Fracture of Nitinol under Quasistatic and Dynamic Loading
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NITINOL is a thermoelastic alloy with a composition of approximately 50 at. pct Ni and 50 at. pct Ti, capable of two successive thermal martensite phase transformations on cooling from its higher-temperature austenite phase. Nitinol has excellent physical and mechanical properties of interest, such as shape-memory effect, biocompatibility, sueperelasticity, and thermal fatigue and corrosion resistance. These characteristics enable Nitinol to have wide utilization as a functional material in medical implants, as well as in electrical, aerospace, and mechanical engineering applications.[1–3] Recently, studies have shown that Nitinol and its composites have potential applications in engineering practice, as structural materials.[3–8] Superelastic Nitinol undergoes a large recoverable strain during loading, due to stress-induced martensitic transformation, and generates a large hysteretic loop upon unloading, which enables Nitinol to absorb significant energy during superelastic deformation. This large strain-energy absorption capability has recently been considered for improving the impact tolerance of composite structures and vibration damping devices, in which the components can be integrated into critical parts of structures that may need protection from impact loads,[9] and used to protect buildings and bridges against earthquake vibration damage.[3] As a consequence, the study of the FENGCHUN JIANG, Project Scientist, and KENNETH S. VECCHIO, Professor, are with the Department of NanoEngineering, University of California, San Diego, La Jolla, CA 92093-0411. Contact e-mail: [email protected] This article is based on a presentation given in the symposium ‘‘Dynamic Behavior of Materials,’’ which occurred February 26– March 1, 2007, during the TMS Annual Meeting in Orlando, FL, under the auspices of the TMS Structural Materials Division and the TMS/ASM Mechanical Behavior of Materials Committee. Article published online October 30, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation and fracture behavior of Nitinol and its composites under various loading conditions becomes critical for the large-scale utilization of Nitinol. The mechanical response of Nitinol has been extensively studied experimentally and theoretically, at both quasistatic[10–17] and high strain rates.[18–23] However, the utilization and the potential application of Nitinol in long-life components as a structural material requires a thorough understanding of the dominant deformation and fracture mechanisms of Nitinol under different loading conditions.[24] Recently, the mechanical response and fracture behavior of Nitinol and its composites under cyclic loading have been studied[25–29] and the effect of stress-induced martensite (SIM) transformation on deformation and fracture behavior of Nitinol[31–42] has received more attention. It is recognized that SIM formation in the crack-tip zone will significantly influence the fracture properties as well as the deformation and fracture mechanisms of Nitinol. Wang et al.,[32] using finite-element a
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