Pseudoelastic Shape Memory Alloys Research Articles

A time-domain modeling of systems containing viscoelastic materials and shape memory alloys as applied to the problem of vibration attenuation

It is widely known that traditional damping materials such as elastomers present a number of interesting characteristics when applied for vibration mitigation, such as inherent stability and good damping performance in relatively broad frequency bands, besides cost effectiveness. However, the behavior of those materials is highly dependent upon environmental and operational parameters such as excitation frequency and temperature. Another typical drawback is the added weight entailed by viscoelastic treatments. Especially regarding environmental influences, uncontrolled temperature variations and moisture can jeopardize the damping capacity and endurance of viscoelastic dampers. On the other hand, shape memory alloys present potential advantages in vibration damping due to their large pseudoelastic hysteresis loop in stress–strain relationship and can be used both as a damping material and structural elements in various engineering applications. Thus, it becomes apparent the convenience of combining both types of materials in such a way to explore the advantageous features of each of them. In this paper, a time-domain modeling procedure of structures containing both viscoelastic materials and shape memory alloys is addressed. The main goal is the development of a finite-element-based methodology intended to perform the analysis of engineering structures treated by passive constraining layer damping and pseudoelastic shape memory alloy wires for vibration mitigation. The viscoelastic behavior is modeled by using a four parameter fractional derivative model. To model the hysteresis response of the shape memory alloy, a phenomenological simplified model suitable for performing the parametric study of such dynamic system is used. After the discussion of various theoretical aspects, the time-domain responses are calculated for a three-layer sandwich beam containing viscoelastic materials and shape memory alloy wires and the main features of the modeling methodology are highlighted.

Passive control of the flutter instability on a two-degrees-of-freedom system with pseudoelastic shape-memory alloy springs.

A passive control of aeroelastic instabilities on a two-degrees-of-freedom (dofs) system is considered here using shape memory alloys (SMA) springs in their pseudo-elastic regime. SMA present a solid-solid phase change that allow them to face strong deformations (∼ 10%); in the pseudo-elastic regime, an hysteresis loop appears in the stress-strain relationship which in turn gives rise to an important amount of dissipated energy. This property makes the SMA a natural candidate for damping undesired vibrations in a passive manner. A 2-dofs system is here used to model the classical flutter instability of a wing section in an uniform flow. The SMA spring is selected on the pitch mode in order to dissipate energy of the predominant motion. A simple model for the SMA hysteresis loop is introduced, allowing for a quantitative study of the important parameters to optimize in view of an experimental design.

On the vibration control capability of shape memory alloy composite beams

In this paper, the vibration control capability of shape memory alloy (SMA) composite beams subjected to impulsive loads is examined. In order to simulate the SMA response, a one-dimensional constitutive model is introduced which is able to reproduce pseudo-elasticity, martensite transformation/orientation and in particular ferro-elasticity effects. A numerical algorithm is presented to solve non-linear SMA constitutive model by means of an elastic-predictor inelastic-corrector return map procedure. The equivalent single layer theory of Rayleigh–Euler–Bernoulli is used to describe displacement field of SMA laminated composite beams. Geometrical non-linearity is also considered in the von Karman sense. Considering rotary inertia effects, finite element equations of motion are developed using the Hamilton principle. Newmark and Newton–Raphson methods are utilized to obtain an incremental solution of the problem. Extensive numerical results are presented to provide an insight into the influence of pre-strain, temperature, location and thickness of SMA layers on the vibration control of SMA composite beams subjected to various blast pulses. Considering ferro-elasticity effect, results reveal the fact that SMA layers with high pre-strain have a passive vibration control capability in low temperatures and yield a better efficiency in comparison with pseudo-elastic SMA layers.

