Superelastic Shape Memory Alloy Bars Research Articles

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

This study examines the seismic behaviour of precast post-tensioned segmental (PPS) bridge piers that are equipped with shape memory alloy (SMA) bars. PPS piers are designed to minimise overall and localised 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 highly 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 framework. The seismic performance of the piers is evaluated using incremental dynamic analysis (IDA) curves, which are obtained from analysing 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 utilised.

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.

Cyclic behavior and seismic control performance of SMA friction damper

Using superelastic shape memory alloy (SMA) bars and non-asbestos organic friction material, this study developed an innovative self-centering friction damper (SCFD). This hybrid passive control device consists of the self-centering device using SMA bars and the friction energy dissipation device which can provide excellent self-centering ability and energy-absorbing ability to meet the requirements of civil engineering applications. To explore the feasibility and hysteretic properties of the SCFD, experimental tests under cyclic loading were conducted. According to the experimental results, the proposed SCFD exhibited a stable and repeatable flag-shaped hysteretic response, which can achieve the recovered displacement of 76.07% and dissipated energy of 6.04 kJ at 42 mm. The finite element model of the SCFD using ABAQUS software was established and validated by experimental results. And a series of numerical simulations with different parameters were performed, which enables a more in-depth interpretation of the SCFD. Additionally, a system-level nonlinear time-history analysis was performed on a three-story steel frame equipped with and without SCFDs. The dynamic analysis results indicated that the SCFDs could effectively reduce structural damage and enhance post-earthquake recoverability under rare earthquakes.

A Novel Hybrid Self-Centering Piston-Based Bracing Fitted with SMA Bars and Friction Springs: Analytical Study and Seismic Simulation

Self-centering piston-based braced frames (SC-PBBFs) are such structural assemblies that can withstand a severe earthquake through passive control. The self-centering motion of the bracing system curtails the seismic permanent damage of a building, thereby considerably mitigating postearthquake repair costs and downtime. This paper proposes a novel hybrid self-centering piston-based bracing (SC-PBB) equipped with friction springs (FS) and superelastic shape-memory alloy (SMA) bars. Analytical formulas are presented for rapid analysis and preliminary seismic design of the SC-PBBFs fitted with SC-PBBs. The general mechanics of the proposed archetypes confirms their symmetric self-centering behavior. To validate the proposed design procedure, a set of SC-PBBFs fitted with tension-only SMA bars and precompressed FS is examined under far-field ground motions. Effects of prestressing of components and earthquake types (crustal, subcrustal, and subduction) are quantified and compared with the responses of buckling-restrained braced frames (BRBFs). The analysis outcomes revealed the effectiveness of the proposed hybrid SC-PBBF subjected to design-level earthquakes.

Seismic collapse assessment of hybrid self-centering piston-based braced frames equipped with SMA bars and friction springs

Hybrid Self-centering Piston-Based Bracing (SC-PBB) is a cutting-edge, resilient earthquake-resistant element that comprises friction springs (FSs) and super-elastic shape memory alloy (SMA) bars. This innovative bracing component is specifically devised to attain a full self-centering response under a design-level earthquake, effectively mitigating the need for post-earthquake repairs and downtime. Nevertheless, concerns surrounding the potential collapse of Self-centering Piston Based Braced Frames (SC-PBBFs) during severe seismic motion have highlighted the necessity for a comprehensive collapse assessment. This paper evaluates the seismic collapse of SC-PBBFs by utilizing the FEMA P695 methodology. A set of buildings ranging from low- to high-rise archetypes is designed with a proposed response modification coefficient of R = 8. Incremental dynamic analyses (IDA) are conducted under 44 far-field ground motions to evaluate Adjusted Collapse Margin Ratios (ACMRs). Additionally, the collapse risk of the archetypes is measured under the influence of system-level uncertainty. The outcomes indicate that the proposed design procedure ensures an adequate margin of safety against collapse for SC-PBBFs.

