Mean strain effects and microstructural observation during in-vitro fatigue testing of NiTi

The non-linear nature of the superelastic phase transformation in NiTi means that conventional fatigue life theory is difficult to apply. The volume fraction of martensite/parent phase and its role in the fatigue mechanism is not understood. This paper considers the fatigue life of superelastic NiTi alloys when tested at 2Hz in Ringers solution. In agreement with other researchers [1] [2] it was found that low alternating strains but high mean strain values resulted in the longest fatigue lives. To support the mechanical testing data, optical and electron microscopy was used to investigate the mechanism of fatigue and the role of the NiTi microstructure