Superelastic Shape Memory Alloy Bars Research Articles

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.

Seismic Performance of Two Story Steel Building Using Shape Memory Alloys (SMAs) Bars

Shape Memory Alloys (SMA) is type of smart materials that have ability to undergo large deformation and return back to their undeformed shape through heating (shape memory effect) or removal of load (superelastic effect). This unique ability is useful to enhance behavior of structure and seismic resistance. In this paper, superelasticity (SE) effect of NiTi alloys is used to improve the structural characteristics of steel building. The finite element analysis of steel building is done using ABAQUS v.2017. In order to compare the structural behavior of the steel building equipped with Shape Memory Alloy bars at beam-column connection, three steel building was modeled with a different combination of high strength steel bars and SMA bars. The steel building was checked for time history analysis by using Vrancea 1977 earthquake data. In order to estimate the recentring ability, residual of roof displacement and energy dissipation. The steel building equipped with SMA bars shows 82.7%, 152.72% recovery in residual roof displacement for steel building equipped with 50% SMA bars and 50% HS steel bars and steel building equipped with 100% SMA bars respectively, and moderate energy dissipation. In general, the frame equipped with 50% superelastic SMA bars and 50% HS steel bars provided better seismic performance.

Experimental and numerical study on the seismic performance of a self-centering bracing system using closed-loop dynamic (CLD) testing

This study investigates the seismic performance of a newly developed self-centering bracing system using a novel experimental technique named as closed-loop dynamic (CLD) testing. The bracing, named piston-based self-centering (PBSC) apparatus, employs Ni-Ti superelastic shape memory alloy (SMA) bars inside a sleeve-piston assembly for its self-centering mechanism. During cyclic tension-compression loading, the SMA bars are only subjected to tension avoiding buckling and leading to flag-shaped symmetric force-deformation hysteresis. Initially, a braced frame building fitted with PBSC is seismically designed and the preliminary sizing of the brace is determined. For testing, considering the lab capability, the brace is fabricated at a reduced scale. The process of “Closed-loop dynamic testing” starts with the brace test (step 1) under strain-rate loading to characterize the numerical model parameters (step 2), which are then scaled-up as per similitude law and implemented in a finite element software, S-FRAME’s PBSC brace model (step 3). Then the braced frame building is analyzed under an earthquake (step 4) and the axial force-deformation response of the brace under consideration is captured (step 5). In order to further understand and validate the actual response of the brace under earthquake type loading, the axial deformation obtained from S-FRAME is scaled-down (step 6) and used as input parameters for testing the reduced scale brace (step 7). The obtained response (step 8) is further scaled-up and used to match the S-FRAME’s PBSC model for validation (step 9). Iterations from step 3 to step 9 will be required until the experimental and numerical results converge. Convergence criteria used for this validation include both the energy dissipation capacity and initial stiffness within 10% accuracy. Reasonable agreement between the numerical and experimental results is achieved in the closed-loop dynamic testing. The PBSC brace shows excellent self-centering capability under various earthquake loadings.

Earthquake resilient RC walls using shape memory alloy bars and replaceable energy dissipating devices

This study presents a novel precast reinforced concrete (RC) wall system using shape memory alloy (SMA) bars and replaceable energy dissipating (ED) devices to achieve earthquake resilience (hereafter referred to as SMA-based RC wall). The major advantages of this structural system include (1) self-centering (SC) capability provided by the unbonded superelastic SMA bars used in the bottom boundary zones of the wall, (2) concentrated energy dissipation and damage in the replaceable steel angles, and (3) an earthquake resilient design that requires minimal repair even after strong earthquakes. The structural details and design considerations of the SMA-based RC wall were first introduced. Subsequently, the mechanical behavior of the SMA bars and ED angles was investigated through cyclic loading tests. The seismic performance of the SMA-based RC walls was computationally investigated using various parameters of interest in seismic applications and evaluated in terms of strength, stiffness, and SC and ED capabilities. The proposed SMA-based RC walls exhibited satisfactory and stable flag-shaped hysteretic loops with excellent SC and satisfactory ED capabilities. Compared with conventional RC walls, the proposed SMA-based RC walls provide a promising solution for high-performance structural systems required by modern resilient and sustainable civil infrastructure.

