Shape Memory Alloy Components Research Articles

Effect of axial preloads on torsional behavior of superelastic shape memory alloy tubes – experimental investigation and simulation/predictions of intricate inner loops

Shape memory alloy (SMA) devices (utilizing wires/rods, springs or tubes) engage unique superelastic/pseudoelastic (SE) phenomenon for large stroke recovery, energy dissipation and damping applications. In particular, SMA components in torsion are also generally subjected to some pre-tension loads and hence understanding their combined effects on the torsional response is vital. In this work, some intricate internal loop and outer loop responses (large twists > 1500°) of SMA tubes under different twisting-untwisting scenarios are investigated to understand the effects of static pre-load on torsional responses. Key SMA hysteretic features like sink point memory (SPM) and return point memory (RPM) have not been well understood under torsion (with and without axial loads) have been investigated here. Further, a modified two variant thermodynamic Preisach modeling approach is proposed to simulate responses of twisted tubes with varying extents of twist and different axial loads. The central principle here is the use of a Gibbs Potential framework to separate the dissipative and thermoelastic parts of the SE responses using well established thermodynamic principles and then utilizing a non-ideal switching type disjunctive Preisach model to fit the nonlinear hysteretic dissipative section of the SE response. By using the outer loop response for model calibration, other complex loading unloading scenarios leading to intricate internal loops and effects of axial loads on these can be predicted. It is also shown that addition of a single interior loop detail for model calibration significantly enhances prediction of other more intricate internal loops. This approach dramatically simplifies the experimental data needed for the simulation of SMA components even under complex loading.

Wire-based electron beam additive manufacturing of NiTiNb shape memory alloy: Microstructure, phase-transformation behavior and mechanical properties

NiTiNb shape memory alloy (SMA) is considered to have great potential in aerospace due to its wide phase-transformation hysteresis. Here, we innovatively used electron-beam additive manufacturing (EBAM) to fabricate a NiTiNb SMA thin-walled structure. The phase-transformation temperature, microstructure evolution, and mechanical properties were investigated in detail. The as-built NiTiNb SMA comprised a NiTi matrix and different morphologies of β-Nb phase at room temperature. The microstructure homogeneity and mechanical properties of NiTiNb prepared by EBAM significantly improved compared with those of NiTiNb powder-based AM technology. Our work provided a promising method for the preparation of defect-free and microstructure homogeneous NiTiNb SMA by AM. This method further stimulated the potential of additively manufactured SMA components for industrial applications.

Fabrication of FeMnSi-based shape memory alloy components with graded-microstructures by laser powder bed fusion

This work demonstrates the tailoring of the microstructure of two FeMnSi-based shape memory alloys during laser powder bed fusion. Significant variations in the microstructure of the parts can be induced by modifying the scanning speed or the scanning strategy during the process. Specifically, the volume phase fractions, texture, and grain size and morphology change with the applied processing parameters as the thermal history experienced by the material is modified. Additionally, the generally undesired Mn evaporation during laser melting can be deliberately exploited to manipulate the phase composition in the final microstructure. Taking advantage of this, graded-microstructure samples with tailored amounts of the two phases (bcc-δ and fcc-γ) and distinct microstructure at different locations can be fabricated. A good combination of pronounced shape memory effect and ultrahigh strength can be achieved in this way. The findings of this study offer valuable insights that can drive future research and advancements in additive manufacturing, opening exciting opportunities for the fabrication of multifunctional and high performance materials.

Microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with different prestrain levels

