Characteristics Of Shape Memory Alloy Research Articles

Simultaneous position and force control of a SMA-actuated continuum robotic module

Continuum manipulators with controllable shape and exerting force are attractive devices. Utilizing great features of shape memory alloys (SMAs), thin continuum modules are developed. SMAs are embedded around an elastic rod and activated through a control algorithm to set a desired shape. However, complex behavior of SMAs in such a multi-input multi-output system, make the control challenging. Additionally, simultaneous control of position and the force applied by the module, as a challenging problem, has not been investigated so far. Handling this problem may increase the application of continuum robots when utilized as manipulators or when pass through narrow and complex canals with sensitive wall. In this research, position and force control of such continuum module is under focus for achieving a more practical tool for better non-invasive medical devices. Further to the position control, the amount of force applied to the environment is adjusted in different locations of the workspace through a novel fuzzy controller. The results indicate the possibility of simultaneous control of the position and force using fuzzy controller with a reasonable accuracy of 0.5° for angle and 0.05 N for the force. The concluded results may be utilized in developing smarter soft robots.

Hybrid dynamical modeling of shape memory alloy actuators with phase kinetic equations

Shape memory alloy morphing actuators are a type of composite soft actuator with many attractive properties such as large deformation, small form factor, self-sensing ability, and physical reservoir computing potential. These actuators are composed of active shape memory alloy wires and a passive material to magnify the overall deflection. However, the dynamic modeling of these actuators is difficult due to both shape memory alloy characteristics and the nonlinearity of the passive layer. Here, a hybrid dynamical model is proposed that couples the phase kinetics and thermal modeling for the shape memory alloy with a dynamic Cosserat beam model. This hybrid model is benchmarked against experimental linear and morphing actuators resulting in a root mean squared error of 0.87 mm for the linear actuator and root mean squared error of 1.34 and 1.42 mm for the two morphing actuator configurations evaluated in this work. This model applies continuous phase kinetic equations in a comprehensive hybrid dynamical model to accurately simulate the hysteretic transition of the alloy, which is then coupled to a high deformation beam model. This work can expand the capability and design of novel morphing actuators to achieve specified dynamic characteristics for increased application in robotic fields.

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Application of SMA materials in aerospace

The various characteristics of shape memory alloys, such as hyperelasticity, memory alloy effect and so on, make shape memory alloys become a new type of material with broad engineering applications. These components developed based on the characteristics of shape memory alloys are not only used in the aerospace field, but also in various fields such as bridges and railways, and can be used for various purposes such as bridge vibration control and intelligent hybrid control. This article mainly introduces several characteristics of shape memory alloys, and explains the practical application and development prospects of shape memory alloys in the aerospace field. Based on these studies, this article studies the characteristics of shape memory alloys through equation calculus and ANSYS simulation experiments modeling. It can be foreseen in the future that with the development of intelligent control technology, shape memory alloy structures will have a larger operating temperature range, more precise structural control, and will be applied in a wider variety of spacecraft structures.

Environment-Induced Degradation of Shape Memory Alloys: Role of Alloying and Nature of Environment.

Shape memory effects coupled with superelasticity are the distinctive characteristics of shape memory alloys (SMAs), a type of metal. When these alloys are subject to thermomechanical processing, they have the inherent ability to react to stimuli, such as heat. As a result, these alloys have established their usefulness in a variety of fields and have in recent years been chosen for use in stents, sensors, actuators, and several other forms of life-saving medical equipment. When it comes to the shape memory materials, nickel-titanium (Ni-Ti) alloys are in the forefront and have been chosen for use in a spectrum of demanding applications. As shape memory alloys (SMAs) are chosen for use in critical environments, such as blood streams (arteries and veins), orthodontic applications, orthopedic implants, and high temperature surroundings, such as actuators in aircraft engines, the phenomenon of environment-induced degradation is of both interest and concern. Hence, the environment-induced degradation behavior of the shape memory alloys (SMAs) needs to be studied to find viable ways to improve their resistance to an aggressive environment. The degradation that occurs upon exposure to an aggressive environment is often referred to as corrosion. Environment-induced degradation, or corrosion, being an unavoidable factor, certain techniques can be used for the purpose of enhancing the degradation resistance of shape memory alloys (SMAs). In this paper, we present and discuss the specific role of microstructure and contribution of environment to the degradation behavior of shape memory alloys (SMAs) while concurrently providing methods to resist both the development and growth of the degradation caused by the environment.

