PSI - Issue 66

6

Pelton/ Structural Integrity Procedia 00 (2025) 000–000

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

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Fig. 4: (a) Bending moment of a superelastic Nitinol tube as a function of average curvature for incremental cycling, and (b) corresponding axial strain field images times labeled in (a). Each image was taken at the maximum curvature for that cycle. Note the nonuniform strain distributions on the outer fiber of the tubing on the tension side of the bent tubing. After (Reedlunn, Churchill et al. 2014). In the context of cardiovascular device deployment (as illustrated in Figure 2), this tension-compression asymmetry can create complications for devices with struts in bending. Specifically, when a device is crimped with bending, plasticity forms earlier on the side of the strut in compression. Upon deployment/unloading, the plasticity from the compression side results in residual tensile stress on the compression side. With enough tensile stress on the compression side, this phenomenon can lead to crack formation during cycling (Figure 2, stage 3) in which cracks form counterintuitively on the compressive side of a strut. 2. Monitoring Cracks: Total Life vs Damage Tolerance 2.1. Total Life Fatigue Crack Initiation and Growth During the past decade, Nitinol material manufacturers have accelerated the pace to provide “ultraclean” Nitinol for the medical device industry. These high-purity Nitinol materials are processed to maintain the mechanical and thermal property requirements of ASTM F2063 (Standard Specification for Wrought Nickel-Titanium Shape Memory Alloys for Medical Devices and Surgical Implants) (ASTM 2018). Reduction in Titanium Oxide (Ti 4 Ni 2 O x ) and Titanium Carbide (TiC) inclusion size and volume fraction is the primary driver of the development of the new purities of Nitinol materials. These oxide and carbide non-metallic inclusions (NMIs) are byproducts of the ingot melting processes and once formed, cannot be removed or reduced by conventional thermal or mechanical treatments. Consequently, the NMI modifications must take place in the melt process. Furthermore, these NMIs are ceramic particles — often associated with voids — that are significantly harder and more brittle than the base Nitinol. Therefore, these inclusions (and/or voids) provide fracture initiation sites (see Figure 5) and thereby limit the fatigue life of Nitinol medical devices under cyclic fatigue conditions as detailed in many publications (Robertson, Pelton et al. 2012, Launey, Robertson et al. 2014, Robertson, Launey et al. 2015, Urbano, Cadelli et al. 2015, Pelton, Pelton et al. 2017, Launey, Ong et al. 2023, Weaver, Sena et al. 2023, Roiko, Cook et al. 2025). These total life fatigue publications indicate that “intrinsic” NMIs are the initiation sites for crack initiation and that the fatigue life of Nitinol dramatically increases with decreasing inclusion size. Specifically, with a maximum inclusion length of ~100µm, the fatigue strain limit (FSL) is ~ 0.5%, whereas, other Nitinol alloys with ~ 40µm length inclusions, the FSL increases to ~ 1%. The greatest microstructural purity with ~ 10µm inclusions has a FSL of ≥ 2% (Robertson, Launey et al. 2015, Pelton, Pelton et al. 2019, Pelton, Pelton et al. 2024).