Ni47Ti44Nb9 shape memory alloy (SMA) is a promising material in the aerospace field due to its wide transformation hysteresis. The application of shape memory effect depends on multistep thermomechanical loading, viz., low-temperature deformation and subsequent heating to recovery. Low-temperature deformation prestrain plays a pivotal role in shape memory properties tailoring of SMA components. However, microstructure evolution and deformation mechanisms of Ni47Ti44Nb9 SMA subjected to various prestrain levels are still unclear. To this end, microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with prestrain levels of 8% - 16% at -28 °C (Ms + 30 °C) were investigated. The results demonstrate that the stress-strain curve of the specimen exhibits four distinct stages at a maximal prestrain of 16%. Whereas stage II and stage III end at prestrains of ∼8% and ∼12%, respectively. In stage II, the stress-induced martensitic transformation is accompanied by the dislocation slip of the NiTi matrix and β-Nb inclusions. In stage III, in addition to the higher density of dislocations and further growth of stress-induced martensite variants (SIMVs), (0 0 1) compound twins are introduced as a result of the (0 0 1) deformation twinning in stress-induced martensite. More {2 0 -1} martensite twins are gradually introduced in stage IV. Correspondingly, after subsequent unloading and heating, a higher density of {1 1 4} austenite twins form in the specimen with a larger prestrain of 16%. With increasing prestrain from 8% to 16%, the recoverable strain εreT upon heating increases first and then decreases. The εreT obtains a maximum of 7.03% at 10% prestrain and decreases to 6.17% at 16% prestrain. The increase of εreT can be attributed to the formation of new SIMVs, the further growth of existing SIMVs, and the recoverable (0 0 1) compound twins. While the decrease of εreT is mainly associated with the irrecoverable strain by {2 0 −1} martensite twins. The effect of β-Nb inclusions on the evolution of SIMVs is also found herein that deformed β-Nb inclusions can significantly hinder the growth and recoverability of adjacent stress-induced martensite.

Influence of the interlayer temperature on structure and properties of CMT wire arc additive manufactured NiTi structures

A potential method for producing intricate Shape Memory Alloy (SMA) structures for uses like actuators and vibration dampers is wire arc additive manufacturing (WAAM). However, excessive heat build-up during the multi-layer deposition procedure in WAAM can result in process instability and departures from the intended mechanical qualities and dimensions. To address these problems, the interlayer temperature was optimized for printing on NiTi walls in this work. Without interlayer temperature control (WITC), 200 °C, and 400 °C were used to create three samples in order to study the impacts on the porosity level, morphology, and mechanical characteristics of the NiTi walls generated by WAAM. The Shape memory behavior of the manufactured NiTi walls was further examined through shape recovery investigations utilizing hot plate actuation. The findings demonstrated that regulating the interlayer temperature below 300 °C, preferably at 200 °C, enhanced the mechanical characteristics, decreased the porosity content, and enhanced the microstructure of the NiTi walls. The development and improvement of WAAM techniques for the manufacture of premium NiTi SMA components can benefit greatly from these findings.

SMA-braced steel frames influenced by temperature: Practical modelling strategy and probabilistic performance assessment

This study presents a practical and efficient modelling approach for shape memory alloy (SMA)-braced steel frames under various temperatures, and examines their probabilistic performance affected by temperature. A one-dimensional constitutive model is developed in OpenSees to capture the superelasticity (SE), partial SE, shape memory effect (SME), and permanent yielding behaviors of SMA elements subjected to varying temperatures. The proposed modelling approach is partly validated against an experimental study newly conducted in the present work. The material-level investigation is followed by a comprehensive probabilistic performance assessment of a typical steel frame employing SMA-based self-centering braces (SMA-SCBs), where a practical and realistic modelling strategy for the SMA-SCB is developed to capture the degradation behavior, especially at low temperatures. Seismic risk assessment is conducted to evaluate the collapse resistance and resilience performance of the considered structures, with the influence of temperature highlighted. Among other important findings, it is shown that the decrease in environment temperature significantly increases the peak inter-story drift (PID) and residual inter-story drift (RID) responses. Under the maximum considered earthquake (MCE), the probability of collapse increases from 2.7% to 19.7% when the temperature decreases from 30 °C to −15 °C. From a self-centering capability point of view, the probabilistic of exceedance of 0.5% RID threshold soars from negligible to nearly 100%, considering the same temperature variation. The study highlights the fact that temperature plays a significant role in the annual rate of exceedance of all damage states, and warns that the idealized flag-shaped modelling approach for SMA components is extremely unsafe at low temperatures.