Emerging superelastic SMA core damping elements for seismic application

Structural and non-structural damage reported after recent strong earthquakes promotes a fundamental shift in structural design target from “collapse resistance” to “fast structural function recovery”. A unique class of metal called superelastic shape memory alloy (SMA) has been emerging as a promising solution to enhance the seismic resilience of structures. SMA core damping elements are the basic constituents of many newly-proposed seismic resistant members or devices, and they are directly responsible for the safety, integrity, and economical efficiency of the structures using them. This mini-review paper offers a comprehensive summary of the emerging SMA core damping elements recently developed, covering SMA monofilament wires, fibers, bars, flat plates, U-shaped plates, angles, cables, rings, helical springs, disc springs, and friction springs. These SMA core elements are suitable for various seismic application scenarios, and their efficiency have been verified through either full-scale or proof-of-concept experimental studies. The basic working principles, advantages, and potential shortcomings of these elements are discussed, and future research needs are outlined.

Experimental Study on Flexural Behavior of Seawater Sea-Sand Concrete Beams Reinforced with Superelastic Shape Memory Alloy Bars

In order to research the flexural behavior of shape memory alloy (SMA)-reinforced seawater sea-sand concrete (SWSSC) beams and improve their self-healing ability, three SMA SWSSC beams and one anti-corrosive steel bar SWSSC beam were designed. The influence of the reinforcement ratio, strength grade of SWSSC and type of reinforcement on the flexural performance of the beam were considered. The failure process, maximum crack width, mid-span deflection, displacement ductility and stiffness degradation of beams were studied by cyclic loading tests. The test results showed that the number of cracks in SMA-reinforced beams were significantly smaller than that in anti-corrosive-reinforced beams, and the crack width and mid-span deflection recovery effect were better after unloading. However, the effect of increasing the SMA reinforcement ratio on crack recovery was not obvious. The increase in SMA reinforcement ratio and the strength grade of SWSSC can significantly improve the bearing capacity of the beam and the stiffness, but the stiffness degradation rate decreased. Moreover, the ductility of concrete beams with SMA bars was significantly increased.

Experimental study on the seismic performance of self-centering bridge piers incorporating ECC and superelastic SMA bars in the plastic hinge regions

The seismic performance of pier columns is important to the safety of bridge structures. To improve the deformation capacity and energy dissipation, reduce the residual deformation and plastic damage, and resume normal functions with no or minimal repair after an earthquake, an innovative pier column with self-centering capability based on shape memory alloy (SMA) and engineered cementitious composites (ECC) was proposed. In the plastic hinge region of the pier column, SMA bars and ECC were used to replace the ordinary longitudinal reinforcements and concrete. The superelasticity of SMA can be used to achieve the self-centering function of the pier column; the strain hardening characteristic of ECC can improve the energy dissipation capacity of the pier column and reduce damage. Five specimens were produced, i.e., ordinary reinforced concrete bridge pier (R/C-BP), reinforced ECC bridge pier (R/ECC-BP), steel strand reinforced concrete bridge pier (SS/C-BP), steel strand reinforced ECC bridge pier (SS/ECC-BP) and SMA bar ECC bridge pier (SMA/ECC-BP). After low cycle repeated loading tests, the seismic performances of different piers, such as failure mode, bearing capacity, ductility and energy dissipation capacity, were compared and analyzed. The test results show that SMA material can significantly improve the deformation ability and ductility of the structure and reduce the residual deformation of the structure; ECC material can effectively improve the ductility and energy dissipation capacity of the structure and reduce the speed of cracking. Compared with ordinary reinforced concrete specimens, SMA/ECC pier columns were less damaged with better ductility and self-centering effects and exhibited excellent seismic performance.

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.

The Seismic Performance of New Self-Centering Beam-Column Joints of Conventional Island Main Buildings in Nuclear Power Plants

