On Superelastic Deformation of NiTi Shape Memory Alloy Micro-Tubes and Wires — Band Nucleation and Propagation
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Applications of shape memory alloys (SMAs) in the medical industry are multiplying rapidly. NiTi polycrystalline SMAs, due to their unique superelastic properties and biocompatibity, have been successfully used to manufacture medical devices in recent years. One of the most dramatically demonstrated examples is the utility of NiTi superelastic micro-tubes and wires in minimal access surgery or less invasive operations as guidewires, guidetubes and stents etc.. High flexibility, large recoverable deformation, good fatigue life and outstanding superelastic behavior at or around the body temperature are qualities that make it possible to reduce and to minimize the size of critical medical device and to perform functions impossible with other materials. It offers the best compromise between engineered plastics and traditional metals. High mechanical reliability (such as kink resistance) and controllability of deformation are the two critical requirements for very long and small diameter surgery instruments [1, 2]. Though some fundamental studies on the bulk superelastic NiTi wire and strips have been performed in recent years , only a few conference reports on tests of NiTi tubes can be found in the open literature . With the decrease in the dimension of medical instruments, size effect on the mechanical behavior of materials becomes another topic of great concern and has received wide attention in last decade. For materials undergoing martensite type phase transformation such as NiTi polycrystalline shape memory alloys, interesting size effect on the macroscopic martensite band nucleation stress of the wire under tension was observed in the authors’ recent experiments, i.e., the smaller the diameter of the wire the higher the nucleation stress.
KeywordsShape Memory Alloy Stress Drop Shape Memory Alloy Wire NiTi Shape Memory Alloy NiTi Wire
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