Effects of pre-strain and cyclic loading on compressive response of Nitinol

Innovation of Nitinol transcatheter implants drives the demand for low-profile crimping. When a Nitinol device is crimped to smaller diameter, fatigue regions subjected to high crimp strain may undergo permanent deformation. This deformation will induce residual stress and change of plateau stress, both of which can alter fatigue stress/strain and impact overall fatigue performance. However, current Nitinol models still face challenges to accurately capture this path-dependent behavior, especially under compressive loading. This limitation is partly due to the lack of understanding Nitinol’s compressive responses under cyclic loading at large pre-strain levels, in contrast to the more studied tensile behavior.

To address this gap, we developed a test method capable of measuring uniaxial compressive behavior of thin-walled Nitinol tubing, commonly used for transcatheter implants. Validation using three-dimensional digital image correlation measurement confirmed the absence of compression buckling up to a stress level of 1.5 GPa. Utilizing this method, we successfully characterized the compression response of Nitinol under repeated loading-unloading with pre-strain levels up to 6%. The results confirmed typical features of Nitinol’s compressive behavior reported in literature. However, the cyclic compressive responses at different pre-strain levels were found to differ from the tensile behavior – particularly in terms of residual strain accumulation and the evolution of lower plateau stress. Limitations of the existing Nitinol model in Abaqus in capturing the effects of pre-strain and cyclic loading under compression are also discussed.