Austenitic Shape Memory Alloys Research Articles

Variable stiffness mechanism of flexible composite laminates implanted with modular SMAHC

In this study, a new flexible hybrid shape memory alloy composites (SMAHCs) with a polydimethylsiloxane (PDMS) matrix containing a shape memory alloy (SMA) of a hybrid Basalt/Kevlar fiber 3D woven composite is developed. The SMAHC is implanted into a glass fiber/PDMS composite laminate in a modular manner to study the stiffness of composite plates in response to temperature. The variable stiffness theory of the SMAHC is derived, and the influence of SMA parameters on the stiffness of the SMAHC is analyzed. A three-point bending test is performed to determine the stiffness of the composite plates at multiple temperature points in the form of Joule heat driving the composite plates. The stress and temperature distributions of the composite plate are analyzed in Abaqus under a temperature-displacement coupling setting. The results shows that the SMAHC stiffness is negatively correlated with SMA fiber diameter and positively correlated with SMA fiber volume fraction. With an increase in driving temperature, the stiffness of the composite plates tends to decrease. When the end temperature of the austenitic phase transformation is reached, the high-modulus austenitic SMA slightly increase the stiffness of the composite plate. The stiffness improvement is more significant when the SMAHC is placed in the middle area of the laminate and the SMA is located in the tension area. This study may provide a reference for the design of variable stiffness SMAHC for plate composite structures.

Optimized Neural Network Prediction Model of Shape Memory Alloy and Its Application for Structural Vibration Control.

The traditional mathematical model of shape memory alloy (SMA) is complicated and difficult to program in numerical analysis. The artificial neural network is a nonlinear modeling method which does not depend on the mathematical model and avoids the inevitable error in the traditional modeling method. In this paper, an optimized neural network prediction model of shape memory alloy and its application for structural vibration control are discussed. The superelastic properties of austenitic SMA wires were tested by experiments. The material property test data were taken as the training samples of the BP neural network, and a prediction model optimized by the genetic algorithm was established. By using the improved genetic algorithm, the position and quantity of the SMA wires were optimized in a three-storey spatial structure, and the dynamic response analysis of the optimal arrangement was carried out. The results show that, compared with the unoptimized neural network prediction model of SMA, the optimized prediction model is in better agreement with the test curve and has higher stability, it can well reflect the effect of loading rate on the superelastic properties of SMA, and is a high precision rate-dependent dynamic prediction model. Moreover, the BP network constitutive model is simple to use and convenient for dynamic simulation analysis of an SMA passive control structure. The controlled structure with optimized SMA wires can inhibit the structural seismic responses more effectively. However, it is not the case that the more SMA wires, the better the shock absorption effect. When SMA wires exceed a certain number, the vibration reduction effect gradually decreases. Therefore, the seismic effect can be reduced economically and effectively only when the number and location of SMA wires are properly configured. When four SMA wires are arranged, the acceptable shock absorption effect is obtained, and the sum of the structural storey drift can be reduced by 44.51%.

Effect of temperature on high strain rate deformation of austenitic shape memory alloys by phenomenological modeling

High strain rate compressive behavior of austenitic shape memory alloys (SMAs) is investigated by finite element analysis at temperatures ranging from the martensitic start temperature (Ms) to the temperature (Md) above which stress-induced martensite no longer forms. It is found that the start stress for phase transformation increases with increasing temperature. The maximum martensitic volume fraction during phase transformation decreases with increasing temperature. When the ambient temperature is higher than Md, phase transformation is no longer observed and the SMA deforms like a pure elastic material according to the model. At temperatures below Md and above Ms, the microscopic band structure is observed during phase transformation. The critical strain for initiation of the band structure increases with increasing temperature. The band structure disappears at temperatures approaching or above Md.

