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Shape Memory Alloys (SMAs) for Engineering Applications
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Shape Memory Alloys (SMAs) for Engineering Applications

A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Smart Materials".

Deadline for manuscript submissions: closed (31 October 2020) | Viewed by 56657

Special Issue Editors

Prof. Dr. Masoud Motavalli

E-Mail Website
Collection Editor
Structural Engineering Laboratory, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
Interests: application of advanced materials (such as fiber-reinforced polymer composites and shape memory alloys in civil engineering); structural rehabilitation and repair; seismic retrofitting; large and full scale laboratory and field experiments
Special Issues, Collections and Topics in MDPI journals
Dr. Christoph Czaderski

E-Mail Website
Collection Editor
Structural Engineering Laboratory, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
Interests: strengthening of reinforced concrete with CFRP (Carbon Fibre Reinforced Polymers), prestressed CFRP, and shape Memory Alloys (SMA) for usage in building industry
Prof. Dr. Moslem Shahverdi

E-Mail Website
Collection Editor
Structural Engineering Laboratory, Empa Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
Interests: shape memory alloys, fatigue and fracture mechanics, composite materials, solid mechanics, reinforced concrete, lightweight structures, numerical simulations, and field experiments

Special Issue Information

Dear Colleagues,

This Special Issue of Materials is dedicated to “Shape Memory Alloys (SMAs) for Engineering Applications”. We are expecting to receive papers dealing with cutting-edge issues on research and application of SMAs in structural engineering. The topics of the Special Issue include, but are not limited to:

  1. Alloy designing of SMAs for structural engineering including:
    1. Nickel-titanium-based SMAs
    2. Copper-based SMAs
    3. Iron-based SMAs
    4. Aluminum-based SMAs
  2. Applications of SMAs for structural engineering using:
    1. Damping capacity of SMAs
    2. Superelasticity of SMAs
  3. Applications in structural engineering using shape memory effect of SMAs for tensioning applications as:
    1. Coupling, Fastener
    2. Tendon
    3. Concrete reinforcement
    4. Near surface mounted reinforcement
    5. Short fibers
  4. Actuator applications of SMAs in structural engineering
  5. Active vibration control in structural engineering using SMAs
  6. Hybrid composites of shape memory alloys and polymers for structural engineering
  7. SMAs as sensors for health monitoring of structural engineering
  8. Modeling of the SMAs applications in structural engineering including
    1. Material constitutive models
    2. Structural behavior models
    3. Long term behavior models

Prof. Dr. Masoud Motavalli
Dr. Christoph Czaderski
Prof. Dr. Moslem Shahverdi
Collection Editors

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Keywords

  • shape memory alloys (SMAs)
  • shape memory effect (SME)
  • superelasticity
  • modeling
  • alloy design
  • structural engineering
  • civil engineering
  • smart materials
  • external strengthening
  • structural rehabilitation
  • constitutive models
  • long term behavior

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Published Papers (15 papers)

