Superelastic Shape Memory Alloy Wires Research Articles

Multifactor experimental investigation of the mechanical performance of a shape memory alloy‐based self‐centering‐energy‐dissipation device

SummaryAn innovative self‐centering‐energy‐dissipation device (SCED) based on a superelastic shape memory alloy (SMA) is developed. This device exhibits asymmetric, flag‐shaped hysteretic behavior and tension–compression symmetry property. The symmetry is attributed to the tension‐only state of the SMA wires in the device during the loading process. The working principle of the device is described, and a theoretical model describing the mechanical properties of the device is proposed. Uniaxial tension–compression cyclic loading tests are carried out to investigate the influence of the section area of the SMA wire, the loading amplitude, and the loading rate on the mechanical performance, including the yield strength, maximum strength, austenite stiffness, martensite stiffness, energy dissipation, and efficiency ratio. Expressions of the correction factors and mechanical performance considering multiple factors are also suggested.

Flexural Behavior of Polypropylene Fiber Reinforced Concrete Beams with Super elastic Wire Under Repeated Loading

This paper investigates the effect of polypropylene fiber (PPM-12) and nitinol (NiTi) superelastic Shape Memory Alloy (SMA) wires in concrete beams under flexural repeated loading. For this purpose, nine of simply supported beams with dimensions of (1000 mm length, 80 mm width and 125 mm depth) were considered. Test specimens were divided into three groups (G1, G2 and G3). Specimens in group G2 and G3 were reinforced with different ratios of PPM-12 fiber (0.5%and 0.75%) by volume of concrete and threaded (NiTi) superelastic SMA wires. Group G1 was reinforced with traditional steel rebar and SMA wires without PPM-12 fiber. The results revealed that the addition of PPM-12 fiber to concrete have a negative effect on the mechanical properties of concrete with respect to load carrying capacity in the other side the addition of fiber led to a controlled crack propagation in concrete and increased the fatigue life of concrete. The (NiTi) superelastic threaded wires were active in some specimens in which some displacement recovery was noticed. The importance of study is how to enhance concrete life and reducing crack propagation in concrete by using polypropylene fiber and NiTi wire as reinforcement in concrete.

Rate dependent free energy formulations for dynamically excited superelastic SMA wires

AbstractSuperelastic shape memory alloys (SMAs) are metallic two‐phase polycrystal alloys. The transformation between austenite and martensite phases allows superelastic SMAs unique hysteretic energy dissipation capacity, which is particularly interesting for the development of new‐type of intelligent vibration control systems for structures. However, in structural control, most of the vibrations occur in high strain rate regimes, which interfere the release of self‐generated heat and cause atomic lattice destabilization and thus influence the hysteretic dissipation. The present work proposes two different strain rate dependent free energy formulations and aims to improve the accuracy of existing one‐dimensional macroscopic models of SMA wires. The approaches are developed and implemented in different continuum thermomechanical frameworks, without impairing the simplicity and robustness of the solution process. The first approach introduces an additional control variable for the entropy evolution. The second approach uses a phenomenological formulation of the latent heat evolution. The proposed formulations are validated by cyclic tensile tests conducted on SMA wires. The results show that the calculations using the new formulations can represent the superelastic SMA wire response more accurately.

Experimental and Numerical Analysis of the Mechanical Properties of a Pretreated Shape Memory Alloy Wire in a Self-Centering Steel Brace

