Superelastic Behavior Of Shape Memory Alloys Research Articles

Strain amplitude effects on the seismic performance of dampers utilizing shape memory alloy wires

Superelastic shape memory alloys (SMAs) are unique smart materials with a considerable energy dissipation potential for the vibration control of structures. The energy dissipation arises from the hysteretic phase transformation. Random vibration patterns, such as by earthquakes and wind, include not only changing strain rates but also different strain amplitudes. Previous studies revealed that the reverse phase transformation behavior of SMAs depend apart from the strain rate also on the strain amplitude. This study confirms these findings by experiments and proposes a modeling approach for an accurate representation of this effect. Both tensile cyclic and seismic responses of Nitinol wires are studied for this purpose. The modeling approach is validated by the experimental results. Finally, the proposed model is applied numerically to a seismically excited 20-story benchmark building incorporating multiple SMAs. An intermediate time stepping approach is employed, which improves both the computational efficiency and the accuracy of the simulation process.

High-performance self-centering steel columns with shape memory alloy bolts: Design procedure and experimental evaluation

Steel moment-resisting frames are popular structural systems used extensively around the world. However, conventional column base connections are vulnerable to large residual deformation after strong earthquakes. By contrast, shape memory alloys (SMAs), which are high-performance metallic materials, can experience large strains and still recover their initial shape through either heating (shape memory effect) or unloading (superelastic effect). The superelastic behavior of SMAs is appealing to the earthquake engineering community because of the material’s excellent self-centering (SC) and energy dissipation capabilities. In this paper, a novel type of steel columns equipped with NiTi SMA bolts was introduced and its potential for achieving earthquake resilience were investigated. Structural details of the column base and mechanical properties of the SMA bolts were described first. Subsequently, an analytical model of the SC column for different limit states and the corresponding design procedure were presented. The seismic behaviors of two steel column specimens were experimentally tested to investigate the effects of the initial prestrain in the SMA bolts and the axial compressive force in the column under cyclic loading. Results showed that the steel columns equipped with SMA bolts exhibited satisfactory and stable flag-shaped hysteresis loops with excellent SC and moderate energy dissipation capabilities. More importantly, SMA bolts with prestrain could still be tightened after removal of lateral force. Therefore, the proposed SC column could achieve seismic resilience design that requires no (or minimal) repair even after strong earthquakes and remains highly functional for aftershocks or future earthquakes. In addition, the analytical model was verified through a comparison with test results obtained at key limit states.

A Proof-of-Concept Study on Self-Centering Column Feet Equipped with Innovative Shape Memory Alloy Ring Springs

This paper presents a feasibility study on a self-centering steel column foot equipped with innovative shape memory alloy (SMA) ring springs. The ring spring system includes a series of SMA outer rings and high-strength alloy inner rings stacked in alternation with mating taper faces, and the system is used in conjunction with high-strength bolts in a typical extended end-plate connection to achieve favorable self-centering and energy dissipation performances while ensuring ease of construction. An experimental investigation was first conducted looking into the basic mechanical performance of individual SMA ring springs under cyclic loading. Driven by the superelastic behavior of SMA, excellent self-centering ability with satisfactory energy dissipation is exhibited. Subsequently, a further experimental study on a proof-of-concept column foot specimen is conducted, where the key properties, including ductility, self-centering ability, and energy dissipation capability, are discussed in detail in this paper.

Innovative use of a shape memory alloy ring spring system for self-centering connections

This paper presents superelastic shape memory alloy (SMA) ring spring systems for seismic applications. The study commences with an experimental investigation looking into the mechanical performance of individual SMA ring springs under cyclic loading. The strength, self-centering ability, and energy dissipation capacity are shown to be dependent on the ring size as well as the treatment of the contacting taper face. Subsequently, a proof-of-concept beam-to-column self-centering connection is physically tested, shedding further light on the practical application of the ring spring systems for high-performance seismic resisting steel frames. The connection shows favorable cyclic behavior, with the majority of the deformation demand resisted by the SMA ring spring systems. The remaining structural components generally stay undamaged. Driven by the superelastic behavior of SMA, excellent self-centering ability with satisfactory energy dissipation is exhibited. A numerical investigation is then conducted to gain a further understanding of the load resistance mechanism of the proposed connection. The numerical model is validated against the experimental results with good agreement observed, enabling a further parametric study to be conducted taking a more in-depth look into the influence of ring pre-compression, taper face friction, ring size, and shear tab bolt preload on the overall performance of the connection. Based on the test and numerical data, some design comments are also made.

