Superelastic Shape Memory Alloy Research Articles

Resistance spot welding of NiTi shape memory alloy sheets: Microstructural evolution and mechanical properties

Resistance spot welding was used for welding superelastic NiTi shape memory alloy sheets. The microstructure of the welded sample showed a fully austenitic NiTi nugget without any visible weld defect due to the insignificant change in the transformation temperature during the welding process. Additionally, the fusion zone has a homogenous microhardness distribution with an average hardness value of ~340 HV, which is below the average hardness value for NiTi base material (~410 HV) because of the large grain size in the weld nugget. The perseverance of the superelastic response as a main functional property of NiTi alloy after the RSW procedure is significant. The welded sample failed via pullout failure mode during tensile shear loading conditions. Face digital image correlation imaging confirmed that strain is localized in the area around the nugget. The dominant fracture mode of the sample during tension-shear loading was ductile, and the EBSD results of the fracture area revealed the excessive deformation around the HAZ resulted in a transgranular fracture. Therefore, resistance spot welding is highly capable of providing a sound, straightforward and reliable joint between NiTi sheets.

Anomalous high-temperature quasi-linear superelasticity of Ni-Fe-Ga-Co shape memory alloy

The superelasticity of shape memory alloys is associated with the martensitic transformation which has been widely used in engineering applications. Here, we clarify quasi-linear superelasticity in Ni-Fe-Ga-Co shape memory alloy exhibiting a recovery strain of 3.58% at 573 K, which is far above the martensitic transformation temperature. Real-time in-situ neutron diffraction measurement was used to explore the underlying quasi-linear superelasticity mechanism via tracing the structural evolution during cyclic compression loading. Neutron diffraction observation showed that the superelasticity is correlated with stress-induced continuous variation of lattice parameter. The in-situ neutron diffraction provides the direct evidence on the anomalous diffraction peak width broadening, which mainly stems from the spatial heterogeneity in strain. The excessive Co doping is responsible for the decrease in stacking-fault energy leading to an increase in stacking faults, which suppresses the martensitic transformation and triggers the nucleation of the weak first-order phase under the external stress. The study provides insights into the interplay between superelasticity and structural transformation in shape memory alloys, and it is also instructive for understanding the anomalous high-temperature quasi-linear superelasticity in functional materials. • The quasi-linear superelasticy of Ni-Fe-Ga-Co shape memory alloy is caused by the weak first-order transformation. • The quasi-linear superelasticity of Ni-Fe-Ga-Co exhibits a recovery strain of ~3.58% at 573 K, which is rarely reported. • The excessive Co is account for trigger the nucleation of the weak first-order phase under the external stress.

Development, characterisation, and modelling of processability of nitinol stents using laser powder bed fusion

Additive manufacturing (AM) of customised vascular or peripheral stents is of great potential for surgeons and patients, enabling the patients to have customised stents and achieving better outcomes from stenting procedures, with further advantages of having a resource efficient manufacturing process. In this study, the potential for AM of superelastic NiTi-based shape memory alloy (Nitinol) stents was investigated. Two stent designs, which are used for the treatment of complex peripheral artery stenosis in the lower limbs, were studied. Laser Powder Bed Fusion (LPBF) of two stent designs was studied to investigate the impact of the process parameters on the stent geometry, strut size, structural integrity and the phase transformations. The study demonstrated the successful manufacture of Nitinol stents via LPBF, with strut sizes in the range between 250 µm and ≈ 560 µm. The elastic modulus of the stents was between 56 and 73 GPa, which matches well with the elastic modulus of standard austenitic Nitinol. Chemical etching was used to reduce the strut diameter and to remove the partially melted particles. It was shown that the laser energy input has a vital role in controlling the Ni-evaporation and the subsequent changes in the transformation temperatures, as well as the morphology of the stents. The lower energy input results in a reduced Ni-evaporation, maintaining the austenite finish temperature at the expected range, in addition to generating a good build morphology.

Mechanical Behavior of Shape Memory Alloy Fibers Embedded in Engineered Cementitious Composite Matrix under Cyclic Pullout Loads.

