STP1559

    Evaluation of Fatigue Performance and Residual Stress of Surface Modification Techniques for Ti-6Al-4V Medical Device Applications

    Published: Sep 2013


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    Abstract

    A variety of surface modification techniques exist that offer the potential to enhance mechanical fatigue performance of medical device components, yet these technologies are not widely applied across the industry. As implantable device designs are continually miniaturized, components are often subjected to higher stresses, requiring improvements in material properties, such as fatigue strength. This paper describes an evaluation of three different surface modification techniques for their potential to enhance the fatigue resistance of Ti-6Al-4V anterior cervical plates: shot peening, laser shock peening, and electropolishing. The primary evaluation criterion was high-cycle bending fatigue testing following ASTM F1717-09, 2009, “Standard Test Methods for Spinal Implant Constructs in a Vertebrectomy Model,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. The relative roles that compressive residual stress and surface roughness play in determining fatigue performance are examined by the measurement of residual stress depth profiles (by the hole-drilling method) and surface roughness. Whereas similar studies have been reported in the literature in the context of engine components, the smaller size of implantable medical device components makes this work unique. There is concern that thin cross sections may give rise to significant sub-surface compensatory tensile stresses when techniques, such as shot peening or laser peening, are used to impart near-surface compressive residual stresses. Results from this study are somewhat counterintuitive: the deeper compressive residual stress profiles of laser shock peening are not necessarily beneficial when they come at the expense of less compressive peak residual stress. This reflects the importance of surface and very-near-surface residual stresses under bending conditions for thin plates. Although increasing shot peen intensity yielded relatively deeper and more compressive residual stress profiles, these higher-intensity peening conditions resulted in lower fatigue performance compared to lower-intensity peened samples (which, in turn, were not conclusively better than the as-fabricated baseline). Electropolishing is shown to be beneficial alone, as well as when combined with prior shot peening.

    Keywords:

    cervical plate, residual stress, fatigue, surface modification, peening, roughness, hole drilling


    Author Information:

    Terry, Scott
    Medtronic, Inc., Minneapolis, MN

    Rouleau, Jeff
    Medtronic, Inc., Minneapolis, MN

    Dace, Mark
    Medtronic, Inc., Minneapolis, MN

    Pelo, Mark
    Medtronic, Inc., Minneapolis, MN


    Paper ID: STP155920130025

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP155920130025


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