Superelastic Shape Memory Alloy Wires Research Articles

The driving characteristics of bidirectional SMA wire actuators - Theoretical modeling and experimental testing

Bidirectional shape memory alloy (SMA) wire actuators play a critical role in driving applications thanks to their characteristics in memory shaping and good overall performance. SMA wire actuators were often studied at a single load type, and there is no comprehensive comparison investigation of different SMA wire actuators in the literature. In this paper, the driving characteristics of four available types of SMA wire actuators, namely, dead-load, biased spring, differential type, and biased superelastic SMA wire actuators are modelled mathematically and then tested in laboratory settings. The characteristics of SMA wire actuators are quantitatively measured and then compared under a range of application scenarios. The output displacement and thermal-electric models of NiTi SMA wires are derived based on the Brinson constitutive model with stress effects in consideration. A customized SMA wire actuator test device is developed to evaluate the theocratical models and benchmark testing four types of SMA wires and their driving characteristics, including structure, design, displacement, response and recovery time, energy efficiency and hysteresis properties. The SMA wire phase transformation characteristics of the four actuators are tested under different activation currents, and then quantitatively compared. In conclusion, this paper adds theoretical and experimental significance to the literature on SMA wire actuators. The reference value on the phase transformation characteristics of SMA wires can be used to guide SMA application in industrial designs and enable theoretical studies in complementary research areas.

Evaluation of rocking seismic isolation performance of railway tall piers with different energy dissipation devices

Conventional pier top isolation measures cannot reduce the inertial force produced by railway gravity tall piers. This may damage the pier column and pile foundation during strong earthquakes. As an effective pier bottom seismic isolation structure, rocking piers have broad application prospects in railway tall pier bridges. This paper considers five energy dissipation devices (EDDs) to constrain the bottom of a free rocking pier (F-RP), namely buckling restrained braces (BRBs), yielding steel cables (YSCs), self-centering buckling restrained braces (SC-BRBs), superelastic shape memory alloy wires (SMAs), and viscous dampers (VDs). In particular, an additional “locking” structure is proposed to prevent excessive rocking of the piers and protect the EDDs. Parameter design methods are proposed for damage control and residual deformation control of piers. Nonlinear static and dynamic analyses are performed. The results show that the fixed base piers based on the ductility design suffered severe damage during strong earthquakes. The longitudinal rebar strain ratio is recommended as the damage index for the rocking pier due to large axial force fluctuations. Based on the design method proposed in this paper, the additional EDD constraints make the pier's expected damage level controllable and improve the post-earthquake predictability of the railway pier. BRBs and VDs are unfavorable to pier bottom shear forces. YSCs may produce irrecoverable residual deformation, while BRBs may have significant residual stresses. SC-BRBs and SMAs require no post-earthquake repair. SC-BRBs have more potential for application in rocking piers in high-intensity earthquake areas.

Identification of Behavior Characteristics of Energy Dissipation Damper with Superelastic Shape Memory Alloy Wire Through Finite Element Analysis

Identification of Behavior Characteristics of Energy Dissipation Damper with Superelastic Shape Memory Alloy Wire Through Finite Element Analysis

A multi-scale diffusional-mechanically coupled model for super-elastic NiTi shape memory alloy wires in hydrogen-rich environment

