LAUSR

The near equi-atomic intermetallic Ni Ti alloy Nitinol is used for medical implants, notably in self-expanding stent grafts and heart valve frames, which are subjected to several hundred million load cycles in service. Increasing the testing frequency to the ultrasonic range would drastically shorten the testing times and make the very-high cycle regime experimentally accessible. Such tests are, however, only meaningful if the material response at ultrasonic frequency is identical to that observed in conventional fatigue tests. A novel fatigue testing setup where superelastic Nitinol dog bone specimens are loaded at ultrasonic cycling frequency is presented. Loading conditions resemble in vivo loading (i.e., repeated cyclic loading with relatively small strain amplitudes, specimens in a pre-strained multi-phase state). Strains and phase transformations during ultrasonic frequency cycling are quantitatively measured in an X-ray diffraction (XRD) synchrotron experiment and compared to the material response at low frequency. The XRD experiment confirms that forward and reverse stress-induced phase transformation from austenite to martensite via the intermediate R-phase occurs during low frequency (0.1 Hz, strain rate ε ˙ ≈ $$ \dot{\varepsilon}\approx $$ 10 −3  s −1 ) and ultrasonic frequency (20 kHz, ε ˙ ≈ $$ \dot{\varepsilon}\approx $$ 10 2  s −1 ) cycling. Since the same deformation mechanisms are active at low and ultrasonic frequency, these findings imply a general applicability of the ultrasonic fatigue testing technique to Nitinol.