Neural network model for hysteretic characteristic of shape memory alloy

Shape memory alloy has obvious hysteretic characteristics, which is crucial for the utilization of shape memory alloy, so a rational model is needed to describe them. In the current paper, a model for shape memory alloy based on the neural network is established, in which the activation function is modified so that it can naturally describe the hysteretic curve of shape memory alloy. The proposed neural network model is used to simulate temperature-induced hysteresis and stress-induced hysteresis, and the results show that the proposed model has a good effect and has certain adaptability. The reason why the proposed neural network model can describe hysteresis is qualitatively explained, and it is further pointed out that the model can be regarded as a generalized model of the Preisach model. The model is further extended to accommodate hysteretic curves under different conditions. The proposed neural network model has a simple form and high accuracy, especially in comparison with traditional models, and can be used for the modelling of shape memory alloys or other materials with hysteretic characteristics.

Seismic Retrofitting of Nonseismically Detailed Exterior Reinforced Concrete Beam-Column Joint by Active Confinement Using Shape Memory Alloy Wires

It is widely accepted that the seismic retrofitting of nonseismically detailed reinforced concrete beam-column joints (BCJs) in reinforced concrete (RC) frame buildings is an urgent necessity due to its high vulnerability and potential consequences in seismic events. As a result, considerable effort has already been put into developing efficient and practical retrofitting solutions for such BCJs. Most of the existing techniques, however, are based on either passive confinement techniques, for example, fiber reinforced polymer (FRP) wrappings, or involve a considerable joint enlargement, which, in many cases, is undesirable. In this study a new technique of retrofitting BCJs is proposed by employing a more effective method of confinement (i.e., active confinement), utilizing the shape recovery feature of shape memory alloys (SMAs). To evaluate the performance of the proposed retrofitting scheme, experimental tests were conducted on full-scale BCJ specimens. The efficacy of the proposed retrofitting scheme is evaluated in terms of enhancement in strength, ductility, energy dissipation capacity, damage reduction in the specimens, and ease of application. The results from this study suggest that the proposed retrofitting scheme could be effectively used in achieving the full capacity of the joints corresponding to beam yielding and consequently enhances the energy dissipation capacity of the system significantly. The test results demonstrated that the proposed retrofitting scheme performs excellently in reducing the joint shear strain (core damage) (to almost zero or negligible) and also in retaining the full axial load carrying capacity of the column, even at very large drift values. The proposed retrofitted scheme could also be conveniently used in cases where the capacity ratio of column-to-beam needs to be improved.

Accurate Position Tracking Control of SMAAs Based on Low-Complexity Self-Sensing Model and Compound Control Strategy

Shape memory alloy actuators (SMAAs) exhibit high strain, high power-to-weight ratio, and self-sensing ability, which are promising characteristics for application in micro-actuator systems. However, the complex self-sensing and nonlinear hysteresis characteristics of shape memory alloy (SMA) seriously affect the position tracking accuracy of SMAAs. In order to realize the accurate position tracking of the SMAAs without sensors, this article proposes a low-complexity self-sensing of modeling methods, which does not require complex mechanical structure, specific prestress, and complex mathematical models. Furthermore, a compound control strategy of generalized Prandtl–Ishlinskii (GPI) inverse model and fuzzy-proportional–integral derivative (PID) was presented to compensate the nonlinear hysteresis and time-varying characteristics of SMAAs. First, macroscopic and microcosmic characteristics of SMA were analyzed to explain the negative effect of the rhombohedral ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> ) phase on the self-sensing characteristics of SMA. Second, a NiTi-based SMA wire without <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}$ </tex-math></inline-formula> phase was selected as a self-sensing modeling object by comparative evaluation experiments, which has similar linear self-sensing properties under various pretension force conditions, a self-sensing model containing only quadratic polynomials is constructed by an offline identification. Third, a fuzzy-PID controller is designed based on the transfer function obtained from the SMAA’s system identification, further, a nonlinear hysteresis feedforward compensation controller is designed based on the GPI model. Finally, comparative experiments with the compound controller and fuzzy-PID controller for multistep and sinusoidal position tracking responses were performed, and the experimental results show that the proposed compound controller has a faster response and higher control accuracy in position tracking control.

