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This regulatory science tool presents a laboratory method that outlines a strategy for estimating the presence of cyclic phase transformation to aid in the determination of appropriate load levels for Fatigue to Fracture (FtF) testing of nitinol components. Please see appendix.
Technical Description
This RST outlines a strategy for estimating the presence of cyclic phase transformation to aid in the determination of appropriate load levels for Fatigue to Fracture (FtF) testing of nitinol components. Specifically, it includes a flowchart (Figure 1 in Appendix) to estimate the low to high cycle fatigue transition using either computational or experimental methods. Based on the output of the analysis, the user can create a test plan for fatigue testing per ASTM F3211 which includes both expected low cycle fatigue fractures and runouts.
Intended Purpose
This RST is intended to assist individuals selecting load levels to test as part of an FtF test plan per ASTM F3211 for medical devices containing a nitinol component. As described in Section 6.8 of ASTM F3211-17, it is often desirable to conduct FtF testing at multiple load levels to induce both fractures and run-outs. To that end, it may be advantageous to estimate the transition (sometimes referred to as the knee in a fatigue life diagram) from low to high cycle fatigue without the need to conduct supplemental ‘pilot’ or ‘scoping’ fatigue tests.
This RST is applicable to implantable medical devices containing a nitinol component that are being tested using the ASTM F3211 FtF approach and is intended to be used in conjunction with ASTM F3211-17 and ASTM F2516-18. It may be used for testing of entire devices, components, coupons, or surrogate samples as long as the cyclically loaded component is constructed from pseudoelastic nitinol. As mentioned in FDA’s Guidance Document “Technical Considerations for Non-Clinical Assessment of Medical Devices Containing Nitinol,” testing and analysis should be conducted on specimens that are representative of the final manufactured device.
Testing
Multiple researchers have observed that low cycle fatigue (less than approximately 10 million cycles) was associated with the presence of cyclic phase transformation in nitinol [Catoor et al. 2019, Paranjape et al. 2020, Rahim et al. 2013, and Weaver et al. 2023]. Thus, load level(s) without evidence of cyclic phase transformation would be expected to survive to approximately 10 million cycles, although fatigue fractures beyond 10 million cycles may occur. Selecting levels both above and below where cyclic phase transformation occurs may aid in the overall characterization of the fatigue performance of nitinol components. Additional detail on the testing completed to support this RST can be found in Weaver et al. 2023.
Limitations
This RST is only applicable to pseudoelastic nitinol because of its unique austenite-martensite phase transformation. The RST is intended to estimate the transition from low to high cycle fatigue and thereby aid in selecting test levels for FtF testing. Some factors that may affect the transition that are not addressed in this RST include: effects of pre-strain (crimping) or other sources of plastic deformation, loading path, nitinol purity (microcleanliness), mean strain, presence of R-phase, and the relative volume of material undergoing high cyclic stress or strain. Lastly, as observed in Weaver et al. 2023, fatigue fracture of nitinol beyond 100 million cycles can occur. This RST does not address how to estimate the loading needed to observe ultra-high cycle fatigue fractures. Note also that prediction of phase transformation will be sensitive to mesh resolution (e.g., see Weaver et al. 2023). Accordingly, mesh refinement studies are recommended to increase the accuracy of the RST for predicting the low to high cycle fatigue transition.
Supporting Documentation
FDA-recognized standards include:
- ASTM F3211-17 Standard Guide for Fatigue-to-Fracture (FtF) Methodology for Cardiovascular Medical Devices
- ASTM F2516-18 Standard Test Method for Tension Testing of Nickel-Titanium Superelastic Materials
FDA Guidance documents include:
- FDA guidance “Technical Considerations for Non-Clinical Assessment of Medical Devices Containing Nitinol”: Section IV.B provides recommendations on experimental mechanical testing as well as computational stress/strain analyses that may be applicable to the use of this RST.
Further recommended reading (studies wherein low cycle fatigue was observed in the presence of cyclic phase transformation):
- Weaver, J.D., Sena, G.M., Aycock, K.I., Roiko, A., Falk, W.M., Sivan, S. and Berg, B.T., 2023. Rotary bend fatigue of nitinol to one billion cycles. Shape Memory and Superelasticity, 9, pp.50-73. https://doi.org/10.1007/s40830-022-00409-7
- Rahim, M., Frenzel, J., Frotscher, M., Pfetzing-Micklich, J., Steegmüller, R., Wohlschlögel, M., Mughrabi, H. and Eggeler, G., 2013. Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys. Acta Materialia, 61(10), pp.3667-3686. https://doi.org/10.1016/j.actamat.2013.02.054
- Paranjape, H.M., Ng, B., Ong, I., Vien, L. and Huntley, C., 2020. Phase transformation volume amplitude as a low-cycle fatigue indicator in nickel–titanium shape memory alloys. Scripta Materialia, 178, pp.442-446. https://doi.org/10.1016/j.scriptamat.2019.12.014
- Catoor, D., Ma, Z. and Kumar, S., 2019. Cyclic response and fatigue failure of Nitinol under tension–tension loading. Journal of Materials Research, 34(20), pp.3504-3522. https://doi.org/10.1557/jmr.2019.254
Contact
Tool Reference
Please reference the use of this tool using RST24OP01.01