Stress-Corrosion Behavior of Nickel Titanium Shape Memory Alloy Under Quasi-Static Loading

Nickel titanium (NiTi) shape memory alloys (SMAs) are well known for their significant shape recovery under thermomechanical loading [1]. Austenitic SMA can recover its original shape upon unloading (pseudoelasticity) while martensitic SMA can do so after subsequent heating (shape memory effect). Understanding the corrosion behavior of NiTi SMAs is important for extending their successful applications. Studying the corrosion of SMAs is challenging due to the inherent complexity of their constitutive response together with the presence of different phases (austenite or martensite) and microstructural states (twinned or detwinned) [2]. In this study, the corrosion behavior of equiatomic NiTi under quasi-static loading in the plastic deformation regime is studied in artificial physiological solution at room temperature. Elelctrochemcial Frequencies Modulation (EFM) technique is used the parameters of which has been found in a previous study by the authors [3]. The corrosion parameters of NiTi are measured at different load levels during plastic deformation where the passive breakdown occurs. Strain distribution on the surface of the plastically deforming NiTi sample corresponding to these load levels is captured by means of Digital Image Correlation (DIC) technique [4]. Metal loss is measured using Faraday’s law and Open Circuit Potential (OCP) measurements show that the repassivation of oxide layer is not taken place. The findings are considered a step towards better understanding of the corrosion behavior of SMAs under thermo-mechanical loading.

Tests on Pretrained Superelastic NiTi Shape Memory Alloy Rods: Towards Application in Self‐Centering Link Beams

Austenitic shape memory alloy has potential applications in self‐centering seismic resistant structural systems due to its superelastic response under cyclic tension. Raw austenitic SMA needs proper pretreatments and pretraining to gain a stable superelastic property. In this paper, tests are carried out to investigate the effects of pretraining, pretreatments, loading rate, and strain amplitude on the mechanical performance on austenitic SMA rods with a given size. The tested rods are to be used in a new concept self‐centering steel link beam. Customized pretraining scheme and heat treatment are determined through the tests. The effects of loading rate and strain amplitude are investigated. A simplified stress‐strain model for the SMA rods oriented to numerical simulations is obtained based on the test results. An example of using the simplified material model in numerical analysis of a self‐centering steel link beam is conducted to validate the applicability of the model.

One-dimensional thermomechanical model for high strain rate deformation of austenitic shape memory alloys

Shape memory alloys (SMAs) exhibit the ability to absorb large dynamic loads and, therefore, are excellent candidates for structural components where impact loading is expected. While most models focus on the shape memory effect and/or pseudoelasticity of polycrystalline SMAs under quasi-static loading conditions; models for dynamic loading are limited. Many of the existing high strain rate models assume that the latent heat generated during deformation contributes to the change in the stress strain behavior during dynamic loading. . The driving force for phase transformation is also related to the phase front velocity, which is not considered in existing models for the constitutive relationship of SMAs at high strain rate deformation. In this paper, the relationship between the driving force for phase transformation and phase front velocity is discussed and a new one-dimensional rate-dependent model is established to model the dynamic loading behavior of SMAs.

Texture and Strain Measurements from Bending of NiTi Shape Memory Alloy Wires

Shape memory alloys (SMAs) are a new generation of materials that exhibit unique nonlinear deformations due to a phase transformation which allows the material to return to its original shape after removal of stress or a change in temperature. These unique properties are the result of a martensitic/austenitic phase transformation through the application of temperature changes or applied stress. Many technological applications of austenitic SMAs involve cyclical mechanical loading and unloading in order to take advantage of pseudoelasticity, but are limited due to poor fatigue life. In this paper, commercial pseudoelastic NiTi SMA wires (50.7 at.% Ni) were placed under different bending strains and examined using scanning electron microscopy and high-energy synchrotron radiation X-ray diffraction (SR-XRD). By observing the microstructure, phase transformation temperatures, surface texture and diffraction patterns along the wire, it is shown that the wire exhibits a strong anisotropic behavior whether on the tensile or compressive side of the bending axis and that the initiation of micro-cracks in the wires is localized on the compression side, but that crack propagation will still happen if the wire is reloaded in the opposite direction. In addition, lattice strains are examined for both the austenite and martensite phases.