As a stimulus-sensitive material, the difference in composition, fabrication process, and influencing factors will have a great effect on the mechanical properties of a superelastic Ni-Ti shape memory alloy (SMA) wire, so the seismic performance of the self-centering steel brace with SMA wires may not be accurately obtained. In this paper, the cyclic tensile tests of a kind of SMA wire with a 1 mm diameter and special element composition were tested under multi-working conditions, which were pretreated by first tensioning to the 0.06 strain amplitude for 40 cycles, so the mechanical properties of the pretreated SMA wires can be simulated in detail. The accuracy of the numerical results with the improved model of Graesser’s theory was verified by a comparison to the experimental results. The experimental results show that the number of cycles has no significant effect on the mechanical properties of SMA wires after a certain number of cyclic tensile training. With the loading rate increasing, the pinch effect of the hysteresis curves will be enlarged, while the effective elastic modulus and slope of the transformation stresses in the process of loading and unloading are also increased, and the maximum energy dissipation capacity of the SMA wires appears at a loading rate of 0.675 mm/s. Moreover, with the initial strain increasing, the slope of the transformation stresses in the process of loading is increased, while the effective elastic modulus and slope of the transformation stresses in the process of unloading are decreased, and the maximum energy dissipation capacity appears at the initial strain of 0.0075. In addition, a good agreement between the test and numerical results is obtained by comparing with the hysteresis curves and energy dissipation values, so the numerical model is useful to predict the stress–strain relations at different stages. The test and numerical results will also provide a basis for the design of corresponding self-centering steel dampers.

A Soft Five-Fingered Hand Actuated by Shape Memory Alloy Wires: Design, Manufacturing, and Evaluation.

This work presents a novel five-fingered soft hand prototype actuated by Shape Memory Alloy (SMA) wires. The use of thin (100 μm diameter) SMA wire actuators, in conjunction with an entirely 3D printed hand skeleton, guarantees an overall lightweight and flexible structure capable of silent motion. To enable high forces with sufficiently high actuation speed at each fingertip, bundles of welded actuated SMA wires are used. In order to increase the compliance of each finger, flexible joints from superelastic SMA wires are inserted between each phalanx. The resulting system is a versatile hand prototype having intrinsically elastic fingers, which is capable to grasp several types of objects with a considerable force. The paper starts with the description of the finger hand design, along with practical considerations for the optimal placement of the superelastic SMA in the soft joint. The maximum achievable displacement of each finger phalanx is measured together with the phalanxes dynamic responsiveness at different power stimuli. Several force measurement are also realized at each finger phalanx. The versatility of the prototype is finally demonstrated by presenting several possible hand configurations while handling objects with different sizes and shapes.

Strain rate dependent formulation of the latent heat evolution of superelastic shape memory alloy wires incorporated in multistory frame structures

Due to their unique hysteretic energy dissipation capacity, shape memory alloy (SMA) wires are particularly interesting for the development of new-type of intelligent vibration control systems for structures. However, in structural control, most of the vibrations occur in high strain rate regimes, which interfere the release of self-generated heat and thus influence the hysteretic dissipation. This paper proposes a strain rate dependent formulation of the latent heat evolution and aims to improve the accuracy of existing macroscopic modeling approaches developed for SMA wires particularly for the dynamic load cases. The proposed formulation is determined phenomenologically and implemented in a continuum thermomechanical framework based constitutive SMA wire model without impairing the simplicity and robustness of the solution process. The proposed formulation is validated by cyclic tensile tests conducted on SMA wires. Results show that the calculations using the formulation can predict the wire response more accurately than the strain rate independent formulation. For the simulation of multistory frame structures incorporating multiple SMA wires, the governing equations are driven. Shaking table tests are conducted on a 3-story frame structure under harmonic and seismic excitation. The responses of the structure are successfully replicated using the strain rate dependent latent heat formulation.

Improvement of impact resistance of plain-woven composite by embedding superelastic shape memory alloy wires

Carbon fiber reinforced polymer (CFRP) composites have excellent mechanical properties, specifically, high specific stiffness and strength. However, most CFRP composites exhibit poor impact resistance. To overcome this limitation, this study presents a new plain-woven CFRP composite embedded with superelastic shape memory alloy (SMA) wires. Composite specimens are fabricated using the vacuum-assisted resin injection method. Drop-weight impact tests are conducted on composite specimens with and without SMA wires to evaluate the improvement of impact resistance. The material models of the CFRP composite and superelastic SMA wire are introduced and implemented into a finite element (FE) software by the explicit user-defined material subroutine. FE simulations of the drop-weight impact tests are performed to reveal the superelastic deformation and debonding failure of the SMA inserts. Improvement of the energy absorption capacity and toughness of the SMA-CFRP composite is confirmed by the comparison results.