Correction: Corrigendum: Nature of hardness evolution in nanocrystalline NiTi shape memory alloys during solid-state phase transition

Due to a distinct nature of thermomechanical smart materials' reaction to applied loads, a revolutionary approach is needed to measure the hardness and to understand its size effect for pseudoelastic NiTi shape memory alloys (SMAs) during the solid-state phase transition. Spherical hardness is increased with depths during the phase transition in NiTi SMAs. This behaviour is contrary to the decrease in the hardness of NiTi SMAs with depths using sharp tips and the depth-insensitive hardness of traditional metallic alloys using spherical tips. In contrast with the common dislocation theory for the hardness measurement, the nature of NiTi SMAs' hardness is explained by the balance between the interface and the bulk energy of phase transformed SMAs. Contrary to the energy balance in the indentation zone using sharp tips, the interface energy was numerically shown to be less dominant than the bulk energy of the phase transition zone using spherical tips.

On the Fracture Toughness of Pseudoelastic Shape Memory Alloys

A finite element analysis of quasi-static, steady-state crack growth in pseudoelastic shape memory alloys is carried out for plane strain, mode I loading. The crack is assumed to propagate at a critical level of the crack-tip energy release rate. Results pertaining to the influence of forward and reverse phase transformation on the near-tip mechanical fields and fracture toughness are presented for a range of thermomechanical parameters and temperature. The fracture toughness is obtained as the ratio of the far-field applied energy release rate to the crack-tip energy release rate. A substantial fracture toughening is observed, in accordance with experimental observations, associated with the energy dissipated by the transformed material in the wake of the growing crack. Reverse phase transformation, being a dissipative process itself, is found to increase the levels of toughness enhancement. However, higher nominal temperatures tend to reduce the toughening of an SMA alloy—although the material's tendency to reverse transform in the wake of the advancing crack tip increases—due to the higher stress levels required for initiation of forward transformation.

Nature of hardness evolution in nanocrystalline NiTi shape memory alloys during solid-state phase transition

Due to a distinct nature of thermomechanical smart materials' reaction to applied loads, a revolutionary approach is needed to measure the hardness and to understand its size effect for pseudoelastic NiTi shape memory alloys (SMAs) during the solid-state phase transition. Spherical hardness is increased with depths during the phase transition in NiTi SMAs. This behaviour is contrary to the decrease in the hardness of NiTi SMAs with depths using sharp tips and the depth-insensitive hardness of traditional metallic alloys using spherical tips. In contrast with the common dislocation theory for the hardness measurement, the nature of NiTi SMAs' hardness is explained by the balance between the interface and the bulk energy of phase transformed SMAs. Contrary to the energy balance in the indentation zone using sharp tips, the interface energy was numerically shown to be less dominant than the bulk energy of the phase transition zone using spherical tips.

Strain mapping of crack extension in pseudoelastic NiTi shape memory alloys during static loading

Crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) compact tension (CT) specimens was examined during static loading. The material composition of 50.7at.% Ni (austenitic, pseudoelastic) was investigated using high-energy synchrotron X-ray diffraction. A miniature CT specimen was developed, which is small enough to allow in situ testing in a synchrotron beam line to identify phases, textures and lattice strains in front of a crack tip. Stress-induced martensite in pseudoelastic NiTi SMAs was mapped in front of the crack of a CT specimen during static loading using synchrotron radiation. The phase volume fraction and lattice microstrain results are discussed and compared with results from thermographic measurements. The Poisson effect is observed by comparing the lattice strains in the loading direction and transverse to the loading direction.

Frequency tunable vibration and shock isolator using shape memory alloy wire actuator

Launch vehicles and satellites experience severe dynamic loads during flight phases. In particular, pyroshock generated from several separation events could result in malfunctions in the electric components in the launch vehicles. Shock isolators are generally applied in order to attenuate these severe shock environments; however, these isolators could amplify the low-frequency vibration generated by the engine thrust and aerodynamic loads and reduce the payload stability. When the natural frequency of the isolator is increased in order to avoid the low-frequency vibration amplification, sufficient shock attenuation could not be obtained. Thus, the isolators used in launch vehicles need to be designed with trade-offs between the low-frequency vibration amplification and the pyroshock attenuation. This article presents a novel frequency tuning method for the isolator in order to achieve both shock attenuation performance and avoidance of the vibration amplification. Compressed mesh washer isolators using the pseudoelasticity of shape memory alloy were adopted for easier attainment of the frequency tuning with a high performance in the shock attenuation.