High-energy synchrotron X-ray diffraction measurements of simple bending of pseudoelastic NiTi shape memory alloy wires

Many technological applications of austenitic shape memory alloys (SMAs) involve cyclical mechanical loading and unloading in order to take advantage of pseudoelasticity. In this paper, we investigated the effect of mechanical bending of pseudoelastic NiTi SMA wires using high-energy synchrotron radiation X-ray diffraction (SR-XRD). Differential scanning calorimetry was performed to identify the phase transformation temperatures. Scanning electron microscopy images show that micro-cracks in compressive regions of the wire propagate with increasing bend angle, while tensile regions tend not to exhibit crack propagation. SR-XRD patterns were analyzed to study the phase transformation and investigate micromechanical properties. By observing the various diffraction peaks such as the austenite (200) and the martensite (${\bar 1}12$), (${\bar 1}03$), (${\bar 1}11$), and (101) planes, intensities and residual strain values exhibit strong anisotropy, depending upon whether the sample is in compression or tension during bending.

Modelling Bonded Shape Memory Alloy Vibration Attenuators Elements Using the Finite-Element Method

Shape memory alloys (SMAs) present the capability to develop large forces and displacements with low power consumption. Due their special characteristics, SMAs have been used in many different applications. Pseudoelastic hysteresis loop observed in austenitic SMAs is associated with energy dissipation. Therefore, pseudoelastic SMA elements can be used as vibration attenuators. Joining methods present some technological challenges for the use of these elements. Welding can strongly affect the properties of the alloy. Mechanical joints using rivets and screws are commonly used but promote stress concentration effects. The use of adhesives offers some benefits, being an alternative to be investigated. This work presents a numerical model based on the finite-element method and experimental procedures to study the behaviour of bonded vibration attenuators with SMA elements. The proposed model considers the pseudoelastic behaviour of SMA elements, and a cohesive zone model was used to study the union between absorber and an aluminium plate. Finally, several loading conditions were analysed with the proposed models to assess the capability of bonded pseudoelastic SMA elements to dissipate energy. The proposed geometry allows the elements to actuate as an efficient vibration attenuator, in particular when submitted to axial loading.

Real-Time Compatible Phenomenological Modelling of the Austenitic Phase in Shape Memory Alloys as an Example for Modelling of Materials with Repeatable Non-Linear Characteristics

Shape Memory Alloys (SMAs) are attractive materials for a variety of applications due to advantages such as high reversible strains, high specific actuation stresses and high energy density. The extreme non-linearity inherent in SMAs, however, makes the design of applications such as position control non trivial. While control strategies benefit greatly from models incorporating these non-linearities, existing models for SMA constitutive behaviour are either too complicated to be used in real-time applications or too simple to accurately predict observed behaviour. When considering the mechanical stress-strain behaviour of austenitic SMAs, for example, it can be observed that the non-linearities follow distinct patterns. These patterns can be exploited to form mathematical models with continuous analytical functions that are extremely computationally effective and thus making it conducive for use in the development of real-time control algorithms and sophisticated industrial applications. This work presents a method for the modelling and identification of non linear materials for control purposes given that the non-linearities are repeatable or hysteretic in nature using pseudoelastic SMAs as an example. The model is validated by stress-strain experiments and the results show good correlation with experimental data.

Nanoscale variation in energy dissipation in austenitic shape memory alloys in ultimate loading cycles

Wavy behaviours of hysteresis energy variation in nanoscale bulk of thermomechanical austenitic NiTi shape memory alloy are reported in ultimate nanoindentation loading cycles. One sharp and two spherical tips were used while two loading–unloading rates were applied. For comparison, another austenitic copper-based shape memory alloy, CuAlNi shape memory alloy, and a metal with no phase transition, elastoplastic Cu, were investigated. In shape memory alloys, the hysteresis energy variation ultimately undergoes a linear decrease with internal wavy fluctuations and no stabilisation was observed. The internal energy fluctuation in these alloys was found dissimilar depending on the loading–unloading rate and the indentation tip geometry. In contrast, there was an absence of both overall and internal variations in hysteresis energy for Cu after the second loading cycle. The underlying physics of these variations is discussed and found to be attributed to both the created dislocations and ratcheting thermal–mechanical behaviour of the phase-transformed volume in shape memory alloys.

