Abstract
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 dot{varepsilon}approx 10−3 s−1) and ultrasonic frequency (20 kHz, dot{varepsilon}approx 102 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.
Highlights
Since its discovery in 1962 [1, 2], the nearly equi-atomic intermetallic Ni-Ti alloy, commonly known as Nitinol, has attracted much interest for industrial applications
A unique new experimental technique is presented to quantitively measure strains during high strain rate cyclic deformation, with the primary goal of comparing ultrasonic and ordinary cyclic loading. The implementation of this method has provided important insights into ultrasonic cyclic loading of Nitinol that are of interest to accelerated fatigue testing of Nitinol medical implants
The newly developed technique is capable of applying cyclic strains at high strain rates of the order of ε ≈ 102 s−1 to superelastic Nitinol dog bone specimens in a pre-strained multi-phase state
Summary
Since its discovery in 1962 [1, 2], the nearly equi-atomic intermetallic Ni-Ti alloy, commonly known as Nitinol, has attracted much interest for industrial applications. The present experiment aims to combine those abilities of XRD with in situ cyclic loading of Nitinol sheet specimens to resolve lattice deformations and phase transitions during cycling at ultrasonic frequency, resulting in small strains at high strain rates. At 9174 frames per full exposure at each of the 50 phase offsets Δφi, with each frame accounting for one load cycle, plus overhead cycles during pulse build up the specimen would quickly be destroyed before the acquisition could be completed This was overcome by only acquiring exposures at the phase positions of minimum and maximum strain, Δφεmin and Δφεmax , respectively, that had been obtained by aligning the ultrasonic signal with the known phasing of the 0.1 Hz loading as shown in Fig. 6 (right). This demonstrates that both the forward and reverse transformation from B2 austenite to the B19’ martensite can occur at ultrasonic loading rates
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