In order to improve the deformation energy consumption and self-centering ability of reinforced concrete (RC) frame beam-column joints for main buildings of conventional islands in nuclear power plants, a new type of self-centering joint equipped with super-elastic shape memory alloy (SMA) bars and a steel plate as kernel components in the core area of the joint is proposed in this study. Four 1/5-scale frame joints were designed and manufactured, including two contrast joints (a normal reinforced concrete joint and a concrete joint that replaces steel bars with SMA bars) and two new model joints with different SMA reinforcement ratios. Subsequently, the residual deformation, energy dissipation capacity, stiffness degradation and self-centering performance of the novel frame joints were studied through a low-frequency cyclic loading test. Finally, based on the OpenSees finite element software platform, an effective numerical model of the new joint was established and verified. On this basis, varying two main parameters, the SMA reinforcement ratio and the axial compression ratio, a simulation was systematically conducted to demonstrate the effectiveness of the proposed joint in seismic performance. The results show that replacing ordinary steel bars in the beam with SMA bars not only greatly reduces the bearing capacity and stiffness of the joint, but also makes the failure mode of the joint brittle. The construction of a new type of joint with consideration of the SMA reinforcement and the steel plate can improve the bearing capacity, delay the stiffness degradation and improve the ductility and self-centering capability of the joints. Within a certain range, increasing the ratio of the SMA bars can further improve the ultimate bearing capacity and energy dissipation capacity of the new joint. Increasing or decreasing the axial compression ratio of column ends has little effect on the overall seismic performance of new joints.

Seismic Performance of Hybrid Corrosion-Free Self-Centering Concrete Shear Walls

Reinforced concrete (RC) walls are extensively used in high-rise buildings to resist lateral loads, while ensuring an adequate level of ductility. Durability problems, including corrosion of conventional steel reinforcements, necessitate exploring alternative types of reinforcement. The use of glass fiber reinforced polymer (FRP) bars is a potential solution. However, these bars cannot be used in seismic applications because of their brittleness and inability to dissipate seismic energy. Superelastic shape memory alloy (SMA) is a corrosion-free material with high ductility and unique self-centering ability. Its high cost is a major barrier to use in construction projects. The clear advantage of utilizing both SMA and FRP to achieve durable self-centering structures has motivated the development of a composite SMA-FRP bar. This paper investigates the hybrid use of FRP bars and either SMA bars or composite SMA-FRP in concrete shear walls. An extensive parametric study was conducted to study the effect of different design parameters on the lateral performance of hybrid RC walls. The seismic behavior of the hybrid walls was then examined. The hybrid walls not only solved the durability problem but also significantly improved the seismic performance.

Enabling shape memory effect wires for acting like superelastic wires in terms of showing recentering capacity in mortar beams

Superelastic shape memory alloy (SMA) wires embedded in mortar beams show a recentering capacity, however, shape memory effect wires can provide a prestressing effect after inducing phase transformation. In this study our goal is to empower shape memory effect wires to show recentering capacity by designing a new wire configuration. These wires will thus be able to achieve both a prestressing effect and a recentering capacity in mortar beams. To this end, various shapes of martensitic SMA wires, including as-received, crimped, dog bone, and anchoring reinforced crimped wires, were employed to examine the potential of displaying recentering capacity while preserving the prestressing force in mortar beams. Several monotonic and cyclic three-point bending tests were also conducted to investigate the crack-closing effect, as well as the load bearing of the specimens. To evaluate the effect of temperature and heating procedure, two different methods, namely, a hot air gun and electric current heating, were used to induce phase transformation in shape memory effect wires. The results generally show that for two types of as-received and dog bone wires, the displacement recovery ratio have been reduced about 65% and 74%, respectively, however, for the crimped wire and anchoring reinforced crimped wire, this ratio is almost the same. More importantly, when the results of displacement recovery ratio of crimped wires heated by electrical heating are compared with the available experimental data for superelastic SMA bars, proves the great ability of SME wires to fulfill recentering capacity.

Experimental study on mechanical properties of large NiTi superelastic shape memory alloy bars

This study intensively examined the mechanical properties of large-sized superelastic shape memory alloy (SMA) bars, mainly focusing on their self-centering and energy dissipation capabilities. A detailed investigation on the effects of the heat treatment strategy, loading rate, strain amplitude, cyclic loading, prestress, and diameter of the SMA bars on their mechanical performance—residual strain, energy dissipation, equivalent viscous damping ratios, strength, and stiffness—was conducted. Furthermore, the fracture microstructure of monotonic tensile specimens was analyzed via scanning electron microscopy. The results indicated that the optimally heat-treated SMA bars show good superelasticity. The mechanical properties were relatively stable under constant strain loading–unloading training, which should be considered in engineering applications. The test results provided basic experimental data support for the engineering application of large SMA bars.

