STP1559

    The Effects of Test Speed on Fatigue Life of Nitinol Wire in Rotary Bend

    Published: Sep 2013


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    Abstract

    In order to acquire durability data for medical devices in a reasonable amount of time, manufacturers often perform fatigue testing at an accelerated rate. However, the effects of increasing test frequency on the fatigue life of nitinol devices have not been thoroughly investigated. In order to better understand how test speed affects nitinol fatigue, superelastic nitinol wire was tested in body-temperature water in a free (unguided) rotary bend fatigue apparatus. Alternating strains of 0.9 %, 1.0 %, 1.1 %, 1.5 %, or 2.0 % were applied at rotational test speeds of 0.6, 6, or 60 Hz. Except for the highest strain level, results trended toward increased fatigue life at higher test frequencies. Fatigue life at 6 and 60 Hz was significantly greater (p < 0.05) than fatigue life at the slowest test speed for alternating strains of 1.0 %, 1.1 %, and 1.5 %. High-speed video of 1.0 % and 1.1 % alternating strain tests at 60 Hz revealed small vibrations that manifested as side-to-side rocking motion of the wire. Because of the unexpected vibrations seen in nitinol wire tests, supplementary testing of cobalt–chromium wire was conducted. High-speed video of cobalt–chromium wire at 0.5 % alternating strain and 333 Hz test speed showed a vertical bouncing motion that appeared to be a second vibration mode. To constrain motion and reduce vibration effects, wire guides were constructed and used in additional testing. Guided tests of cobalt–chromium wire showed reduced vibration, resulting in a lowered fatigue life that was nearer to values expected based on literature. Although further testing is needed, vibrations appear to artificially increase fatigue life by altering the imposed radius of curvature and reducing the amount of strain wires undergo in each cycle. Rotary bend wire fatigue testing remains a relatively simple and useful test; however, caution is recommended when the test is conducted without wire guides, as vibrations may affect fatigue life.

    Keywords:

    fatigue, nitinol, speed, frequency, rotary bend, wire


    Author Information:

    Weaver, Jason D.
    Office of Science & Engineering Laboratories, Center for Devices & Radiological Health, Food & Drug Administration, Silver Spring, MD

    Gupta, Shikha
    Office of Science & Engineering Laboratories, Center for Devices & Radiological Health, Food & Drug Administration, Silver Spring, MD

    Woods, Terry O.
    Office of Science & Engineering Laboratories, Center for Devices & Radiological Health, Food & Drug Administration, Silver Spring, MD


    Paper ID: STP155920120213

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP155920120213


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