Evaluation of quasi-static indentation response of superelastic shape memory alloy embedded GFRP laminates using AE monitoring

In this paper, the potential of superelastic shape memory alloy (SE-SMA) wire embedded architectures to increase the quasi-static indentation properties of a laminated glass/epoxy composite material was evaluated. Three types of SE-SMA configurations namely straight independent, meshed and anchored wires were embedded in the glass/epoxy composite laminates via a vacuum bag resin infusion technique. Throughout this investigation, the changes in the quasi-static indentation behavior and allied damage mechanisms due to these embedments were compared with the homogenous glass/epoxy laminates. Real time acoustic emission (AE) monitoring technique was employed to characterize the damage profile of the different glass/epoxy specimens during the quasi-static indentation tests. The experimental results showed that SE-SMA embedments play a vital role in increasing the penetration resistance by enhancing redistribution of the indentation load all across the laminates. In particular, the meshed specimens restricted penetration of an indenter and delayed the critical fiber fracture unlike homogeneous and straight wired ones, whereas the anchored specimens further restricted extensive SMA/matrix pull-out, unlike meshed ones and provided the most excellent balance among rigidity, rear face fiber breakage, and SMA/matrix pull-out. Straight, meshed and anchored SE-SMA wires increased the load-carrying capacity approximately by 31%, 79%, and 100%, respectively, in comparison to the homogeneous ones.

Reconstruction of Heat Sources Induced in Superelastically Loaded Ni-Ti Wire By Localized Deformation Processes

Shape memory alloys (SMAs) are phase transforming materials featuring strong thermomechanical couplings. Infrared thermography and heat source reconstruction (HSR) enable to track the calorific signature of deformation processes. The objective was to characterize the transformation processes in a superelastic nickel-titanium SMA wire subjected to a force-controlled superelastic tensile cycle. In-situ recorded thermographs were converted into spatiotemporal maps of heat sources using an in-house developed post-processing method based on the heat diffusion equation resolved numerically for unknown heat sources. Sequentially appearing patterns of localized transformation events of four types were identified and associated with martensite bands nucleations and their subsequent merging upon tensile loading. Analogically, the events associated with austenite bands nucleations and their subsequent merging were identified upon unloading. In addition, weak heat sources observed before and after the localized transformation events were associated with the homogeneous martensitic transformation. The intrinsic dissipation heat associated with the nucleation and merging events is estimated to be ~ 25% of the released/absorbed latent heat.

Development of a novel deformation-amplified shape memory alloy-friction damper for mitigating seismic responses of RC frame buildings

Super-elastic shape memory alloys (SMAs) are widely used to mitigate seismic responses of engineering structures because of their self-centring and energy dissipation characteristics. However, the energy dissipation capacity of SMAs is limited when compared to other displacement-dependent dampers. In this paper, a novel deformation-amplified SMA-friction damper (DASMAFD) is proposed to control the seismic responses of reinforced concrete (RC) frame buildings. In this damper, a lever system is employed to amplify the deformation that is transferred from the structure to the SMA wires, and a friction adjusting system is adopted to adjust the friction force in the damper. Hence, the proposed novel system is advantageous due to the excellent self-centring capacity of SMAs and the high damping capacity of the friction adjusting system. To demonstrate the effectiveness of the proposed system, the design and mechanism of the device are presented in detail, the self-centring and damping capacity of the damper are investigated through experimental studies, and the influences of deformation amplification ratio (DAR) and friction ratio (FR) on the self-centring and hysteresis characteristics of the damper are systematically examined. The damper is then applied to control the seismic responses of a 10-storey RC frame building, and numerical simulations are performed to examine the control effectiveness of the proposed system. The calculated responses of the building controlled by a conventional SMA damper are compared with those of a SMA-friction damper. Furthermore, parametric studies are performed to examine the effects of key parameters of the damper, namely DAR and FR, on the seismic responses of the building. Numerical results indicate that DASMAFD exhibits the most evident control effectiveness when compared to other dampers.