Ductility and overstrength of shape-memory-alloy reinforced-concrete shear walls

The unique properties of superelastic shape-memory-alloy (SMA) bars have motivated researchers to investigate their use as reinforcing bars for concrete elements. They were found to decrease seismic residual deformations, while increasing seismic inelastic deformations. This characteristic deformation behaviour requires an assessment of the seismic design parameters of SMA reinforced concrete walls. This paper addresses this requirement by evaluating their ductility and overstrength factors. A total of 972 walls were analyzed under a quasi-static lateral load. Suitable values for the overstrength and ductility factors were estimated for two proposed locations of SMA bars. FEMA P695 was then utilized to evaluate the seismic safety margin for case study buildings, which were designed based on the estimated seismic design parameters.

Performance of self-centering devices containing superelastic SMA bars and their application via finite element analysis

This paper aimed to use finite-element analysis to evaluate the seismic performance of self-centering devices, containing superelastic shape memory alloy (SMA) bars and their application. It should be noted that the devices could perform bending behaviors combining with tensions or compression. Cyclic performance of the devices under pulling and pushing actions have been modelled and verified by previous experimental results. Then, parametric studies were conducted to investigate the effect of design parameters on the stiffness of the self-centering devices containing SMA bars. An application of the device to a steel frame was proposed and modelled. Results shown that the increasement of diameters and number of SMA bars have positive effects while the increase of bars length and moment arms have negative effects on the effective stiffness of the devices. Besides, with the same area, the use of SMA bars in the rectangular shape gives higher effective stiffness than the use of those in the circular shape. However, with the same moment of inertia, the former gives lower effective stiffness than the latter. The hysteretic responses of a steel frame equipped with SMA devices showed perfect self-centering capacity with symmetric behavior. The devices are effective to enhance the seismic performance of steel frames.

Uncertainty analysis of a shape memory alloy model for dynamic analysis

Superelastic shape memory alloys (SMAs) are capable of recovering large inelastic deformation and dissipating energy under loading reversals, and therefore present promising application in vibration control of engineering structures subjected to dynamic loads. Properties of SMAs are often optimised based on experimental results and treated as deterministic for dynamic analysis of structures with SMA-based devices. This study applies the Metropolis–Hasting algorithm to characterise the uncertainties within an SMA material model and to provide insight into its parameter uncertainty. Cyclic tests of SMA bars are first conducted and the experimental data are analysed using the Markov chain Monte Carlo method. The statistical properties of SMA model parameters are calculated based on posterior parameter distributions. The influence of SMA model parameter uncertainty is further explored on energy dissipation capacity under cyclic tests. The first four L-moment method is applied to transform the posterior parameter distributions into standard normal distribution to further evaluate the influence of model parameter uncertainty on energy dissipation in dynamic analysis.

Long-stroke shape memory alloy restrainers for seismic protection of bridges

To prevent unseating failures and pounding problems in simply supported bridges, steel restrainers that can limit relative displacements of adjacent spans have commonly been used. This study proposes a shape memory alloy (SMA)-based restrainer that can serve as both energy dissipater and restrainer under cyclic tension and compression loading. The proposed device named as, long-stroke SMA restrainer (LSR), consists of a superelastic SMA bar, a steel tube, and a filler material. The SMA bar provides re-centering force and dissipates energy, while steel tube and the infill material prevent buckling of SMA bar under compression. Taking advantages of large strain capacity of SMAs, the LSRs are designed to accommodate large displacement demands for bridge restrainers. The mechanical response of NiTi SMA bars with a diameter of 12 mm was first studied through uniaxial tension and compression testing. Buckling and post-buckling response of the SMA bars were also characterized. Then, a high-fidelity finite element (FE) model was developed and validated to represent the behavior of SMA bars. Next, the buckling mechanism of proposed LSR was studied through FE simulations and a design methodology for the LSR was developed to prevent buckling. Results shows that the LSR can exhibit stable energy dissipation capabilities with excellent self-centering ability both under tension and compression loading and can serve as an effective restrainer in simply supported bridge structures.