SMA-based adaptive tuned mass dampers: Analysis and comparison

Different types of adaptive tuned mass dampers have been recently proposed in the literature. One of the most promising approaches to make tuned mass dampers adaptive is the use of shape memory alloys. In this class of tuned mass dampers, different layouts have been proposed. This paper aims at comparing the two main layouts (wire-based and beam-based) in terms of adaptation capability, exerted force and electrical power consumption. To this purpose, the models of the two layouts are developed. These models minimise the number of required inputs, which basically are only related to device geometry, shape memory alloy characteristics, and vibration input. After an experimental validation, the models are employed for the mentioned comparisons between the two considered layouts.

Evolution of hysteresis characteristic of shape memory alloys at incomplete phase transformation cyclic loading

The phase transformation ratchetting of shape memory alloys (SMAs) at incomplete phase transformation cyclic loading is experimentally and theoretically investigated. To this end, two different kinds of incomplete phase transformation cyclic loading tests on NiTi wires are performed, i.e. incomplete transformation cyclic loads are respectively applied at the stages of forward martensitic transformation and reverse martensitic transformation. When the cyclic load of incomplete transformation is applied in the forward martensitic transformation stage, a novel phenomenon is discovered: although there is no greater stress to drive the anstenite turn to martensite, the SMAs can still gradually undergo martensitic transformation and accumulation until martensite reaches saturation. The hysteretic behavior finally reaches a shakedown state where the strain–stress curve no longer changes with the number of cycles. When the cyclic load of incomplete transformation is applied in the reverse martensitic transformation stage, a similar phenomenon is obtained. According to the analysis of the temperature evolution during the deformation process of the SMAs, combined with the relationship between the phase transformation yield stress and the temperature of SMAs, the experimental results are reasonably explained. This research is of great significance for a more comprehensive grasp of the mechanical behavior of SMAs.

Spatial characteristics of nickel-titanium shape memory alloy fabricated by continuous directed energy deposition

Additive manufacturing has been adopted to process nickel‑titanium shape memory alloys due to its advantages of flexibility and minimal defects. The current layer-by-layer method is accompanied by a complex temperature history, which is not beneficial to the final characteristics of shape memory alloys. In this study, a continuous directed energy deposition method has been proposed to improve microstructure uniformity. The spatial characterization of nickel‑titanium shape memory alloy fabricated by continuous directed energy deposition is investigated to study the temperature history, phase constituent, microstructure, and mechanical properties. The results indicate that the fabricated specimen has a monotonic temperature history, relatively uniform phase distribution and microstructure morphology, as well as high compressive strength (2982 MPa–3105 MPa) and strain (37.7%–41.1%). The reported method is expected to lay the foundation for spatial control during the printing of functional structures.

Part II.: Dissimilar friction stir welding of nickel titanium shape memory alloy to stainless steel – microstructure, mechanical and corrosion behavior

In the present work, nickel-titanium (NiTi) shape memory alloys (SMAs) is joined to 304 stainless steel (SS) using friction stir welding (FSW) using process parameters of 400 rpm and a constant tool translation speed of 75 mm/min. The macrostructure of the dissimilar weld showed some surface tunneling defects but there were regions of the weld that were defect free and without evidence of intermetallics at the interface. The weld stir zone (SZ) showed grain refinement compared to both base materials (BM). NiTi side of the joint showed very fine thermomechanical affected zone (TMAZ) and absence of a distinct heat affected (HAZ) zone, resulting in very little variation in microhardness. SS side of the weld showed typical FSW microstructural zones with higher microhardness in the SZ (270 HV) compared to BM (185 HV). Tensile testing of the weld showed a small initial plateau region, characteristic of shape memory alloys (SMAs) arising from NiTi part of the weld. Importantly, the transformation temperatures of NiTi, measured by differential scanning calorimetry (DSC) from different weld zones, varied by no more than 4 °C. Corrosion properties of the weld zone investigated in 3.5 wt. % NaCl solution showed moderately inferior corrosion resistance of dissimilar weld, compared to individual base metals, as characterized by the surface film impedance and corrosion current density. The decreased corrosion resistance was attributed to inhomogeneous passive film formed across the weld due to zonal heterogeneities. The predicted and measured galvanic parameters of the galvanically coupled base metals suggested that there appeared to be negligible galvanic effects. The results from the study show that with further development, FSW can be successfully utilized to create dissimilar joints between NiTi and stainless steel. • Nickel titanium shape memory alloy joined to stainless steel using friction stir welding process. • Refined grained along with absence of intermetallics was observed in the weld zone. • The transformation temperature on NiTi side across the different weld zones shows very little variation. • Under tensile loading, dissimilar joint showed small plateau region, characteristic of SMAs in martensitic phase. • Weld zone showed slightly inferior corrosion property compared to individual base material, while negligible galvanic effect was observed.