A 1D strain-amplitude- and strain-rate-dependent model of super-elastic shape memory alloys for structural vibration control

The recently increasing interest in using super-elastic shape memory alloys (SMAs) for structural vibration control creates the need for a suitable mechanical model that considers the material’s close correlation with the strain amplitude and the strain rate. This paper proposes an improved Graesser-Cozzarelli (G-C) model for predicting the hysteretic responses of the super-elastic SMAs at various strain rates and strain amplitudes. A series of uniaxial tensile tests of super-elastic SMA wires are conducted first, and the effects of the strain amplitude and the strain rate on the characteristic parameters are quantified. Then, based on the original G-C model and experimental test results, an improved version is proposed. The revised model considers the nonlinear hardening behaviour of martensite and divides the complete hysteretic curve into two stages: a loading stage and an unloading stage. Finally, numerical simulations are conducted to evaluate the model’s performance in predicting the hysteresis responses at various strain amplitudes and strain rates. High accuracy is realized in all comparisons with the experimental results; thus, the improved G-C model is highly suitable for the simulation of super-elastic SMA-based dampers in applications for vibration control.

Uniaxial Compressive Behavior of Concrete Columns Confined with Superelastic Shape Memory Alloy Wires.

Superelastic shape memory alloy (SMA) exhibits the ability to undergo large deformations before reverting back to its undeformed shape following the removal of the load. This unique property underlies its great potential in the seismic design and retrofitting of structure members. In this paper, superelastic SMA wires were utilized to confine concrete cylinders to enhance their axial compressive behavior. The axial carrying and deformation capacities of SMA-confined concrete cylinders are assessed by uniaxial compression testing on a total of eight SMA-confined concrete columns and one unconfined column. The influence of the amount of SMA and the prestrain level of SMA wires, as well as the reinforcing mode, on the axial carrying and deformation capacity of confined concrete columns were considered. The analysis focuses on the axial carrying capacity and deformation performance of concrete columns reinforced with superelastic SMA under different loading conditions. Based on the experimental data and analysis results, it is found that superelastic SMA wires can increase the axial loading capacity and enhance deformation performance of concrete columns. Under the same loading condition, the ultimate bearing capacity of SMA-confined concrete columns increases as the increasing of the amount of SMA wire. The results of this study verify the effectiveness of superelastic SMA in enhancing the loading capacity and deformation behavior of concrete cylinders.

Seismic Behavior of Superelastic Shape Memory Alloy Spring in Base Isolation System of Multi-Story Steel Frame.

Owing to excellent re-centering capability and good damping behavior, superelastic shape memory alloys (SMAs) are emerging as a potential new material to enhance the seismic resilience of civil structures. This paper focuses on using base isolation with SMA device for isolated structures. SMA springs are deemed to be promising candidate as the damper in the base isolation system, due to the compact form, damping contribution, restoring capability and flexible stiffness. This paper reported the concept of an innovative spring which is made of superelastic SMA wire. Then cyclic loading tests were carried out to evaluate the interested cyclic properties. Parametric analyses based on finite element simulations were conducted to reveal the versatile performance of SMA springs. To further examine its seismic behavior in the base isolation system, the SMA spring was later installed at the isolation level of a multi-story steel frame, based on the finite element model built in the earthquake engineering simulation platform OpenSees. An ordinary elastic spring is included for comparison to highlight the features of SMA springs. Both isolated frames were subjected to real earthquakes. The comparisons indicated that using SMA spring is more effective in controlling maximum and residual deformation for the protected superstructures. Thus, this paper well demonstrated the feasibility and merits of using SMA springs in the isolated frames.

