Fatigue Behavior of Generation II and Generation III Nitinol

During the past decade, manufacturers have accelerated the pace to provide "ultra-clean" Nitinol for the demanding requirements of next generation Nitinol neurovascular and structural heart implants. Previous research established that the low fatigue performance of Generation I Nitinol is the limiting factor for these critical applications. For example, Robertson, et al. (2015) showed that Generation I Nitinol (VAR and VIM/VAR) have statistically significant lower 10M-cycle fatigue strain limits than Generation II Nitinol (High Purity VAR, VIM, and Process Optimized VIM/VAR). The researchers tested diamond coupons from 8mm tubing with 6% crimp strain and 3% mean strain to demonstrate the profound effects of device inclusion size to predict the probability of fracture under given test conditions. HP VAR had the greatest 10M-cycle fatigue strain limit due to its low average device inclusion size (40µm) compared with Generation I materials with > 80µm. These published results afford great insight into the potential of even higher purity microstructures, such as found in Generation III Nitinol, the so-called HCF material with <7µm inclusion size. The present study provides a statistical comparison of the fatigue performance of Generation II and Generation III Nitinol with custom-designed diamonds manufactured from 10mm diameter Nitinol tubing. Test parameters of 6-10% crimp strain, with varying mean and alternating strains are used to characterize the effects of inclusion size and distribution on the fatigue behavior. Results demonstrate > 25% improvement in the 10M-cycle fatigue strain limit of HCF compared with Generation II Nitinol.