Numerical simulation and experimental investigation of the elastocaloric cooling effect in sputter-deposited TiNiCuCo thin films

The exploitation of the elastocaloric effect in superelastic shape memory alloys (SMA) for cooling applications shows a promising energy efficiency potential but requires a better understanding of the non-homogeneous martensitic phase transformation. Temperature profiles on sputter-deposited superelastic \({\mathrm {Ti_{55.2}Ni_{29.3}Cu_{12.7}Co_{2.8}}}\) shape memory alloy thin films show localized release and absorption of heat during phase transformation induced by tensile deformation with a strong rate dependence. In this paper, a model for the simulation of the thermo-mechanically coupled transformation behavior of superelastic SMA is proposed and its capability to reproduce the mechanical and thermal responses observed during experiments is shown. The procedure for experiment and simulation is designed such that a significant temperature change from the initial temperature is obtained to allow potential cooling applications. The simulation of non-local effects is enabled by the use of a model based on the one-dimensional Muller–Achenbach–Seelecke model, extended by 3D mechanisms such as lateral contraction and by non-local interaction, leading to localization effects. It is implemented into the finite element software COMSOL Multiphysics, and comparisons of numerical and experimental results show that the model is capable of reproducing the localized transformation behavior with the same strain rate dependency. Additionally to the thermal and the mechanical behavior, the quantitative prediction of cooling performance with the presented model is shown.

Exploitation of Large Recoverable Deformations Using Weaved Shape Memory Alloy Wire-Based Sandwich Panel Configurations

A new class of truss structure based on superelastic shape memory alloy (SMA) wire has been developed by weaving superelastic SMA wire through two perforated facesheets. A gap was maintained between the facesheets while weaving and the ends of wire forming the truss legs are anchored in each facesheet. The resulting structure has a modified pyramidal configuration and is capable of undergoing large recoverable deformations typical of superelastic SMA. A four-unit cell truss specimen has been tested under static load cycles to investigate the compressive response. The truss specimen underwent a hysteretic loop and demonstrated minimal permanent deformation closely resembling the behavior of bulk SMA. A finite element model of the truss was generated and the analysis results were compared with the experimental response. The present work is an attempt to demonstrate an SMA-based truss structure having energy absorption capabilities with minimum permanent deformation. These truss structures may be applied for damage mitigation in composites subjected to impact and blast loads.

Influence of high austenite stiffness of shape memory alloy on the response of base-isolated benchmark building

The influence of high austenite stiffness of shape memory alloy (SMA) on the response of base-isolated benchmark building has been investigated. Dynamic analysis was performed by solving the governing equations of motion using Newmark-beta method under three near-fault earthquake motions. The super-elastic behavior of SMA was modeled by the Grasser–Cozzarelli model. Structural response parameters such as top-floor acceleration, base displacement, and base shear were chosen for investigation. Three near-fault earthquakes with bidirectional ground motions were considered. It was observed that because of high austenite stiffness of SMA, higher accelerations associated with high frequencies are transmitted to the superstructure. The isolation device with high austenite stiffness of SMA excites the higher modes of the base-isolated structure and thus magnifies the floor accelerations. This phenomenon can be detrimental to the high-frequency internal equipments mounted in the base-isolated structure. On the other hand, the high austenite stiffness of SMA does not affect the base displacement and base shear of the base-isolated structure significantly. Furthermore, the influence of high austenite stiffness of SMA is dependent on the transformation strength of SMA and flexibility of the isolator. Copyright © 2016 John Wiley & Sons, Ltd.

