Pseudoelastic Shape Memory Alloys Research Articles

Application and potential of shape memory alloys for dowel-type connections in timber structures

ABSTRACT The present study aims at identifying and analyzing challenges and prospects related to the use of pseudoelastic shape memory alloys for well-established double shear connections in timber engineering. The unique behavior of shape memory alloys will allow to bear large reversible deformations without loss of cohesion. Thereby, the main objective of the present work is to study the behavior of a connection that is able to stay completely reversible even upon very large deformations. In this context, the stiffness of the connection as well as functional and structural integrity and damping behavior have been improved by stepwise adaption of the detailing. Proof of concept is provided, and current limitations are discussed, based on the application of bolts made from a commercially available shape memory alloy.

Quantification of Shape Memory Alloy Damping Capabilities Through the Prediction of Inherent Behavioral Aspects

In this work the time response of pseudoelastic Shape Memory Alloy (SMA) wires is numerically simulated. In particular, the effect of their operation under partial phase transformation is investigated and quantified. Additionally, the effect of the thermomechanical coupling under cyclic operation is evaluated both under adiabatic and natural convection conditions. To this end, proper finite element models are generated considering a low-frequency harmonic sinusoidal excitation. The effect of the partial transformation and thermomechanical coupling on the operation of the SMA is highlighted by comparison with respective results acquired by finite element models which neglect the modified hardening function that accounts for the partial loops. The results suggest that the latent heat produced during forward transformation highly affects the energy dissipation potential of SMAs. The hardening behavior also affects the transformation evolution and therefore impacts the amount of heat generation/absorption. Although both phenomena, when accounted for, result in the prediction of an altered hysteresis area and consequently different dissipation capabilities, the scope of the paper is to highlight their importance on the calculated values of dissipated energy and loss factor. These quantities are of particular interest since they constitute crucial design parameters for the development of smart dampers employing SMA materials.

Static finite element analysis of three-closed box thin-wall beam based on Pseudo-elastic SMA hybrid composite material ANSYS

The Auricchio model based on shape memory alloy and the finite element analysis software ANSYS Workbench firstly simulated the pseudo-elastic characteristics of SMA bar during the stretching cycle loading and unloading. Secondly, based on ANSYS Material Designer module, a SMA/glass/epoxy resin composite Material model was established and its Material parameters were obtained. ANSYS ACP module was used to establish the finite element model of pseudo-elastic SMA hybrid composite box thin-walled beam. Finally, the static response of SMA composite box thin-walled beam under transverse load is studied, and the influences of SMA, layout Angle, width to height ratio and volume content of SMA layer on the static response of box girder are discussed. The results show that the transverse displacement of the free end of the thin-walled beam increases with the increase of the laying Angle whether there is SMA fiber or not. After embedding SMA, the transverse displacement of the free end of the box thin-wall beam of hybrid composite was smaller. The transverse displacement of the free end of the thin-walled beam with the change curve of the lay-up Angle under the two con Figurations is consistent and basically consistent. At the same laying Angle, the transverse displacement of the free end of SMA hybrid box thin-walled beam decreases with the increase of the width to height ratio and the volume content of SMA monolayer.

Thermomechanical modeling of nonlinear internal hysteresis due to incomplete phase transformation in pseudoelastic shape memory alloys

This paper presents a thermomechanical model for pseudoelastic shape memory alloys (SMAs) accounting for internal hysteresis effect due to incomplete phase transformation. The model is developed within the finite-strain framework, wherein the deformation gradient is multiplicatively decomposed into thermal dilation, rigid body rotation, elastic and transformation parts. Helmholtz free energy density comprises three components: the reversible thermodynamic process , the irreversible thermodynamic process and the physical constraints of both. In order to capture the multiple internal hysteresis loops in SMA, two internal variables representing the transition points of the forward and reverse phase transformation, $$\phi _s^f$$ and $$\phi _s^r$$ , are introduced to describe the incomplete phase transformation process. Evolution equations of the internal variables are derived and linked to the phase transformation. Numerical implementation of the model features an Euler discretization and a cutting-plane algorithm. After validation of the model against the experimental data, numerical examples are presented, involving a SMA-based vibration system and a crack SMA specimen subjected to partial loading–unloading case. Simulation results well demonstrate the internal hysteresis and free vibration behavior of SMA.

