figure 2, there is a strong path dependent behavior of superelastic Nitinol for which pre-strains intensify texture, increase martensite volume fraction, and induce plasticity. During pre-straining, the affected regions in the Nitinol devices are exposed to large forward (crimp) and reverse (deploy) mechanical strain excursions even before encountering any physiological cyclic deformation. This single large thermomechanical strain cycle (or multiple cycles for retrievable/re-positional devices) may result in a measurable permanent set ( i.e ., permanent changes in local or global dimensions), as well as changes in the stress–strain behavior. Collectively, both macroscale and microscale observations indicate that pre strain tends to increase the FSL with a delay of crack formation even in the presence of NMIs. Although the effects of mean strain on fatigue life is a bit more controversial, the vast majority of the publications cited in (Pelton, Berg et al. 2022, Launey, Ong et al. 2023) as well as experimental data in (Launey, Ong et al. 2023) show credible evidence that increasing mean strain ( e.g. , amount of oversizing) increases the volume fraction of martensite with a concomitant decrease in modulus. The compelling reason for these observations of enhanced FSL is that for a given strain amplitude the stress amplitude decreases with the decrease in cyclic modulus (for reference, the modulus of Austenite in medical grade Nitinol is approximately 60GPa; in contrast, the corresponding modulus of Martensite is approximately 30GPa) while cycling in the linear elastic regime. 2.2. Damage Tolerance Crack Growth In contrast to the total life approach to fatigue of superelastic Nitinol (many to support medical device regulatory submissions), relatively few studies have been conducted to evaluate the damage tolerance properties, i.e. the fatigue crack growth and fracture toughness behavior (Dauskardt, Duerig et al. 1989, McKelvey and Ritchie 1999, McKelvey and Ritchie 2001, Robertson and Ritchie 2007, Robertson, Mehta et al. 2007, Stankiewicz, Robertson et al. 2007, Robertson and Ritchie 2008, Robertson, Ritchie et al. 2008, Robertson, Pelton et al. 2012, LePage 2018, LePage, Ahadi et al. 2018, LePage, Shaw et al. 2021). The main reason for this disparity in test methods is that the dimensions of most Nitinol medical devices are too small to monitor crack growth rates. However, damage tolerance