Superelastic NiTi SMA cables: Thermal-mechanical behavior, hysteretic modelling and seismic application

This paper reports a comprehensive study on the mechanical behavior, annealing (heat treatment) scheme, hysteretic modelling strategy, and potential seismic application of superelastic shape memory alloy (SMA) cables. The study commenced with the thermal-mechanical characterization of monofilament SMA wires, and in particular, the influence of annealing scheme on the mechanical and phase transformation characteristics of the material was revealed. A series of 7 × 7 SMA cable specimens were subsequently tested at room temperature under various cyclic loading protocols. It is observed, among other findings, that the SMA cables are able to reasonably “scale up” the satisfactory properties of the SMA wires, and the mechanical behavior of the SMA cables may be improved by annealing. Moderate annealing temperature and duration (i.e., 350–400 °C for 15 min) can generally increase the stiffness, energy dissipation, and form setting ability of the SMA cables considered in this study, whereas an overly high annealing temperature tends to compromise these characteristics. Following the experimental study, an effective numerical modelling approach is proposed which reliably captures the basic mechanical behavior of the SMA cables. A model bridge, where SMA cables are adopted as restrainers, is finally designed and analyzed to demonstrate the efficiency of the SMA components for seismic damage mitigation. The analysis result shows that the SMA-cable restrainers can effectively control the peak and residual displacements of the bridge girder, and make the bridge more resilient.

Numerical Simulation and Experimental Study of a Simplified Force-Displacement Relationship in Superelastic SMA Helical Springs.

This paper proposes a new force-displacement model for superelastic shape memory alloy (SMA) springs under complex loading and unloading. For the SMA wires used to make superelastic springs, a new multilinear constitutive model based on a modification of the 1D Motahari model is developed. In the modified model, the stress-strain relation curves are changed to fit the experimental results. Furthermore, the established force-displacement relationship of the springs considers the impact of not only the torque but also the moment on the cross sections of the SMA wires. Afterwards, a series of tension tests are performed on four NiTi helical spring specimens under various loading conditions. From the numerical simulations and experimental results, it is shown that, compared with the force-displacement curves for the SMA springs simulated by the Motahari model, those simulated by the proposed model can better approximate the experimental results. The new model inherits the advantage of simple computation of the multilinear constitutive model and can predict the force-displacement relation for superelastic SMA springs very well. Furthermore, due to the self-sensing properties of the SMA springs, the new model is very significant for establishing a new strategy for measuring the displacements or forces of SMA springs under complex loading and unloading.

The Low Velocity Impact Response of Foam Core Sandwich Panels with a Shape Memory Alloy Hybrid Face-Sheet.

Most foam core sandwich panels are sensitive to the impact load because of the poor toughness of thin composite face-sheets and the low strength of foam core. Superelastic shape memory alloy (SMA) wires have been applied to enhance the impact damage resistance of composite laminates in recent decades. To improve the impact damage resistance of foam core sandwich panels and to protect the foam core, SMA wires were incorporated into the face-sheets of foam core sandwich panels in this work. Eight new types of SMA hybrid sandwich panels were designed, and low-velocity impact tests were carried out at an impact energy of 35 J. The damage morphology of the impacted sandwich panels was identified by visual inspection and scanning electron microscope technology. Results indicate that the impact damage resistance of the SMA hybrid sandwich panels is enhanced. The damage area in the hybrid sandwich panels is greatly reduced and a decrease of 85.63% can be reached in the bottom face-sheet. The maximum contact force has an improvement of 28.15% when the two layers of SMA wires are incorporated into the bottom face-sheet.

The Low Velocity Impact Response of Shape Memory Alloy Hybrid Polymer Composites.

Polymer composites are sensitive to impact loading due to their low impact resistance. Shape memory alloy (SMA) wires have been used to improve the impact resistance of the polymer composite materials because of their unique superelasticity performance. In this study, a new SMA hybrid basalt fiber-reinforced polymer composite embedded with two perpendicular layers of superelastic SMA wires is designed and the low-velocity impact behavior is experimental investigated. For contrast, the conventional polymer composite without SMA wires is also tested as the reference laminate. The tests are carried out at three different impact energy levels (30, 60 and 90 J). Moreover, to find out indications for manufacturing of SMA hybrid composites with high impact resistance, four different SMA wires embedded modes are investigated. Visual inspection and scanning electron microscope methods are adopted to identify the damage modes of the impacted samples. Results show that the impact resistance of the hybrid laminates is improved due to the hybridization of SMA wires. The most effective impact resistance of the SMA hybrid composites can be obtained by incorporating the SMA wires with one layer between the front two plies and another layer between the bottom two plies into the composite structure.