The incorporation of superelastic shape memory alloy (SMA) fibers into engineered cementitious composite (ECC) materials can provide high seismic energy dissipation and deformation self-centering capabilities for ECC materials. Whether the SMA fibers can be sufficiently bonded or anchored in the ECC matrix and whether the mechanical properties of the SMA fibers in the ECC matrix can be effectively utilized are the key scientific issues that urgently need to be studied. In order to study the mechanical behavior of SMA fiber embedded in ECC matrix, four groups of semi-dog-bone pullout specimens were fabricated, and the cyclic pullout tests were conducted in this paper. The pullout stress, displacement, and self-centering capability were analyzed, and different influencing factors were discussed. The results show that the knotted ends can provide sufficient anchorage force for SMA fibers, and the maximum pullout stress of SMA fiber can reach 1100 MPa, thus the superelasticity can be effectively stimulated. The SMA fibers show excellent self-centering capability in the test. The minimum residual deformation in the test is only 0.29 mm, and the maximum self-centering ratio can reach 0.93. Increasing bond length can increase the ultimate strain of SMA fibers with knotted ends, but reduce the maximum pullout stress. Increasing fiber diameter can increase both the ultimate strain and the maximum stress of knotted end SMA fibers. While neither bond length nor fiber diameter has significant effect on the self-centering ratio. This paper provides a theoretical basis for further study of the combination of SMA fibers and ECC materials.

Role of super-elastic shape memory alloy (SE-SMA) embedment designs on energy absorption in GFRP composites

In this paper, we experimentally investigate the energy absorption characteristics of super‐elastic shape memory alloy (PE‐SMA) integrated in a flexible glass/epoxy composites fabricated using vacuum‐assisted resin infusion (VARI) process. The composite samples have three different PE-SMA configurations, characterized by length variation (35, 70, 150 mm) and anchored/unanchored (35 and 70 mm anchored and 150 mm unanchored). Quasi-static (tensile and indentation (QSI)) and dynamic (low-velocity impact (LVI)) mechanical tests were performed on the composite samples, and the changes in energy absorption characteristics and damage tolerance at strain rates ranging from 8.41 to 952.1/s were compared with the homogeneous glass/epoxy samples. Experiments performed on three SMA-based samples reveal that the 70 mm SMA wire samples, due to their relatively higher SMA deformation and restricted pull-out, show around 13- and a 27-fold increase in absorbed energy under QSI and LVI, respectively, compared with homogeneous glass/epoxy samples.

Life Cycle Thinking–Based Decision Making for Bridges under Seismic Conditions. II: A Case Study on Bridges with Superelastic SMA RC Piers

Life Cycle Thinking–Based Decision Making for Bridges under Seismic Conditions. II: A Case Study on Bridges with Superelastic SMA RC Piers

Seismic performance of self-centering beam-column joints reinforced with superelastic shape memory alloy bars and engineering cementitious composites materials

This paper presents a preliminary experimental investigation of a self-centering beam-column joint reinforced with superelastic Ni-Ti SMA bars and ECC materials. As an alternative to the longitudinal bars in the beam plastic hinge, SMA is used to reduce the residual deformations and dissipate energy, whereas the ECC materials used in the critical region of the joints can improve the ductility of the structural member. Five 1/2 scale frame joints were designed and manufactured. The failure phenomenon, bearing capacity, energy dissipation capacity, displacement ductility, residual deformation, and self-centering performance of the joints were comparatively analyzed through a low cyclic loading experiment. A finite element model of SMA-ECC joints was established, and the influence of the SMA and ECC on the seismic behavior of the joints was investigated. The study results show that SMA bars can dramatically enhance their self-centering capability, enabling self-retrofitting damage. The ECC material is helpful for optimizing the development of the plastic hinge at the end of the beam and improving the ductility and energy dissipation capacity of the structure. The SMA-ECC composite materials exhibit good supplement to significantly improve the ductility, energy dissipation, and self-centering capabilities and delay the stiffness degradation of the structure under extreme events.

Thermomechanical modeling on cyclic deformation and localization of superelastic NiTi shape memory alloy

Stress-induced martensitic transformation of superelastic NiTi leads to inhomogeneous deformation under primarily tensile state. In this study, a constitutive model is developed to describe thermomechanical coupling of cyclically deformed NiTi at various strain rates. Martensitic transformation, strain localization, and the evolution of mechanical properties upon cycling are taken into consideration. The model is implemented implicitly into a finite element code, so that the global thermomechanical response, i.e. stress–strain response and average temperature, and the local thermomechanical response, i.e. band pattern and temperature contour, are achieved. It is demonstrated that band pattern is not only affected by strain rate but also by cycling number. The consistence between simulation and experiment verifies that the proposed approach reproduces the complicated thermomechanical response of cyclically deformed NiTi reasonably.