Ballistic Impact Behaviour of Glass/Epoxy Composite Laminates Embedded with Shape Memory Alloy (SMA) Wires.

This paper aims to estimate the enhancement in the energy absorption characteristics of the glass fiber reinforced composites (GFRP) by embedding prestrained pseudo-elastic shape memory alloy (SMA) that was used as a secondary reinforcement. The pseudo-elastic SMA (PE-SMA) embedded were in the form of wires and have an equiatomic composition (i.e., 50%–50%) of nickel (Ni) and titanium (Ti). These specimens are fabricated using a vacuum-assisted resin infusion process. The estimation is done for the GFRP and SMA/GFRP specimens at four different impact velocities (65, 75, 85, and 103 m/s) using a gas-gun impact set-up. At all different impact velocities, the failure modes change as we switch from GFRP to SMA/GFRP specimen. In the SMA/GFRP specimen, the failure mode changed from delamination in the primary region to SMA-pull out and SMA deformation. This leads to an increase in the ballistic limit. It is observed that energy absorbed by SMA/GFRP specimens is higher than the GFRP specimens subjected to the same levels of impact energy. To understand the damping capabilities of SMA embedment, vibration signals are captured, and the damping ratio is calculated. SMA dampens the vibrations imparted by the projectile to the specimen. The damping ratio of the SMA/GFRP specimens is higher than the GFRP specimens. The damping effect is more prominent below the ballistic limit when the projectile got rebounded (65 m/s).

Experimental free vibrations test of steel braced frames reinforced with NiTi shape memory alloy wires

Shape memory alloys (SMA) have just recently made their way into structural design, and through studies have proved to be quite effective in increasing a structures strength and performance. Earthquakes are one of the most chaotic natural disasters that occur around the world, significantly impacting infrastructure to the point where cities lose their ability to function as bridges and hospitals are destroyed. The main objective of this paper is to investigate the natural behaviour of steel braced frames reinforced with pseudoelastic (PE) NiTi SMA wires. A free vibrations test was conducted to determine the natural frequency and damping ratio of two different systems. The tests of the steel and PE SMA frames showed that the stiffness of the brace does not affect the natural frequency of the system but rather, the connection of the system to the rigid floor controls the natural behaviour. Findings of this research are expected to add valuable knowledge to the field of seismic retrofitting of structures and widen the potential applications of the SMA in the structural engineering field.

Compression after high-velocity impact behavior of pseudo-elastic shape memory alloy embedded glass/epoxy composite laminates

The compression after high-velocity impact (CAI) behavior of pseudo-elastic shape memory alloy (PE-SMA) embedded glass/epoxy composite laminates manufactured via vacuum-assisted resin infusion (VARI) technique was investigated experimentally. High-velocity impact tests were performed on the glass/epoxy composites using an in-house build gas-gun set-up at four different velocities (70, 85, 95, and 105 m/s). To elucidate, the series of damage events undergone on the composites under the high-velocity impact, a high-speed camera was used. Quasi-static compression-after-impact (CAI) tests were performed to evaluate the damage tolerance of different composites subjected to high-velocity impact. Real-time damage progression during the CAI as well as the extent of damage was characterized by using a 2D Digital Image Correlation (DIC) technique. Three different types of embedment configurations of PE-SMA wire, namely, with length variation (35, 70, and 150 mm) and also with and without anchors, were used. The impact test results show that the ballistic limit of anchored PE-SMA composites with an embedment length of 35 mm is 72.72% higher than that of the homogeneous glass/epoxy composites. CAI results indicate that the PE-SMA based composites have better damage tolerance with respect to the homogeneous ones at 70 m/s. In contrast, at higher velocities, the delamination area decreases in the homogenous glass/epoxy composites due to more localized damage and present a higher compressive strength.

