Shape Memory Alloy Elements Research Articles

Research and Development of Applied Devices utilizing the Buckling Properties of Tape-shaped Shape Memory Alloy Elements

An elongated Shape memory alloys (SMAs) elements which shows superelastic properties can exhibit the quasi-zero or negative stiffness during post-buckling deformation. Since the buckling deformation is recovered by the superelastic effect upon unloading, the quasi-zero or negative stiffness can be used continuously. Our research group has devised, studied and reported on passive vibration isolator devices that utilize this property, but this property can be applied not only to passive vibration isolator but also to various other possible applications. This paper reports on the characteristics of a force-limiting device using the buckling properties of shape memory alloy elements, which was devised and prototyped by our research group. We also report on the newly devised convex-tape shape memory alloy element and the movement characteristics of a rehabilitation mechanism using its buckling characteristics.

Effects of Curvature of the Cross-section on the Buckling Characteristics and Buckling Shape of the Convex Tape-shaped Ti-Ni Shape Memory Alloy Element

The negative stiffness exhibited by shape memory alloys (SMA) during buckling can be applied to passive vibration isolators and force-limiting mechanisms. However, to apply the negative stiffness of SMA, it is necessary to calculate the negative tangential stiffness value and buckling load of the shape memory alloy after buckling by design. In our previous study, we have shown that it is possible to increase the negative tangential stiffness and buckling load after buckling by increasing the curvature of a plate-like SMA material in the form of a convex tape with a given cross-sectional curvature. In this study, it is considered that the cause of the negative tangential stiffness after buckling is the change in martensitic phase transformation behavior during buckling deformation due to the addition of curvature to the cross-section. The results showed that the phase transformation behavior of a flat SMA element and a convex tape SMA element with curvature in its cross section changed. As a result, it was found that the flat SMA element does not exhibit negative stiffness immediately after buckling because martensitic phase transformation does not occur in the center of the specimen immediately after buckling, while the convex-tape SMA element tends to exhibit negative stiffness from immediately after buckling due to martensitic phase transformation in the center of the specimen.

Effects of Microstructure on the Buckling Fatigue and Functional Degradation Characteristics of the Tape-shaped Cu-Al-Mn Shape Memory Alloy Element

Shape memory alloys (SMAs) are functional materials with high shape memory and super elasticity effects that are used in a wide range of fields, including medical and engineering fields. Our research group has focused on the negative stiffness characteristics of plate-shaped SMA during buckling deformation and has attempted to apply them to vibration-isolation devices. The buckling properties of Cu-Al-Mn SMA, which exhibits almost the same super elastic properties as Ti-Ni SMA, were evaluated in comparison with those of Ti-Ni SMA. The results showed that Cu-Al-Mn SMA is more suitable for vibration isolators because of its smaller change in post buckling properties due to temperature change and smaller hysteresis compared with Ti-Ni SMA. In addition, the buckling fatigue and functional degradation properties of Cu-Al-Mn SMA were studied. As a result, it was clarified that fatigue failure is basically caused by the propagation of cracks originating from grain boundaries, and that the increase of these cracks is the cause of the decrease in buckling load. In this study, the buckling fatigue properties of Cu-Al-Mn SMA material in which bainite phase is formed at the grain boundary by changing the heat treatment conditions were investigated, and the effects of the differences in internal microstructures on the buckling fatigue and functional degradation properties of plate-like Cu-Al-Mn SMA with different internal microstructures were compared with those of conventional Cu-Al-Mn material. The effects of different internal microstructures on buckling fatigue and functional degradation properties of plate Cu-Al-Mn SMA with different internal structures were investigated. The results show that the specimens with bainite phase have superior fatigue properties, but in terms of functional degradation properties, they increase the rate of decrease in Pcr and increase the rate of increase in tangential stiffness after buckling.

Nonlinear Mechanics of a Smart Biotensegrity Human Foot Prosthesis

This work deals with the nonlinear mechanics of smart bioinspired tensegrity structures. A minimal regular tensegrity prism actuated by shape memory alloy (SMA) elements is investigated to represent a human foot. A formulation considering the force density matrix approach is used to model the equilibrium equations of the tensegrity structure based on node mapping. Lagrange multipliers are employed to represent constraints. The SMA thermomechanical behavior is described by considering a modified polynomial constitutive model. Numerical simulations are developed from an optimization procedure employing the Levenberg–Marquardt method. An investigation of the tensegrity capability to model a human foot is carried out analyzing either mechanical or physiological aspects of the tensegrity prosthesis. The mechanical performance is compared with high performance prostheses available on the market, showing that it is an interesting alternative with respect to mechanical resistance. Regarding physiology, foot movements are properly mimicked from SMA actuation.

