Superelastic Shape Memory Alloy Research Articles

Cyclic performance of I-beam-to-SHS column joint using through SMA bolts

Shape memory alloys (SMAs) are increasingly used in steel structures. Three I-beam to square hollow section column joints with super-elastic SMA bolts were tested. A finite element model was subsequently developed, and a parametric study was conducted. The results show that all the joints can be classified as semi-rigid or partial-strength joints. With an increase in the SMA bolt length, the residual story drift and energy dissipation of the joint decreased. Increasing the SMA bolt diameter improved the energy dissipation capacity. In terms of energy dissipation and economic cost, installing the SMA bolts only outside the beam flanges is a good choice.

Experimental investigation of buckling stability of superelastic Ni-Ti tubes under cyclic compressive loading: Towards defining functionally stable tubes for elastocaloric cooling

Elastocaloric cooling technology recently attracted significant attention as an environmental friendly alternative to vapor-compression technology. It is based on the elastocaloric effect, which occurs in superelastic shape memory alloys (SMAs) during stress-induced martensitic transformation. To date, several proof-of-concept devices (mostly based on tensile loading) have been developed, but limited fatigue life was shown to be one of the major issues. Compressive loading improves the fatigue life of such devices significantly, but in return buckling of SMA structure might occur. To overcome this challenge, it is crucial to understand the buckling phenomena of SMA structures, especially for thin-walled tubes that seem to be ideal candidates for application in elastocaloric cooling devices. Here, we experimentally investigated the effects of the diameter-to-thickness ratio (Dout/t) and slenderness (λ) on buckling stability of Ni-Ti tubes that were subjected to cyclic compressive loading. In total, 161 superelastic Ni-Ti tubes with outer diameter (Dout) ranging from 2 mm to 3 mm, Dout/t ranging from 5 to 25 and gauge lengths (Lg) ranging from 6 to 20 mm were tested. The loading procedure consisted of 3 parts: (I) 1 isothermal full-transformation loading cycle, (II) 50 training cycles, and (III) 20 adiabatic cycles to simulate loading conditions in elastocaloric device. We constructed experimental phase diagrams of buckling modes in λ - Dout/t space for constant Dout and in λ - Dout space for constant Dout/t ratio. Marked areas of functionally stable tubes in these phase diagrams give the design guidance for future developments of durable and efficient elastocaloric devices and other applications, e.g. actuators and dampers.

Fatigue life study of superelastic NiTi Shape Memory Alloys using self-heating under cyclic loading method

This paper concerns the High Cycle Fatigue (HCF) properties of superelastic NiTi Shape Memory Alloys (SMA). Up till today, HCF fatigue of SMAs has not been much studied and, therefore, there is a lack of tools to design the SMA pieces and structures against fatigue. In this study, it is proposed to investigate the HCF properties of NiTi SMAs by using a new approach based, on one hand, on self-heating tests under cyclic loading and, on the other hand, on a probabilistic two-scale model. The proposed model permits to establish a link between self-heating phenomena and the fatigue properties of the material. Finally, our approach is validated by comparing the model predictions to the experimental results obtained with classical HCF fatigue tests.

Bending fatigue life enhancement of NiTi alloy by pre-strain warm surface mechanical attrition treatment

Many structures like medical stents made of superelastic NiTi shape memory alloy (SMA) are subjected to cyclic bending loads and show a limited fatigue life due to crack nucleation and growth in the surface layers under local tensile stress. Here, we enhance the bending fatigue life of NiTi plates by pre-strain warm surface mechanical attrition treatment (pw-SMAT) where the austenite phase is directly subjected to severe plastic deformation and grain refinement without inducing phase transformation. Amorphous and grain size gradient (5–100 nm) microstructures, as well as a maximum compressive residual stress of 1093 MPa are produced in the surface layer of the NiTi plates via the pw-SMAT. The compressive residual stress notably reduces the surface tensile stress from bending. The grain size gradient layers with improved hardness and reduced hysteresis have high fatigue crack nucleation resistance, while the middle large-grained layers of the plates have high fatigue crack growth resistance. The combined effects of the gradient nanostructure and the compressive residual stress substantially increase the bending fatigue life of the NiTi plates from an original 103 cycles to over 1.3 × 104 cycles. The results open up a new route to improve the bending fatigue life of NiTi plates by heterogenous nanostructures.

