Finite Element Analysis Of Sputtered Nitinol Specimen For Medical Applications And Experimental Evaluation Of Their High Cycle Mechanical Fatigue Behavior

Because of its unique properties, Nitinol is widely used to design medical devices for minimal-invasive surgery and temporary or permanent vascular implants. In this field of applications, the mechanical fatigue behavior of the material is crucial and must be tested and evaluated adequately. In recent years, a fabrication technique based on microsystem technology processes has been developed that allows to fabricate complex Nitinol based material systems for next generation medical products, and simultaneously promises improvements in fatigue resistance due to a high purity Nitinol and lack of inclusions. In order to assess the fatigue behavior a fatigue test rig has been developed. This test rig addresses 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 low cost components. Hence, several samples can be tested simultaneously at different parameters in order to obtain a thorough characterization within reasonable test duration. FEA is used to derive maximum principle strains of test specimen during cycling loading. Therefore, two superelastic, multi-axial material models, capable of considering tension/compression asymmetry and temperature effects are realized and implemented using Comsol Multiphysics and its general PDE-Interface. These material models are compared and validated against proprietary implemented shape memory material models and experimental data from tensile tests. This paper describes the components and functional principle of the test rig and shows experimental fatigue data for different mean strains and strain amplitudes of diamond shaped specimen and their fatigue safety limit.