Study of a Torque Limiter Based on NiTi Pseudoelastic Tapes

Herein, a new torsional device based on shape memory alloy elements and operating as torque limiter is presented. It consists of a set of pseudoelastic NiTi tapes working in series configuration. The tapes are assembled radially with respect to the rotational direction and work sequentially in beam‐like bending mode. The trend of the resulting torque may span from pulsating to continuous by changing the number of tapes. Furthermore, the use of NiTi material enables a temperature‐dependent response. Due to the particular mounting, the flag‐shaped pseudoelastic mechanical behavior typical of NiTi is partially hidden; however, the use of pseudoelastic elements provides a stable cyclic response and enables the possibility to tune the output torque.

Experimental and numerical investigation of thermomechanical cycling of notched NiTi shape memory ribbon using SMA model accounting for plastic deformation

Shape memory alloys (SMAs) are being increasingly applied as thermally driven actuators. The commercially available SMA elements, however, frequently contain stress risers, either due to internal or surface defects or due to the required component shape. Stress risers represent a potential danger for the preliminary fatigue failure of such actuators but its actual mechanism is not very clear. This paper presents a combined experimental (2D DIC analysis of surface strains) and numerical analysis (SMA model with plastic deformation) of the stress, strain and phase fraction fields evolving in a thin NiTi shape memory ribbon with an artificial notch subjected to cyclic cooling-heating through transformation range under constant external force. It appeared that, even if only very low tensile stress is externally applied upon thermal cycling, local tensile stress at the notch tip sharply increases during the first cooling due to the forward martensitic transformation (MT) proceeding heterogeneously in space. This heterogeneity gives rise to plastic deformation at the notch-tip, which gradually accumulates upon thermal cycling and shields the notch tip from tensile overloading. Hence, fatigue performance of thermal NiTi actuator with stress riser depends very much on the plastic deformability of the alloy.

Damping properties of innovative NiTi elements: development of proof of concept and demonstrators

Thanks to their functional properties, NiTi and NiTi-based shape memory alloys (SMAs) exhibit both pseudo-elastic and intrinsic damping ability finding applications in several fields. The use of these alloys in damping devices or systems allows the attenuation of different kinds of vibration such as noise or more intense oscillations. In this work, the damping behaviour of different NiTi semifinished products has been evaluated by means of dynamic thermomechanical measurements in order to have a preliminary indication for the integration of these SMA elements in damper demonstrators. The evolution of the internal friction parameter with respect to temperature, frequency and strain extent is evaluated. Moreover, some possible demonstrators’ concepts, i.e. a cladding panel, some dampers and a shock absorber, are proposed and characterized. The investigated SMA elements are a plain-woven mesh made of NiTi and steel thin wires, wave-shaped NiTi ribbons and NiTi single-turn wave springs. The elements have been thermally treated in order to achieve the required shapes and thermomechanical conditions. The perspectives and potential of these semifinished products in the field of damper devices are investigated by a preliminary characterization and discussed.

Актуатор с последовательным соединением стержня из сплава с памятью формы и упругого элемента смещения

One of the most promising applications of shape memory alloys (SMA) is their use for creating working bodies of power exciters (actuators). The working body of the actuator must perform certain movements due to shape memory phenomena (heating, working stroke of the actuator) and the accumulation of direct transformation deformations (cooling, idling of the actuator). It is known that the process of deformation of SMA during cooling occurs only in the presence of mechanical action, while the return to the original form during heating occurs in the absence of a corresponding mechanical action and even in the presence of a sufficiently large counteraction. To ensure the possibility of not only working, but also idling of the actuator, the working body of the SMA is connected to the elastic bias element so that both of these elements are deformed together. In this case, when the SMA element is heated, it is deformed due to the shape memory phenomenon, which leads to the deformation of the bias element associated with the working body and the appearance, both in this element and in the working body, of mechanical stresses that increase with the temperature of the SMA element. It is these stresses, which continue to act during the cooling of the working body, that ensure its deformation in the opposite direction at the stage of cooling of the working body and the corresponding direct transformation into SMA. In this paper, the behavior of an actuator consisting of a SMA rod and an elastic rod (an bias element) connected in series, the total length of which is assumed to be unchanged, is analytically investigated. The influence of the system parameters on the stresses in the SMA rod and the value of the working stroke of the actuator is investigated. The conditions for the implementation of the closed two way shape memory effect in this system are determined.