The functional devices made by NiTi shape memory alloys (SMAs) are often serviced in hydrogen (H)-rich environment. Recently experimental results showed that the martensite transformation (MT) of NiTi SMA changed strongly after charging H, which originates from the H diffusion in the material and an additional transformation resistance caused by the absorbed H atoms. In this work, a multi-scale diffusional-mechanically coupled constitutive model is constructed to describe the super-elastic deformation of NiTi SMA wires in H-rich environment. In the grain scale, a crystal plasticity-based constitutive model is developed within the framework of irreversible thermodynamics. Four deformation mechanisms, i.e., elasticity, MT, transformation-induced plasticity (TRIP) and H expansion are taken into account. Since the H atoms can be trapped by the lattice defects, the total H concentration is further decomposed into two parts, i.e., the lattice hydrogen concentration and the trapped one. The generalized thermodynamic forces of MT and TRIP are derived from the Gibbs free energy and Clausius-Duhem inequality. The evolution of H concentration field is derived by combining the chemical potential, diffusion balance equation and the Fick's law. In the polycrystalline aggregate scale, in order to measure the interaction among the grains and obtain the overall response of the polycrystalline aggregate, a diffusional-mechanically coupled self-consistent homogenization method is developed. Meanwhile, the finite volume method is employed to calculate the H concentration in each grain. An assumption of uniform stress field in the macroscopic scale is adopted to achieve the scale transition from the polycrystalline aggregate to the whole wire. To validate the prediction capability of the proposed model, the predictions are compared with the experiments. Moreover, the influences of loading rate on the deformation of the wires in the process of in-situ charging H are predicted and discussed.

Enhanced Fatigue Resistance of Nanocrystalline Ni50.8Ti49.2 Wires by Mechanical Training

In this paper, the fatigue resistance of superelastic NiTi shape memory alloy (SMA) wires was improved by combining mechanical training and nanocrystallization. Fatigue tests were performed after mechanical training with a peak stress of 600 MPa for 60 cycles of nanocrystalline (NC) NiTi wires, and the associated microscopic mechanism was investigated by using transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD). The results showed that stress-controlled training effectively improved the functional stability (the accumulated residual strain decreased by 83.8% in the first 5000 cycles) of NC NiTi SMA wires, as well as increased the average structural fatigue life by 187.4% (from 4538 cycles to 13,040 cycles). TEM observations and TKD results revealed that training-induced dislocations resulted in lattice rotation and preferential grain orientation. The finite element method (FEM) simulation results indicated that the training-induced preferential grain orientation tended to decrease the local stress concentration and strain energy density. Combined with fractography analysis, the uniform deformation caused by mechanical training changed the crack growth mode from multi-regional propagation to single-regional propagation, improving the structural fatigue life.

Pseudo-static tests of reinforced concrete pier columns confined with pre-tensioned superelastic shape memory alloy wires

To enhance the seismic performance and self-centring capacity of reinforced concrete (RC) pier columns, a novel self-centring shape memory alloy (SMA) wire-confined–RC-composite column was proposed and experimentally tested in this study. One common RC-pier specimen and eight SMA-wire-constrained–RC-pier specimens were constructed for experimentation. The effects of the axial-compression ratio, SMA pre-strain level, and SMA configuration rate on the seismic performance of the RC piers were investigated. The failure mode, hysteresis curve, skeleton curve, bearing capacity, ductility, stiffness, energy-dissipation capacity, and self-centring capacity of the novel column are illustrated and discussed. The test results show that, compared with the shear failure of normal RC-pier columns, the failure mode of the SMA-wire-confined–RC-pier columns were transformed into bending failure or bending-shear failure with certain ductility. With an increase in the SMA-wire configuration rate, the shear bearing capacity, ductility, energy-dissipation capacity, and self-centring capacity of the RC-pier columns were improved. Pre-tensioned SMA wires were attributed to the earlier activation of force restraint and provided a greater binding force, providing more shear restraint and improving the deformation capacity of the core concrete. The ductility and bearing capacity of the strengthened specimens decreased with an excessively large axial-compression ratio; however, they were higher than those of unreinforced specimens. The research results provide new ideas and an experimental basis for the external strength gain and rapid recovery of existing RC piers after an earthquake.

