Ultrasonic fatigue testing of thin MP35N alloy wire

AbstractMP35N (35 % Co, 35 % Ni, 20 % Cr, 10 % Mo; weight fraction) in the form of thin wires (100 µm diameter) is commonly used as conductors in cardiac leads, which require excellent corrosion resistance and high fatigue strength, in particular in the very-high cycle fatigue (VHCF) regime. This becomes apparent when one assumes a typical adult human heart rate of 72 beats per minute, which over 10 years of implant deployment will roughly yield 3.8 × 108 cycles. Tensile properties of MP35N wire are considerably enhanced through extensive cold-working. Static strength of the material is in the range of 2 GPa (Prasad et al., 2014). Fatigue testing of very thin wires is time consuming with conventional fatigue testing methods. In earlier investigations (Prasad et al., 2014) the wire was stressed at a cyclic frequency of 30 Hz in monotonic loading tests, which would require about 193 days for one single specimen to complete 5 × 108 cycles. A complete characterization of a material’s fatigue properties however requires many specimens to be tested well into the VHCF regime, calling for an accelerated testing method, especially with regard to development of new implant materials. For the first time, a method to test thin wires with the ultrasonic fatigue testing method is presented. Rather than vibrating in resonance as in conventional ultrasonic fatigue tests, the wire is stressed with cyclic tension loads. Results of fatigue tests at a cycling frequency of around 20 kHz up to lifetimes of 109 cycles at load ratio R = 0.3 are shown. The influence of secondary phase particles on crack initiation is discussed. Microstructural observations and lifetimes measured at 30 Hz from earlier studies and 20 kHz are compared and discussed

High Speed In Situ Synchrotron Observation of Cyclic Deformation and Phase Transformation of Superelastic Nitinol at Ultrasonic Frequency

AbstractThe 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 $$\dot{\varepsilon}\approx$$ε˙≈ 10−3 s−1) and ultrasonic frequency (20 kHz, $$\dot{\varepsilon}\approx$$ε˙≈ 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.</jats:p

High Speed In Situ Synchrotron Observation of Cyclic Deformation and Phase Transformation of Superelastic Nitinol at Ultrasonic Frequency

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˙ ≈ 10−3 s−1) and ultrasonic frequency (20 kHz, ε˙ ≈ 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