Investigating Self-Centering Capacity of Superelastic Shape Memory Alloy Fibers with Different Anchorages Through Pullout Tests.

This study investigated the pull-out resistance of superelastic shape memory alloy (SMA) short fibers in mortar with consideration of various end-anchorages that provide different anchoring actions. For the purpose, four types of SMA fibers were prepared using NiTi SMA wires with a diameter of 1.0 mm and the following four end shapes: straight (ST), L-shaped (LS), N-shaped (NS), and spearhead-shaped (SH). The straight-ended fiber was a reference with no working on the end, and the fiber with the spearhead-shaped end was crimped to make the end part flat. The fibers with L- and N-shaped ends were bent with single or double bending. The results showed that only the spearhead-shaped fibers showed self-centering behavior because of the superelasticity of the SMA after slip occurred. This paper discusses the reasons that the ST, LS, and NS fibers do not show self-centering behavior and proposes a concept to induce superelastic behavior in SMA fibers in mortar or concrete.

Multifeedback Control of a Shape Memory Alloy Actuator and a Trial Application

Shape memory alloy (SMA) actuators exhibit high strain, high energy density, and self-sensing ability, which are promising characteristics for application in soft robot systems. This paper proposes a new SMA actuator with high accuracy that is driven by antagonistic SMA and super-elastic SMA (SSMA) wires or pairs of SMA wires that is intended to be used in miniature flexible structures. A multifeedback control methodology is presented for the actuator that uses feedback from both the SMA and SSMA wires. The prestress was chosen to minimize the gap between the heating and cooling paths of the strain-to-resistance (S-R) curve for both antagonistic SMA and SSMA. A radial basis function neural network was used to establish the S-R curve model for both self-sensing feedback and SSMA feedback. To achieve better system accuracy and stability, a data-fusion algorithm (based on the support function) was used to combine the multifeedback data. Accurate and stable motion control is demonstrated through multistep response, sinusoidal tracking, and force-disturbance tests. The experimental results show that the combination model using all the feedback has superior accuracy compared to the models that use self-sensing feedback or SSMA feedback. In addition, the maximum standard deviation within the SMA-SSMA antagonistic platform and the paired-SMA antagonistic platform can be reduced to 0.036% and 0.026%, respectively. A soft joint with two degrees of freedom is illustrated to demonstrate how the current research concept could potentially be implemented in bionic soft robots.

Pullout behavior of superelastic SMA fibers with various end-shapes embedded in cement mortar

This study investigated the effects of superelastic shape memory alloy (SMA) fibers end-shape on pullout resistance through hysteretic pullout testing. Superelastic NiTi SMA wire of 1.0 mm diameter was employed in manufacturing short fibers. SMAs were fabricated with four end-shapes: 1) prismatic and straight end, 2) L-shaped end, 3) N-shaped end, and 4) crimped end with a spearhead. The embedded length of a fiber into the mortar matrix having compressive strength of 50.0 MPa was 18.0 mm, except for the N-shaped end fiber for which the embedded length was 21.0 mm. The pullout test was conducted with displacement control to obtain hysteretic pullout behavior by four cycling loadings. The results showed that crimped-end fibers considerably magnified the pullout strength, and the deformation was recovered by their superelastic behavior. As N-shaped end fibers could not show flag-shaped behavior, another test was performed through which fibers were annealed to induce superelasticity, so, a perfect strain recovery was obtained. Results showed that additional annealing of the N-shaped fibers after the manufacturing improved superelastic behavior at the bended part.