title: 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.
authors.affiliation: U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, MD, USA.
authors.forename: Shikha
authors.initials: S
authors.lastname: Gupta
authors.affiliation: G. RAU Inc., Fremont, CA, USA.
authors.forename: Alan R
authors.initials: AR
authors.lastname: Pelton
authors.affiliation: U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, MD, USA.
authors.forename: Jason D
authors.initials: JD
authors.lastname: Weaver
authors.affiliation: Medical Implant Mechanics, Irvine, CA, USA.
authors.forename: Xiao-Yan
authors.initials: XY
authors.lastname: Gong
authors.affiliation: U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, MD, USA. Electronic address: srinidhi.nagaraja@fda.hhs.gov.
authors.forename: Srinidhi
authors.initials: S
authors.lastname: Nagaraja
mesh_terms: D000497:Alloys; D057785:Catheters; D019245:Compressive Strength; D004548:Elasticity; D057510:Endovascular Procedures / Q000295:instrumentation*; D008422:Materials Testing; D013314:Stress, Mechanical; D013499:Surface Properties
chemical_list: D000497:Alloys; C013616:nitinol
keywords: Bending; Crimp; Endovascular; Fatigue; Nitinol; Pre-strain; Pseudoelastic; Shape memory