Quantitative energy storage and ejection release in superelastic shape memory alloy wire

Superelastic shape memory alloy (SMA) wire is a memorable deformation material with large resilience and high energy density. In this paper, a revolutionary and yet explainable property of the SMA is investigated and confirmed: superelastic SMA energy storage and release can be quantitatively measured using electrical resistance. This finding boosted the SMA with significant advantages and potential in the field of mechanical energy storage and ejection release. A state-of-the-art energy storage ejection device is designed to test the relationship among SMA wires' stress, strain, and electrical resistance. The resistance change rate, ejection energy density and energy conversion efficiency are studied in the SMA wire energy tests. The experimental results confirm that the resistance change rate of the superelastic SMA wire has a stable linear relationship with the strain. The ejection energy density increases with the resistance change rate. A theoretical model of the relationship between the ejection energy density and resistance change rate is established based on the experimental results. In conclusion, the superelastic SMA wire can achieve sophisticated tasks that require ‘programmable’ and predictable energy storage or ejection.

Impact Energy Absorption Analysis of Shape Memory Hybrid Composites

Shape memory hybrid composites are hybrid structures with fiber-reinforced-polymer matrix materials. Shape memory wires due to shape memory/super-elastic properties exhibit a pseudo-elastic response with good damping/energy absorption capability. It is expected that the addition of shape memory wires in the glass-fiber-reinforced-polymer matrix composite (GFRP) will improve their mechanical and impact resistant properties. Stainless-steel wires are also expected to improve the impact resistance properties of GFRPs. In this research work, we investigated the effect of addition of shape memory wires and stainless-steel wires on the impact resistance properties of the GFRP and compared our results with conventional GFRPs. Super-elastic shape memory alloy wires and stainless-steel wires were fabricated as meshes and composites were fabricated by the hand-layup process followed by vacuum bagging and the compression molding setup. The shape-memory-alloy-wires-reinforced GFRP showed maximum impact strength followed by stainless-steel-wires-reinforced GFRPs and then conventional GFRPs. The effect of the energy absorption capability of super-elastic NiTi wires owing to their energy hysteresis was attributed to stress-induced martensitic transformation in the isothermal regime above the austenite transformation temperature. The smart shape memory wires and stainless-steel-wires-based hybrid composites were found to improve the impact strength by 13% and 4%, respectively, as compared to the unreinforced GFRPs. The shape-memory-reinforced hybrid composite also dominated in specific strength as compared to stainless-steel-wires-reinforced GFRPs and conventional GFRPs.

Experimental study and modification of the constitutive model of superelastic SMA considering multi-factor effects

The constitutive model is the basis and key for the application of materials in practical engineering. Numerical simulation has become one of the most important methods of scientific research, and an accurate constitutive model provides guarantees for the efficiency of numerical simulation. The multi-linear constitutive model is one of the most useful models for engineering. In this paper, different diameters of superelastic Shape Memory Alloy (SMA) wires are selected to investigate mechanical properties through experimentation. The influences of diameter, loading rate, temperature, and loading amplitude on the phase transformation stress, elastic modulus, residual strain, and energy dissipation properties are researched individually, and then the expressions of the influence law are given. Correction coefficients considering the effects of diameter, loading rate, temperature, and loading amplitude are employed to modify the multi-linear constitutive model. A multi-linear dynamic constitutive model considering multiple influence factors is given. Finite element analysis is carried out by introducing the modified multi-linear dynamic constitutive model into the finite element software, and the simulation results are compared to the testing results. The modified multi-linear dynamic constitutive model has high accuracy and can guide the engineering and numerical studies.

Concept of Elastocaloric Granular Material Made from SMA Wires in Bending

Shape memory alloys (SMAs) are promising materials for the creation of heating or cooling systems due to their elastocaloric character. The paper proposes a concept of elastocaloric “porous” SMA beam working in bending. The beam was made with superelastic nickel-titanium SMA wires of different diameters placed in a flexible tube. While water was flowing through the tube, bending was manually applied using 3D printed wavy profiles with portions of arcs with constant curvatures. Preliminary results showed an oscillation of the fluid temperature at the outlet of the flexible tube (containing the SMA wires) at the same frequency as the mechanical loading, validating therefore the concept of elastocaloric porous SMA beam operating in bending.