A modified one-dimensional constitutive model of pseudoelastic SMAs and its application in simulating the force–deformation relationship of SMA helical springs

The Graesser–Cozzarelli constitutive model of shape memory alloys (SMAs) has been widely applied in seismic engineering due to its concise functions with well-defined material parameters. However, this model has some limitations, e.g. it fails to model the hardening behavior of SMAs under relatively large strain amplitudes and the effects of ambient temperature. In this paper, a modified Graesser–Cozzarelli constitutive model of pseudoelastic SMAs is proposed, through consideration of the changes of the elastic moduli of SMAs induced by different martensite fractions, the effects of ambient temperature on the pseudoelastic characteristics, the hardening behavior and the phenomenon of degradation of the pseudoelasticity of SMAs under large strain amplitudes. Furthermore, the modified model was validated by the results of experiments on SMA wires under different strain amplitudes and ambient temperatures. The modified model was then utilized to simulate the force–deformation relationship of SMA helical springs for the first time. In the procedure used to simulate the springs, the torsional and bending moments on the cross-section of the coiled wire of the springs were considered simultaneously, and the effect of the pitch angle was also included. Moreover, the spring simulation procedure was verified by tensile tests of SMA helical springs with complex loading patterns and existing simulated results of the effects of ambient temperature. The research results indicate that the spring simulation procedure based on the modified model can simulate the force–deformation relationship of the springs under different complex loading conditions and ambient temperature reasonably well. Furthermore, the effects of the spring index and the initial height of the springs on the characteristics of the force–deformation relationship were investigated in detail.

Nonlinear free vibration and damping analysis of a microbeam with pseudoelastic shape memory alloy layer based on the modified couple stress theory

The aim of this investigation is to evaluate the size effects on the nonlinear free vibration of the sandwich composite microbeam with an extensible shape memory alloy layer in the midplane. A one-dimensional constitutive model is considered to simulate the pseudoelastic behavior of the shape memory alloy layer. The governing equation of motion is derived using the Euler–Bernoulli beam theory together with the modified couple stress theory through the Hamilton’s principle. Midplane stretching and phase transformation of the shape memory alloy which are sources of nonlinearity were considered, and a numerical solution method is presented. A damped response of the sandwich composite microbeam is observed because of the hysteresis behavior of the extensible shape memory alloy layer in the midplane. Results are appraised by comparing with the available literature. The influence of material length scale, temperature, and initial velocity on the loss factor and other pivotal vibrational behavior is evaluated. Results show that increasing the material length scale to thickness ratio has a decreasing effect on damping capacity.

On the Importance of Structural and Functional Fatigue in Shape Memory Technology

The present work provides a brief overview on structural and functional fatigue in shape memory alloys (SMAs). Both degenerative processes are of utmost technological importance because they limit service lives of shape memory components. While our fundamental understanding of these two phenomena has improved during the last two decades, there are still fields which require scientific attention. NiTi SMAs are prone to the formation of small cracks, which nucleate and grow in the early stages of structural fatigue. It is important to find out how these micro-cracks evolve into engineering macro-cracks, which can be accounted for by conventional crack growth laws. The present work provides examples for the complexity of short crack growth in pseudoelastic SMAs. The importance of functional fatigue has also been highlighted. Functional fatigue is related to the degeneration of specific functional characteristics, such as actuator stroke, recoverable strain, plateau stresses, hysteresis width, or transformation temperatures. It is caused by the accumulation of transformation-induced defects in the microstructure. The functional stability of SMAs can be improved by (1) making phase transformations processes smoother and (2) by improving the material’s resistance to irreversible processes like dislocation plasticity. Areas in need of further research are discussed.

Optimal design of a hanging truss with shape memory alloy wires

In this study, we discuss the dynamic problem of a column-type hanging truss structural system with pseudo-elastic shape memory alloy (SMA) bracing wires. In the case where the sectional area values of the bracing SMA wire members are small enough to be negligible, it is close to the situation where there are no braces. In the case where the sectional area values of the bracing SMA wire members are large enough not to be negligible, the vibration amplitude of the peripheral end apparatus is suppressed from the deformation point of view. In addition, energy attenuation efficiency is improved with larger sectional area values because of the hysteretic characteristic of SMA. Small sectional area values of the bracing SMA wire members near the support ceiling or peripheral end are beneficial to vibration transmission reduction. These findings indicate that the placement and sectional area values of SMA wire members are both significant from the viewpoint of suppression of influence of the support ceiling vibration of the hanging truss. In this study, we obtain the optimal sectional area values of the SMA wire bracing members for the objectives of vibration isolation and attenuation. We discuss influences of different vibration conditions on the optimal solutions.

