Small crack growth thresholds for nitinol in fully reversed ultra-high cycle fatigue, comparing standard and VAR/EBR wire

Fatigue fracture in nitinol components used in medical devices depends on the initiation and growth of small cracks before conventional propagation and final fracture. Understanding small crack threshold and growth is key to durability. This study examines the size of inclusions or defects causing crack initiation and the link between this size and the stress intensity threshold in two different nitinol variants.

A distinct smooth area around certain defects generally after over 10 million loading cycles is identified and studied. This area is thought to result from a small crack growing from a defect until it reaches a critical size and then conventional fatigue crack propagation begins. The relationship between the defect size and the smooth area is quantified, generating a damage curve based on the ratio of small crack size (Region of Reduced Roughness, RRR) to original inclusion size versus cycles to fracture.

This curve estimates the maximum allowable defect size for fully-reversed loads into the ultra-high cycle domain, (N>10^7). Combining this curve with a probability law associated with the largest inclusion size in the high load volume may predict the probability of fracture.

The analysis reveals that while standard nitinol inclusions initiate cracks that grow sub-threshold before conventional propagation, VAR/EBR (vacuum arc remelted/electron beam refined) wire RRR features are observed but often without inclusions at the initiation sites. This comparative assessment highlights the importance of material selection and defect size effects on the fatigue life and reliability of nitinol used in medical device applications.