Magnetically Levitated Linear Drive Using an Active Gravity Compensation Based on Hybrid Shape Memory Actuators

This article presents a magnetically levitated linear drive for precision engineering with integrated active gravity compensation. The gravity compensation combines permanent magnets with novel hybrid shape memory actuators. The magnetically levitated system uses a homopolar linear drive as well as nine reluctance actuators to levitate a passive armature with no mechanical connection between the stator and armature. The mechanical design is based on geometrically decoupled horizontal and vertical guidance axes. A decoupled Cartesian control scheme is developed, which compensates for the actuator and system nonlinearities in order to achieve high bandwidths. The active gravity compensation adjusts the air gap between the permanent magnets and armature to the required gravity compensation force. This helps to keep the dissipated power and temperature increase in the magnetically levitated drive low (and thus, system precision high). To avoid the generation of particles in clean room applications, the actuators positioning the permanent magnets are solely based on solid-state effects: thermal shape memory alloys actively generate motion, while passive magnetic shape memory alloy elements realize a multistable behavior based on their internal twinning force. The resulting actuator is compact, energy efficient, and abrasion-free. The implementation of the gravity compensation in the levitated linear drive is verified and the functionality of the system is demonstrated in experiments.

A Double Shape Memory Alloy Damper for Structural Vibration Control

Shape memory alloy (SMA) dampers are widely investigated passive control systems for structural vibration mitigation. However, the damping robustness of conventional austenite SMA dampers may be affected by environmental temperature. In this study, an innovative double SMA damper (DSD) system is presented to improve the temperature robustness of the SMA dampers. In the proposed system, double SMA hysteretic elements with different phase transition temperatures are arranged in parallel, where the SMA element with lower transition temperature behaves as austenite under room temperature, and the other with higher transition temperature behaves as martensite. To study the vibration control effect, both single-degree-of-freedom (SDOF) and multiple-degree-of-freedom (MDOF) structures with DSD systems are employed. The thermal and mechanical behaviors of the SMA elements and the working principle of DSD are also introduced. Thereon, the equivalent linearization method for SMA’s output force and the motion-governing equations for SDOF structure with DSD are derived. Moreover, parametric studies are conducted to investigate the performance of the proposed DSD system in both frequency and time domains. Also, numerical analysis for the MDOF structure with DSD systems is carried out to illustrate the trend in response reduction with an increasing number of degrees of freedom. The analytical results show that the DSD can mitigate the structural seismic response more effectively than the conventional one with acceptable residual deformation, and is capable of delaying the degradation of SMA’s energy dissipation capacity. Less SMA material is required for the proposed DSD to fulfill the same mitigation requirement, and it is suitable for general applications for temperature robustness.

Effects of the Volume Fraction of Martensitic-phase during Buckling Deformation on Post-buckling Behavior of Tape-shaped Ti-Ni Shape Memory Alloy Element

Effects of the Volume Fraction of Martensitic-phase during Buckling Deformation on Post-buckling Behavior of Tape-shaped Ti-Ni Shape Memory Alloy Element

Development of an Actuator for Translatory Movement by Means of a Detented Switching Shaft Based on a Shape Memory Alloy Wire for Repeatable Mechanical Positioning

Actuators based on the shape memory effect have recently become more and more economically important due to the many advantages of shape memory alloys (SMAs), such as their high energy density. SMAs are usually used to control the end/maximum positions, thus the actuators always move between two positions. The repeatable control of intermediate positions has so far proven difficult, because in most cases, external sensors are necessary to determine the length of the SMA element. Additionally control strategies for SMA actuators are rather complex due to the non-linear behavior of this material. The SMA actuator presented here is able to control intermediate positions with repeatable accuracy without the need of a separate control technology. The integrated control unit is based on a mechanical principle using a shaft with a circumference groove. This groove has a height profile that turns the shafts rotation, generated by the SMA, into a translational movement. Therefore, the SMA wire generates a partial stroke at each complete activation, turning the shaft partially. With several activation cycles in a row, the stroke adds up until reaching the maximum. A further activation cycle of the wire resets the actuators stroke to its initial position. Each part of the stroke can, thereby, be controlled precisely and repeatedly within the scope of each complete cycle of the actuator. Based on an integrated ratchet, each stroke of the actuator can hold energy free.

