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Robertson S.W.,Fathom Engineering | Launey M.,Glassimetal Technology Inc. | Shelley O.,Medina Medical | Ong I.,Nitinol Devices and Components NDC | And 5 more authors.
Journal of the Mechanical Behavior of Biomedical Materials | Year: 2015

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 107 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 107-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 107-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. © 2015 Elsevier Ltd.

Launey M.,Glassimetal Technology Inc. | Robertson S.W.,Fathom Engineering | Vien L.,Nitinol Devices and Components NDC | Senthilnathan K.,Nitinol Devices and Components NDC | And 2 more authors.
Journal of the Mechanical Behavior of Biomedical Materials | Year: 2014

The bending fatigue resistance of commercially-available Standard versus High Purity Nitinol was evaluated at 3% mean strain and a range of strain amplitudes with the simple wire Z-specimen geometry. The Standard grade Nitinol demonstrated a 107-cycle fatigue strain limit of 0.50% alternating strain, comparable to results reported elsewhere in the literature. Conversely, the High Purity grade VAR Nitinol demonstrated a 5-fold improvement in fatigue resistance with an impressive 107-cycle fatigue strain limit of 2.5% alternating strain. The High Purity Nitinol has an oxygen+nitrogen content of 60wppm, maximum wrought-material inclusion length of 17μm, and inclusion volume fraction of 0.28%, all substantially less than industry standards. With all processing variables held constant except for inclusion content, it is clear that this marked fatigue superiority is due exclusively to the reduction in both size and area fraction of inclusions. © 2014 Elsevier Ltd.

Robertson S.W.,Nitinol Devices and Components NDC | Pelton A.R.,Nitinol Devices and Components NDC | Ritchie R.O.,University of California at Berkeley
International Materials Reviews | Year: 2012

Nitinol, a near equiatomic intermetallic of nickel and titanium, is the most widely known and used shape memory alloy. Owing to its capacity to undergo a thermal or stress induced martensitic phase transformation, Nitinol displays recoverable strains that are more than an order of magnitude greater than in traditional alloys, specifically as high as 10%. Since its discovery in the 1960s, Nitinol has been used for its shape memory properties for couplings and actuators, although its contemporary use has been in for medical devices. For these applications, the stress induced transformation ('superelasticity') has been used extensively for self-expanding implantable devices such as endovascular stents and vena cava filters, and for tools such as endodontic files. Most of these applications involve cyclically varying biomechanical stresses or strains that drive the need to fully understand the fatigue and fracture resistance of this alloy. Here we review the existing knowledge base on the fatigue of Nitinol, both in terms of their stress or strain life (total life) and damage tolerant (crack propagation) behaviour, together with their fracture toughness properties. We further discuss the application of such data to the fatigue design and life prediction methodologies for Nitinol implant devices used in the medical industry. © 2012 Institute of Materials.

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