Tailoring superelasticity of soft magnetic materials

Embedding magnetic colloidal particles in an elastic polymer matrix leads to smart soft materials that can reversibly be addressed from outside by external magnetic fields. We discover a pronounced nonlinear superelastic stress-strain behavior of such materials using numerical simulations. This behavior results from a combination of two stress-induced mechanisms: a detachment mechanism of embedded particle aggregates and a reorientation mechanism of magnetic moments. The superelastic regime can be reversibly tuned or even be switched on and off by external magnetic fields and thus be tailored during operation. Similarities to the superelastic behavior of shape-memory alloys suggest analogous applications, with the additional benefit of reversible switchability and a higher biocompatibility of soft materials.

Seismic behavior of SMA–FRP reinforced concrete frames under sequential seismic hazard

Accumulation of plastic deformation under excessive loads, is one of the most critical drawbacks in steel reinforced concrete structures. Permanent plastic deformation of steel rebars is among the main reasons for the disruption of the functionality of RC structures after major seismic events. It can also pose life threatening risks in case of strong aftershock occurrence. In an attempt to address the problem of excessive permanent deformations and their impact on the post-earthquake functionality of concrete moment resisting frame (MRF) structures, this paper studies analytically a new type of reinforcing bars made of fiber reinforced polymer (FRP) with embedded superelastic shape memory alloy (SMA) fibers. SMA–FRP reinforcement is characterized with both ductility and pseudo-elasticity which are two important characteristics that are sough in this study to enhance the ability of RC moment frames to withstand strong sequential ground motions (i.e. main shock followed by one or more aftershocks). In this study, experimentally validated SMA–FRP material models are used in structural level models to assess the performance of RC frame structures under seismic loading. Three-story, one-bay prototype RC MRFs, reinforced with steel and SMA–FRP composite reinforcements are first designed using performance based criteria and then subjected to incremental dynamic analysis under sequential ground motions. Comparison is drawn between steel and SMA–FRP reinforced frames based on accumulation of damage and residual drifts. Numerical results show superior performance of SMA–FRP composite reinforced MRF in terms of dissipation of energy and accumulation of lower residual drifts. Increased demands from the effects of aftershock causes accumulation of residual drifts in steel reinforced frames which is mitigated in SMA–FRP reinforced frame through re-centering capability.

Numerical simulation for the behavior of superelastic shape memory alloys

Superelastic shape memory alloys (SMAs) are unique metallic materials that undergo substantial plastic deformations and recover their original conditions when stresses are only removed without any heat treatment. SMAs have currently become prevalent for application in structural engineering because this superelastic property contributes to entire construction system by mitigating the problem of permanent deformation. Notwithstanding many structural advantages, there exist relatively few investigations on the numerical modeling of these smart materials, which had been mostly used for nonlinear analyses. For this reason, this study mainly focuses on a one-dimensional (1D) constitutive model able to simulate the inherent behavior of superelastic SMAs, taking into account phase transformation between austenite and martensite. After discussing a possible approach for the solution scheme, numerical simulation results are compared to experimental data obtained from pull-out tests that are performed on SMA bars in order to validate the adequacy for the 1D constitutive material model presented. Furthermore, the user material model based on the solution algorithm of reproducing this superelastic behavior is applied to the structural analysis with a view to assure adequacy in practical use.

Effect of notches on the behavior of superelastic round-bar NiTi-specimens

Several studies have recently been dedicated towards understanding failure mechanisms inshape memory alloys. Many of these investigations indicate that triaxial stress andstress-concentration affect the fracture behavior of these materials. However, few havemade an attempt to characterize the effect of these features on the superelastic behavior ofshape memory alloy specimens. In this work, notched round-bar NiTi-specimens have beensubjected to a simple set of experimental tests and an extensive numerical investigation. Boththe cases of tensile loading and one-cycle loading have been studied. A superelastic–plasticconstitutive model is used in this work. The main results show that notches can have anegative effect on the superelastic behavior in NiTi, and that this is related to thedevelopment of plastic strains in the material. It was expected that introducingnotches would induce residual strain at an early stage. However, for strains up to ∼ 4%, i.e. just exceeding the transformation length, only the smallest notch investigatedherein developed residual strain. By employing the onset of plastic deformation asa criterion for superelastic deterioration, a design window was established todetermine the deformation level at which notches become detrimental to superelasticrecovery.

