High compressive pre-strains reduce the bending fatigue life of nitinol wire

Abstract

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 10(8) cycles. The imposition of 8% or 10% bending pre-strain 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 10(8) cycles at 0.6% and 0.4% strain amplitude, respectively, 10% pre-strain wires continued to fracture at less than 10(5) cycles, even at 0.3% strain amplitude. Furthermore, over 70% fatigue cracks were found to initiate on the compressive pre-strain surface in pre-strained wires. In light of the texture-dependent tension-compression asymmetry in Nitinol, this reduction in fatigue life and preferential crack initiation in pre-strained wires is thought to be attributed to compressive pre-strain-induced plasticity and tensile residual stresses as well as the formation of martensite variants. Despite differences in fatigue life, SEM revealed that the size, shape and morphology of the fatigue fracture surfaces were comparable across the pre-strain levels. Further, the mechanisms underlying fatigue were found to be similar; despite large differences in cycles to failure across strain amplitudes and pre-strain levels, cracks initiated from surface inclusions in nearly all wires. Compressive pre-strain-induced damage may accelerate such crack initiation, thereby reducing fatigue life. The results of the present study indicate that large compressive pre-strains are detrimental to the fatigue properties of Nitinol, and, taken together, the findings underscore the importance of accounting for thermo-mechanical history in the design and testing of wire-based percutaneous implants.

In Situ Neutron Diffraction Studies of Increasing Tension Strains of Superelastic Nitinol

“…Furthermore, this precise microstructural account of strain accommodation mechanisms of superelastic Nitinol, including plasticity and transformational/reorientation behavior, may explain the enhancement of fatigue behavior with mean strains between *1.5-6 % [41]. Together with our parallel report of the large tension-compression asymmetry in the critical mechanism transition points [16], these new data also affirm previous speculation of why bending pre-strains of 8 % or more degrade fatigue life [8] while analogous tensile strains improve it [3,6]. The defect structures imparted by tensile pre-strains evoke tensile residual stresses, which shifts the unloaded state of the material Fig.…”

“…Despite the differences in manner of actuation (thermal versus mechanical) as well as composition and thermomechanical processing, the current results and those from, for example, Benafan et al [37], are comparable findings. They concur with patents [3][4][5][6][7] and personal communications with Launey et al that shows fatigue enhancement with tensile pre-strain in Nitinol wires [8]. Hence, we propose that pre-strain of actuators to this same extent, in a microstructural sense, would increase their fatigue performance.…”

“…In a recent rotary bend fatigue study of medical-grade Nitinol wire, it was found that fatigue life does not correlate with the amount of pre-& Aaron P. Stebner astebner@mines.edu strain. Instead the data indicate that the optimal pre-strain appears to lie within a narrow deformation region and that compressive bending pre-strains have a negative effect on subsequent rotary bend fatigue life [8].…”

“…For martensite, all difference IPFs for Sample 8 j-n) are taken with respect to peak load (i), while for Sample 3, x is taken with respect to (w), and all remaining difference IPFs y-dd are taken with respect to (x). Note that the scale bars are different for the B19' phase relative to the B2 phase as the distributions are more sharply focused degradation, together with possibly additional surface damage in the case of bending that act to promote crack propagation, as observed by Gupta et al [8]. This mechanistic shift serves to set up strong competition between martensite reorientation, phase transformation, and plasticity.…”

Influences of granular constraints and surface effects on the heterogeneity of elastic, superelastic, and plastic responses of polycrystalline shape memory alloys

“…A modification of the low-amplitude loading, as performed in this work could potentially be more beneficial in introducing moderate compressive stresses on the surface than the conventional methods. A low-amplitude cycling regime is less likely to introduce large plastic deformation and thus larger flaws on the surface (Gupta et al, 2015;Pelton et al, 2015).…”