Damping enhanced novel re-centering seismic isolator incorporating superelastic SMA for response control of bridges under near-fault earthquakes

To enhance the bridge resilient performance, a novel re-centering seismic isolator (RSI) is developed by incorporating damping enhanced (DE)-sliding-lead rubber bearing (LRB) with superelastic shape memory alloy (SMA), which functions with the yielding-sliding hysteretic mode. The numerical model of the novel RSI is developed in OpenSees platform and validated by comparing the numerical and experimental hysteretic loops of the superelastic SMA and DE-sliding-LRB. A parametric design procedure is proposed to determine the optimum parameters of the novel RSI system. A three-span continuous girder bridge is selected to investigate the response mitigation efficiency of the novel RSI system compared with LRB system under near-fault ground motions. A systematic parametric study is conducted to design the optimal parameters of the novel RSI system for bridges. Case study is conducted to investigate the effectiveness of the proposed design procedure and the novel RSI system for response mitigation of bridges. Results show that the damping capability is effectively enhanced by designing the friction coefficient of the sliding element. The novel RSI system can achieve dual mitigation of the displacement responses and base force in piers for bridges. Case study demonstrates the effectiveness of the novel RSI and parametric design procedure.

Seismic response sensitivity and optimal design of an isolated multi-span continuous highway bridge with self-centering SMA RC bridge piers and superelastic SMA restrainers

The use of superelastic shape memory alloy (SMA) in reinforced concrete (RC) pier and restrainers between girder and pier/abutment has been proposed in an earlier study to improve the seismic performance of highway bridges. This paper aims at identifying the significant factors that affect the seismic response of such a novel bridge system. A sequential fractional factorial design method is performed to statistically evaluate the effects of six design factors (three geometry-related and three material-related) as well as their interactions. Additionally, a multi-criteria optimization technique is implemented to determine the most efficient combination of the design parameters for SMA RC piers and SMA restrainers. Results demonstrate that the geometry-related factors and their interactions have large effects (with a contribution greater than 91%) on the relative displacement between the girder and pier. The target residual drift of the pier, design target displacement of the restrainer, and their interaction are the three most significant factors (with contributions approximately 30%–68%) affecting the base shear of the pier. The residual drift of the pier is sensitive to the design target displacement of the restrainer, its interaction with the forward transformation stress of SMA, and material-related factors with regard to the energy dissipation of SMA.

Drift limit state predictions of rectangular reinforced concrete columns with superelastic shape memory alloy rebars

This study derives empirical drift limit state expressions for rectangular concrete columns reinforced with nickel–titanium shape memory alloy (SMA) bars using numerical simulation results obtained from well-calibrated nonlinear finite element models. A parametric study is conducted to examine the effects of various design parameters, such as the axial load ratio, section information, and material properties of concrete and SMA on the drift limit states of the columns. For each limit state (LS), a multivariate stepwise regression analysis was performed. An expression was derived to estimate the associated drift capacity of the SMA-reinforced concrete columns. In addition, the precondition for the SMA-reinforced concrete columns to ensure ductile behavior was investigated. It turned out that the axial load and the amount of transverse reinforcement are the most influential parameters on the ductility of the columns. Finally, nonlinear static and dynamic analyses were conducted for an SMA-reinforced concrete building frame to demonstrate the application of the proposed drift LSs. Both analysis results revealed that the building model tends to experience core crushing or loss of lateral load capacity prior to SMA failure.

Seismic performance assessment of steel frames upgraded with shape memory alloy re-centering dampers for passive protection of structures subjected to seismic excitations using high-performance NiTiHfPd material

This study explores a super-elastic memory alloy re-centering damper device and investigates its performance in improving the response of steel frame structures subjected to multi-level seismic hazard. The configuration of the device was initially proposed by the authors in a different paper. The proposed super-elastic memory alloy re-centering damper (SMARD) counts on high-performance shape memory alloy (SMA) bars for re-centering capability and employs friction springs to augment its deformation capacity. First of all, this study explores the super-elastic response of NiTiHfPd SMAs under various conditions and illustrates their application into seismic applications. In order to collect experimental data, uniaxial tests are conducted on super-elastic NiTiHfPd SMAs in the temperature range of −35 °C–25 °C and at the loading frequencies of 0.05 Hz to 1.0 Hz with four different strain amplitudes. The effects of loading rate and temperature on super-elastic characteristics of NiTiHfPd SMAs are examined. Then, an analytical model of six-story and nine-story steel special moment frame buildings with installed SMARDs is developed to determine the dynamic response of the building. Finally, nonlinear response time history analyses are conducted to assess the behavior of controlled and uncontrolled buildings under 44 ground motion records. Results show that SMARDs can enormously mitigate the dynamic response of steel frame structures at different seismic hazard levels and, at the same time, enhance their post-earthquake functionality.

