Ultrasonic fatigue testing of thin MP35N alloy wire

Abstract MP35N (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 × 10 8 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 × 10 8 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 10 9 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.