M. Fonte, A. Saigal, Tufts University, Medford, MA
As Nitinol emerges to find more applications in engineered medical device products, understanding the effects of material processing becomes increasingly important. Its mechanical behavior is highly non-linear and is strongly dependent on alloy composition, heat treatment history and mechanical work. Published Nitinol literature is almost exclusively related to processing and testing of thin wall tubing and wire devices, usually exhibiting superelastic characteristics. There is a dearth of information in the public domain regarding the compressive deformation mode of solid blocks of Nitinol and whether or not “bulk” Nitinol products can exhibit shape recovery effects and superelastic characteristics. The potential for strain recovery of compressed solid Nitinol products, combined with the material's low modulus and biocompatibility can enable the design of improved medical devices, specifically in the orthopedic realm. The motivation for this research is to provide the first characterization of the shape recovery effects of “bulk" size, solid Nitinol material under compressive deformation versus the often practiced and well understood tensile loading of wire and thin wall tubing.
Summary: Nitinol exhibits a shape memory effect (SME) in which an apparent plastic strain is recovered through heating the material to a temperature above which the material transforms from a ductile, lower stiffness martensitic phase to a higher stiffness austenitic phase. The shape recovery is associated with a relatively large change in stiffness resulting in relatively high actuation stresses. It is well understood that Nitinol wire products are pulled and bent in tension in order to prepare and set the material for shape recovery, whereas Nitinol tubing is often drawn over an inner mandrel to expand the diameter circumferentially. Due to intense study efforts, the mechanism of deformation is well characterized in polycrystalline NiTi shape memory alloys subjected to monotonic tensile loading conditions. However, the characteristics of deformation in polycrystalline NiTi subjected to compression deviate from the well-documented monotonic tensile response and until recently, there have been few attempts to quantitatively understand the compression shape memory effect of “bulk” size polycrystalline NiTi.