Molecular dynamics simulations of ultralow hysteretic behavior in super-elastic shape memory alloys

Shape memory alloys (SMAs) that exhibit superelasticity with large recoverable strain and small hysteresis are in demand for practical applications, although their synthesis remains a challenge. We introduce metastable engineering to dope conventional SMA solid-solution atoms of relatively high concentration with "weak" local lattice distortion to realize ultralow hysteretic superelasticity. Large-scale molecular dynamic (MD) simulations of NiTi-based SMAs are performed to demonstrate how the presence of 2 ∼ 4 at.% Nb dopants lead to a stress-induced transition from a metastable pretransitional state to a strain-glass state. This is facilitated by a macroscopically homogeneous and continuous phase transformation in the course of superelastic loading and unloading. This spinodal decomposition-like phase transformation process endows SMAs with anhysteretic superelasticity that is insensitive to loading direction and grain size (below 15 nm). These findings show promise of achieving ultralow hysteretic superelasticity with large recoverable strain for SMAs.

U-Type Honeycomb Based on Super-Elastic Shape-Memory Alloy for One-Dimensional Large Wing Morphing

Flexible skin is an important part of morphing wings, which must comply with the altered geometry during wing morphing. Especially for some applications of one-dimensional large morphing wing such as chord-change, flexible skin is demanded to be capable of large deformation. In this paper, we present a U-type honeycomb structure of shape-memory alloy (SMA) for flexible skin that can undergo large deformation owing to the super-elastic Nickel Titanium (NiTi)-SMA, and the mechanical properties are studied by numerical and experimental methods. In the stage of linear elasticity, U-type honeycomb structure is analyzed theoretically based on the virtual work principle, and its mechanical behavior in the stage of large deformation is studied by finite element method with considering geometric and material nonlinearity. In-plane tensile tests and out-of-plane load-bearing tests are performed, and the experimental results agree well with the numerical results. The experimental results have proved that the U-type honeycomb structure based on NiTi-SMA has large deformation capability, of which the recoverable global strain can reach at least 60%, and has great potential in the application of morphing wings.

Experimental Study on Self-Centering Performance of the SMA Fiber Reinforced ECC Composite Beam.

The combination of superelastic shape memory alloy fibers and ECC materials can form a new SMA fiber reinforced ECC composite material (SMAF-ECC) with good self-centering performance. In order to study the self-centering performance of the new composite material, 6 groups of pre-notch beam specimens were made for three-point bending cyclic loading tests, and the failure phenomenon, hysteresis curve, self-centering effect and influencing factors of the specimens were analyzed. The research results show that when the SMA fibers are effectively anchored in the ECC matrix, the SMA fibers can exert the superelastic properties to provide the ECC beams with recoverying force, and realize the crack self-closure and deflection self-recovery function for the beams, with the minimum residual crack width and deflection is only 0.9 mm and 1.3 mm respectively. Increasing fiber content can cause a small increase in the self-centering ability of the beams. However, only when the fiber diameter is appropriate, better self-centering effect can be achieved, but the difference caused by fiber diameter in the test was only 5%. SMA Fiber end forms have significant influence on self-centering performance. The knotted end beam can get a more than 70% self-centering ratio, while the straight end beams and bended end beams have no self-centering ability. The research results provide important reference for the research and application of this new self-centering materials and their structures.

Passive Attenuation of Mechanical Vibrations with a Superelastic SMA Bending Springs: An Experimental Investigation

This work presents an experimental study related to the mechanical performance of a special design spring fabricated with a superelastic shape memory alloy (SMA-SE). For the experimental testing, the spring was coupled in a rotor machine, aiming to attenuate the mechanical vibration when the system went through a natural frequency without any external power source. It was verified that the reduction in instabilities stemmed from the better distribution of vibration force in the proposed device, as well as the damping capacity of the spring material. These findings showed that the application of the M-Shape device of SMA-SE for three different cases could reduce vibration up to 23 dB when compared to the situations without, and with, 1.5 mm of preload. The M-Shape device was shown to be efficient in reducing the mechanical vibration in a rotor system. This was due to the damping capacity of the SMA-SE material, and because the application did not require any external source of energy to generate phase transformation.