Experimental investigation of the effects of aging and cryogenic treatments on the mechanical properties of superelastic nickel-titanium shape-memory alloys

In this study, electropolishing and two different heat treatments were applied to wires made of superelastic nickel-titanium (NiTi) shape-memory alloy (SMA) and their mechanical properties and stress-induced deformations were investigated. In experimental studies, cryogenic and aging heat treatments were applied to NiTi SMA wire samples and tensile test experiments were carried out to determine the effect of the heat treatments on their mechanical properties. Following the tensile test experiments conducted at room temperature (23°C), the study investigated changes in the elemental composition, fracture modes, micro cracks, and phase structures and in the mechanical properties formed in the fracture region. Intermetallic phase structures (Ti2Ni, Ni3Ti, and Ni4Ti3) were observed in the X-ray diffraction (XRD) analyses. It was concluded that the aging heat treatment had directly affected the reduction in hardness. In particular, in samples without the aging heat treatment, a stress-induced decrease in the Ni and Ti ratios and an increase in the carbon (C) ratio were observed in the chemical composition of the fracture surface of the superelastic NiTi SMA wires. It was determined that the changes in the chemical composition caused by stress had affected the mechanical properties negatively. In the fractography of the NiTi SMA wires, the samples exhibited mostly ductile fracture behavior with small dimples.

Design, manufacturing, and performance evaluation of a novel smart roller bearing equipped with shape memory alloy wires

In this study, a new type of seismic isolation device, called shape memory alloy (SMA) wire-based roller bearing (SMA-RB) is developed and introduced. The SMA-RB has been designed, manufactured, and experimentally tested. This bearing consists of cylindrical roller bearings and SMA wires with straight or cross configurations, as supplementary damping elements. In such a smart bearing, the superelastic (SE) SMA wires are passed through the hooks/pulleys attached to the supporting plates of the bearings in different configurations. The rollers provide lateral flexibility, and SMA wires supply energy dissipation and self-centering (SC) properties. In the manufacturing stage, a new mechanism for coupling wires (i.e. SMA wire joiner/coupler) is proposed. The results show that SMA wires, made of nickle titanium (NiTi), provide a SC damping mechanism with almost zero residual deformation which can effectively control the device from over-displacement. While using pulleys and newly designed wire joiners in the SMA-RB, the bearing can experience a stable cyclic behavior. Since the rollers generate a negligible amount of frictional force, the SE NiTi wires with a flag-shaped hysteresis mainly contribute to the overall shear hysteretic response of the SMA-RB. A triangular-shaped constitutive model can be used to accurately describe the hysteretic behavior of SMA-RB with different wire configurations.

Machine Learning Enhanced Dynamic Response Modelling of Superelastic Shape Memory Alloy Wires.

Superelastic shape memory alloy (SMA) wires exhibit superb hysteretic energy dissipation and deformation capabilities. Therefore, they are increasingly used for the vibration control of civil engineering structures. The efficient design of SMA-based control devices requires accurate material models. However, the thermodynamically coupled SMA behavior is highly sensitive to strain rate. For an accurate modelling of the material behavior, a wide range of parameters needs to be determined by experiments, where the identification of thermodynamic parameters is particularly challenging due to required technical instruments and expert knowledge. For an efficient identification of thermodynamic parameters, this study proposes a machine-learning-based approach, which was specifically designed considering the dynamic SMA behavior. For this purpose, a feedforward artificial neural network (ANN) architecture was developed. For the generation of training data, a macroscopic constitutive SMA model was adapted considering strain rate effects. After training, the ANN can identify the searched model parameters from cyclic tensile stress–strain tests. The proposed approach is applied on superelastic SMA wires and validated by experiments.

