High compressive pre-strains reduce the bending fatigue life ofnitinol wire

摘要

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. Though such large thermo-mechanical excursions are known to impact the microstructural and mechanical properties of Nitinol, their effect on fatigue properties is still not well understood. The present study investigated the effects of large thermo-mechanical pre-strains on the fatigue of pseudoelastic Nitinol wire using fully reversed rotary bend fatigue (RBF) experiments. Electropolished Nitinol wires were subjected to a 0%, 8% or 10% bending pre-strain and RBF testing at 0.3−1.5% strain amplitudes for up to 10⁸ cycles. The imposition of 8% or 10% bending prestrain resulted in residual set in the wire. Large pre-strains also significantly reduced the fatigue life of Nitinol wires below 0.8% strain amplitude. While 0% and 8% pre-strain wires exhibited distinct low-cycle and high-cycle fatigue regions, reaching run out at 10⁸ cycles at 0.6% and 0.4% strain amplitude, respectively, 10% pre-strain wires continued to fracture at less than 10⁵ cycles, even at 0.3% strain amplitude. Furthermore, over 70% fatigue cracks were found to initiate on thecompressive pre-strain surface in pre-strained wires. In light of thetexture-dependent tension−compression asymmetry in Nitinol, thisreduction in fatigue life and preferential crack initiation in pre-strainedwires is thought to be attributed to compressive pre-strain-induced plasticityand tensile residual stresses as well as the formation of martensitevariants.Despite differences in fatigue life, SEM revealed that the size, shapeand morphology of the fatigue fracture surfaces were comparable across thepre-strain levels. Further, the mechanisms underlying fatigue were found to besimilar; despite large differences in cycles to failure across strain amplitudesand pre-strain levels, cracks initiated from surface inclusions in nearly allwires. Compressive pre-strain-induced damage may accelerate such crackinitiation, thereby reducing fatigue life. The results of the present studyindicate that large compressive pre-strains are detrimental to the fatigueproperties of Nitinol, and, taken together, the findings underscore theimportance of accounting for thermo-mechanical history in the design and testingof wire-based percutaneous implants.