Abstract
One of the contentious issues associated with the high-cycle fatigue of Nitinol, a nominally equiatomic alloy of nickel and titanium, is the claim that increasing the applied mean strain can increase, or at least have no negative impact, on the fatigue lifetime, in conflict with reported behavior for the vast majority of other metallic materials. To investigate this in further detail, cyclic fatigue tests in bending were carried out on electropolished medical grade Nitinol at 37 °C for lives of up to 400 million cycles of strain involving various levels of the mean strain. A constant life model was developed through statistical analysis of the fatigue data, with 90% reliability at a confidence level of 95% on the effective fatigue strain. Our results show that the constant life diagram, a plot of strain amplitude versus mean strain, is monotonic yet nonlinear for lives of 400 million cycles of fatigue loading. Specifically, we find that in contradiction to the aforementioned claim, the strain amplitude limit at zero mean strain is 0.55% to achieve a 400 million cycle lifetime, at 90% reliability with 95% confidence; however, to achieve the same lifetime, reliability and confidence level in the presence of a 3% or more mean strain, the required strain amplitude limit is decreased by over a factor of three to 0.16%. Moreover, for mean strains from 3% to 7%, the strain amplitude limit that allows a 400 million cycle lifetime, at 90% reliability with 95% confidence, is ~ 0.16%, and essentially independent of mean strain. We conclude that the debatable claim that an increase in the applied mean strain can increase the fatigue life of Nitinol components is not supported by the current data.
Highlights
Nitinol (NiTi) is a shape memory alloy composed of nominally equiatomic proportions of nickel and titanium, with metallurgical characteristics and properties associated with an austenite-martensite phase transition from a cubic lattice to a monoclinic one (Duerig et al, 1990; Otsuka and Ren, 2005)
Fatigue life data from the initial campaign of tests, series-austenite start temperature (As), are presented in Fig. 9a in the form of a Goodman diagram, where results for all 216 specimens are shown in terms of strain amplitude versus mean strain with a symbol indicating their fracture status
Our work indicates that careful study of the effect of mean strain on the fatigue life of medical implant grade Nitinol is essential, and future work should focus attention on the mean strain range within which the intended design will operate in order to achieve the highest precision in its life prediction
Summary
Nitinol (NiTi) is a shape memory alloy composed of nominally equiatomic proportions of nickel and titanium, with metallurgical characteristics and properties associated with an austenite-martensite phase transition from a cubic lattice to a monoclinic one (Duerig et al, 1990; Otsuka and Ren, 2005). In addition to its thermomechanical behavior consistent with its shape-memory characteristics, at selected temperatures Nitinol is superelastic ( termed pseudoelastic), enabling large recoverable strains through combinations of the austenite-martensite phase transformation and twinning/detwinning of the martensite. Such behavior is associated with stress-strain hysteresis loops (Pelton et al, 2000). It is predominantly the superelastic phenomena of Nitinol that are utilized, as it enables transcatheter delivery followed by spontaneous expansion of the device into the patient’s anatomy (Duerig et al, 1999; Stöckel et al, 2004) Examples of such devices made from Nitinol are self-expanding arterial stents (Pelton et al, 2008), self-expanding inferior vena cava filters (Grassi, 1991), and self-expanding heart valves (Lanz et al, 2019 )
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