Modeling and transient dynamic analysis of pseudoelastic SMA hybrid composite beam

This study deals with the dynamic analysis of a continuous SMA hybrid composite beam subjected to impulse load, taking into account the instantaneous phase transformation at any time and also material nonlinearity effects, for every point along the beam for the first time. Due to the phase transformation, the equations of motion are coupled with the phase transformation’s kinetic equations of SMA wires which make the problem more complicated. The one-dimensional constitutive equation of SMA proposed by Brinson is employed to model the pseudoelastic behavior of SMA wires. A transient finite element along with an iterative incremental method is employed to investigate the dynamic response of pseudoelastic SMA hybrid composite beam and also full SMA beam. The Newmark time integration method is used in solving the dynamic finite element equations. Results show the efficiency of the proposed model and the relevant solution algorithm. Furthermore, damped response of SMA hybrid composite beam and full SMA beam is observed, which is due to the hysteresis behavior of SMAs. In the case of full SMA beam, the phase transformation is considered for every point along the length and also through the thickness of the beam. A result shows that the through-the-thickness variation of the stress is nonlinear due to the material nonlinearity. Finally, several numerical examples upon the effect of amplitude of the loading, different boundary conditions and thermal loading have been analyzed.

Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys

In the present work we show how different oxygen (O) and carbon (C) levels affect fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation fatigue (up to cycle numbers >108) and scanning electron microscopy (for investigating microstructural details of crack initiation and growth) to study fatigue behavior. High cycle fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of structural fatigue of pseudoelastic NiTi shape memory alloys.

A robust model of pseudoelasticity in shape memory alloys

SUMMARYA model of pseudoelasticity in shape memory alloys is developed within the incremental energy minimization framework. Three constitutive functions are involved: the Helmholtz free energy and rate‐independent dissipation that enter incrementally the minimized energy function, and the constraint function that defines the limit transformation strains. The proposed implementation is based on a unified augmented Lagrangian treatment of both the constitutive constraints and nonsmooth dissipation function. A methodology for easy reformulation of the model from the small‐strain to finite‐deformation regime is presented. Finite element computations demonstrate robustness of the finite‐strain version of the model and illustrate the effects of tension–compression asymmetry and transversal isotropy of the surface of limit transformation strains. Copyright © 2012 John Wiley & Sons, Ltd.

Finite element analysis of the plane strain crack-tip mechanical fields in pseudoelastic shape memory alloys

The plane strain mechanical fields near a stationary crack tip in a pseudoelastic shape memory alloy (SMA) are analyzed via the finite element method. The small scale transformation assumption is employed for the calculations using displacement boundary conditions on a circular region that encloses the stress-induced phase transformation zone. The constitutive law used adopts the classical rate-independent small strain flow theory for the evolution equations of both the transformation and plastic strains. Results on the size and shape of the stress-induced transformation and plastic zone formed near the stationary crack are obtained and a fracture toughness criterion based on the J-integral is discussed in view of the observed path-dependence of J. Moreover, the obtained results are discussed in relation to results for stationary cracks in conventional elastic–plastic materials.

Non-linear dynamic analysis of a sandwich beam with pseudoelastic SMA hybrid composite faces based on higher order finite element theory

In the present work a non-linear dynamic response of a continuous sandwich beam with SMA hybrid composite face sheets and flexible core is analyzed taking into account the phase transformation and also the material non-linearity effects, for every point along the face sheets. The one-dimensional constitutive equation of SMA proposed by Brinson is employed. Equations of motion are derived using Hamilton’s principle and a new finite element is proposed based on a higher order sandwich panel theory. Due to the phase transformation, the equations of motion are coupled with the phase transformation’s kinetic equations of SMA wires. A new finite-element-based approach along with an iterative incremental method is developed to study the dynamic response of sandwich beam with SMA hybrid composite face sheets and flexible core. A damped response of the sandwich beam is observed, which is due to the hysteresis behavior of SMA wires. The influence of the SMA wires on the vibration suppression related to the resonance phenomena of a sandwich beam as well as the effect of the through thickness location of the SMA wires inside the composite face sheets and also the effect of different boundary conditions on the dynamic response are analyzed.

