Apex Forming Stresses and Strains In As-Drawn Cold-Worked Ni-Ti Wire

The fabrication of stent rings for self-expanding endovascular medical implant devices frequently involves the creation of bends (apexes) in as-drawn heavily cold-worked Ni-Ti wire prior to shape setting.  The wire diameter and apex bend radii relevant to these components create large fiber bending strains at the intrados and extrados of an apex.  Excessive strains can create tensile surface cracks in the extrados or compression-induced cracking damage in the intrados that could compromise fatigue performance.  However, the stress-strain behavior of as-drawn cold-worked Ni-Ti wire under bending or even uniaxial tension has not been widely studied since in this state it has cold-worked behavior (rather than the post-heat treatment flag-shaped superelastic properties that dictate device performance).

This paper compares the uniaxial tensile behavior of the as-drawn wire to calculated bending stresses and strains generated during apex formation.  While the calculated bending strains are found to be adequately modeled by classical elastic beam theory, bending stresses are calculated via a finite element analysis using a material model fit to the as-drawn wire uniaxial tensile behavior.  The implications of the uniaxial tensile elongation to failure for apex forming is discussed.  The magnitude of the calculated compressive intrados bending strains is compared to previously cited thresholds of around 25% for compression-induced cracking damage (James et al., Proc. Int. Conf. SMST 2004).