Superelastic Shape Memory Alloy Research Articles

Design and Experimental Evaluation of Passive Ankle-Foot Orthoses Hinges Using Superelastic NiTi Shape Memory Alloy

Design and Experimental Evaluation of Passive Ankle-Foot Orthoses Hinges Using Superelastic NiTi Shape Memory Alloy

Seismic performance of resilient self-centering bridge piers equipped with SMA bars

This study examines the seismic behavior of precast post-tensioned segmental (PPS) bridge piers that are equipped with Shape Memory Alloy (SMA) bars. PPS piers are designed to minimize overall and localized damage to bridges by incorporating natural hinges and rocking mechanisms. The introduction of super-elastic SMA bars has the potential to enhance the use of PPS piers in high-seismic regions. A parametric study is conducted to identify piers with the highest energy dissipation capacity, which are then subjected to dynamic analysis. The aim is to assess the energy dissipation capability of these systems by subjecting the selected piers, both with and without SMA bars, to far-field ground motions using a Finite Element (FE) framework. The seismic performance of the piers is evaluated using Incremental Dynamic Analysis (IDA) curves, which are obtained from analyzing the piers under 44 far-fault ground motions. The IDA results indicate a significant reduction in the drift responses of the piers when SMA bars are utilized.

Investigating Pullout Behavior of Inclined Superelastic SMA Fiber in a Mortar Matrix Based on Finite Element Method

Investigating Pullout Behavior of Inclined Superelastic SMA Fiber in a Mortar Matrix Based on Finite Element Method

The driving characteristics of bidirectional SMA wire actuators - Theoretical modeling and experimental testing

Bidirectional shape memory alloy (SMA) wire actuators play a critical role in driving applications thanks to their characteristics in memory shaping and good overall performance. SMA wire actuators were often studied at a single load type, and there is no comprehensive comparison investigation of different SMA wire actuators in the literature. In this paper, the driving characteristics of four available types of SMA wire actuators, namely, dead-load, biased spring, differential type, and biased superelastic SMA wire actuators are modelled mathematically and then tested in laboratory settings. The characteristics of SMA wire actuators are quantitatively measured and then compared under a range of application scenarios. The output displacement and thermal-electric models of NiTi SMA wires are derived based on the Brinson constitutive model with stress effects in consideration. A customized SMA wire actuator test device is developed to evaluate the theocratical models and benchmark testing four types of SMA wires and their driving characteristics, including structure, design, displacement, response and recovery time, energy efficiency and hysteresis properties. The SMA wire phase transformation characteristics of the four actuators are tested under different activation currents, and then quantitatively compared. In conclusion, this paper adds theoretical and experimental significance to the literature on SMA wire actuators. The reference value on the phase transformation characteristics of SMA wires can be used to guide SMA application in industrial designs and enable theoretical studies in complementary research areas.

Research on Superelasticity of Shape Memory Alloys Based on Reproducing Kernel Particle Method

Shape memory alloys (SMAs), known for their unique superelastic effect, exhibit superior mechanical properties compared to traditional materials. In this study, the reproducing kernel particle method (RKPM) is applied to investigate the superelastic effect of the SMAs. A segmented linearization RKPM model is developed to establish the stress–strain relationship for the superelastic effect, and relevant formulas of superelasticity for the SMAs are derived. Finally, the applicability and accuracy of the RKPM in analyzing the superelastic effect of the SMAs are demonstrated through numerical examples.

Investigating the Application of Smart Materials for Enhanced Maintenance of Rubber Expansion Joints in Bridge Expansion Joint Systems

In South Korea, the public infrastructure encompasses 172,111 facilities, with bridges accounting for a significant segment (totaling 34,199). These bridges undergo expansion due to traffic, vehicular loads, and temperature fluctuations. Expansion joint devices are installed to maintain vehicle stability and driving performance across expansion gaps. While these devices effectively ensure vehicular stability and performance, they do not address issues such as leakage and debris fall; therefore, rubber expansion joints should be installed. However, these rubber joints are prone to damage from various factors, resulting in secondary issues such as girder corrosion and accidents under bridges. Because of these inherent vulnerabilities, these joints require frequent replacements, making continuous bridge maintenance challenging. Therefore, this study explores the development of novel expansion joints using superelastic shape memory alloys to overcome the limitations of traditional rubber expansion joints. A comparative finite element analysis was conducted on the developed superelastic shape memory alloy and traditional rubber expansion joints. This study also assessed the long-term usability of these novel joints, particularly their ability to revert to their original shape post load removal. This research presents a promising alternative to conventional expansion joints and holds potential implications for enhancing the durability and safety of bridge infrastructure.