Seismic Performance of Self-centering Steel Frames with SMA-viscoelastic Hybrid Braces

ABSTRACT This study presents a novel hybrid self-centering system aiming to overcome critical shortcomings identified in the existing self-centering solution. Two types of hybrid brace incorporating shape memory alloy elements and integrated viscoelastic dampers are first introduced, followed by a system-level analysis on a series of prototype buildings. The results show that using viscoelastic material to reach a moderate damping ratio is highly effective in peak and residual deformation control. Floor acceleration is also effectively controlled by the hybrid solution. A parametric study is then conducted, and design recommendations are given. A probability-based residual deformation prediction model is finally proposed.

Deployment of Solar Sails by Joule Effect: Thermal Analysis and Experimental Results

Space vehicles may be propelled by solar sails exploiting the radiation pressure coming from the sun and applied on their surfaces. This work deals with the adoption of Nickel-Titanium Shape Memory Alloy (SMA) elements in the sail deployment mechanism activated by the Joule Effect, i.e., using the same SMA elements as a resistance within suitable designed electrical circuits. Mathematical models were analyzed for the thermal analysis by implementing algorithms for the evaluation of the temperature trend depending on the design parameters. Several solar sail prototypes were built up and tested with different number, size, and arrangement of the SMA elements, as well as the type of the selected electrical circuit. The main parameters were discussed in the tested configurations and advantages discussed as well.

Synergistic use of piezoelectric and shape memory alloy elements for vibration-based energy harvesting

The efforts for clean and renewable energy have been encouraging a growing interest for vibration-based energy harvesting devices. Piezoelectric materials are remarkable elements to promote electro-mechanical coupling allowing the conversion of the mechanical vibration into electrical power through piezoelectric direct effect. Nevertheless, this promising application is associated with the key challenge to enhance and expand the energy harvesting capacity. In this regard, this work proposes the synergistic use of smart materials, combining piezoelectric and shape memory alloy (SMA) elements. Experimental and numerical analyses are performed showing the enhanced capabilities of the system due to the adaptability provided by shape memory alloys. A piezoelectric beam excited by an electrodynamics shaker is connected with a shape memory alloy element that allows to exploit its remarkable characteristics in order to change system properties with temperature variations through Joule’s effect. Thermomechanical tests are performed for SMA characterization. Afterward, nonlinear dynamics of the energy harvesting system is investigated exploiting the SMA adaptive behavior. Results show that the synergistic use of smart materials is able to increase the device bandwidth, improving the system performance for energy harvesting purposes.

Prestress state evolution during thermal activation of memory effect in concrete beams strengthened with external SMA wires

The memory effect of shape-memory alloys (SMAs) has opened interesting perspectives to create prestress states in concrete elements. However, the procedure has not been yet fully resolved due to the complex thermomechanical behavior of these alloys, in addition to the practical difficulties of mechanical coupling between SMA and concrete elements. The present study deals with tests on the development of prestressing forces in concrete beams during the thermal cycle required in the procedure. Pre-stretched nickel–titanium wires were externally placed on concrete prismatic beams equipped with strain gauges. As concrete rupture may occur during the heating by the Joule effect, a compromise must be found between the SMA pre-stretch level and the maximum temperature to be applied before returning to ambient temperature. A macroscopic model was developed to analyze this compromise. The complex thermomechanical response of SMAs implies a particular attention in the definition of the ambient temperature and heating conditions for the creation of prestress states in concrete components.

Investigations on laser actuation and life cycle characteristics of NiTi shape memory alloy bimorph for non-contact functional applications