A hybrid inter-module connection for steel modular building systems with SMA and high-damping rubber components

The recent technological advancements achieved in modular construction have accelerated the trend of building taller self-standing steel modular building systems (MBSs), leading to a consensus among researchers regarding the vital role that inter-module connections (IMCs) play in the structural performance of MBSs subjected to extreme lateral loading. However, existing IMCs are typically designed such that the global structural system heavily relies on the hysteresis of the steel framing members, leading to severe sustained damage and costly, impractical retrofitting programmes. To unlock the full “disassembly and reuse” potential of steel MBSs, IMCs can be designed to contribute more effectively to the global damage distribution mechanism, by engaging specific “fuse” components which are easy to replace, improving the reuse prospects of volumetric modules. In this regard, the present study proposes a novel, hybrid IMC using custom corner fittings, a high-damping rubber (HDR) core and a shape-memory alloy (SMA) bolt. Calibrated and validated material models using data from experimental material characterisation tests have facilitated the full characterisation of the hybrid mechanical response, determining the deformation modes, stress states, hysteresis loops and mechanical parameters. The parametric FEA included the variation of bolt preload, endplate thickness, axial load magnitude and the vertical layout of the HDR core. The study represents a preliminary, proof-of-concept investigation, showcasing the favourable cyclic performance of the proposed IMC under the main deformation modes expected in tall self-standing MBSs during lateral loading. Due to the effective contribution of each component to the combined hybrid response, the connection succeeds in preventing the formation of significant plastic damage in the MBS’s corner fittings to facilitate reusability of modules.

Development and prototyping of SMA-metamaterial biaxial composite actuators

Shape memory alloys (SMA) are excellent candidates for implementation in actuator systems due to their ability to recover their original shape after high-strain loading through a thermally-induced phase transition. In this work, we propose and develop a novel SMA-metamaterial actuator which is capable of exhibiting a reversible, global elongation in multiple directions induced by the unidirectional contraction upon heating of a single SMA component. This actuator consists of (a) an SMA component, (b) a bias component and (c) the metamaterial geometry, with each component having a distinct function: (a) actuation activation, (b) reversibility of actuation upon deactivation and (c) amplifying and re-directing the uni-directional SMA actuation globally throughout the actuator, respectively. A prototype actuator was designed and tested in various configurations over multiple activation/deactivation cycles in order to demonstrate the functionality and reusability of this system. Furthermore, a theoretical model which predicts the actuation stroke of the system on the basis of the material properties of the SMA and bias components as well as the geometry of the metamaterial system was developed and validated. The findings of this work demonstrate the considerable potential of SMA-metamaterial actuators for implementation in systems requiring a multi-axial actuation output.

Processing parameters and martensitic phase transformation relationships in near defect-free additively manufactured NiTiHf high temperature shape memory alloys

Additive Manufacturing (AM) has quickly emerged as a promising technology to manufacture Shape Memory Alloy (SMA)-based components of complex geometries. 3D-printed High Temperature SMAs (HT-SMAs) components can enable applications that demand operating temperatures well beyond conventional NiTi SMAs. Here, a detailed printability assessment of NiTiHf HT-SMAs fabricated using laser powder bed fusion (LPBF) AM is conducted for the first time. Specifically, the regions associated with lack of fusion, keyholing, and balling regimes are quantitatively classified through an efficient printability assessment framework. Nearly porosity-free specimens are achieved from the predicted good printable region. The effects of key processing parameters (laser power, scanning speed, and hatch spacing) on the microstructure variation and phase transformation characteristics are studied systematically. A positive correlation is observed between the transformation temperatures and volumetric energy density (EV). Beyond a critical EV of 100 J/mm3, the transformation temperatures become insensitive to further increases in EV. At identical or similar energy density levels, the individual change of laser power, scanning speed, or hatch spacing alters the transformation behavior to a certain extent. It is hypothesized that to a large extent, this behavior is due to the effects that processing conditions have on the differential evaporation of Ni from the melt pool. A model connecting thermal histories, scanning strategy and chemistry changes is found to be consistent with this hypothesis. It is shown that the transformation temperatures in AM NiTiHf HT-SMAs can be varied over a range of more than 160 °C by controlling Ni evaporation through altering processing parameters.

Heat Treatment of NiTi Alloys Fabricated Using Laser Powder Bed Fusion (LPBF) from Elementally Blended Powders.