Enabling shape memory effect wires for acting like superelastic wires in terms of showing recentering capacity in mortar beams

Superelastic shape memory alloy (SMA) wires embedded in mortar beams show a recentering capacity, however, shape memory effect wires can provide a prestressing effect after inducing phase transformation. In this study our goal is to empower shape memory effect wires to show recentering capacity by designing a new wire configuration. These wires will thus be able to achieve both a prestressing effect and a recentering capacity in mortar beams. To this end, various shapes of martensitic SMA wires, including as-received, crimped, dog bone, and anchoring reinforced crimped wires, were employed to examine the potential of displaying recentering capacity while preserving the prestressing force in mortar beams. Several monotonic and cyclic three-point bending tests were also conducted to investigate the crack-closing effect, as well as the load bearing of the specimens. To evaluate the effect of temperature and heating procedure, two different methods, namely, a hot air gun and electric current heating, were used to induce phase transformation in shape memory effect wires. The results generally show that for two types of as-received and dog bone wires, the displacement recovery ratio have been reduced about 65% and 74%, respectively, however, for the crimped wire and anchoring reinforced crimped wire, this ratio is almost the same. More importantly, when the results of displacement recovery ratio of crimped wires heated by electrical heating are compared with the available experimental data for superelastic SMA bars, proves the great ability of SME wires to fulfill recentering capacity.

Shape Memory Alloy Hybrid Composites for Improving Impact Properties

This paper investigates the method to improve the property that can decrease the impact response of composite plate. Embedding the super-elastic shape memory alloy wires into composite plates has increased the attention of material researchers. Super-elastic shape memory alloy has the properties of absorbing mechanical energy, large recoverable deformation and so on. In this study, experiments were conducted to analyze the impact properties of composite plates with Ni-Ti SMA wires. Composite plates with Ni-Ti SMA wires and without Ni-Ti SMA wires were subjected to two impacts respectively. This study measured the responses of two impacts. The results showed that the composite plate with Ni-Ti SMA wires were subjected to a second impact with a peak deflection of 5.47 mm, which was only 0.22 mm larger than the first impact. The relevant data of the composite plate without Ni-Ti SMA wires were 9.02 mm, 1.22 mm, and serious damage occurred. It was verified that the Ni-Ti SMA wires improved the impact resistance of the composite plate. After studying the impact tests of variable diameters of SMA wires embedded at the low layer of composite plate, it was shown that as the diameter of SMA wires increased, the impact resistance of composite plates was improved.

Modeling bending behavior of shape memory alloy wire-reinforced composites: Semi-analytical model and finite element analysis

In this study, we propose a novel and simple exact semi-analytical model for superelastic Shape Memory Alloy (SMA) wire reinforced composites subjected to bending loads. In order to study the mechanical response of the composite during loading/unloading, a Representative Volume Element (RVE) is extracted to examine the bending response of the composite. Analytical moment–curvature, and shear force-shear strain relations are derived based on a 3-Dimensional (3D) thermomechanical SMA model and Timoshenko beam theory. The composite Simpson’s rule is adopted to numerically solve the exact analytical moment–curvature and shear force-shear strain relationships. Results including the moment–curvature response, axial stress distribution along the vertical and longitudinal directions, martensite volume fraction, and the tip deflection are reported and validated against 3D finite element simulations. The influence of temperature, martensite volume fraction distribution, and matrix stiffness on the mechanical performance of the composite is also investigated. In particular, the composite is found to behave superelastically under certain conditions of temperature, SMA volume fraction, and elastic stiffness of the matrix. Such behavior is advantageous in applications requiring large recoverable strains or high energy dissipation density.

Multifactor experimental investigation of the mechanical performance of a shape memory alloy‐based self‐centering‐energy‐dissipation device

SummaryAn innovative self‐centering‐energy‐dissipation device (SCED) based on a superelastic shape memory alloy (SMA) is developed. This device exhibits asymmetric, flag‐shaped hysteretic behavior and tension–compression symmetry property. The symmetry is attributed to the tension‐only state of the SMA wires in the device during the loading process. The working principle of the device is described, and a theoretical model describing the mechanical properties of the device is proposed. Uniaxial tension–compression cyclic loading tests are carried out to investigate the influence of the section area of the SMA wire, the loading amplitude, and the loading rate on the mechanical performance, including the yield strength, maximum strength, austenite stiffness, martensite stiffness, energy dissipation, and efficiency ratio. Expressions of the correction factors and mechanical performance considering multiple factors are also suggested.

