High-Cycle Mechanical Fatigue Performance of Sputtered Nitinol

This paper investigates the fatigue performance of Nitinol thin-film devices used in medical applications. Freestanding films are fabricated and structured by microsystem technology processes (magnetron sputtering, UV lithography and wet chemical etching). A test rig is developed to address the requirements of Nitinol thin-film samples in terms of force, stroke and precision and also allows the multiplication of test rigs due to its inexpensive components. Hence, several samples can be tested simultaneously at different parameters in order to obtain a thorough characterization within reasonable test duration. Finite element analysis (FEA) is used to derive maximum principle strains of test specimen during cycling loading. Therefore, a superelastic, multiaxial material model with two different kinetic transformation mechanisms being capable of considering tension/compression asymmetry and temperature effects is realized and implemented using the FEA software Comsol Multiphysics. Good agreement between simulation and experimental tensile tests is shown. An excellent fatigue resistance with a high fatigue safety limit of 1.75% pulsatile strain amplitude for mean strains up to 2.5% with sputtered Nitinol diamond specimen is observed.