Fatigue Crack Initiation of High-Purity Nitinol in Fractures Between 100 Million and 2 Billion Cycles

Recent metallurgical advancements in the production of nitinol have created a new generation of material with fewer and smaller non-metallic inclusions that promises to deliver improved fatigue performance compared with more conventional nitinol. Since high-purity nitinol is explicitly designed for improved fatigue performance at very high cycle counts, in order to avoid unexpected high-cycle fractures it is essential to improve our understanding of what might lead to fatigue crack initiation beyond 100 million cycles. High-purity nitinol wires were straightened and electropolished prior to testing in rotary bend fatigue. Fatigue tests were conducted at a range of alternating strain between approximately 0.6% and 2.6% in phosphate buffered saline (PBS) to a runout cycle count of 1 or 2 billion. Similar to conventional nitinol, a bimodal distribution in fatigue life was observed with fractures occurring either prior to 100,000 or after 100 million cycles. High cycle fractures were observed between approximately 0.6% and 1.2% alternating strain and scanning electron microscopy (SEM) was used to characterize the fatigue crack initiation site of those specimens. Nearly all the initiation sites had a region of reduced roughness (RRR) present. Of the 12 very high cycle fractures with clear RRR analyzed to date, fatigue crack initiation ‘catalysts’ were roughly split evenly among manufacturing surface defect (5), < 1 µm inclusion/void (3), or unknown (4). These results will provide designers with important information to ensure safety against fatigue fracture of high-purity nitinol for fatigue-critical applications beyond 100 million cycles.