A laser source was used as non-contact mode of heating to actuate NiTi/kapton polyimide shape memory alloy (SMA) bimorph. The bimorph was fabricated by depositing NiTi thin film over Kapton polyimide sheet of dimension 5 × 2 cm2 using thermal evaporation. The laser parameters such as laser power, scanning speed and number of passes were optimized to ensure actuation without damaging the bimorph. The actuation was carried out at laser powers and scanning speeds of 14 to 16 W and 13–16 mm/s respectively. The displacement of SMA bimorph was found to increase with increase in laser power whereas it decreases with increase in scanning speeds. At lower laser power, the scanning speeds have influenced the actuation behavior to larger extent. Notably at the laser power of 14 W, the influence of scanning speed was significant and registered 53 % reduction in displacement with increase in scanning speeds from 13 mm/s to 16 mm/s. A minimum and maximum displacement of 0.35 mm and 3.8 mm was obtained during laser actuation of bimorph. The temperature experienced during laser actuation has been simulated using COMSOL Multiphysics and corroborated with the actuation behavior of the SMA bimorph. The displacement range and the actuation speed of laser actuation were found to be higher than the conventional electrical actuation. Furthermore, the life cycle analysis has been performed up to 100 laser passes and the phenomenon for reduction in displacement has been discussed in detail. Laser actuation of SMA bimorph could be utilized in thermal switches, adaptive optics and remote actuation of SMA elements etc.

Development of a double-tee flange connection using shape memory alloy rods

This study investigated the out-of-plane shear performance of a newly developed shape memory alloy (SMA) connector and a commonly used shear/alignment connector. The innovation lies in developing a durable and easily installed and maintained flange-to-flange connector between precast concrete double-tee members. The proposed connector consists of a superelastic SMA curved bolt inserted into a duct that is then cast into the precast concrete member. Two types of sealant materials were used: polyurethane elastomeric sealant and nonshrink cementitious grout. The shear/alignment and SMA connectors were tested under monotonic vertical shear. The tests were conducted on 4 ft × 4 ft × 4 in. (1.2 m × 1.2 m × 100 mm) slab specimens. The resulting capacities and associated damage were summarized. Higher stiffness and lower ductility were observed for the SMA connector with nonshrink grout compared with the shear/alignment connector with polyurethane elastomeric sealant. The average stiffnesses of the SMA and shear/alignment connector specimens were 116,681 lb/in. (20,434 kN/m) and 31,300 lb/in. (5481 kN/m), respectively. The ductility of the SMA connector was improved when using polyurethane elastomeric sealant; however, more tests should be done to confirm this behavior. The SMA rod was reused in several tests through reheating of the SMA element. The shear/alignment connector cannot be reused.

On the microstructure and superelastic evolution of laser annealed thin NiTinol wires

Shape setting is a crucial step of the production route of shape memory alloys (SMAs) elements for fixing their functional properties. For thin SMA wires, this peculiar setting can be performed by a laser beam scanning the wire length. In the present work the correlation among the functional properties, such as stress–strain curves, the microstructural properties and the laser power was studied on 100 μm pseudoelastic NiTi wire. A comparison between the performances of the laser treated and the commercial wires was discussed. It can be stated that the wire responses can be modulated as a function of the laser power; the optimal laser condition can induce functional properties at least comparable to the ones of the wire conventionally treated in a furnace. Laser induced superelasticity was obtained at room temperature and the corresponding microstructure suggests a texture effect associated with the directional and fast heating induced by the laser beam scan. A favorable condition for extended stress induced plateau lengths, compared to the one of the commercially available furnace treated wires, was induced by the laser scan.

Usage of shape memory alloy actuators for large force active disassembly applications

Shape memory alloys (SMAs) possess inherent superior properties that make their applications in active disassembly an emerging and interesting field of research. This is because extremely large forces can be generated repeatedly using a small compact-sized element, such as an SMA actuator. To ensure the ability of the SMA actuator to generate a repeated large force or withstand repeated load, several factors should be considered. These include factors that affect the value of the generated recovery forces, such as the amount of strain used, activation temperature, activation time, and cross-sectional area of the SMA element. In general, the compressive strain can be considered as the most influential factor that affects the value of the generated recovery force. The present research investigates the possible use of the SMA actuator in large-force active disassembly applications. To the best of the authors' knowledge, all the studies conducted in this field are concerned with implementing active disassembly in applications requiring small disassembly forces. The present research was conducted in three phases. First, the behaviour of the SMA element upon exposure to different repetitive compressive strains was studied, and the generated recovery force and strain hardening induced in the material were considered to ensure the continuous generation of large recovery forces with the least amount of residual strain induced in the material. Second, the optimum value of the compressive strain required to generate the maximum force with the least amount of residual strain induced in the material was estimated. Third, a practical case study was presented to validate the possible implementation of SMA actuators in large force active disassembly applications. The study successfully estimated the optimum compressive strain value that generated the required recovery force to disassemble the conducted case study using active disassembly technique.