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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Keywords: Ntinol; Austenite; Martensite; shape memory; superelasticity; cracks; inclusions

1. Introduction Nitinol, a nearly equiatomic nickel-titanium alloy known for its unique shape memory, superelasticity, and biocompatibility, has proven particularly successful in biomedical applications during the past two decades. It is now common practice to employ Nitinol in endovascular stents, vena cava filters, transcatheter valve repair and replacement implants, neurovascular occlusion devices, orthodontic files, orthopedic implants, and guidewires for minimally invasive procedures (Stöckel, Pelton et al. 2004). Shape memory and superelasticity are driven by a temperature- and/or stress-induced martensitic phase transformations, respectively, that allow recoverable accommodation of up to 8% strain (Otsuka and Ren 2005). With this magnitude of strain recovery, Nitinol enables minimally invasive procedures and thereby reduces unnecessary patient trauma (Stöckel, Pelton et al. 2004). The durability of medical devices is regulated by international standards (for example, the three part ISO 5840 (ISO5840-1 2021, ISO5840-2 2021, ISO5840-3 2021), multiple ASTM standards (ASTM 2013, ASTM 2017) as well as U.S. FDA guidelines (FDA 2015, FDA 2021). In particular, more stringent requirements for fatigue assessment have been implemented during the recent years. For example, ISO 5840 outlines five areas of fatigue analysis, including: 1. accurate anatomical and physiologic boundary conditions, 2. stress/strain analysis (generally through validated Finite Element Analysis), 3. material fatigue strength analysis, 4. component fatigue demonstration, and finally, 5. fatigue safety factor calculation (ISO5840-1 2021, ISO5840-2 2021, ISO5840-3 2021). Once implanted, Nitinol medical devices are subjected to a variety of cyclic biomechanical motions, including blood flow with the cardiac cycle, respiratory expansion and contraction through breathing/diaphragm motions, and limb movement (Cheng 2019). Such devices experience millions-to-billions in vivo cyclic mechanical motion, and as such, lifetime predictions of components are critical for the design and optimization of devices manufactured from Nitinol. Design of structural devices relies on a “safe-life” design approach (Suresh 1998). In the safe-life approach to fatigue design the typical cyclic load spectra, which are imposed on a device in service, are first determined. On the basis of this information, the devices are analyzed or tested in-vitro under load conditions that are typical of service spectra, and a useful fatigue life is estimated for the component. The estimated fatigue life, suitably modified with a factor of safety, provides a prediction of “safe-life” for the component. This approach depends on achieving a specified life without the development of a fatigue crack with the emphasis on the prevention of crack initiation rather than crack growth (Gurney 1998). Given that: 1. the small dimensions of stents, heart valve frames and other Nitinol implants, and that 2. superelastic Nitinol has a low fatigue crack growth resistance in terms of high growth rates, da/dN , and low fatigue threshold values, ∆ K th , as compared to other biomedical materials (as discussed in section 2.2) the fraction of the total-life of a strut spent in propagating a crack is small compared with the initiation phase (Robertson, Pelton et al. 2012). As such, this approach forms the basis of most cardiovascular device fatigue and durability analyses, such as found in the standards and regulations noted above; i.e., the stress-based S-N approach or strain based ε -N approach. Damage tolerance is an alternative, or complementary fatigue analysis method, and may be applicable for some medical devices as discussed in (ISO5840-2 2021). The importance of understanding crack growth rates, an output of such fracture mechanics-based approaches, is to account for crack propagation from pre-existing flaws from the material purity ( i.e ., non-metallic inclusions), manufacturing ( i.e ., pits and/or microcracks), or device handling ( i.e ., needle scratches). While this approach has been successfully applied to linear-elastic Cobalt-Chromium coronary stents (Marrey, Burgermeister et al. 2006), this methodology has not been readily applied to Nitinol medical devices due to the complexity and poor understanding of the damage tolerance of thermo-mechanically processed (cold worked and aged) Nitinol used in self-expandable medical devices. It is expected that a detailed review of fatigue crack initiation and growth of cracks in medical-grade Nitinol may clarify the roles of intrinsic factors such as metallurgical composition, grain size, texture, transformation temperatures, non-metallic inclusion size and volume