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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1.1. Sources of Stress/Strain in Nitinol Medical Devices The majority of Nitinol medical devices have an A f temperature between ~0  C and <37  C and therefore, follow the general stress-strain relationship shown in Figure 1(c) at body temperature. More specifically, the strain path for “self-expanding” (superelastic) Nitinol medical devices follows the schematic shown in Figure 2. This figure shows that the Nitinol device is first reduced in diameter (crimped) in order to fit into the delivery catheter; after initial linear elasticity of the Austenite, there is a phase transformation from Austenite-to-Martensite and subsequent linear elasticity of the Martensite. Finite Element Analysis (FEA) is generally used to quantify the maximum pre-strains (~6-12%) of the device sheathing process. The device is then deployed into the diseased site (for example, artery, vein) whereby the stress-strain state follows the unloading path from Martensite-to-Austenite. Nitinol devices are oversized ( e.g. , diameter greater than the intended site) and the corresponding strains could be ~1 to ~8% across the device, also determined by FEA. The in vivo cardiac cycles and musculoskeletal motions establish the cyclic stress strain state. The cyclic motions could range from ~0.1% up to ~2%. Based on this strain analysis, cracks could form at each stage including, for example, stage 1 if the device is able to be crimped-deployed-retracted-redeployed multiple times to allow for more precise positioning in the diseased site. The most likely condition for crack formation is, of course, stage 3 during the cycling. For the FDA-regulated lifetime for stents, filters, etc ., the cardiac pulsations amount to nearly 400,000,000 cycles. Consequently, benchtop tests require at least 400,000,00 cycles with a sample size commiserate with the risk profile. These cardiac cycles generally produce comparatively low strain amplitudes, on the order of <0.1% to <1%. Other in vivo deformations, such as those from musculoskeletal or respiratory motions could result in greater strain amplitudes although fewer cycles (for example, less than 100,000,000 cycles). The FDA stipulates benchtop testing to a 15-year equivalent or 600,000,000 cycles for heart valve frames (Aguel, Hillebrenner et al. 2011). These devices are more complicated in design than for example arterial stents, and are deployed in anatomical regions that induce multiaxial deformations, the range of strain amplitudes in a single device could range from <0.1% to >2%.

Fig. 2: schematic stress-strain diagram that illustrates the three major sources of stress/strain for a self-expanding Nitinol medical device that includes: 1. crimping to a pre-strain of the device to fit into the delivery catheter; 2. deployment of the device to the diseased anatomical site; and then 3. cycling due to cardiac cycles and other in vivo deformations. After (Launey, Ong et al. 2023).

1.2. Microstructural Analysis of Biomedical Nitinol In order to predict crack formation and growth in Nitinol, it is imperative to understand the effects of texture (grain alignment) on mechanical properties in medical-grade Nitinol. Numerous high-resolution investigations have reported on texture of different Nitinol product forms in various deformation directions (Robertson, Gong et al. 2006, Barney, Xu et al. 2011, Reedlunn, Churchill et al. 2014, Reedlunn, LePage et al. 2020, LePage, Shaw et al. 2021). These studies included investigations with synchrotron x-ray µdiffraction with x-ray beams of ≤ 1µm 2 that