Sound Radiation of Orthogonal Antisymmetric Composite Laminates Embedded with Pre-Strained SMA Wires in Thermal Environment

The interest of this article lies in the sound radiation of shape memory alloy (SMA) composite laminates. Different from the traditional method of avoiding resonance sound radiation of composite laminates by means of structural parameter design, this paper explores the potential of adjusting the modal peak of the resonant acoustic radiation by using material characteristics of shape memory alloys (SMA), and provides a new idea for avoiding resonance sound radiation of composite laminates. For composite laminates embedded with pre-strained SMA, an innovation of vibration-acoustic modeling of SMA composite laminates considering pre-stain of SMA and thermal expansion force of graphite-epoxy resin is proposed. Based on the classical thin plate theory and Hamilton principle, the structural dynamic governing equation and the frequency equation of the laminates subjected to thermal environment are derived. The vibration sound radiation of composite laminates is calculated with Rayleigh integral. Effects of ambient temperature, pre-strain, SMA volume fraction, substrate ratio, and geometrical parameters on the sound radiation were analyzed. New laws of SMA material and pre-strain characteristics on sound radiation of composite laminates under temperature environment are revealed, which have theoretical and engineering functional significance for vibration and sound radiation control of SMA composite laminates.

Evolution of transformation characteristics of shape memory alloys during cyclic loading: transformation temperature hysteresis and residual martensite

This paper investigates the evolutions of transformation temperatures and residual martensite of shape memory alloys during cyclic loading. Pseudoelastic NiTi specimens were firstly subjected to stress-controlled cyclic loading; then differential scanning calorimetry and x-ray diffraction characterizations were conducted on samples cut from specimens after different cycles as well as virgin one. Results show that (i) transformation temperatures rise while the transformation temperature hysteresis increases at initial cycles and then remarkably decreases when the SMA is highly distorted; (ii) due to the accumulation of residual martensite, the more fatigue cycles applied to the SMAs, the less latent heat absorbed/released during the transformation. Furthermore, residual martensite volume fraction was calculated with respect to plasticity and a linear character was observed.

A Temperature-Dependent Model of Shape Memory Alloys Considering Tensile-Compressive Asymmetry and the Ratcheting Effect.

Tensile-compressive asymmetry and the ratcheting effect are two significant characteristics of shape memory alloys (SMAs) during uniaxial cyclic tests, thus having received substantial attention in research. In this study, by redefining the internal variables in SMAs by considering the cyclic accumulation of residual martensite, we propose a constitutive model for SMAs to simultaneously reflect tensile-compressive asymmetry and the cyclic ratcheting effect under multiple cyclic tests. This constitutive model is temperature dependent and can be used to reasonably capture the typical features of SMAs during tensile-compressive cyclic tests, which include the pseudo-elasticity at higher temperatures as well as the shape-memory effect at lower temperatures. Moreover, the proposed model can predict the cyclic mechanical behavior of SMAs subjected to applied stresses with different peak and valley values under tension and compression. Agreement between the predictions obtained from the proposed model and the published experimental data is observed, which confirms that the proposed novel constitutive model of SMAs is feasible.

A constitutive level-set model for shape memory alloys

The present work proposes a phenomenological level-set model for shape memory alloys within the framework of internal variable theory and continuum thermomechanics. While the majority of constitutive models for shape memory alloys assume the martensitic volume fraction as an internal variable, in this study a level-set function is chosen to be the internal variable. Such a choice allows to account, on average, for the microstructural changes within the material by representing in an implicit manner the interfaces between different phases. Utilizing consistent thermodynamical arguments within the framework of canonical thermomechanics, the evolution equation for the level-set function is derived. Then, the model is implemented in a one dimensional case, where simple evolution equations are proposed reproducing certain characteristics of shape memory alloys, such as the pseudoelastic effect, the strain-temperature hysteresis, the one-way shape memory effect and hardening effects. Also, application of the model on functional fatigue phenomenon is thoroughly inspected. This new constitutive level-set model provides extreme flexibility and seems to be very promising for future research.

