PSI - Issue 66

15

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

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

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The investigators state that when a defect is being loaded, the stresses/strains adjacent to the defect are in tension when the remote loading is in compression. As such, the compressive portion of the loading will then also contribute to the damage accumulation or small crack initiation and growth by sharpening the crack tip and smoothing out the crack faces that are interacting. The decrease in surface roughness that the authors fittingly define as RRR, as measured by optical profilometry, is suggestive of a kind of slow self-burnishing process where the two faces of the RRR smoothed each other out over millions of loading cycles (Roiko, Cook et al. 2025). The authors differentiate RRR that was identified in this work from smooth regions on fatigue fracture surfaces that are generated when two opposing surfaces rub against each other after fracture. Those features, differ from the RRR in that they often include visible lines in the smooth region that identify the direction the two surfaces translated against each other and the smooth regions are not typically centered on the fatigue-crack initiating NMI. The limiting RRR size from Roiko, et al . (Roiko, Cook et al. 2025) is in accord with the transition shown by Malito, et al. in their Kitagawa-Takahasi diagram (Malito, Haghgouyan et al. 2024) at ~ 10µm size. Clearly, additional investigations are required that focus on the specific small crack paths and sizes. 4. Summary and Conclusions  Even for isothermal cases (e.g. stable body temperatures for in vivo devices), crack initiation and propagation in Nitinol are complicated by additional deformation mechanisms (phase transformation, detwinning, etc .) and microstructural phenomena (crystallographic texture, anisotropic mechanical behavior, tension compression asymmetry, etc.).  In the case of Nitinol with non-metallic inclusions NMIs larger than about 10 µm, most fatigue cracks in small struts (e.g. those found in cardiovascular devices) initiate and propagate from the inclusions. However, new high-purity Nitinol processing routes are opening the door to other crack initiation sites.  Fatigue crack growth threshold stress intensities are significantly lower for small cracks compared to those conventionally measured for long cracks, yet in most practical structures, not just medical devices, the total life is dominated by crack propagation when the cracks are small. Modified methods to combine aspects of total life fatigue and damage-tolerant fatigue to investigate small crack growth thresholds have been proposed as suitable tools for fatigue life prediction and device safety.  Opportunities for future work include development of new fatigue-resistant shape memory alloys, new surface treatments and device deployment routes that leverage residual stress effects, and foundational understanding of mechanical properties towards high-fidelity modeling and fatigue predictions. Aguel, F., M. Hillebrenner, S. F. C. Stewart, J. Swain, V. Hampshire and B. Zuckerman (2011). "U.S. Regulatory Considerations for Heart Valves Implanted by Minimally Invasive Surgery." Cardiovascular Engineering and Technology 2(2): 62-65. Ahadi, A. and Q. Sun (2013). "Stress hysteresis and temperature dependence of phase transition stress in nanostructured NiTi—Effects of grain size." Applied Physics Letters 103(2). ASTM (2013). F2942-13 Standard Guide for in vitro Axial, Bending, and Torsional Durability Testing of Vascular Stents. ASTM (2016). Standard Test Method for Determination of Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery, ASTM International. F2082 – 16. ASTM (2017). "F3211 − 17 Standard Guide for Fatigue-to-Fracture (FtF) Methodology for Cardiovascular Medical Devices." ASTM (2017). Standard Test Method for Determination of Transformation Temperature of Nickel-Titanium Shape Memory Alloys by Thermal Analysis, ASTM International. F 2004-17. ASTM (2018). F2063 Standard Specification for Wrought Nickel-Titanium Shape Memory Alloys for Medical Devices and Surgical Implants. 5. References