Assessment of the impact of inclusions on the 10 million cycle fatigue constant-life of two Nitinol tubing materials

It is generally recognized that non-metallic inclusions have a direct impact on fatigue resistance of Nitinol materials. Several publications also suggested that the fatigue resistance was dramatically improved on the Nitinol materials with smaller and fewer inclusions. However, little data on the effect of non-metallic inclusions on the Nitinol fatigue constant-life is available in the literatures over a broad range of applied mean strain and strain amplitude conditions.

Preliminary assessment was made on the fatigue resistance of two types of commercially available Nitinol tubing. One group, used as the control group, was a standard tubing made of a VIM/VAR melting process and the other group was manufactured with a refining melting process aimed at reducing the inclusions and improving the fatigue property. Fatigue tests were performed at 0%, 3% and 5% mean strains and the strain amplitudes up to 2.5%, and up to 10⁷ cycles. The inclusions in both materials were examined and measured. The phase transformation temperature and mechanical property of both materials were also evaluated.