The use of elemental metallic powders and in situ alloying in additive manufacturing (AM) is of industrial relevance as it offers the required flexibility to tailor the batch powder composition. This solution has been applied to the AM manufacturing of nickel-titanium (NiTi) shape memory alloy components. In this work, we show that laser powder bed fusion (LPBF) can be used to create a Ni55.7Ti44.3 alloyed component, but that the chemical composition of the build has a large heterogeneity. To solve this problem three different annealing heat treatments were designed, and the resulting porosity, microstructural homogeneity, and phase formation was investigated. The heat treatments were found to improve the alloy’s chemical and phase homogeneity, but the brittle NiTi2 phase was found to be stabilized by the 0.54 wt.% of oxygen present in all fabricated samples. As a consequence, a Ni2Ti4O phase was formed and was confirmed by transmission electron microscopy (TEM) observation. This study showed that pore formation in in situ alloyed NiTi can be controlled via heat treatment. Moreover, we have shown that the two-step heat treatment is a promising method to homogenise the chemical and phase composition of in situ alloyed NiTi powder fabricated by LPBF.

Shape Memory Alloys via Halide‐Activated Pack Equilibration

Fabrication of shape memory alloy (SMA) components based on NiTi is challenging due to the precision with which elemental composition and microstructure must be controlled during processing to achieve desired shape memory behavior. Herein, a method to control chemistry in an NiTi SMA via halide‐activated pack equilibration (SHAPE) against a constant chemical potential reservoir is described. To demonstrate the efficacy of the SHAPE process, an initially titanium‐deficient specimen (pure nickel foam) has been equilibrated against an excess of an intimately mixed two‐phase pack (NiTi + Ti2Ni) in the presence of a vapor phase transport agent (iodine). The two‐phase pack regulates chemical potentials in this two‐component system in accordance with Gibbs’ phase rule. Ti‐rich NiTi foams thus produced exhibit reproducible and well‐defined phase transformation behaviors. The SHAPE process is advantageous for the fabrication of shape memory components of varying areal dimension, shape, and/or complexity owing to independence of the equilibrium state of the system from either the initial state of the specimen or the details of the process kinetics. Current limitations and prospects for the application of this method to improve the quality of SMA components are briefly discussed.

Phase transformation-driven artificial muscle mimics the multifunctionality of avian wing muscle.

Skeletal muscle provides a compact solution for performing multiple tasks under diverse operational conditions, a capability lacking in many current engineered systems. Here, we evaluate if shape memory alloy (SMA) components can serve as artificial muscles with tunable mechanical performance. We experimentally impose cyclic stimuli, electric and mechanical, to an SMA wire and demonstrate that this material can mimic the response of the avian humerotriceps, a skeletal muscle that acts in the dynamic control of wing shapes. We next numerically evaluate the feasibility of using SMA springs as artificial leg muscles for a bipedal walking robot. Altering the phase offset between mechanical and electrical stimuli was sufficient for both synthetic and natural muscle to shift between actuation, braking and spring-like behaviour.

A novel structural health monitoring approach in concrete structures using embedded magnetic shape memory alloy components

In the field of structural health monitoring, innovative methods of non-destructive evaluation (NDE) are being investigated to enable safer, longer lasting structures. In concrete structures, stress-induced phase transformations in embedded Magnetic Shape Memory Alloy (MSMA) components caused by failure in a surrounding concrete matrix can be correlated to changes in measurable magnetic properties. In this paper, a computational framework is developed to evaluate the development of stress-induced martensite due to damage in a load-bearing concrete-MSMA composite, which alters an externally applied magnetic field. The computational results support a new approach in NDE of concrete structures using embedded phase transforming materials.

Analytical and numerical study on the cyclic behavior of buckling-restrained SMA-based self-centering damper

