J. M. Crank, Boston Scientific, Maple Grove, MN
This objective of this study was to characterize the stress relaxation of linear elastic NiTi wire in accelerated conditions of bending. The response surface experiment had the following factors and levels: wire diameter (.005”, .009”, .013”), time (3 hrs, 73.5 hrs, 144 hrs), temperature (120 °F, 160 °F, 200 °F), and input strain (.2%, .5%, .8%). The test specimens, 27 runs with 5 samples/run, were wrapped around cylinders and placed in an oven for the requisite time. When removed from the oven and unwrapped they relax to some final curvature over a period of time on the order of hours. The key output in this study was the strain present immediately after removal from the oven, which was called the initial output strain. The curvature of the samples was calculated from x-y position data obtained by floating them on the surface of a water bath containing a grid of scales just below the surface. The data fit and compared to real time data.
Summary: Measurements of the stress relaxation of linear elastic NiTi wire in bending were done to determine if this material would be suitable for use in guidewires, which are packaged in hoops. Samples ranging in diameter from .005" to .013" were wrapped around cylinders to create strains ranging from .2% to .8% and held at temperatures ranging from 120F to 200F for times ranging from 3 hrs to 144 hrs to obtain a model for real-time, room temperature stress relaxation. To measure the resulting curvature of the wires, a new method was developed that involved floating the wires on the surface of water, taking x-y coordinates of the wire with a grid of scales just below the water's surface, and fitting a circle to the coordinates.
