title: A statistical approach to understand the role of inclusions on the fatigue resistance of superelastic Nitinol wire and tubing.
abstract: Superelastic wires and diamond-shaped stent surrogates were manufactured from Nitinol rods and tubing, respectively, from five different mill product suppliers - Standard VAR, Standard VIM, Standard VIM+VAR, Process-Optimized VIM+VAR, and High-Purity VAR. High-cycle fatigue tests up to 10(7) cycles were conducted under tension-tension conditions for wires and bending conditions for diamonds. These materials were compared under both testing methods at 37°C with 6% prestrain and 3% mean strain (unloading plateau) with a range of alternating strains. The High-Purity VAR material outperformed all alloys tested with a measured 10(7)-fatigue alternating strain limit of 0.32% for wire and 1.75% for diamonds. Process-Optimized VIM+VAR material was only slightly inferior to the High Purity VAR with a diamond alternating bending strain limit of 1.5%. These two "second generation" Nitinol alloys demonstrated approximately a 2× increase in 10(7)-cycle fatigue strain limit compared to all of the Standard-grade Nitinol alloys (VAR, VIM, and VIM+VAR) that demonstrated virtually indistinguishable fatigue performance. This statistically-significant increase in fatigue resistance in the contemporary alloys is ascribed to smaller inclusions in the Process-Optimized VIM+VAR material, and both smaller and fewer inclusions in the High-Purity VAR Nitinol.
authors.affiliation: Fathom Engineering, 600 Addison Street, Berkeley, CA 94710, USA. Electronic address: scott@fathomengineering.com.
authors.forename: Scott W
authors.initials: SW
authors.lastname: Robertson
authors.affiliation: Glassimetal Technology, Inc., 2670 E Walnut Street, Pasadena, CA 91107, USA.
authors.forename: Maximilien
authors.initials: M
authors.lastname: Launey
authors.affiliation: Medina Medical, 937 Hamilton Avenue, Menlo Park, CA 94025, USA.
authors.forename: Oren
authors.initials: O
authors.lastname: Shelley
authors.affiliation: Nitinol Devices & Components (NDC), 47533 Westinghouse Dr, Fremont, CA 94539, USA.
authors.forename: Ich
authors.initials: I
authors.lastname: Ong
authors.affiliation: Nitinol Devices & Components (NDC), 47533 Westinghouse Dr, Fremont, CA 94539, USA.
authors.forename: Lot
authors.initials: L
authors.lastname: Vien
authors.affiliation: Nitinol Devices & Components (NDC), 47533 Westinghouse Dr, Fremont, CA 94539, USA.
authors.forename: Karthike
authors.initials: K
authors.lastname: Senthilnathan
authors.affiliation: Endologix, 2 Musick Road, Irvine, CA 92618, USA.
authors.forename: Payman
authors.initials: P
authors.lastname: Saffari
authors.affiliation: ATI Wah Chang, 1600 Old Salem Road NE, Albany, OR 97321, USA.
authors.forename: Scott
authors.initials: S
authors.lastname: Schlegel
authors.affiliation: G. RAU Inc., 704 Rancho Arroyo Parkway, Fremont, CA 94536, USA.
authors.forename: Alan R
authors.initials: AR
authors.lastname: Pelton
mesh_terms: D000497:Alloys; D004548:Elasticity; D008422:Materials Testing; D015607:Stents; D013314:Stress, Mechanical
chemical_list: D000497:Alloys; C013616:nitinol
keywords: Fatigue; Inclusions; Microstructure; NMIs; Nitinol