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Research

13 pages, 6982 KiB  
Article
Effect of Phase Changes on the Axial Modulus of an FeMnSi-Shape Memory Alloy
by Yajiao Yang, Matteo Breveglieri and Moslem Shahverdi
Materials 2021, 14(17), 4815; https://doi.org/10.3390/ma14174815 - 26 Aug 2021
Cited by 18 | Viewed by 2680
Abstract
The axial modulus ESMA(κ) of FeMnSi-based shape memory alloys (FeMnSi-SMAs) is a parameter introduced in this study to characterize the relationship between stress and strain behavior at the early stage of tensile loading. ESMA(κ) can [...] Read more.
The axial modulus ESMA(κ) of FeMnSi-based shape memory alloys (FeMnSi-SMAs) is a parameter introduced in this study to characterize the relationship between stress and strain behavior at the early stage of tensile loading. ESMA(κ) can be used to correctly estimate and model the interaction forces between FeMnSi-SMAs and other materials. Unlike the conventional Young’s modulus, which is usually given at room temperature, the ESMA(κ) is evaluated at different temperatures and strongly depends on phase transformation and plastic deformation. This study investigated the evolution of ESMA(κ) during and after pre-straining as well as in the course of the activation processes. The effect of different factors (e.g., phase transformation and plastic deformation) on the magnitude of ESMA(κ) is discussed. The result shows that the ESMA(κ) can differ significantly during activation and thus needs to be modified when interaction forces between FeMnSi-SMAs and other substrates materials (e.g., concrete) must be modeled and evaluated. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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22 pages, 13853 KiB  
Article
Numerical Modeling of Unreinforced Masonry Walls Strengthened with Fe-Based Shape Memory Alloy Strips
by Moein Rezapour, Mehdi Ghassemieh, Masoud Motavalli and Moslem Shahverdi
Materials 2021, 14(11), 2961; https://doi.org/10.3390/ma14112961 - 30 May 2021
Cited by 26 | Viewed by 3004
Abstract
This study presents a new way to improve masonry wall behavior. Masonry structures comprise a significant part of the world’s structures. These structures are very vulnerable to earthquakes, and their performances need to be improved. One way to enhance the performances of such [...] Read more.
This study presents a new way to improve masonry wall behavior. Masonry structures comprise a significant part of the world’s structures. These structures are very vulnerable to earthquakes, and their performances need to be improved. One way to enhance the performances of such types of structures is the use of post-tensioning reinforcements. In the current study, the effects of shape memory alloy as post-tensioning reinforcements on originally unreinforced masonry walls were investigated using finite element simulations in Abaqus. The developed models were validated based on experimental results in the literature. Iron-based shape memory alloy strips were installed on masonry walls by three different configurations, namely in cross or vertical forms. Seven macroscopic masonry walls were modeled in Abaqus software and were subjected to cyclic loading protocol. Parameters such as stiffness, strength, durability, and energy dissipation of these models were then compared. According to the results, the Fe-based strips increased the strength, stiffness, and energy dissipation capacity. So that in the vertical-strip walls, the stiffness increases by 98.1%, and in the cross-strip model’s position, the stiffness increases by 127.9%. In the vertical-strip model, the maximum resistance is equal to 108 kN, while in the end cycle, this number is reduced by almost half and reaches 40 kN, in the cross-strip model, the maximum resistance is equal to 104 kN, and in the final cycle, this number decreases by only 13.5% and reaches 90 kN. The scattering of Fe-based strips plays an important role in energy dissipation. Based on the observed behaviors, the greater the scattering, the higher the energy dissipation. The increase was more visible in the walls with the configuration of the crossed Fe-based strips. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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18 pages, 4988 KiB  
Article
Straining Behavior of Mortar Reinforced by Cold Drawn Crimped and Dog-Bone-Shaped Fibers under Monotonic and Cyclic Compressions
by Ha Vinh Ho, Eunsoo Choi, Duhyeon Kim and Joowon Kang
Materials 2021, 14(6), 1522; https://doi.org/10.3390/ma14061522 - 20 Mar 2021
Cited by 16 | Viewed by 2395
Abstract
The straining behavior of the shape memory alloy (SMA) fibers-reinforced mortar was investigated in this study by the monotonic compressive and cyclic compressive tests. Two types of SMA fibers with a crimped and dog-bone shape were used due to the high pullout resistance [...] Read more.
The straining behavior of the shape memory alloy (SMA) fibers-reinforced mortar was investigated in this study by the monotonic compressive and cyclic compressive tests. Two types of SMA fibers with a crimped and dog-bone shape were used due to the high pullout resistance capacity, which guaranteed that the fibers and mortar matrix were composited well. The plain mortar was mixed with two different compositions to create the higher elastic modulus mortar matrix and the lower elastic modulus mortar matrix compared with the elastic modulus of SMA fibers. The results of the experimental test indicated that the non-heated SMA fibers could control the strains in both elastic and plastic phases; in which, the crimped fiber was more effective in precracking due to the higher composite capacity while the dog-bone-shaped fiber had a higher effect in post-cracking. After heating, the dog-bone-shaped fiber slipped more than that of the crimped fiber; thus, the heated crimped fiber was more effective than the heated dog-bone-shaped fiber in controlling strains after cracking. The effect of SMA fibers on the elastic modulus depended on both the elastic modulus of mortar matrix and the property of SMA fibers. In the plastic phase, the fibers were effective on reducing the speed of damage in monotonic case. An equation using reinforcing index was suggested for damage evolution in the cyclic case. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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20 pages, 3962 KiB  
