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Berkeley, CA, United States

Kuo W.T.,Stanford University | Robertson S.W.,Fathom Engineering
Journal of Vascular and Interventional Radiology | Year: 2015

A 46-year-old woman underwent inferior vena cava filter placement before bariatric surgery and returned within 6 months for routine removal. She complained of a 1-week history of severe chest pain, and during retrieval, two fractured filter components were identified including one arm in the right ventricle. The filter body and one fragment were successfully retrieved, but the fragment in the right ventricle was refractory to percutaneous retrieval. During open-heart surgery, the fragment was found traversing through the ventricular wall resulting in cardiac tamponade. Electron microscopic fragment analysis revealed high-cycle metal fatigue indicating the filter design failed to withstand this patient's natural inferior vena cava biomechanical motions. © 2015 SIR. Source

Kaynak Y.,Marmara University | Robertson S.W.,Fathom Engineering | Karaca H.E.,University of Kentucky | Jawahir I.S.,University of Kentucky
Journal of Materials Processing Technology | Year: 2015

This study investigates the effects of fabrication (vacuum arc remelting (VAR) melted vs. vacuum induction melting combined with VAR (VIM + VAR)), processing (hot rolled + fully annealed vs. cold worked + superelastic anneal) and machining conditions (dry, cryogenic, and minimum quantity lubrication (MQL)) of NiTi alloys on their progressive tool-wear behavior. Experimental findings reveal that cryogenic machining substantially improves the performance of cutting tools by reducing the progressive tool-wear in machining of the room-temperature austenitic NiTi alloys. Therefore, cryogenic machining could result in improved productivity and reduced manufacturing costs compared to dry and MQL machining. Experimental evidence suggests that cold working did not alter the progressive tool-wear substantially; however, the presence of carbide inclusions increased the progressive tool-wear in machining NiTi. Surface quality of machined samples under cryogenic machining presents promising improvement upon short-duration machining compared to dry and MQL machining, but all three techniques resulted in comparable quality after 4 min of machining. © 2014 Elsevier B.V. Source

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. Source

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. Source

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