Small crack growth thresholds for nitinol in ultra-high cycle fatigue

Fatigue fracture of small nitinol components commonly used in medical devices is dominated by the initiation and/or growth of small cracks preceding conventional fatigue crack growth and fracture. Therefore, the threshold and growth of small cracks is critical to understand and measure to properly inform the fatigue performance of the device. In this paper we examine the size of the inclusion from which the fatigue crack initiates and the dependency of the size of the inclusion with respect to the stress intensity threshold calculated.

The size of the inclusions is also compared to the size of a unique feature which appears as a smooth area surrounding the inclusion and is found only around inclusions which led to fracture after >10 million loading cycles. The hypothesis presented in this paper is that the smooth feature around the inclusion is the growth of a small crack around the inclusion which happens below the conventional fatigue crack growth threshold. This small crack growth continues until it reaches a size large enough to cause conventional fatigue crack growth based on the increase in size of the crack. The relationship between inclusion size and small crack size measured from the smooth feature creates a damage curve based on the ratio of damage area (small crack size) to the original inclusion size with respect to cycles to fracture. This damage curve is useful to calculate the critical size of the largest defect allowed based on the length of the design life of the nitinol component.