A mode I fracture analysis of a center-cracked infinite shape memory alloy plate under plane stress

The problem of a center plane crack in an infinite, thin, pseudoelastic Shape Memory Alloy (SMA) plate subjected to an in-plane uniform tensile stress at infinity is analyzed. The analysis follows closely the Dugdale–Barenblatt model developed for conventional metals. It is found for low remote stress values—less than a critical value—that the SMA is not fully transformed in the vicinity of a crack tip. Closed form expressions for the size of the partial transformation zone, crack opening displacement and J-integral are given for this case. For remote stress levels above the critical value, the fully-transformed material near a crack tip is assumed to yield plastically. The sizes of the transformed (both partially and fully) and plastic regions are numerically evaluated by solving a system of integral equations and their sensitivity to the transformation characteristics (i.e., maximum transformation strain and temperature) is determined. Moreover, a relationship between the J-integral and the crack-tip opening displacement is derived. The results obtained are important in understanding the effect of stress-induced phase transformation in the fracture behavior of SMAs in the presence of static cracks, and subsequently in formulating conditions for initiation of crack propagation.

Seismic control of concrete shear wall using shape memory alloys

Shape memory alloy is a new functional material, which has found increasing applications in many engineering areas. Research efforts have been extended to use shape memory alloy in controlling civil structures. In this article, both pseudoelastic and shape memory effect properties of Nitinol shape memory alloy are implemented for control of concrete shear wall structure when subjected to seismic excitation. First, the behavior of the concrete shear walls equipped with shape memory reinforcements in its pseudoelastic characteristics is assessed, and the results are compared with the behavior of the ordinary concrete shear wall utilizing steel reinforcements. A finite element time history analysis is employed, and the results indicate that the proposed enhancement of the concrete shear wall with shape memory alloy reinforcements is able to reduce the permanent residual strain in the structure and hence attain reasonable improvement in seismic response. Then, the memory effect characteristics of shape memory alloys by imposing pretension in SMA rebar in concrete shear wall are evaluated. The results show that the concrete shear wall equipped with pretension rebars is much stiffer than either the concrete shear wall equipped with pseudoelastic shape memory alloy rebars or the ordinary concrete shear wall with steel.

Nonlinear Model of Pseudoelastic Shape Memory Alloy Damper Considering Residual Martensite Strain Effect

Recently, there has been increasing interest in using superelastic shape memory alloys for applications in seismic resistant-design. Shape memory alloys (SMAs) have a unique property by which they can recover their original shape after experiencing large strains up to 8% either by heating (shape memory effect) or removing stress (pseudoelastic effect). Many simplified shape memory alloy models are suggested in the past literature for capturing the pseudoelastic response of SMAs in passive vibration control of structures. Most of these models do not consider the cyclic effects of SMA's and resulting residual martensite deformation. Therefore, a suitable constitutive model of shape memory alloy damper which represents the nonlinear hysterical dynamic system appropriately is essential. In this paper a multilinear hysteretic model incorporating residual martensite strain effect of pseudoelastic shape memory alloy damper is developed and experimentally validated using SMA wire, based damper device. A sensitivity analysis is done using the proposed model along with three other simplified SMA models. The models are implemented on a steel frame representing an SDOF system and the comparison of seismic response of structure with all the models is made in the numerical study.