Article
The Application of Ni–Ti SMA Wires in the External Prestressing of Concrete Hollow Cylinders
by Aleksandra Dębska, Piotr Gwoździewicz, Andrzej Seruga, Xavier Balandraud and Jean-François Destrebecq
Materials 2021, 14(6), 1354; https://doi.org/10.3390/ma14061354 - 11 Mar 2021
Cited by 7 | Viewed by 2241
Abstract
An innovative method for prestressing structural elements through the use of shape memory alloys (SMAs) is gaining increasing attention in research as this method does not require the use of mechanical anchorages for tendons. The activation of the memory effect by means of [...] Read more.
An innovative method for prestressing structural elements through the use of shape memory alloys (SMAs) is gaining increasing attention in research as this method does not require the use of mechanical anchorages for tendons. The activation of the memory effect by means of temperature variations (Joule effect) in effect produces high stresses in SMA components attached to concrete components as reported in the literature. This paper presents the work performed for the purpose of prestressing concrete hollow cylinders with the use of nickel–titanium (Ni–Ti) SMA wires. In the tests, a variety of hollow cylinders were made using the same concrete mix and with the same wall thickness (20 mm), but with different external diameters (200 mm, 250 mm, and 300 mm). Their prestressing was achieved by the means of Ni-Ti SMA wires of different diameters (1 mm, 2 mm, and 3 mm) wrapped around the cylinders. Longitudinal and circumferential strain during the thermal activation of the SMA wires by Joule heating was measured using gauges located on the internal surface of the hollow cylinders. The experimental protocol, recorded observations, and discussion of the effectiveness of the prestressing of concrete elements as a function of the test parameters are included in the text in detail. Comments on the conditions for effective prestressing of concrete cylinders with SMA wires are proposed in the conclusions of the paper. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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25 pages, 10252 KiB  
Article
RC Structures Strengthened by an Iron-Based Shape Memory Alloy Embedded in a Shotcrete Layer—Nonlinear Finite Element Modeling
by Neda Dolatabadi, Moslem Shahverdi, Mehdi Ghassemieh and Masoud Motavalli
Materials 2020, 13(23), 5504; https://doi.org/10.3390/ma13235504 - 3 Dec 2020
Cited by 36 | Viewed by 3231
Abstract
Shape memory alloys (SMAs) have been widely used in civil engineering applications including active and passive control of structures, sensors and actuators and strengthening of reinforced concrete (RC) structures owing to unique features such as the shape memory effect and pseudo-elasticity. Iron-based shape [...] Read more.
Shape memory alloys (SMAs) have been widely used in civil engineering applications including active and passive control of structures, sensors and actuators and strengthening of reinforced concrete (RC) structures owing to unique features such as the shape memory effect and pseudo-elasticity. Iron-based shape memory alloys (Fe-SMAs) have become popular in recent years. Use of iron-based SMAs for strengthening RC structures has received attention in the recent decade due to the advantages it presents, that is, no ducts or anchor heads are required, friction losses do not occur and no space is needed for a hydraulic device to exert force. Accordingly, Fe-SMAs embedded in a shotcrete layer have been used for pre-stressing RC beams at Empa. The aim of this study is to present an approach to model and analyze the behavior of RC members strengthened and pre-stressed with Fe-SMA rebars embedded in a shotcrete layer. The lack of research on developing finite element models for studying the behavior of concrete structures strengthened by iron-based shape memory alloys is addressed. Three-dimensional finite element models were developed in the commercial finite element code ABAQUS, using the concrete damaged plasticity model to predict the studied beams’ load–displacement response. The results of the finite element analyses show a considerably good agreement with the experimental data in terms of the beams’ cracking load and ultimate load capacity. The effects of different strengthening parameters, including SMA rebar diameter, steel rebar diameter and pre-stressing force level on the beam behavior, were investigated based on the verified finite element models. The results were compared. The load-displacement response of an 18-m concrete girder strengthened and pre-stressed with iron-based SMA bars was examined by the developed finite element model as a case study. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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12 pages, 4420 KiB  
Article
Embedded NiTi Wires for Improved Dynamic Thermomechanical Performance of Silicone Elastomers
by Umut D. Çakmak, Ingrid Graz, Richard Moser, Michael Fischlschweiger and Zoltán Major
Materials 2020, 13(22), 5076; https://doi.org/10.3390/ma13225076 - 11 Nov 2020
Cited by 6 | Viewed by 2059
Abstract
The extraordinary properties of shape memory NiTi alloy are combined with the inherent viscoelastic behavior of a silicon elastomer. NiTi wires are incorporated in a silicon elastomer matrix. Benefits include features as electrical/thermal conductivity, reinforcement along with enhanced damping performance and flexibility. To [...] Read more.
The extraordinary properties of shape memory NiTi alloy are combined with the inherent viscoelastic behavior of a silicon elastomer. NiTi wires are incorporated in a silicon elastomer matrix. Benefits include features as electrical/thermal conductivity, reinforcement along with enhanced damping performance and flexibility. To gain more insight of this composite, a comprehensive dynamic thermomechanical analysis is performed and the temperature- as well as frequency-dependent storage modulus and the mechanical loss factor are obtained. The analyses are realized for the composite and single components. Moreover, the models to express the examined properties and their temperature along with the frequency dependencies are also presented. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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Graphical abstract