Seismic performance of precast CFDST segmental column with hybrid energy dissipation devices

Precast segmental column has various advantages as compared to traditional cast in-situ monolithic column. The major issues of such column are the concrete compressive damages at the toes of the segments and limited energy dissipation due to the rocking mechanism. This study explores a new design of precast segmental column with damage resistant features and hybrid energy absorption system. Super elastic shape memory alloy (SMA), external energy dissipation devices and concrete-filled double-skin steel tubes (CFDST) segments are used to form a seismic resilient precast segmental column. Specifically, the SMA rebars provide the column with good self-centring ability; the external replaceable energy dissipation devices provide extra energy dissipation capacity; the use of CFDST segments mitigates the concrete damage. It is found that the proposed column performs well with small residual displacement, satisfying energy dissipation and minor damage.

Parabolic deployable mesh antenna with a hingeless system of superelastic SMA ribs and composite tape springs

This paper describes an experimental and numerical investigation of a compact parabolic deployable mesh antenna with a hingeless deployment system of superelastic SMA ribs and composite tape springs. One of the key features of the proposed antenna is the primary reflector with SMA ribs. The SMA's superelasticity allows the stowed reflector to deploy without plastic deformation with high damping and fatigue resistance. Thus, the antenna can be folded compactly like lotus flowers. Furthermore, composite tape springs with inherent hysteresis make the mast structure folded. Due to these hingeless deployment mechanisms, high reliability is achieved with a moderate level of packaging efficiency, lightweight, and low deployment shock.The effectiveness of the mechanism design was demonstrated by validation tests such as a basic characteristic test of superplastic SMA ribs, a deployment test of the antenna, and surface error measurement, as well as a deployment simulation with the aid of the finite element method.

Evaluating the Symmetric Behavior of Single Superelastic SMA Rings with Circular and Elliptic Shapes

Evaluating the Symmetric Behavior of Single Superelastic SMA Rings with Circular and Elliptic Shapes

Assessment of constant force properties for various superelastic shape memory alloy structure

In the biomedical materials applications, biocompatibility of implanted prostheses and clamped medical devices leads to devices failures that frequently occur. Embedding constant force elements based on superelastic shape memory alloys in implanted prostheses and clamped medical devices to achieve constant force clamping of soft tissues can improve biocompatibility issues. This paper proposes combining shape memory alloy superelasticity and structural optimization to design constant force elements with special stress-strain relationships. The design criteria for constant force element and evaluation method for constant force characteristics are proposed. According to the design criteria and evaluation methods of in-vivo constant force elements, I-shaped structure, heart-shaped structure, M-shaped structure, V-shaped structure, O-shaped structure and C-shaped structure were designed based on shape memory alloy. Then the finite element models of various structures are established to compare and analyze their constant force characteristics. The results show that the C-shaped may obtain a force-displacement curve with large constant force region by changing their geometric parameters, and it is easier to find the characteristics geometric parameters with a large constant force range.

Equivalent modeling for a finite element analysis of the mechanical properties of a bolted joint with a superelastic shape memory alloy

In this study, a beam element model with tension–compression asymmetry is constructed by using the improved Brinson model. A comparison between the calculated and experimental results verifies the computational effectiveness of the beam element. Then, a new mechanical property analysis model of a shape memory alloy (SMA) bolted joint based on the beam element model is proposed. Combined with a constructed convergence function, the mechanical equilibrium state of the SMA bolted joint under preload and external load is solved iteratively by the bisection method. Consequently, the stress–strain response curve of the SMA bolt rod and the root of the first thread, the deformation law of each thread and the separation conditions of the contact surface of members are analyzed by the proposed model. It is shown that the new proposed method can greatly improve the calculation efficiency with high calculation precision compared with the traditional calculation method.

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.