Mean strain effect on bending fatigue behavior of Nitinol

Fatigue strength expressed as strain amplitude at different mean strains is commonly used to assess the long-term reliability of Nitinol cardiovascular implantable devices. The mean strain in critical locations in vivo after the device’s preconditioning history – crimping, delivery & deployment – is not only a function of the material mechanical behavior, but also a function of the design of implant and delivery system. Consequently, the effect of mean strain on fatigue strength (strain amplitude limit) of Nitinol must be considered in device design and durability assessment.

In our prior study using C-shaped specimens under simple bending in the very high cycle fatigue regime, the fatigue strength was found to decrease as mean strain increases up to 3%. To understand this effect, we conducted further fatigue testing with constant and step-increasing amplitudes at lower cycle counts below 50 million cycles. Results show consistent trends: (1) constant-amplitude testing revealed fracture probability is reduced at lower mean strains, where life distribution becomes bimodal. (2) increasing-amplitude testing was used to identify the strain amplitude threshold above which fatigue life is shorter than 10 million. These strain amplitude thresholds decreased with increasing mean strain – a trend that is consistent with our prior results. These results support the hypothesis that fracture probability is related to the austenite-martensite interface motion and their interaction with defects.