A hybrid self-centering seismic damper: Finite element modeling and parametric analysis

This study presents a finite element model for a hybrid self-centering damper considering the rate and temperature effects and explores the effects of different design parameters on the damper response. The damper, called as superelastic friction damper (SFD), consists of superelastic shape memory alloy (SMA) cables and a frictional energy dissipation mechanism. The experimental response of the SMA cables, frictional unit and overall damper at different loading frequencies and temperature are used to develop numerical model of the damper. Once a validated numerical model is obtained, parametric studies are carried out to evaluate force-displacement response of the damper when the design parameters are altered. The effects of damper design parameters on the equivalent stiffness, dissipated energy, equivalent viscous damping and self-centering capabilities of the damper are analyzed. Based on the findings, the recommendations for the design of the damper are presented.

Self-centering steel beam-to-column connections with novel superelastic SMA angles

This paper proposed a novel type of self-centering (SC) steel beam-to-column connection using superelastic shape memory alloy (SMA) angles and steel angles. Unlike conventional post-tensioned SC connections, the proposed SC connection utilized SMA angles to provide SC ability and moderate energy dissipation, enabling easy installation and replacement. Experimental and numerical studies were conducted to investigate the cyclic behavior of the connection. Two types of steel connections with different configurations were tested: one equipped with SMA angles only and the other with a combination of SMA and steel angles. The cyclic behavior of the SC steel beam-to-column connections was investigated through a series of experiments. A numerical study was also conducted to provide more insight into the mechanical behavior of the connections and the local strain condition of SMA angles. Numerical results were validated with experimental results. Comparative results demonstrated that both connections exhibited desirable flag-shaped hysteresis behavior with SC and energy dissipation. No obvious damage and residual deformation were observed in SMA angles after cyclic loads in both the experiments and numerical simulations, indicating that the SC connection using SMA angles can be reused for successive mainshocks and aftershocks without replacement. Therefore, the proposed SC connection can help achieve the goal of earthquake resilience in structural design.

Experimental investigation of mechanical properties of NiTi superelastic shape memory alloy cables

As a self-centering element in a recoverable functional structure, shape memory alloys (SMAs) usually require a large diameter to provide sufficient bearing capacity, and pre-strain is applied to obtain good self-centering performance and initial stiffness. This study aims to explore the mechanical properties of large-diameter NiTi SMA cables and their potential applications in recoverable functional structures. Through the uniaxial tensile test of 11 SMA cables with 7 × 7 × 1.0 mm, the mechanical properties of SMA cables under different strain amplitudes, cyclic loading effects, loading rates, and different pre-strain modes were evaluated. The results indicate that the energy dissipation capacity of SMA cable increases with the increase of strain amplitude, but at the same time, it will reduce its self-centering capabilities. The equivalent viscous damping ratio decreases after the SMA cable enters the martensitic hardening stage. The residual strain of SMA cable depends on the combined effect of strain amplitude and number of cycles. The larger the strain amplitude and the number of cycles, the more significant the degradation of self-centering performance. After 20 times of constant amplitude loading and unloading, the mechanical properties of SMA cable are relatively stable. The reverse phase transformation stress of the pre-strained SMA cable gradually decreases with the increase of the tensile strain amplitude. The pre-strained SMA cable exhibits a stable restoring force strength in the sub-cycle, and the residual strain is zero. The test results support experimental data for applying large-diameter SMA cables in recoverable functional structures.