Owing to the unique superelasticity and satisfactory energy dissipation capacity, shape memory alloys (SMAs) show great potential in earthquake engineering. This study focused on establishing analytical method and finite element (FE) model for the buckling-restrained SMA (BRSMA) based self-centering (SC) dampers. For the BRSMA-based SC damper, the lateral deformation of the slender SMA component is constrained by the surrounding buckling-restrained plates (BRPs). Hence, the SMA component avoids buckling induced instability and the damper accordingly exhibits stable cyclic behavior under tension–compression loadings. Although the compressive behavior has been obtained through cyclic loading tests, an in-depth understanding on the corresponding force-displacement relationship is required. As such, a simple analytical method was proposed for estimating the compressive strength capacity of the BRSMA-based SC damper. Besides, the FE model was built to provide additional information that was difficult to obtain in physical model tests. The accuracy of the analytical method and FE model was confirmed by the testing results. The verified FE model was further used to conduct the parametric analysis. The parameters of interest included the shape of the reduced section, the dimension of the BRSMA components, the gap between the BRSMA components and BRPs and the number of bolts. The findings of this paper not only promote understanding on the cyclic behavior of this new damper, but also provide suggestions and guidelines for future design.

Controlled shape-morphing metallic components for deployable structures

Transformational advances in additive manufacturing combined with the unique functional behavior of shape memory alloys (SMAs) has propelled the field of 4D printing “smart” materials. In this study, we leverage multiple processing pathways with additive manufacturing to design and fabricate SMA components capable of precise, self-guided shape change that could actuate large-scale (up to 25 × 25 × 33 cm3) structures under external thermal stimuli. The dual benefits of minor alloy doping (<2 at%) and laser processing parameters (laser hatch spacing) were identified to be the most effective method to engineer SMA joints and hinges with locally tailored transformation temperatures over a 90 °C range. Custom-designed plywood-style NiTi hinges with self-regulating features enabled tight bend radii and compact packing sizes. Implementation of these novel SMAs hinges with large deployment structures, mimicking solar panel arrays, demonstrated a 75–90% reduction in cross-sectional area compared to the fully deployed structure. The fundamental understanding and demonstration of tailored and controlled complex shape-morphing kinematics, without the use of specialized and heavy external motors, lays the groundwork for future SMA-enabled deployable structures in remote environments.

On the fatigue life enhancement due to periodic healing of a NiTi shape memory alloy

Fatigue failure in NiTi based shape memory alloys (SMAs) that are in the austenitic state is accelerated by the progressive accumulation of stress-induced martensite (SIM) under cyclic loading, even when the maximum stress of the fatigue cycle is well below that required for stress-induced martensitic transformation. Wagner et al. (2008) [1] have shown that periodic annealing of the fatigued specimens at temperatures well above the austenitic finish temperature, which they termed as ‘healing’, can enhance the fatigue life of the SMAs that are cyclically loaded in the austenitic state. In this paper, the optimum interval at which healing must be performed is investigated. Experimental results show that considerable improvement in the total life of the SMA component can be realized if the fatigued specimens are healed periodically right after 20% of their service life has lapsed. Healing later (at 40% and 60% of the fatigue life) does not lead to any significant improvement, indicating that irreversible damage has already set in. Real-time infrared thermography technique was used to study the thermal signatures during tensile and fatigue testing. Results show that it is possible to monitor the formation of SIM during cyclic loading using thermography.

The Application of Ni-Ti SMA Wires in the External Prestressing of Concrete Hollow Cylinders.

An innovative method for prestressing structural elements through the use of shape memory alloys (SMAs) is gaining increasing attention in research as this method does not require the use of mechanical anchorages for tendons. The activation of the memory effect by means of temperature variations (Joule effect) in effect produces high stresses in SMA components attached to concrete components as reported in the literature. This paper presents the work performed for the purpose of prestressing concrete hollow cylinders with the use of nickel–titanium (Ni–Ti) SMA wires. In the tests, a variety of hollow cylinders were made using the same concrete mix and with the same wall thickness (20 mm), but with different external diameters (200 mm, 250 mm, and 300 mm). Their prestressing was achieved by the means of Ni-Ti SMA wires of different diameters (1 mm, 2 mm, and 3 mm) wrapped around the cylinders. Longitudinal and circumferential strain during the thermal activation of the SMA wires by Joule heating was measured using gauges located on the internal surface of the hollow cylinders. The experimental protocol, recorded observations, and discussion of the effectiveness of the prestressing of concrete elements as a function of the test parameters are included in the text in detail. Comments on the conditions for effective prestressing of concrete cylinders with SMA wires are proposed in the conclusions of the paper.