Modeling and experimental validation of the hysteretic dynamics of shape memory alloy springs

Abstract In this work, the hysteretic dynamics of a pseudoelastic shape memory alloy (SMA) spring is modeled and tested. A constitutive model of SMA spring is constructed by combining the theory of mechanical spring and the derived SMA constitutive relation. The SMA spring recovery force is modeled by employing the Euler–Lagrange equation based on a non-convex potential function. A macroscopic differential model is formulated to describe the hysteretic dynamics and the pseudoelasticity of the SMA spring. A theoretical method for estimating the equivalent stretch ratio at a specific temperature of SMA spring is given. Numerical simulations were presented in studying the influence of the stretch ratio at a temperature of 50, 70, and 90°C. The experimental tests are performed and presented together with their numerical counterparts. The comparison is made between numerical and experimental results to validate the proposed model for SMA spring.

An integrated model to predict the microstructure evolution and the mechanical behaviour of a two-phases pseudo-elastic SMA

Shape memory alloys (SMAs) are more and more used in many fields of mechanics and medicine. The reason for the success is the ability to recover the initial shape also after high values of deformation. During the deformation, the SMAs are able to change the structure from an initial one, often named as austenite, to a final one, named martensite. This transformation takes place at low temperature, and no recrystallization occurs, but the bulk of grains is interested by a structure changing. For that, this phenomenon cannot be qualified as a traditional structure transformation, but it is much more correct to talk about the transition of phase. The austenite-martensite transition is reversible, allowing a perfect recovery of the shape shown in the austenite phase.In this work, a model able to describe the microstructure evolution related to the mechanical behaviour of a SMA has been proposed. Results of FEM simulations have been compared with experimental ones both in terms of microstructure modifications and mechanical behaviour.

The effects of shape-setting on transformation temperatures of pseudoelastic shape memory alloy springs

Abstract Since shape memory alloy (SMA) wires can hardly ever be reliably employed under compressive loadings, SMA springs are developed as axial actuators with the ability of withstanding both tension and compression. In this paper, shape memory alloy helical springs are produced by shape-setting two types of wires: one with shape memory effect (SME) and the other with pseudoelasticity (PE) at the ambient temperature. Phase transformation temperatures of the produced springs are measured by differential scanning calorimetry (DSC), and the influences of effective parameters including cold work, heat treatment temperature and duration, and cooling rate are investigated on transformation temperatures of the products. The results show that phase transition temperatures of the fabricated springs can be tuned by performing cold work and by adjusting temperature and duration of the conducted heat treatment as well as the subsequent cooling rate. It is found that transformation temperatures of the springs fabricated using the SME wire increase as the heat treatment temperature increases. However, for samples manufactured using PE wire, transformation temperatures first increase and then decrease with the increase in the heat treatment temperature. An increase in the cooling rate leads to a decrease in the austenite final temperature (Af), and an increase in the extent of cold work leads to the increase in transformation temperatures especially Af.

Linear-superelastic metals by controlled strain release via nanoscale concentration-gradient engineering

The elastic strain limit of most metals are less than 0.2% except for whiskers or freestanding nanowires whose elastic strain limit could reach 4–7%. Ferroelastic metals such as shape memory alloys (SMAs) do exhibit giant recoverable strains (up to ∼13%). However, the strong non-linear pseudo-elasticity of SMAs leads to mechanical instability. By taking advantage of the strong composition-dependent critical stress for stress-induced martensitic transformation (MT) in NiTi SMA, this work demonstrates a novel design approach to achieve linear-superelasticity (∼4.6%) and ultralow modulus (8.7 GPa) of a NiTi single crystal. These unprecedented properties are realized through precisely controlling strain release during the MT via nanoscale concentration-gradient engineering. The computer simulation results and theoretical analyses reveal that the stress–strain behavior of NiTi and other SMAs can be regulated effectively by fine-tuning the concentration gradient. This may open a new avenue for the design of next generation ferroelastic materials.

Theoretical study of tensegrity systems with tunable energy dissipation

This letter centers on the study of novel structures with tunable energy dissipation based on the tensegrity system known as the D-bar. Through an analytical approach, it is found that the geometry and topology of D-bar systems can be designed to tune the ratio of the stress experienced by the overall D-bar systems to that of their internal members. Pseudoelastic shape memory alloy (SMA) wires are then incorporated as the tensile members of the D-bars to introduce energy dissipation capabilities, and the formulas to determine the energy dissipation of these SMA-based D-bars are provided. The global–local stress formulas are used to design the properties of the D-bars such that a target maximum stress which the SMA wires experience in a loading/unloading cycle is obtained. Our results show that this target maximum stress can be tuned such that it is large enough to allow the SMA wires to undergo full pseudoelastic phase transformations and thus provide maximum energy dissipation, while being below their failure stress value.