Behavior of Smart Beams Reinforced with Superelastic Shape Memory Alloy Rebar (SMA)Subjected to Repeated Loading

A reinforced concrete beam with shape memory alloy rebar (SMA) is a novel form of smart beam used in smart seismic structural systems to reduce the effects of earthquakes while keeping nearly the same load-carrying capability as conventional concrete beams. The experimental research investigation is carried out to study beams' flexural behavior by replacing some steel rebars with shape memory alloy rebars (SMAs) in the longitudinal reinforcement zone. The experimental program included testing three beams to investigate the effects of the replacement of longitudinal steel rebar by shape memory alloy rebar on the flexure behaviour of beams. The beams were tested by the repeated load. The study was focused on determining ultimate load, maximum deflection, and load-deflection behaviour. Experimentally, the flexure behaviour beams are significantly affected by changing the number of reinforcing bars with shape memory alloys (SMA) longitudinal direction. However, for using one rebar of shape memory alloy as a longitudinal reinforcement, the replacement of one bar and two bars of shape memory alloy SMA beams have less yield load than the control beam by about (12.5%) and (37.5%) respectively. The replacement of one bar and two bars of shape memory alloy SMA beams having less ultimate load compared with reference beam by about (9.82%) and (21.94%) respectively. Because of its superelasticity quality, the introduction of superelastic SMA bars to the beam shows excellent recentering ability.

Seismic Performance of Steel-SMA Reinforced Columns: Numerical Investigation

The residual displacement of a bridge column governs its functionality after a severe earthquake. A box girder bridge was selected as a case study. Pushover analysis, quasi-static cyclic analysis, incremental dynamic analysis and fragility analysis were conducted to compare the seismic performance of the reinforced concrete column, the unbonded prestressed reinforced concrete column (UBPRC column) and the unbonded prestressed reinforced concrete column using superelastic shape memory alloy (UBPRC-SMA column). The threshold loading plateau axial load ratio was recommended in designing a UBPRC-SMA column, thereby ensuring the self-centering performance of a bridge column with fewer superelastic SMA bars. The self-centering performance was considered in the fragility analysis, and the results show that the UBPRC-SMA column suffers the least damage for the collapse damage state.

Superelastic Shape Memory Alloy Honeycomb Damper

The relative displacements between the girders and piers of isolated bridges during intense earthquakes are usually so large that traditional restrainers cannot accommodate the resulting deformation. A novel superelastic shape memory alloy (SMA) honeycomb damper (SHD) is proposed as a means to combine the large strain capacity of SMA and the geometrical nonlinear deformation of honeycomb structures. As a result, the large deformation capacity of the novel damper satisfies the requirements for bridge restrainers. The proposed device consists of a superelastic shape memory alloy (SMA) honeycomb structure, which enables a self-centering capability, along with steel plates that serve to prevent the buckling of the SMA honeycomb. An examination of the SHD was undertaken initially from theoretical perspectives. A multi-cell SHD specimen was subsequently manufactured and evaluated. Following this, numerical simulation analyses of the SHDs using a three-dimensional high-fidelity finite element model were employed to examine the experimental results. In the end, a technique for improving the SHD was suggested. The results indicate that the SHD is able to demonstrate superior self-centering capabilities and stable hysteretic responses when subjected to earthquakes.

Cyclic load behaviour of reinforced concrete columns with SMA and ECC in the critical regions

Design techniques that can reduce the residual deformation are necessary to ensure that reinforced concrete (RC) structures function properly after earthquakes. The use of superelastic materials in the plastic hinge region permits the structure to undergo large displacements with less residual displacement upon load removal. The present study deals with the incorporation of two innovative materials, super-elastic shape memory alloy (SMA) and engineered cementitious composites (ECC), in the critical region or plastic hinge zone of the column. This numerical study focused on the seismic performance evaluation of full-scale RC circular cantilever columns reinforced with two types of nickel-based and two iron-based SMA and ECC in the critical region. The specimens were subjected to a reverse cyclic lateral loading with a constant axial load at the top. The performance of these hybrid columns was compared with those of conventional steel-reinforced columns and steel-reinforced columns with ECC in the critical region. The results of the study revealed that the columns reinforced with SMA and ECC in the critical region showed negligible residual displacement compared to the normal and ECC columns. The column reinforced with the FeNCATB SMA and ECC in the critical region showed better behaviour in terms of a higher load-carrying capacity and ultimate displacement/rotation under a higher drift ratio. From the longitudinal rebar strain profile, it is evident that this hybrid column specimen did not reach the elastic strain limit at a higher drift ratio than the other specimens.