Adaptive resetting of SMA actuators

Shape memory alloys (SMAs) have essential advantages compared with conventional actuators, in particular their high-power density and their silent mode of operation. However, this material has not yet gained acceptance in technical applications. The main reasons are the missing simulation tools and a lack of knowledge of materials as well as the companies’ uncertainty as to how to handle SMA. The resetting of the SMA element to generate a repeatable movement is often a defined problem. In this context, reset springs made of steel are conventional solutions, although their characteristics are a disadvantage. To reach a high level of power output and hence a high degree of efficiency, a reduction of the preload is necessary. A solution for this problem is an adaptive resetting. One main possibility to generate an adaptive resetting is given by the agonist–antagonist principle where two SMA elements work against each other. Here, the reset force can be applied if necessary. The advantage of this type of design is that a conventional return spring or a mechanical brake for clamping the position (electrically operated) is not necessary. Another possibility for adaptive resetting is to change the spring characteristics of a pseudoelastic SMA element by heating. The aim of this publication is to sum up the different possibilities of adaptive resetting of shape memory actuators. It also provides methods and the knowledge to support the development process of such resetting principles. The development of these methods is based on the analysis of different designs and requirements. Based on the experimental results, a conclusion of the possibilities is given.

Processing and characterization of braided NiTi microstents for medical applications

AbstractIn the present exploratory study we investigate the processing of NiTi microstents, which were produced by braiding. We give a short description of the braiding of microstents using 0.1 mm pseudoelastic NiTi wires. We compare our braided microstents with microstents which were produced, using the traditional tube making and laser cutting technology. We outline why braiding may avoid some of the inherent problems related with the traditional procedure and highlight its cost effectiveness. We report on the thermal, chemical and mechanical properties of braided microstents. Most importantly, we provide information on the elementary damage processes which characterize displacement controlled fatigue loading up to 30·106 cycles. The results presented in this work provide information on a research field, which links the engineering manufacturing field braiding (as a potential new processing technology for NiTi stents) with state‐of‐the‐art materials science techniques for the characterization of pseudoelastic NiTi shape memory alloys.

Finite-Element Simulation of the Anti-Buckling-Effect of a Shape Memory Alloy Bar

Shape memory alloys (SMA) are characterized by an intricate stress-strain curve modified by temperature, posing thermomechanically coupled problems. A model able to address this feature is the Muller-Achenbach-Seelecke (MAS) model which had been ported into the user material interface in the finite-element (FEM) simulation software ABAQUS. The literature on this model mainly focuses on pseudo-elasticity of SMA at elevated temperature. We address a numerical investigation in the low-temperature pseudo-plastic regime. The present publication deals with the little-known anti-buckling effect which occurs in de-twinned and pre-bent martensitic bars under axial compression. It was experimentally demonstrated by Urushiyama et al. (JSME (The Japan Society of Mechanical Engineers) Int. J. Ser. A, Solid Mech. Mater. Eng., 2003, 46(1), p 60-67). This study reveals that the origin of this effect roots in an interplay of inhomogeneous stress states and mechanically induced twin-twin phase transformations. The proper explanation of the anti-buckling effect can be inferred from the explicit knowledge of the martensitic phase composition of the bar during the process. We show that the MAS model is capable to resolve this matter in detail, hence addressing the reliability of this particular model also in the pseudo-plastic regime of SMA. The study thereby implies that the MAS model is an excellent modeling tool for the analysis of complex, thermomechanically coupled processes.

Mathematical modelling of rate-independent pseudoelastic SMA material

The paper presents a procedure for the formulation of constitutive equations for rate-independent pseudoelastic SMA material models. The procedure applies a rheological scheme representing mechanical properties of the material. An additive decomposition of strains into two parts is proposed. The first part describes strains of a perfectly elastic body while the second part may be represented by a combination of a rigid perfectly elastic body and a rigid perfectly plastic body. It is demonstrated that the key problem of formulation of constitutive relationships is to derive the 1st order differential equation with respect to the tensor describing the second part of the strain field. This equation may be obtained in explicit form starting from the variational inequalities defining non-elastic parts of rheological model. The uniqueness of the obtained differential equation has been proved. A numerical implementation of the constitutive relationships of SMA material was done through the user subroutine module VUMAT within the FE commercial code ABAQUS/Explicit. As an example we analyzed the problem of vibration of a simple 3D structure made of SMA.