FEA Study of the Influence of Modified Surface Layers on Local Mechanical Properties of Nitinol

摘要

The microstructure of a material directly affects the large-scale mechanical behavior of a structure. The shape memory effect of Nitinol for example is purely based on changes on the atomic scale. During Nitinol stent manufacturing, the material undergoes various processes like laser cutting or heat treatment that influence its microstructure. While heat treatment changes the material behavior throughout the volume, processes like laser cutting, blasting or electropolishing only affect the material's surface up to a certain depth. Heat-affected zones (HAZ) and recast layers formed during laser cutting alter the microstructure in the subsurface, which consequently reduces the tensile and fatigue performance of the material [1]. In finite element analyses, the material is regarded as homogeneous and modified surface layers are in general not considered. This study investigates the influence of modified surface layers on the local mechanical properties of stent-like structures. In a first step the material properties of the modified surface layer are determined. Therefore, surrogate samples with bulk-mechanical properties similar to those of the modified surface layer are manufactured. Experimental and simulated tensile tests are performed and compared in order to determine the parameters for the surface layer material model. In a second step, stent-like structures with modified surface layers are modeled and the stress and strain fields during expansion, compression and fatigue loading are analyzed in the respective areas.