Development of novel SMA-based D-type self-centering eccentrically braced frames

In this study, a novel D-type self-centering (SC) eccentrically braced frame (EBF) based on superelastic shape memory alloy (SMA) is proposed. Unlike traditional SC-EBFs using a posttensioned mechanism, the proposed EBF utilizes only SMA angles and steel angles, which can vastly simplify the construction of the whole structure. The behavior of the novel SMA-based SC-EBF is studied through numerical models, and its effectiveness is partially verified by the previous testing results. The study is divided into two parts. First, the behavior of the core component, i.e., link beams (LBs), is investigated. The cyclic behavior of the LB indicates that the SMA angles and steel angles can effectively prevent the LB from any damages. The shear key construction can guarantee the centerline-rocking behavior of the LB, greatly decreasing the stress concentration in angles and protecting the corner of the LB. Subsequently, parametric studies on three important design parameters, namely, steel-to-SMA elastic stiffness ratio, prestrain level in bolts, and angle leg length, are conducted via 12 different designs. All three parameters influence the structural stiffness and strength to some extent. The steel-to-SMA stiffness ratio governs the SC and energy dissipation capacity, and a ratio in the range of 0.35–1.5 is recommended in this study. Second, the behavior of a one-story one-bay SMA-based SC-EBF is investigated to illustrate its unique SC performance. The proposed SMA-based SC-EBF presents a promising SC structure alternative, which may simultaneously achieve replaceability, SC capacity, and simple construction.

Concept of Elastocaloric Granular Material Made from SMA Wires in Bending

Shape memory alloys (SMAs) are promising materials for the creation of heating or cooling systems due to their elastocaloric character. The paper proposes a concept of elastocaloric “porous” SMA beam working in bending. The beam was made with superelastic nickel-titanium SMA wires of different diameters placed in a flexible tube. While water was flowing through the tube, bending was manually applied using 3D printed wavy profiles with portions of arcs with constant curvatures. Preliminary results showed an oscillation of the fluid temperature at the outlet of the flexible tube (containing the SMA wires) at the same frequency as the mechanical loading, validating therefore the concept of elastocaloric porous SMA beam operating in bending.

Seismic Performance Assessment of a Single-Layer Spherical Lattice Shell Structure with Multifunctional Friction Pendulum Bearings

The dynamic responses of spatial lattice shell structures with friction pendulum bearings (FPBs) under multidimensional seismic excitations are complex. In addition, FPBs may experience uplift and separation of the bearing components owing to excessive displacements. In this study, a novel multifunctional FPB (MFPB) with a multi-defense system was developed, and its effectiveness in reducing and controlling the seismic responses of spherical lattice shell structures was evaluated. The proposed MFPB comprises an FPB, superelastic shape-memory alloy cables, and sleeve restrainers. A mechanical model of the MFPB was established, and its isolation and control behaviors were investigated through numerical simulations. Furthermore, the main characteristics and advantages of the isolation system were analyzed. Subsequently, the MFPB system was applied to a single-layer spherical lattice shell structure with surrounding columns. A computational model of the controlled structure was developed using the OpenSees software. Finally, nonlinear time history analyzes were conducted to analyze the seismic performance of controlled and uncontrolled lattice shells. The results demonstrate that the MFPB isolation system can effectively control the structural responses of isolated spatial lattice shell structures under horizontal and vertical seismic excitations and improve their seismic resilience.

Sliding-LRB incorporating superelastic SMA for seismic protection of bridges under near-fault earthquakes: A comparative study

To enhance the resilient performance of the sliding-lead rubber bearing (sliding-LRB), the sliding-LRB combining superelastic shape memory alloy (SMA), termed as superelastic sliding-LRB, is developed for bridge seismic isolation. A comparative study is conducted to demonstrate the advantages of the superelastic sliding-LRB by comparing with the SMA-based friction pendulum bearing (SMA-based FP) considered in the current study. A parameter-seeking procedure is proposed to optimize the parameters of the two superelastic isolation systems, which are conducted considering the equivalent characteristic strength and post-yield stiffness. A three-span seismically isolated bridge is employed to demonstrate the seismic mitigation capability and cost efficiency of the two superelatic isolation systems with various isolation periods. Results show that the parameter-seeking procedure is effective in achieving the parameters of the two superelastic isolation systems. The superelastic sliding-LRB can effectively balance the displacement and base force for isolated bridges and using a relatively smaller amount of SMA, demonstrating higher cost efficiency than that of the SMA-based FP system with relatively long isolation period. The case studies revealed the effectiveness of superelastic sliding-LRB for enhancing the response mitigation and recentering performance with higher cost efficiency for bridges by using NiTi (50.8% Ni) wire. • The superelastic sliding-LRB system is suggested to improve the seismic resilience of bridges. • A parameter-seeking procedure is suggested to achieve the optimum parameters of two superelastic isolation systems. • A comparative study is conducted to demonstrate the effectiveness of superelastic sliding-LRB compared with SMA-based FP. • Case study demonstrated seismic mitigation performance and cost efficiency of the superelastic sliding-LRB system. • Superelastic sliding-LRB can achieve an optimum improvement in balancing the displacement and base force using less amount of SMA.