High Compressive Pre-strain Reduces the Bending Fatigue Life of Nitinol Wire

Prior to implantation, Nitinol-based transcatheter endovascular devices are subject to a complex thermo-mechanical pre-strain associated with constraint onto a delivery catheter, device sterilization, and final deployment. However, the effect of such large thermo-mechanical excursions on fatigue properties is still not well understood. The present study investigated the effects of bending pre-strains on the fatigue of pseudoelastic Nitinol wire using rotary bend fatigue (RBF) subject to 0, 8 or 10% bending pre-strains at 0.3% to 1.5% strain amplitudes for up to 10⁸ cycles. The large pre-strains resulted in residual set in the wire and a significant reduction in the fatigue life below 0.8% strain amplitude.  The 0% and 8% pre-strain wires reached run out at 10⁸ cycles at 0.6% and 0.4% strain amplitude, respectively, whereas 10% pre-strain wires fractured at less than 10⁵ cycles, even at 0.3% strain amplitude. Furthermore, over 70% of fatigue cracks initiated on the compressive pre-strain surface. A model is presented to explain these observations that takes into account texture-dependent tension-compression asymmetry with different martensite variants in combination with compressive pre-strain-induced plasticity and tensile residual stresses.