Thermal mechanical behavior of a functionally graded shape memory alloy cylinder subject to pressure and graded temperat
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As a new functional composite material, functionally graded shape memory alloy (FG-SMA) holds particular properties of both functionally graded materials and shape memory alloys. By bringing in a new concept of conical surface assumption, considering the axial deformation, a macroconstitutive model which can describe the thermal mechanical properties of a FG-SMA cylinder subjected to pressure and graded temperature loads is established in this work. Furthermore, a new layered finite element model (FEM) which can avoid the direct assumption of the macroproperties of the FG-SMA cylinder is provided. The theoretical results display a good agreement with the FEM results, which indicates that both the macroconstitutive model and the FEM provided here are valid. The obtained results show that the stress in the cylinder distributes complexly, and it decreases remarkably as a result of the martensite transformation. This research can provide a base for the design and in-depth investigation of FG-SMA materials.
I. INTRODUCTION
The functionally graded shape memory alloy (FGSMA) is a new kind of functional composite, which holds the particular properties of both functionally graded materials (FGMs) and shape memory alloys (SMAs). According to the components and structures of this material, FG-SMA can be divided into two categories: one is after special procedures, the SMA whose microstructure varies gradually and thus the macroproperties vary gradually, so as to create the FG-SMA1,2; the other is composed of SMA and other materials (such as ceramic and so on), and the component ratio varies gradually, thus producing the FG-SMA.3–6 Such as NiTi–TiC FG-SMA, the NiTi SMA can reduce the maximum stress, satisfying the needs of large deformation; while the TiC has high strength and wear resistance. The FG-SMA composed of these two materials has both the phase transformation characteristics of SMA at one side and the abrasion resistant properties of TiC at another side. Furthermore, the material failure caused by stress concentration can be relieved due to the gradient variation of the component materials. As the growing demand in high and new technology applications, a deep understanding of the mechanical properties of the FG-SMA is required, and several literature studies which investigate the properties of this a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.130
new functional composite have been published recently. Birnbaum provided a novel mechanism and process that can control the shape memory response spatially within monolithic NiTi thin film structures.1 This technique can effectively control the temperature of martensitic phase transformation and thus control the mechanical and shape memory responses as well. Mahmud introduced a new heat treatment approach which can create the functionally graded near-equiatomic NiTi SMA.2 In this approach, SMA is annealed with a graded temperature after cold work, and thus create a graded structure in the matrix of SMA. Tian reported
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