13 pages, 4914 KiB  
Article
In Search of the Optimal Conditions to Process Shape Memory Alloys (NiTi) Using Fused Filament Fabrication (FFF)
by Pedro Carreira, Fábio Cerejo, Nuno Alves and Maria Teresa Vieira
Materials 2020, 13(21), 4718; https://doi.org/10.3390/ma13214718 - 22 Oct 2020
Cited by 14 | Viewed by 3304
Abstract
This research was performed so as to investigate the additive manufacturing of NiTi shape memory alloys, which is associated with direct processes, such as selective laser melting. In addition to its expensive production costs, NiTi readily undergoes chemical and phase modifications, mainly as [...] Read more.
This research was performed so as to investigate the additive manufacturing of NiTi shape memory alloys, which is associated with direct processes, such as selective laser melting. In addition to its expensive production costs, NiTi readily undergoes chemical and phase modifications, mainly as a result of Ni loss during processing as a result of high temperatures. This research explores the potential usefulness of NiTi as well as its limitations using indirect additive processes, such as fused filament fabrication (FFF). The first step was to evaluate the NiTi critical powder volume content (CPVC) needed to process high-quality filaments (via extrusion). A typical 3D printer can build a selected part/system/device layer-by-layer from the filaments, followed by debinding and sintering (SDS), in order to generate a near-net-shape object. The mixing, extruding (filament), printing (shaping), debinding, and sintering steps were extensively studied in order to optimize their parameters. Moreover, for the sintering step, two main targets should be met, namely: the reduction of contamination during the process in order to avoid the formation of secondary phases, and the decrease in sintering temperature, which also contributes to reducing the production costs. This study aims to demonstrate the possibility of using FFF as an additive manufacturing technology for processing NiTi. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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12 pages, 3396 KiB  
Article
Investigation of Mechanical Properties of Large Shape Memory Alloy Bars under Different Heat Treatments
by Liping Kang, Hui Qian, Yuancheng Guo, Chenyang Ye and Zongao Li
Materials 2020, 13(17), 3729; https://doi.org/10.3390/ma13173729 - 24 Aug 2020
Cited by 28 | Viewed by 3562
Abstract
Shape memory alloys (SMAs) are a class of functional materials that possess unique thermomechanical properties, such as shape memory effect (SME), superelasticity (SE), damping, and good fatigue and corrosion resistance, which enable them to become ideal materials for applications in earthquake engineering. Numerous [...] Read more.
Shape memory alloys (SMAs) are a class of functional materials that possess unique thermomechanical properties, such as shape memory effect (SME), superelasticity (SE), damping, and good fatigue and corrosion resistance, which enable them to become ideal materials for applications in earthquake engineering. Numerous studies have shown that the mechanical properties of superelastic SMAs mainly depend on the wire form, or the relationship between the microstructure and thermally induced phase transitions. However, extremely few studies have elucidated the effects of the heat-treatment strategy, size effect of large diameters, and cyclic loading. Herein, the mechanical properties of SMA bars, such as residual strain, energy dissipation, and equivalent damping ratio, were studied with different heat-treatment strategies, cyclic loadings, and strain amplitudes; this was achieved by conducting cyclic tensile tests on SMA bars with four different diameters. The results indicate that the maximum phase transformation stress was obtained with a 14 mm SMA bar subjected to heat treatment at 400 ℃ for 15 min. The mechanical properties were relatively stable after five loading–unloading cycles, which should be considered in engineering applications. The test results provide a mechanical basis for using large SMA bars in self-centering structures in seismic regions. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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24 pages, 14240 KiB  
Article
Seismic Assessment of RC Bridge Columns Retrofitted with Near-Surface Mounted Shape Memory Alloy Technique
by Ammar Abbass, Reza Attarnejad and Mehdi Ghassemieh
Materials 2020, 13(7), 1701; https://doi.org/10.3390/ma13071701 - 5 Apr 2020
Cited by 15 | Viewed by 5162
Abstract
From past earthquakes, it has been found that the large residual displacement of bridges after seismic events could be one of the major causes of instability and serviceability disruption of the bridge. The shape memory alloy bars have the ability to reduce permanent [...] Read more.
