Superelasticity, Shape Memory and Stability of Nitinol Honeycombs under In-plane Compression
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1188-LL05-04
Superelasticity, Shape Memory and Stability of Nitinol Honeycombs under In-plane Compression John A. Shaw1 , Petros A. Michailidis1, Nicolas Triantafyllidis1, and David S. Grummon2 1 2
University of Michigan, Aerospace Engineering, Ann Arbor, Michigan, 48109-2140 Michigan State Univ., Chemical Engin. & Materials Science, East Lansing, Michigan, 48824
ABSTRACT Low density Nitinol shape memory alloy honeycombs were fabricated using a new Nb-based brazing method [1], which demonstrated enhanced shape memory and superelastic properties under in-plane compression [2]. Adaptive, light-weight cellular structures present interesting possibilities for design of new architectures and novel applications. This paper presents an overview of ongoing work to address the multi-scale stability of superelastic, thin-walled, SMA honeycombs and the need for design and simulation tools. BACKGROUND Shape memory alloys (SMAs) are a material class that exhibit the shape memory effect and superelasticity [3], two strain recovery phenomena occurring with changes in temperature and/or stress, that can enable novel adaptive and energy absorption applications. Popular commercial SMAs are NiTi-based (near equiatomic Nitinol, or NiTiX alloys), which have the robust strain recovery and structural properties as polycrystals. On the other hand, cellular metals, made of conventional materials, like aluminum, are desirable in applications where low density, high stiffness, and energy absorption are needed [4]. Papka and Kyriakides [5] performed interesting in-plane crushing experiments of thin-walled aluminum honeycombs, where structures exhibited an initially stiff response, followed by a load plateau with localized row-by-row elasto-plastic collapse and permanent deformation, and then stiffening from internal cell contact. We were inspired to explore whether adaptive honeycomb structures, using SMAs, could be made that recover deformation after load removal (superelasticity) or upon heating (shape memory). Historically, joining Nitinol to itself has been difficult, usually requiring engineers to use mechanical fasters or adhesives [6, 7]. Metallurgical bonding required special welding techniques and conventional brazing usually resulted in weak properties. A new metallurgical bonding method for NiTi, however, was discovered [1] that has good strength, ductility, corrosion resistance, and biocompatibility [8]. Figure 1 shows photographs and micrographs of Nitinol tubes that were brazed together using Niobium as a melting point suppressant, where a quasi-eutectic resulted in aggressive wetting and formation of a classical lamellar microstructure
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Figure 1: Niobium-based brazing of Nitinol tubes. of NiTi and bcc-Nb. This enabled the construction of the first NiTi honeycomb specimens (near 5 % dense) with useful adaptive properties. Hexagonal and lens-like cell geometries were produced by shape setting NiTi strips into corrugations, bonding them together at high temperature using Nb, and then heat-treating near
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