Variation of mechanical properties of shape memory alloy bars in tension under cyclic loadings

It is well-known that the behavior of shape memory alloys (SMAs) depends on varying the temperature and dynamic loading frequency. However, the relationship between SMA behavior and cyclic loads has not been clearly discussed. This relationship is very important in the field of seismic application of SMAs. Therefore, this study arranges several dynamic tensile tests of martensitic and austenitic SMA bars. The parameters of the tests are the initial strain of the SMA bars, the number of cycles, and the loading frequency. The hysteretic curves of the stress–strain relation are measured, and the mechanical properties of the residual strain, Young's modulus is varied at first, but becomes stable after 250 cyclic loading. However, the damping ratio decreases continuously when the number of loading cycles increased. It is found that the initial strain and loading frequency do not affect the mechanical properties significantly. Only the residual strain is influenced by the amount of initial strain but not the loading frequency.

Multiaxial Shape Memory Effect and Superelasticity

Abstract: The specific behaviour of shape memory alloys (SMA) is due to a martensitic transformation [Shape Memory Materials. Cambridge University press, Cambridge]. This transformation consists mainly in a shear without volume change and is activated either by stress or temperature. The superelastic behaviour and the one‐way shape memory effect are both due to the partition between austenite and martensite. The superelastic effect is obtained for fully austenitic SMA: loaded up to 5% strain, a sample recovers its initial shape after unloading with a hysteretic loop. The one‐way shape memory effect is obtained when a martensitic SMA, plastically deformed, recovers its initial shape by simple heating. Superelasticity and one‐way shape memory effect are useful for several three‐dimensional applications. Despite all these phenomena are well known and modelled in 1D, the 3D behaviour, and especially the one‐way shape memory effect, remains quite unexplored [Mater. Sci. Res. Int., 1 (1995) 260]. Actually, the development of complex 3D applications requires time‐consuming iterations and expensive prototypes. Predictive phenomenological models are consequently crucial objectives for the design and dimensioning of SMA structures. Therefore, a series of 2D proportional and non‐proportional, isothermal and non‐isothermal tests have been performed. This database will be used to build a phenomenological model within the framework of irreversible processes.

The behavior of concrete cylinders confined by shape memory alloy wires

The purpose of this study was to propose a new method to confine concrete cylinders orreinforced concrete columns using martensitic or austenitic shape memory alloy (SMA)wires. The prestrained martensitic SMA wire was wrapped around a concrete cylinder thenheated by a heating jacket. In the process, the confining stress around the cylinder wasdeveloped in the SMA wire due to the shape memory effect, which can increase thestrength and ductility of the cylinder under axial compressive load. For austenitic shapememory wires, the wires were prestrained as they were wrapped around the concretecylinders on which post-tensioning stress was generated. In this study, martensiticand austenitic SMA wires of 1.0 mm in diameter were used for the confinement.Recovery tests were conducted on the martensitic wire to assess the recoverystress. Also, a superelastic behavior test was performed for the austenitic wire.The confinement by martensitic SMA wires increased the strength slightly and greatlyincreased the ductility compared to the strength and ductility of plain concrete cylinders.The austenitic SMA wires showed a similar effect on concrete cylinders to that of themartensitic wires. This study showed the potential of the SMA wire jacketingmethod to retrofit reinforced concrete columns and protect them from seismic risks.