From past earthquakes, it has been found that the large residual displacement of bridges after seismic events could be one of the major causes of instability and serviceability disruption of the bridge. The shape memory alloy bars have the ability to reduce permanent deformations of concrete structures. This paper represents a new approach for retrofitting and seismic rehabilitation of previously designed bridge columns. In this concept, the RC bridge column was divided into three zones. The first zone in the critical region of the column where the plastic hinge is possible to occur was retrofitted with near-surface mounted shape memory alloy technique and wrapped with FRP sheets. The second zone, being above the plastic hinge, was confined with Fiber-Reinforced Polymer (FRP) jacket only, and the rest of the column left without any retrofitting. For this purpose, five types of shape memory alloy bars were used. One rectangular and one circular RC bridge column was selected and retrofitted with this proposed technique. The retrofitted columns were numerically investigated under nonlinear static and lateral cyclic loading using 2D fiber element modeling in OpenSees software. The results were normalized and compared with the as-built column. The results indicated that the relative self-centering capacity of RC bridge piers retrofitted with this new approach was highly greater than that of the as-built column. In addition, enhancements in strength and ductility were observed. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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11 pages, 4501 KiB  
Article
Resistance Characteristics of SMA Actuator Based on the Variable Speed Phase Transformation Constitutive Model
by Yifan Lu, Rongru Zhang, Ye Xu, Lei Wang and Honghao Yue
Materials 2020, 13(6), 1479; https://doi.org/10.3390/ma13061479 - 24 Mar 2020
Cited by 18 | Viewed by 3517
Abstract
The shape memory alloy (SMA)-based actuators have been increasingly used in different domains, such as automotive, aerospace, robotic and biomedical applications, for their unique properties. However, the precision control of such SMA-based actuators is still a problem. Most traditional control methods use the [...] Read more.
The shape memory alloy (SMA)-based actuators have been increasingly used in different domains, such as automotive, aerospace, robotic and biomedical applications, for their unique properties. However, the precision control of such SMA-based actuators is still a problem. Most traditional control methods use the force/displacement signals of the actuator as feedback signals, which may increase the volume and weight of the entire system due to the additional force/displacement sensors. The resistance of the SMA, as an inherent property of the actuator, is a dependent variable which varies in accordance with its macroscopic strain or stress. It can be obtained by the voltage and the current imposed on the SMA with no additional measuring devices. Therefore, using the resistance of the SMA as feedback in the closed-loop control is quite promising for lightweight SMA-driven systems. This paper investigates the resistance characteristics of the SMA actuator in its actuation process. Three factors, i.e., the resistivity, the length, and the cross-sectional area, which affect the change of resistance were analyzed. The mechanical and electrical parameters of SMA were obtained using experiments. Numerical simulations were performed by using the resistance characteristic model. The simulation results reveal the change rules of the resistance corresponding to the strain of SMA and demonstrate the possibility of using the resistance for feedback control of SMA. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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14 pages, 4466 KiB  
Article
Study on the Mechanical Properties of Bionic Protection and Self-Recovery Structures
by Xue Guo, Xinju Dong, Zhenglei Yu, Zhihui Zhang, Xinyu Xie, Xiebin Wang, Renlong Xin and Wei Yan
Materials 2020, 13(2), 389; https://doi.org/10.3390/ma13020389 - 15 Jan 2020
Cited by 11 | Viewed by 3496
Abstract
A novel protective structure, based on shrimp chela structure and the shape of odontodactylus scyllarus, has been shown to improve impact resistance and energy absorption. A finite element model of NiTi alloy with shape memory was constructed based on the basic principles of [...] Read more.
