“Laser Cut Nitinol Tubing Fatigue Coupon: Design, Testing, and Endurance Limit”

Towards the goal of determining the strain based endurance limit of medical grade super-elastic Nitinol tubing, a coupon was designed, evaluated via FEA, and fatigue tested to a maximum of 10 million cycles.     The coupons underwent the same processing as a stent-like medical implant would, including laser cutting, honing, and electropolishing.    The coupon was designed and analyzed via finite element analysis (FEA) software.   The material model used in the FEA program was calibrated in the strain ranges tested using force results from a tension/compression cycle test of coupons on a tensile test machine.    All coupons were measured on a comparator to ensure meeting dimensional  tolerances.   FEA was used to determine the maximum strain versus alternating displacement amplitude for the coupon.    Error due to dimensional tolerances was determined.     The coupons where then fatigue tested in a 37C temperature water bath at alternating strain levels ranging from 0.75% to 4.0% at zero mean strain.     Sample replication was greater than 90%, and the median alternation strain fatigue limit was determined via two methods (two point method, and probit method).    Confidence and reliability with maximum likelihood statistics are used to present a strain based endurance limit for the material.    The results differ significantly from previous published values.