Reducing human‐induced vibration of cross‐laminated timber floor—Application of multi‐tuned mass damper system

As a vibration control technique, tuned mass damper (TMD) system has been shown to be effective in reducing the human‐induced vibration of a cross‐laminated timber (CLT) floor. However, the lightweight property of such a floor means there could be off‐tuning when its mass varies. This study therefore developed a steel‐based multi‐TMD (MTMD) system and a shape memory alloy (SMA)‐based MTMD system to reduce human‐induced vibration of the CLT floor. The superelastic SMA components can give the MTMD system more resilience and thus improve the robustness. Two 3‐TMD systems in different locations and 5‐TMD systems were designed to be effective within a certain bandwidth. The results show that SMA‐based 5‐TMDs are the most effective in reducing human‐induced vibration, for example, single‐person and two‐person slow walking, fast walking and running, as they can cover a wider frequency band. By contrast, the effectiveness of the steel‐based MTMD systems was unsatisfactory owing to permanent deformation of the steel components. When the loads on the CLT floor changed, the SMA‐based 5‐TMDs exhibited high robustness and were able to maintain the response at a low level. Test results show that a high‐frequency excitation could degrade the effectiveness of the MTMD, as this is beyond the effective bandwidth. Therefore, future investigations should focus on developing strategies to enlarge the bandwidth of the MTMD.

Intergrated Shape Memory Alloys Soft Actuators with Periodic and Inhomogeneous Deformations by Modulating Elastic Tendon Structures

The soft actuators similar to the periodic motion of biological organisms are accomplished based on assembling multiple unidirectional memory effect shape memory alloys (SMAs) components or introducing biasing elements, which give rise to the complex structure and difficult control of soft actuators. Herein, inspired by the power‐amplified biological systems, beneficial with the advantages of 3D integrated molding technology, an integrated SMA‐polydimethylsiloxane (PDMS) composite structure (SPCS) is proposed, which can achieve periodic heterogeneous actuation only by controlling the SMA phase transformation and the PDMS strain potential energy distribution states. To test the feasibility of the mechanism, a theoretical model of SPCS deformation is conducted. The results of numerical feasibility analysis show that the factors affecting SPCS deformation mainly involve the excitation current strength of SMA, PDMS structure thickness and its distribution state. The experimental results show that the current intensity mainly affects the deformation rate of SPCS, and the thickness of PDMS is not only the key to realize the periodic deformation of SPCS but also the orderly arrangement of PDMS structure thickness is helpful for SPCS to achieve periodic heterogeneous deformation. These demonstrate that the proposed mechanism can inspire the design of soft actuators, smart wearable equipment, and medical devices.

Functional Behavior of Pseudoelastic NiTi Alloy Under Variable Amplitude Loading

Abstract Cyclic loading of superelastic NiTi shape memory alloy (SMA) causes forward and reverse austenite–martensіte transformations, and also increases the volume of stabilized martensite. This appears in the change of stress-strain curve form, the decrease of dissipation energy, and increase of residual strain, that is, named transformation ratcheting. In real structures, the SMA components in most cases are under the action of variable amplitude loading. Therefore, it is obvious that the loading history will influence the functional fatigue. In the present work, the effect of stress ratio on the functional properties of superelastic NiTi shape memory alloy under variable amplitude loading sequence with two blocks was investigated. The studies were carried out under the uniaxial tension of cylindrical specimens under load-full unload and load-part unload. The change of residual strain, strain range, dissipation, and cumulative dissipation energy density of NiTi alloy related to load sequences are discussed. Under both stress ratios, the residual strain in NiTi alloy is increased depending on the number of loading cycles on the high loading block that is similar to the tests at constant stress or strain amplitude. An unusual effect of NiTi alloy residual strain reduction with the number cycles is found at a lower block loading. There was revealed the effect of residual strain reduction of NiTi alloy on the number of loading cycles on the lower amplitude block. The amount of decrement of the residual strain during a low loading block is approximately equal to the reversible part of the residual strain due to the stabilized martensite. The decrease of the residual strain during the low loading block is approximately equal to the reversible part of residual strain due to the stabilized martensite. A good correlation of the effective Young’s modulus for both load blocks with residual strain, which is a measure of the volume of irreversible martensite, is observed.