Superelastic SMA–FRP composite reinforcement for concrete structures

For many years there has been interest in using fiber-reinforced polymers (FRPs) asreinforcement in concrete structures. Unfortunately, due to their linear elastic behavior,FRP reinforcing bars are never considered for structural damping or dynamic applications.With the aim of improving the ductility and damping capability of concrete structuresreinforced with FRP reinforcement, this paper studies the application of SMA–FRP, arelatively novel type of composite reinforced with superelastic shape memory alloy (SMA)wires. The cyclic tensile behavior of SMA–FRP composites are studied experimentally andanalytically. Tests of SMA–FRP composite coupons are conducted to determine theirconstitutive behavior. The experimental results are used to develop and calibrate auniaxial SMA–FRP analytical model. Parametric and case studies are performed todetermine the efficacy of the SMA–FRP reinforcement in concrete structures andthe key factors governing its behavior. The results show significant potential forSMA–FRP reinforcement to improve the ductility and damping of concrete structureswhile still maintaining its elastic characteristic, typical of FRP reinforcement.

Phenomenological modeling of the cyclic behavior of superelastic shape memoryalloys

The cyclic hysteretic behavior of superelastic shape memory alloy (SMAs) under loadingand unloading is desirable for mitigating the vibration of structures. Thus, it is necessaryto investigate the cyclic behavior of SMAs. To obtain experimental data, tests of NiTi wiressubjected to quasi-static cyclic loading are performed, with the completion of thetransformation. The cycling effects on the hysteretic loops are formulated using theleast-squares method. A modification to Lagoudas’ simplified model is proposed to includethe cycling effects. Based on the analysis of the experimental results, the revised cyclicmodel takes into consideration that the slopes of the phase transformation lines in thestress–temperature phase diagram, the residual strain and the austenite elasticmodulus are cycle-dependent. Numerical simulations show that the proposedmodel can capture the ‘classical’ characteristics of the cyclic behavior of SMA, andthe simulated loops closely agree with the experimental results for each cycle.

Rate-dependent Thermo-mechanical Modelling of Superelastic Shape-memory Alloys for Seismic Applications

Experimental tests performed on superelastic shape-memory alloys (SMAs) show a significant dependence of the stress—strain relationship on the loading—unloading rate, coupled with a not negligible oscillation of the material temperature. This feature is of particular importance in view of the use of such materials in earthquake engineering, where the loading rate affects the structural response. Motivated by this observation and by the limited number of available works on the modelling of SMAs for seismic applications, the present article addresses a uniaxial constitutive model for representing the system rate-dependent thermo-mechanical behavior of superelastic SMAs. The model is based on a single internal scalar variable, the martensite fraction, for which different rate-independent evolutionary equations in rate form are proposed. Moreover, it takes into account the different elastic properties between austenite and martensite. The whole model is then thermo-mechanically coupled with a thermal balance equation. Hence, it considers mechanical dissipation as well as latent heat and includes temperature as a primary independent variable, which is responsible for the dynamic effects. The article also provides a description for the integration, in time, of the constitutive equation and presents the solution algorithm of the corresponding time-discrete problem. Finally, results from numerical analyses are reported and the ability of the model to simulate experimental data obtained from uniaxial tests performed on superelastic SMA wires and bars at frequency levels of excitation typical of earthquake engineering is assessed.

Shape-memory alloys: modelling and numerical simulations of the finite-strain superelastic behavior

Shape-memory alloys (SMA) show features not present in materials traditionally used in engineering; as a consequence, they are the basis for innovative applications. The present work proposes a step toward the development of a computation tool to be used during the design of SMA-based devices. To reach this goal, we develop a constitutive model which reproduce the superelastic behavior of shape-memory alloys at finite strains. For an isothermal case we discuss in detail the numerical implementation within a finite-element scheme as well as the form of the algorithmically consistent tangent. To assess the viability of the proposed approach we simulate the response of some simple SMA typical structures (uniaxial test, four-point bending test) as well as an application with possibly a very high impact in different medical fields.