STP1559

    The Effect of Inclusions on Fatigue Properties for Nitinol

    Published: Sep 2013


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    Abstract

    ASTM F2063-05 covers microcleanliness in wrought nickel-titanium shape memory alloys for medical devices and surgical implants. Microcleanliness is evaluated in mill products with section sizes of between 6.3 and 94 mm in diameter, thickness, width, height, wall thickness, and so forth. These dimensions exclude most of the finished components for biomedical applications. Thus, the inclusion assessment is conducted at an intermediate manufacturing process step. To our knowledge, no rigorous method has been developed to correlate the inclusion dimensions in the final product to the dimensions in precursor semi-finished materials. Moreover, the method of inclusion analysis must be based on a robust statistical approach in order to define accurately the inclusion content. Finally, specifications about microcleanliness should rely on studies explaining the correlation between inclusion size and material performance, in particular fatigue. We propose to consider extreme-value inclusion rating, already applied to rate inclusions in hard steels, for shape memory alloy. To this end, a study of the effect of inclusions on nitinol fatigue was undertaken. Here we present the results of the first part of the study, concerning rotating-bending fatigue on 0.3-mm-diameter superelastic wires. Materials from seven ingots were tested. A comprehensive experimental campaign was conducted involving tests at different strain amplitudes. The fracture surfaces of failed wires were analyzed through scanning electron microscopy, and data regarding the presence of particles and their morphology were recorded. Collected data were subsequently analyzed. By means of a simple interpretative model, we compare the fatigue performances of four different processes and show that they are different. Then we show qualitatively how inclusion dimensions affect fatigue. Finally, by means of extreme value statistics, we rate the particle dimensions of the four processes and correlate them to fatigue performance. An outline of ongoing and future work is also provided.

    Keywords:

    extreme value inclusion rating, nitinol, superelasticity, bilinear uniform scatter band


    Author Information:

    Urbano, Marco Fabrizio
    SAES Getters S.p.A., Lainate,

    Coda, Alberto
    SAES Getters S.p.A., Lainate,

    Beretta, Stefano
    Politecnico di Milano, Milano,

    Cadelli, Andrea
    SAES Getters S.p.A., Lainate,

    Sczerzenie, Frank
    SAES Smart Materials, New Hartford, NY


    Paper ID: STP155920120189

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP155920120189


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