Flexural performance and crack closing capacity of double-hooked-end superelastic shape memory alloy fibre-reinforced concrete beams under cyclic loading using digital image correlation

This study investigated the flexural response of hooked-end superelastic shape memory alloy (SMA) fibre-reinforced concrete beams under cyclic loading. The fibres were fabricated from heavily coldworked 1 mm diameter NiTi alloy followed by heat treatment at 350 °C for 40 min. Fibre volume fractions of 0.25 %, 0.5 %, 0.75 % and 1 % in normal strength (NS) concrete and 0.5 % and 1 % in high performance (HP) concrete were investigated. The fibre-reinforced prisms were tested under four-point bending. Digital image correlation (DIC) techniques were used to measure the cracking behaviour of the SMA FRC beams. The results demonstrated that increasing the volumetric ratio of the superelastic fibres as well as the compressive strength of the matrix led to higher flexural strength and toughness, deflection recovery, and crack closing performance. HP concrete matrix with 1 % SMA showed the highest toughening and displacement recovery performance, with a re-centering ratio of more than 55 % at L/140 mid-span displacement. An SMA fibre volume fraction between 0.75 % and 1 % in NS mixtures and at least 1 % in HP composites can be adopted for better flexural resistance.

NUMERICAL MODELLING OF REINFORCED CONCRETE FRAMES RETROFITTED WITH BUCKLING RESTRAINED BRACE INCORPORATING STEEL AND SUPERELASTIC SHAPE-MEMORY ALLOY CORES

The numerical response of one-storey reinforced concrete frames designed following design standards before the enactment of modern seismic provisions was investigated in this study using the nonlinear finite element method. An unbraced frame and two frames retrofitted with a buckling restrained brace (BRB) incorporating either a stainless steel or chrome molybdenum core bar were modelled. The nonlinear static reverse cyclic loading performance of the frames was assessed. Modifications to the BRB are proposed to mitigate deficiencies identified with the original BRB and steel core bars. The modified BRB incorporating a superelastic shape-memory alloy (SE-SMA) core bar was further modelled. The results illustrate the benefits of implementing SE-SMA as a retrofit strategy to control permanent displacements. The modified BRB with a SE-SMA core exhibited improved lateral strength and displacement capacities relative to the frames retrofitted with steel cores and the control bare frame.

A study on performance evaluation of hybrid dampers

In this study, the experimental performance evaluation of the hybrid damper with an innovative structure was performed as a preceding study to apply the self-centering braced frame to the structure. Hybrid dampers used a structural novel material with recovery and semi-permanent characteristics to recenter the displacement caused by external loads. Structural novel materials were used polyurethane and superelastic shape memory alloy(SSMA), which provide recentering capacity through recovery properties, and neodymium permanent magnets, which enhance energy dissipation through friction properties. SSMA could recover the displacement generated at room temperature due to shape memory effect and superelastic behavior, and initial compression was applied to polyurethane to give recovery. In order for each material to play a role appropriate for its purpose, the hybrid damper was designed so that SSMA was applied in tension, polyurethane in compression, and neodymium permanent magnet in friction. In addition, a structural experiment was conducted on the damper by setting the design variables to the initial compression of polyurethane and the presence or absence of neodymium permanent magnet. As a result of analyzing the maximum force, residual displacement, recentering force, and energy dissipation capacity for the behavior response derived from experiments on each model, the model that included all three new structural materials and applied the largest initial compression showed the best performance. Therefore, it is expected that the development of a self-centering braced frame will be possible if a hybrid damper is applied as a result of performance evaluation through structural experiment.

Seismic response analysis of single-degree-of-freedom systems with superelastic monocrystalline CuAlBe SMAs

Although the potential of shape memory alloys (SMAs) in seismic applications has been well recognized, the majority of the past research was directed to NiTi SMAs. This study aims to investigate the feasibility of using a novel type of SMA, termed superelastic monocrystalline CuAlBe SMA, in seismic vibration control. According to prior loading tests, CuAlBe SMAs had an excellent superelasticity behavior with the largest recoverable strain over 25%. Within the superelastic range, CuAlBe SMAs shown unloading behavior which was dependent on the strain amplitude in the present cycle, which could not be captured by current constitutive models. Hence, a new constitutive model was firstly proposed to simulate the hysteretic behavior of CuAlBe SMAs, and a good agreement is found between the experimental data and numerical simulations. And then, to explore the characteristics and advantages in terms of seismic applications, case studies on single-degree-of-freedom (SDOF) systems with NiTi and CuAlBe SMAs were carried out. Finally, two design methods were used to size the NiTi and CuAlBe SMAs, and the corresponding SDOF systems were subjected to two ensembles of earthquakes associated with frequently occurred and design basis seismic hazard levels. The simulation results indicated that, compared with NiTi SMAs, CuAlBe SMAs could more effectively control the displacement and acceleration demands, if their ductility capacity was fully utilized in the design procedure.

