Effect of Cyclic Loading and Strain Rate on Nitinol Stress/Strain Prediction in the Clinically Relevant Strain Range

Tuesday, August 9, 2011: 9:20 AM
Salon B (Hilton Minneapolis )
Dr. Paul Briant , Exponent, Inc. , Menlo Park , CA
Ryan Siskey , Exponent, Inc. , Menlo Park , CA
Sarah Easley , Exponent, Inc. , Menlo Park , CA
Brad James , Exponent, Inc. , Menlo Park , CA
Nitinol stress-strain behavior can be affected by both multiple loading cycles and the rate at which the specimens are loaded.  Nevertheless, most nitinol implant stress analyses utilize material properties obtained from a one- or two-cycle, low strain-rate tensile test. Any differences between in vivo and test conditions, such as the effect of multiple cycles or strain rate, could affect prediction of implant fatigue performance.

Implantable nitinol devices typically undergo a single a high-strain event as they are loaded into a catheter, after which they are deployed into a mechanical environment that typically applies both a relatively constant mean strain and relatively low-cyclic strain amplitudes.  The regions of highest strain amplitude in a device therefore typically undergo small cyclic strains off the lower plateau of the nitinol stress-strain curve.  To better understand the effect of cyclic loading and strain rate on predictions of in vivo stress/strain levels in nitinol devices, the results of multiple finite element analyses using constitutive properties obtained from nitinol wire specimens subjected to multiple loading cycles over a range of strain rates were compared.  Our results indicate that significant variation in predicted in vivo stresses and strains can occur over the range of cyclic loading and strain rates tested.