Experimental Investigation on Buckling and Post-buckling Behavior of Superelastic Shape Memory Alloy Bars

This paper examines the buckling and post-buckling behavior of superelastic shape memory alloy (SMA) bars. A NiTi SMA bar with a diameter of 12 mm was used in all the experimental tests. First, the tensile and compression responses of NiTi bar were characterized under monotonic loading up to failure. A total of 15 specimens with slenderness ratios that range from 25 to 115 were tested to study the critical buckling load and post-buckling behavior of SMA bars. Digital image correlation (DIC) system was implemented to monitor full-field surface displacements. The interaction between material nonlinearity due to phase transformation and geometric nonlinearity was explored. Data obtained from the DIC measurement system were further analyzed to identify the onset of buckling and to extract experimental critical buckling loads. The effect of loading rate on buckling response of SMAs was investigated by conducting additional testing at higher loading rates on the specimens with three selected slenderness ratios. The temperature field on the surface of the specimens was recorded by an infrared camera. The analytical critical buckling loads were computed and compared with experimental results. All specimens exhibited a unique buckling behavior characterized with almost a complete shape recovery upon unloading.

Behavior and application of self-centering dampers equipped with buckling-restrained SMA bars

This study proposes a new type of self-centering damper equipped with novel buckling-restrained superelastic shape memory alloy (SMA) bars. The new solution aims to address some critical issues related to degradation and loss of superelasticity observed in existing tension-only SMA-based self-centering devices, and in addition, to encourage enhanced material utilization efficiency. The cyclic tension-compression behavior of individual SMA bars is experimentally studied first, and subsequently, two proof-of-concept self-centering dampers are manufactured and tested. A simple yet effective numerical model capturing the flag-shaped response of the dampers is then established, and a preliminary system-level analysis is finally conducted to demonstrate the effectiveness of the proposed damper in structural seismic control. The individual SMA bar specimens show asymmetrical flag-shaped hysteretic responses with satisfactory self-centering capability and moderate energy dissipation. Through a specially designed configuration, the proposed damper shows desirable symmetrical and stable hysteretic behavior, and maintains excellent self-centering capability at 6% bar strain. The system-level dynamic analysis indicates that the dampers, as a means of retrofitting, could effectively reduce both the peak and residual inter-story drift ratios of a six-story steel frame. In particular, the mean residual inter-story drift ratio is reduced from over 0.5% to below 0.2% under the maximum considered earthquake, implying elimination of necessary structural realignment even after strong earthquakes.

Probabilistic seismic fragility and loss analysis of concrete bridge piers with superelastic shape memory alloy-steel coupled reinforcing bars

Concrete bridge piers with conventional steel reinforcing bars are vulnerable to strong earthquakes by inducing significant residual deformations, which substantially weakens the seismic resilience of bridges. Superelastic shape memory alloy (SMA) bars showing superior self-centering capacities are desirable substitutes to steel reinforcements to minimize the seismically-induced residual deformations of piers. Nevertheless, high cost, difficult machining, and lack of sufficient energy dissipation are the primary restraining factors to a wide implementation of SMA reinforcements. This study proposes a novel SMA-steel coupled reinforcement for concrete bridge piers, which is intended to achieve the balance between self-centering and energy dissipation capacities. Probabilistic seismic fragility analyses are conducted on the prototype bridge with either pure steel, SMA-steel coupled, or pure SMA reinforcements to evaluate their probability of damage at different limit states. Seismic loss analyses are further performed to compare the relative cost-effectiveness of different patterns of reinforcements. The results indicate that an optimal amount ratio between SMA and steel bars can be found for the coupled reinforcements, which shows lower vulnerabilities and higher resilience under earthquakes than the other reinforcement patterns. The direct repair loss and the indirect downtime loss after earthquakes are considerably reduced when the SMA reinforcing bars are introduced. The coupled reinforcement with the optimal SMA-steel amount ratio shows the most effectiveness in mitigating the long-term economic impacts induced by seismic hazards within the lifetime of the bridge.