Numerical simulation of the mechanical behaviour of shape memory alloys based actuators

In the last decades, the interest in Shape Memory Alloys (SMA) materials has been considerably increased due to their peculiar morphing capabilities, which can be relevant for many different engineering fields such as automotive and aerospace. Among the characteristics of shape memory alloys, their super-elastic behavior and their capability to recover an initial state, once a deformation has occurred, when subjected to a temperature above a predefined threshold, can be considered the most attractive ones. Possible applications of SMA includes, but are not limited to, switch actuators and morphing structures. In this work, the SMA materials super-elasticity effect and their capability to recover an initial state, due to the heating, are described and numerically reproduced by means of a user material routine implemented in the ABAQUS Standard FEM environment. A test-case representative of a mechanical actuator operated by SMA springs is introduced to assess the robustness of the developed numerical procedure for the simulation of Shape Memory Alloys material behavior.

A numerical study of the indentation mechanics of shape memory alloys in different temperature regimes

Instrumented indentation is a versatile technique to quantify not only the mechanical properties but also the phase transformation and recovery characteristics of shape memory alloys (SMAs). The objective of this work is to investigate the effect of temperature on the mechanics of spherical indentation of SMAs as manifested through stress induced martensite transformation (SIMT) and plastic yielding. To this end, finite element simulations of spherical indentation response of Ni–Ti based SMAs are carried out using a constitutive model that incorporates the combined effects of superelasticity and plasticity. A range of temperatures from well below to above the austenite finish temperature Af is considered. It is found that while SIMT is the governing deformation mode during indentation at temperatures below Af, plastic yielding becomes significant at temperatures close to and above Af. The load and mean contact pressure increase with temperature above Af at a given indentation depth. Also, the remnant depth ratio for a given load is lowest close to Af. SIMT and plastic deformation influence the stress distribution differently depending on the temperature.

Development of a cascade position control system for an SMA-actuated rotary actuator with improved experimental tracking results

Hysteresis and significant nonlinearities in the stress–strain–temperature characteristics of shape memory alloy (SMA) actuators prevent effective utilization of these actuators and make them difficult to control. Due to these effects, the position control of SMA actuators has been a great challenge for many industrial applications. In this study, a cascade control for position tracking using torque and position measurement of an SMA-actuated rotary actuator is considered. The effects of the SMA characteristic model on control performance are also focused when simulation consequences are directly applied in practice. Based on nonlinear and unknown dynamics of an SMA actuator, a torque regulation adaptive controller using modified Brinson model is developed that requires to approximate the parameters of the system by only measuring SMA force and actuator rotation. Although it is shown that only an approximate model of the system is sufficient to develop the proposed controller, the system dynamic is simulated using MATLAB software, and the characteristics of SMA wire is considered by both modified Brinson constitutive model and Liang and Rogers constitutive model. The controller gains are regulated for both of the mentioned models using simulation. Finally, the performance of both controllers is further evaluated experimentally, and the accuracy of the proposed controller based on the modified Brinson model is compared to the controller based on the Liang and Rogers model. It is shown that since the Brinson model has a better prediction of the SMA wire behavior, the simulation results based on the modified Brinson model have better conformity to reality.

Modeling and characterization of the shape memory alloy–based morphing wing behavior using proposed rate-dependent Prandtl-Ishlinskii models

Unique features of shape memory alloys make them a proper actuation choice in various control systems. However, their nonlinear hysteresis behavior negatively affects wide utilization of such materials in structure actuation. In this study, the frequency effect on the hysteresis behavior of a shape memory alloy–actuated structure is experimentally investigated, and also two proposed versions of rate-dependent Prandtl-Ishlinskii (modified rate-dependent Prandtl-Ishlinskii and revised modified rate-dependent Prandtl-Ishlinskii) are presented, which are capable of characterizing this phenomenon. Experimental results show that increasing excitation frequency leads to bigger hysteresis loops. It is also proven that rate-dependency cannot be predicted by generalized Prandtl-Ishlinskii model. In addition, a comparison between the dead zone function-based rate-dependent Prandtl-Ishlinskii model as an only benchmark model and the proposed models have been done that proves the proposed models’ superiority. In addition, genetic algorithm is exploited to identify unknown parameters of all models. Trained models performance is also experimentally evaluated at different input frequencies. Comparison between simulation and experimental results indicates that the proposed models can reliably predict saturated, asymmetric, rate-dependent hysteresis behavior, and minor loops in shape memory alloy–embedded actuators.