Low cycle fatigue lifetime prediction of superplastic shape memory alloy structures: Application to endodontic instruments

In this paper we propose a methodology for a fast numerical determination of low cycle fatigue lifetime of superelastic shape memory alloy structures. This method is based on the observation that generally, in low cycle fatigue, shape memory alloy (SMA) structures are subject to loadings that lead to a confined non-linear behaviour at stress concentration points, such as notches. Numerical fatigue lifetime prediction requires the computation of the mechanical state at critical points. However, classical computational methods, like the non-linear finite element method, lead to a prohibitive computation time in a non-linear cyclic framework. To overcome this issue, we propose to use fast prediction methods, based on localization laws. Following the determination of the stabilized behaviour, an energetic fatigue criterion is applied. The numerical fatigue life prediction model is validated experimentally on SMA endodontic instruments.

Development and preliminarily tests of superelastic Ni-Ti SMA ribbed bars for reinforced concrete applications

The research on superelastic shape memory alloy (SMA) bars reinforced concrete members and structures has received a great deal of attention recent years. In this study, novel superelastic Ni-Ti SMA ribbed bars were developed and investigated to improve the bonding performance and to avoid the problem that the threaded or mechanical anchorage parts of the SMA plain round bars (used in many previous studies) were easy to break in advance. A comprehensive experimental program was conducted to characterize the cold-rolled superelastic Ni-Ti SMA ribbed bars, including scanning electron microscopy to observe the micromorphology, differential scanning calorimeter tests to analyze the phase transformation temperatures, and cyclic tensile tests to examine their mechanical properties. Moreover, five reduced-scale concrete beams were prepared and tested to verify the advantages of the superelastic SMA ribbed bars as bottom longitudinal reinforcements. Results shown that the cyclic tensile response of the cold-rolled SMA ribbed bar subjected to some degree of annealing can be similar with that of the annealed SMA plain round bar. Compared with the beam specimen incorporating ECC and superelastic SMA plain round bars and the beam specimen incorporating normal concrete and superelastic SMA ribbed bars, the specimen incorporating ECC and superelastic SMA ribbed bars exhibited a significantly improved deformation ductility, ultimate bearing capacity, energy dissipation ability, self-centering capability, and cracking performance under the cyclic flexural loads. Besides, no reinforcement slippage or fracture were observed during the entire loading process in all beams, suggesting satisfactory bonding effect and uniformity.

Novel two-stage superelastic SMA bars with enhanced ductility and graded pseudo-yielding for seismic applications

This study proposed a novel design concept of two-stage superelastic shape memory alloy (SMA) elements to achieve graded pseudo-yielding and enhanced ductility that are suitable for multi-level seismic design. This design concept utilizes the natural stiffness hardening of superelastic SMA after martensite finish stress to activate two transformation stress plateaus (i.e., graded pseudo-yielding points) of two-stage SMA elements. Consequently, the small- and large-diameter parts can be activated under small and rare earthquakes, respectively. This study discussed the design concept, experimental tests, and numerical simulations of two-stage SMA bars with two different diameters. The cyclic behaviour and failure modes of the two-stage SMA bar observed in the tests successfully validated the expected merits. The ductility of the tested two-stage SMA bar increased by 50% compared with that of a normal one-stage SMA bar. Meanwhile, the self-centring (SC) capacity was not compromised when parts of the bar were in the martensite phase during the graded pseudo-yielding. The two-stage SMA bar showed desirable cyclic performance during the whole loading process. Finally, a finite element model was established based on the testing results to facilitate parametric and comparative studies. The two-stage SMA bar showed obvious graded pseudo-yielding behaviour, larger ductility, and greater post-pseudo-yielding stiffness, which can potentially benefit seismic applications.