Fatigue performance of superelastic NiTi near stress-induced martensitic transformation

The structural fatigue of superelastic NiTi was studied with special attention paid to the drop in fatigue performance commonly observed at the onset of the stress-induced martensitic transformation. We processed the superelastic NiTi wires into an hourglass shape as a result of which the stress induced martensitic transformation in tension is not localized as confirmed by digital image correlation measurements. The hourglass shaped samples were subjected to force controlled pull-pull fatigue tests at two distinct temperatures selected to investigate the fatigue of NiTi with and without the R-phase transformation involved, and to capture the effect of varying critical stress for the onset of B19′ martensitic transformation. Resulting fatigue curves expressed in terms of both the stress and the strain show a first degradation of fatigue performance at stresses more than 200 MPa below the onset of the martensitic transformation. Furthermore, fatigue curves showed a temperature dependence which was successfully implemented into a modified Basquin’s power law model. Notably, the fatigue life drops down to 104 cycles before the nominal loadings reach the B19′ transformation regime. We further found that fatigue life is shorter for the B2-R-phase transformation regime than for the elasticity of the parent B2 austenite phase at low stresses when strains remain below 1%, however at higher strains the B2-R-phase transforming wire is better than the elastic one. Fatigue crack observations revealed crack initiation at the surface and its propagation towards the bulk resulting in a reduction of the cross-section and substantial increase in normal stresses. Martensitic transformation was triggered during the crack growth in samples nominally loaded in elastic or R-phase transformation regimes as confirmed by in-situ infrared thermography. As the crack grows with increasing speed, the activity of transformation processes at the tip gradually increases till unstable crack growth and final rupture occurs, as confirmed by in-situ thermography and stress hysteresis observations. The analysis of fracture surfaces revealed five different crack growth regimes - from quasi-cleavage at early crack growth, through propagation stage evidenced by striations, till the final ductile fracture evidenced by the typical fracture surface morphology.