A novel protective structure, based on shrimp chela structure and the shape of odontodactylus scyllarus, has been shown to improve impact resistance and energy absorption. A finite element model of NiTi alloy with shape memory was constructed based on the basic principles of structural bionics. The protective structure utilizes NiTi alloy as the matrix, a material with many advantages including excellent compression energy absorption, reusability after unloading, and long life. The mechanical properties of the single-layer model were obtained by static crushing experiments and numerical simulations. Building upon the idea of the monolayer bionic structure design, a two-layer structure is also conceived. Both single-layer and double-layer structures have excellent compression energy absorption and self-recovery capabilities. Compared with the single-layer structure, the double-layer structure showed larger compression deformation and exhibited better energy absorption capacity. These results have important academic and practical significance for improving the impact resistance of protective armor. Our study makes it possible to repair automatic rebounds under the action of pressure load and improves the endurance and material utilization rate of other protective structures. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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19 pages, 2096 KiB  
Article
Active Adjustment of Surface Accuracy for a Large Cable-Net Structure by Shape Memory Alloy
by Xiangjun Jiang, Fengqun Pan, Yesen Fan, Jingli Du, Mingbo Zhu and Zhen Chen
Materials 2019, 12(16), 2619; https://doi.org/10.3390/ma12162619 - 16 Aug 2019
Cited by 13 | Viewed by 3783
Abstract
The high surface accuracy design of a cable-net antenna structure under the disturbance of the extremely harsh space environment requires the antenna to have good in-orbit adjustment ability for surface accuracy. A shape memory cable-net (SMC) structure is proposed in this paper and [...] Read more.
The high surface accuracy design of a cable-net antenna structure under the disturbance of the extremely harsh space environment requires the antenna to have good in-orbit adjustment ability for surface accuracy. A shape memory cable-net (SMC) structure is proposed in this paper and believed to be able to improve the in-orbit surface accuracy of the cable-net antenna. Firstly, the incremental stiffness equation of a one-dimensional bar element of the shape memory alloy (SMA) to express the relationship between the force, temperature and deformation was effectively constructed. Secondly, the finite element model of the SMC antenna structure incorporated the incremental stiffness equation of a SMA was established. Thirdly, a shape active adjustment procedure of surface accuracy based on the optimization method was presented. Finally, a numerical example of the shape memory cable net structure applied to the parabolic reflectors of space antennas was analyzed. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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14 pages, 2060 KiB  
Article
Thermal Cycling Effect on Transformation Temperatures of Different Transformation Sequences in TiNi-Based Shape Memory Alloys
by Shyi-Kaan Wu and Yi-Ching Chang
Materials 2019, 12(16), 2512; https://doi.org/10.3390/ma12162512 - 7 Aug 2019
Cited by 16 | Viewed by 3626
Abstract
In TiNi-based shape memory alloys (SMAs), the effects of thermal cycling on the transformation peak temperatures of B2 ↔ B19′, B2 ↔ R, B2 ↔ B19, B2 ↔ R ↔ B19′, and B2 ↔ B19 ↔ B19′ one-stage and two-stage transformations have been [...] Read more.
In TiNi-based shape memory alloys (SMAs), the effects of thermal cycling on the transformation peak temperatures of B2 ↔ B19′, B2 ↔ R, B2 ↔ B19, B2 ↔ R ↔ B19′, and B2 ↔ B19 ↔ B19′ one-stage and two-stage transformations have been investigated and compared. Experimental results of the differential scanning calorimeter and hardness tests indicate that the alloy’s intrinsic hardness and the shear strain, s, associated with martensitic transformation, are two important factors, due to their relation to the ease of introducing dislocations during cycling. The temperature decrease by cycling for one-stage transformation was in the order of B2 ↔ B19′ > B2 ↔ B19 > B2 ↔ R according to the orders of magnitude of their s values. This phenomenon also affected the suppression of B19 ↔ B19′ and R ↔ B19′ transformation peak temperatures in two-stage transformation. Both Ti50Ni48Fe2 and Ti48.7Ni51.3 SMAs aged at 450 °C for 4 h exhibited B2 ↔ R ↔ B19′ transformation, but the hardness of the latter was much higher than that of the former due to the precipitation hardening of the Ti3Ni4 precipitates. This causesd the decrease of the R ↔ B19′ transformation peak temperature in the Ti50Ni48Fe2 SMA to be much higher than that in Ti48.7Ni51.3 SMAs aged at 450 °C for 4 h, which directly affected the sequential B2 ↔ R transformation of Ti50Ni48Fe2 SMA in the next thermal cycle and decreased this transformation peak temperature. The Ti48Ni52 SMA aged at 600 °C for 150 h underwent B2 ↔ B19′ transformation and then B2 → R → B19′/B19′ → B2 transformation as the cycle number increased, in which the B2 ↔ R transformation peak temperature raised slightly by cycling. This characteristic is uncommon and may have resulted from the strain field around the thermal-cycled dislocations favoring the formation of the R-phase. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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18 pages, 6454 KiB  