Flexural Behavior of Polypropylene Fiber Reinforced Concrete Beams with Super elastic Wire Under Repeated Loading

This paper investigates the effect of polypropylene fiber (PPM-12) and nitinol (NiTi) superelastic Shape Memory Alloy (SMA) wires in concrete beams under flexural repeated loading. For this purpose, nine of simply supported beams with dimensions of (1000 mm length, 80 mm width and 125 mm depth) were considered. Test specimens were divided into three groups (G1, G2 and G3). Specimens in group G2 and G3 were reinforced with different ratios of PPM-12 fiber (0.5%and 0.75%) by volume of concrete and threaded (NiTi) superelastic SMA wires. Group G1 was reinforced with traditional steel rebar and SMA wires without PPM-12 fiber. The results revealed that the addition of PPM-12 fiber to concrete have a negative effect on the mechanical properties of concrete with respect to load carrying capacity in the other side the addition of fiber led to a controlled crack propagation in concrete and increased the fatigue life of concrete. The (NiTi) superelastic threaded wires were active in some specimens in which some displacement recovery was noticed. The importance of study is how to enhance concrete life and reducing crack propagation in concrete by using polypropylene fiber and NiTi wire as reinforcement in concrete.

Rate dependent free energy formulations for dynamically excited superelastic SMA wires

AbstractSuperelastic shape memory alloys (SMAs) are metallic two‐phase polycrystal alloys. The transformation between austenite and martensite phases allows superelastic SMAs unique hysteretic energy dissipation capacity, which is particularly interesting for the development of new‐type of intelligent vibration control systems for structures. However, in structural control, most of the vibrations occur in high strain rate regimes, which interfere the release of self‐generated heat and cause atomic lattice destabilization and thus influence the hysteretic dissipation. The present work proposes two different strain rate dependent free energy formulations and aims to improve the accuracy of existing one‐dimensional macroscopic models of SMA wires. The approaches are developed and implemented in different continuum thermomechanical frameworks, without impairing the simplicity and robustness of the solution process. The first approach introduces an additional control variable for the entropy evolution. The second approach uses a phenomenological formulation of the latent heat evolution. The proposed formulations are validated by cyclic tensile tests conducted on SMA wires. The results show that the calculations using the new formulations can represent the superelastic SMA wire response more accurately.

Experimental and Numerical Analysis of the Mechanical Properties of a Pretreated Shape Memory Alloy Wire in a Self-Centering Steel Brace

As a stimulus-sensitive material, the difference in composition, fabrication process, and influencing factors will have a great effect on the mechanical properties of a superelastic Ni-Ti shape memory alloy (SMA) wire, so the seismic performance of the self-centering steel brace with SMA wires may not be accurately obtained. In this paper, the cyclic tensile tests of a kind of SMA wire with a 1 mm diameter and special element composition were tested under multi-working conditions, which were pretreated by first tensioning to the 0.06 strain amplitude for 40 cycles, so the mechanical properties of the pretreated SMA wires can be simulated in detail. The accuracy of the numerical results with the improved model of Graesser’s theory was verified by a comparison to the experimental results. The experimental results show that the number of cycles has no significant effect on the mechanical properties of SMA wires after a certain number of cyclic tensile training. With the loading rate increasing, the pinch effect of the hysteresis curves will be enlarged, while the effective elastic modulus and slope of the transformation stresses in the process of loading and unloading are also increased, and the maximum energy dissipation capacity of the SMA wires appears at a loading rate of 0.675 mm/s. Moreover, with the initial strain increasing, the slope of the transformation stresses in the process of loading is increased, while the effective elastic modulus and slope of the transformation stresses in the process of unloading are decreased, and the maximum energy dissipation capacity appears at the initial strain of 0.0075. In addition, a good agreement between the test and numerical results is obtained by comparing with the hysteresis curves and energy dissipation values, so the numerical model is useful to predict the stress–strain relations at different stages. The test and numerical results will also provide a basis for the design of corresponding self-centering steel dampers.