Novel insight into the strain-life fatigue properties of pseudoelastic NiTi shape memory alloys

The whole low-to-high cycle fatigue behavior of a pseudoelastic NiTi SMA was analyzed. Global and local strains measurements were carried out by an extensometer and digital image correlation. Significant differences were observed at the two scales, due to the localized nature of stress-induced transformations. Local mechanisms play a crucial role on fatigue damage and have a direct effect on the Z-shaped strain-life curves observed in previous works. The effective nature of this unusual behavior was revealed by systematic analysis of local strain. Finally, strain decomposition was made and data were analyzed by a recent modified Coffin–Manson approach.

Gradient-enhanced thermomechanical 3D model for simulation of transformation patterns in pseudoelastic shape memory alloys

Stress-induced martensitic transformation in polycrystalline NiTi under tension often proceeds through formation and propagation of macroscopic phase transformation fronts, i.e., diffuse interfaces that separate the transformed and untransformed domains. A gradient-enhanced 3D finite-strain model of pseudoelasticity is developed in this work with the aim to describe the related phenomena. The underlying softening response is regularized by enhancing the Helmholtz free energy of a non-gradient model with a gradient term expressed in terms of the martensite volume fraction. To facilitate finite-element implementation, a micromorphic-type regularization is then introduced following the approach developed recently in the 1D small-strain context. The complete evolution problem is formulated within the incremental energy minimization framework, and the resulting non-smooth minimization problem is solved by employing the augmented Lagrangian technique. In order to account for the thermomechanical coupling effects, a general thermomechanical framework, which is consistent with the second law of thermodynamics and considers all related couplings, is also developed. Finite-element simulations of representative 3D problems show that the model is capable of representing the loading-rate effects in a NiTi dog-bone specimen and complex transformation patterns in a NiTi tube under tension. A parametric study is also carried out to investigate the effect of various parameters on the characteristics of the macroscopic transformation front.

Effects of Temperature on Fatigue Crack Propagation in Pseudoelastic NiTi Shape Memory Alloys

The effects of temperature on fatigue crack propagation in a pseudoelastic NiTi shape memory alloy (SMA) were analyzed. Single edge crack specimens were used and near crack tip displacements were captured by in situ digital image correlation (DIC). The effective stress intensity range was estimated from displacement data by a fitting procedure involving the William’s solution. Stress intensity range was also estimated using a recent analytical model that accounts for the complex thermo-mechanical response of SMAs. In addition, comparisons with the linear elastic fracture mechanics (LEFM) solution were made. Results revealed an important role of temperature on crack propagation rate, that is, the higher the temperature the longer the fatigue life. However, it was demonstrated that these effects are attributed to the marked influence of temperature on near crack tip fields and, consequently, on the effective stress intensity range. This trend is correctly captured by the DIC method as well as by the analytical model. On the contrary, LEFM does not consider the effects of temperature and, consequently an apparent change in the material properties is observed. Therefore, a novel approach is proposed to analyze crack propagation in SMAs, where both stress and temperature are considered as significant loading parameters.

Compression after ballistic impact response of pseudoelastic shape memory alloy embedded hybrid unsymmetrical patch repaired glass-fiber reinforced polymer composites

This paper investigated the influence of embedding pseudoelastic shape memory alloy within the external bonded patch made up of glass fibers on the compression after impact response of adhesively bonded external patch repaired glass/epoxy composite laminates. Unsymmetrical patch repair was employed in the current studies. Three innovative pseudoelastic shape memory alloy configurations (straight wired, meshed and anchored) were embedded inside the patch and the changes in high-velocity impact response and damage tolerance at four impact velocities (70, 85, 95, 105 m/s) were compared with the conventional glass/epoxy (glass fiber-reinforced polymer) patch. Anchored specimens showed the best response by improving the compressive strength by 25% under non-impacted conditions and restoring it by 88%, 77%, 29%, and 28% at the impact velocity of 70, 85, 95, and 105 m/s, respectively, in comparison to the conventional normal specimens.