Article
Strengthening of Reinforced Concrete Beams with Externally Mounted Sequentially Activated Iron-Based Shape Memory Alloys
by Emanuel Strieder, Christoph Aigner, Gabriele Petautschnig, Sebastian Horn, Marco Marcon, Michael Schwenn, Oliver Zeman, Pablo Castillo, Roman Wan-Wendner and Konrad Bergmeister
Materials 2019, 12(3), 345; https://doi.org/10.3390/ma12030345 - 22 Jan 2019
Cited by 38 | Viewed by 5949
Abstract
Iron based shape memory alloys (Fe-SMA) have recently been used as active flexural strengthening material for reinforced concrete (RC) beams. Fe-SMAs are characterized by a shape memory effect (SME) which allows the recovery of previously induced plastic deformations through heating. If these deformations [...] Read more.
Iron based shape memory alloys (Fe-SMA) have recently been used as active flexural strengthening material for reinforced concrete (RC) beams. Fe-SMAs are characterized by a shape memory effect (SME) which allows the recovery of previously induced plastic deformations through heating. If these deformations are restrained a recovery stress is generated by the SME. This recovery stress can be used to prestress a SMA applied as a strengthening material. This paper investigates the performance and the load deformation behavior of RC beams strengthened with mechanical end anchored unbonded Fe-SMA strips activated by sequentially infrared heating. The performance of a single loop loaded and a double loop loaded SMA strengthened RC beam are compared to an un-strengthened beam and a reference beam strengthened with commercially available structural steel. In these tests the SMA strengthened beam had the highest cracking load and the highest ultimate load. It is shown that the serviceability behavior of a concrete beam can be improved by a second thermal activation. The sequential heating procedure causes different temperature and stress states during activation along the SMA strip that have not been researched previously. The possible effect of this different temperature and stress states on metal lattice phase transformation is modeled and discussed. Moreover the role of the martensitic transformation during the cooling process on leveling the inhomogeneity of phase state in the overheated section is pointed out. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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14 pages, 41518 KiB  
Article
The Analysis of Superelasticity and Microstructural Evolution in NiTi Single Crystals by Molecular Dynamics
by Hung-Yuan Lu, Chih-Hsuan Chen and Nien-Ti Tsou
Materials 2019, 12(1), 57; https://doi.org/10.3390/ma12010057 - 24 Dec 2018
Cited by 13 | Viewed by 4384
Abstract
Superelasticity in shape memory alloys is an important feature for actuators and medical devices. However, the function of the devices is typically limited by mechanical bandwidth and fatigue, which are dominated by the microstructures. Thus, in order to correlate the mechanical response and [...] Read more.
Superelasticity in shape memory alloys is an important feature for actuators and medical devices. However, the function of the devices is typically limited by mechanical bandwidth and fatigue, which are dominated by the microstructures. Thus, in order to correlate the mechanical response and the microstructures, the microstructural evolution in NiTi single crystals under the compression, tensile, and shearing tests is simulated by molecular dynamics (MD) in the current study. Then, the martensite variant identification method, which identifies the crystal variants/phases of each lattice based on the transformation matrix, is used to post-process the MD results. The results with the detailed information of variants and phases reveal many features that have good agreement with those reported in the literature, such as X-interfaces and the transitional orthorhombic phase between the austenite and monoclinic phases. A new twin structure consisting of diamond and wedge-shaped patterns is also discovered. The macroscopic behavior, such as stress-strain curves and the total energy profile, is linked with the distribution of dislocation and twin patterns. The results suggest that the loading cases of shear and compression allow a low critical strain for the onset of martensitic transformation and a better superelasticity behavior. Therefore, the two loading cases are suitable to apply to the NiTi actuators. The current work is expected to provide insight into the mechanical responses and design guideline for NiTi shape memory alloy actuators. Full article
(This article belongs to the Special Issue Shape Memory Alloys (SMAs) for Engineering Applications)
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