Mean strain effects on the fatigue behavior of superelastic Nitinol alloys: An experimental investigation

Experimental fatigue study has been conducted in this paper to investigate the effects of mean strain on the fatigue behavior of superelastic Nitinol in short life regime. Nitinol alloys are used in many applications such as aerospace and bioengineering in which these alloys may be subjected to cyclic loading with mean strains/stresses. For traditional metals, presence of tensile mean strain/stress can significantly reduce fatigue lives. For superelastic Nitinol alloys, however, a more complex behavior is observed under tensile mean strains and ironically some beneficial effects of tensile mean strains have been reported in the literature. This paper aims to experimentally study the effects of tensile mean strains on the fatigue resistance of superelastic Nitinol alloys. Fatigue tests were conducted at room temperature (similar to 24 degrees C) where the material shows superelastic response. Strain -controlled uniaxial fatigue tests with and without tensile mean strains were conducted on standard solid circular specimens. Cyclic deformation and fatigue of the material was studied. Results from this study demonstrates the detrimental effects of mean strain on the fatigue of superelastic Nitinol at least for comparatively large strain amplitudes. Scanning electron microscopy (SEM) was used to investigate the fracture surface and observe the crack initiation sites. Inclusions such as TiC particles located at or near the surface were found to serve as the main crack initiation sites. (C) 2015 Published by Elsevier Ltd.