Seismic Vibration Control Using Superelastic Shape Memory Alloys

Superelastic NiTi shape memory alloy (SMA) wires and bars are studied to determine their damping and recentering capability for applications in the structural control of buildings subjected to earthquake loadings. These studies improve the knowledge base in regard to the use of SMAs in seismic design and retrofit of structures. The results show that the damping properties of austenitic SMAs are generally low. However, the residual strain obtained after loading to 6% strain is typically <0.75%. In general, it is shown that large diameters bars perform as well as wire specimens used in non-civil-engineering applications. The results of a small-scale shake table test are then presented as a proof of concept study of a SMA cross-bracing system. These results are verified through analytical nonlinear time history analysis. Finally, a three-story steel frame implementing either a traditional steel buckling-allowed bracing system or a SMA bracing system is analyzed analytically to determine if there is an advantage to using a SMA bracing system. The results show that the SMA braces improve the response of the braced frames.

Strain Self-Sensing Property and Strain Rate Dependent Constitutive Model of Austenitic Shape Memory Alloy: Experiment and Theory

By using both the pseudoelasticity and strain self-sensing ability of shape memory alloy (SMA) material, an innovative smart SMA-based damper can be developed. This damper has comprehensive energy dissipation and strain self-sensing abilities (i.e., electric resistance versus applied strain relationship) simultaneously. Before shaking table test on a building installed with such SMA-based dampers, the tests were conducted on the hysteresis stress–strain-electric resistance relationship of SMA wires (diameter 1.2mm). These tests were carried out under sinusoidal excitations with different loading frequencies at different ambient temperatures, to investigate the energy dissipation ability and strain self-sensing property of SMA wires. The sensitivity coefficient of electric resistance versus applied strain of the SMA wires was identified to be 5.948 from the test results, which is larger than that of the conventional strain gage and is independent of loading frequency. The experimental results indicate that the pseudoelastic hysteresis loops of the SMA wires are dependent on loading frequencies, particularly in the case of lower frequency. Further tests aimed at revealing the mechanism of the strain rate dependence of pseudoelasticity of SMA wire were conducted. The results indicate that the phase transformation temperature is dependent on strain rate, i.e., both the martensitic phase transformation finish temperature and the austenitic phase transformation start temperature decrease with increasing of strain rate. Based on the results of these tests, a one-dimensional strain rate dependent constitutive model of SMA wires was developed. This model predicts pseudoelastic behavior of SMA under various loading frequencies. Finally, a numerical simulation using the proposed model was performed, and the simulated pseudoelastic hysteresis loops agree well with test results presented not only by writers but also by other researchers.

Transient Thermal Stresses in a Superelastic Shape Memory Alloy Strip.

The austenitic Shape Memory Alloy (SMA) can be loaded up to a large strain without any plastic deformation and behaves superelastically (pseudelastically) above its characteristic transition temperature. When loaded, the superelastic SMA undergoes a stress-induced martensitic transformation that will result in a large recoverable strain up to 8%. On unloading, the superelastic SMA experiences a large hysteresis loop that makes the alloy an useful candidate for strain energy absorption. By taking advantage of this effect, SMA is used in various fields. The SMA can be used to improve the impact damage tolerance of composite materials and to induce the relaxation of stress distribution. This investigation deals with the transient thermoelastic problem for a superelastic SMA strip under the heat exchange. In order to simplify the SMA model, a linearization of one dimensional constitutive equations is made. The piecewise linear stress-strain relation is separated into two parts : before and after the stress-induced martensitic phase transformation. Another assumption is the identical SMA stress-strain relation in tension and in compression. Results for the temperature and stresses distributions are obtained numerically. We discuss about the variation of the martensitic phase and the thermal stress relaxation.

Development of a shape memory alloy FeMnSi

An austenitic shape memory alloy containing about 30 wt.% Mn and an appropriate amount of silicon has been developed. It is a cost-saving alloy and can be used for practical applications. When the strain is less than approximately 5%, the alloy exhibits a complete shape memory effect (SME) of one-way type. Contrary to most shape memory alloys which have ordered structures, the FeMnSi alloy without an ordered structure has a hysteresis as large as 500 K. SME of the alloy originates in the f.c.c.⇄h.c.p. martensitic transformation. The macroscopic properties and microscopic mechanisms involved in the martensitic transformation are reviewed.