Fatigue-to-Fracture of Generation III Nitinol

The fatigue of Nitinol for use in biomedical applications has been dominated over the last several decades by the size and distribution of non-metallic inclusions (NMIs). As a result, manufacturers have pushed the boundaries on increasing Nitinol material purity by iterating on different melting practices. Currently there are two major third generation (Gen. III) Nitinol material purities commercially available. These materials are marketed as High-Cycle-Fatigue (HCF) and Enduro. Each of these Gen. III Nitinol material purities implores different melt practices to aim to reduce the growth of inclusions from the melt pool to improve fatigue resistance.

To date, there is no study that compares the two major Gen. III material purities head-to-head for fatigue resistance. Therefore, the aim of this study is to conduct fatigue-to-fracture testing of Gen. III HCF, and Enduro Nitinol diamonds manufactured from 6mm OD ´ 0.5mm WT tubing. In addition, we aim to compare these Gen. III Nitinol materials to standard VIM-VAR material. Diamond specimens are pre-strained to 9% and unloaded to 3% mean strain and cycled off the lower plateau stress. The goal of the study is to compare how these different melt practices influence the fatigue limit of Nitinol compared to their commercial scalability.