Superelastic shape memory alloy flag-shaped hysteresis model with sliding response from residual deformation: Experimental and numerical study

Superelastic shape memory alloy exhibits flag-shaped hysteresis with self-centering capability. Nevertheless, shape memory alloy undergoes some residual deformation after large plastic strain, especially under repeated cyclic loading. In order to accurately simulate this behavior during nonlinear dynamic time-history analysis, a shape memory alloy flag-shaped hysteresis model with sliding response has been developed. This article shows the gradual development process of this new hysteresis model and provides analysis and verification results to support this claim. A MATLAB-based superelastic uniaxial shape memory alloy material hysteresis model has been developed and was incorporated into a finite element program specifically designed for the piston-based self-centering bracing. This piston-based self-centering bracing system uses superelastic shape memory alloy bars for its energy dissipation and self-centering capability. A proof-of-concept brace specimen was fabricated and tested where numerical and experimental results showed excellent matching. The finite element program was utilized to capture the varying nonlinear quasi-static response of the piston-based self-centering brace. Finally, the piston-based self-centering brace responses from this analysis were used to develop a novel shape memory alloy flag-shaped hysteresis model with sliding response, which was implemented in finite element analysis and design software, S-FRAME. Nonlinear dynamic time-history analysis proves the effectiveness of such bracing in steel frames in reducing interstory drift.

Ductile corrosion-free self-centering concrete elements

Corrosion is a major factor in the deterioration of reinforced concrete (RC) structures. To mitigate this problem, steel bars can be replaced with glass-fiber-reinforced-polymer (GFRP) bars. However, the lack of ductility of GFRP-RC elements has prevented their use in many structural applications, especially in seismic areas. Superelastic shape memory alloy (SMA) bars have been proposed to be used in seismic areas because of their self-centering characteristics. Also, they have the added advantage of being corrosion resistant. This paper examines the combined use of SMA and GFRP bars to achieve ductile self-centering and corrosion-free elements. The first challenge for such a proposal relates to designing concrete frames, reinforced with SMA and GFRP bars, that have adequate lateral performance in terms of initial stiffness, ductility, and strength. A comprehensive parametric study is conducted to better understand the structural behavior of concrete elements reinforced with SMA and/or GFRP bars. Results from the study are utilized to develop design equations that allow designing an SMA/GFRP RC section to replace a steel RC section, while maintaining lateral strength, stiffness, and ductility. To examine the adequacy of the developed equations, a six-storey concrete frame is designed, and its lateral performance is examined using pushover analysis.

Cyclic behavior of hybrid RC columns using High-Performance Fiber-Reinforced Concrete and Ni-Ti SMA bars in critical regions

Thanks to their excellent deformation capacity and ductility, smart materials are now being increasingly used in the construction industry, one example being superelastic Shape Memory Alloys (SMA) such as Ni-Ti, which can recover from large strains without generating significant residual strains.The purpose of this work was to study the behavior under cyclic loading of hybrid-reinforced concrete columns containing super-elastic Ni-Ti bars and High-Performance Fiber-Reinforced Concrete (HPFRC) in their critical end regions, where plastic hinges may form (column ends). This critical region is a dissipative zone of the column where the most adverse combination of actions occurs.An experimental study was carried out to simulate inter-story columns in which the effects of the axial level and continuous or discontinuous beam-column joints were analyzed. The specimens with continuous joints were found to display greater lateral strength, ductility and energy dissipation, while those with discontinuous joints underwent smaller residual displacements and less damage in the critical regions of the columns.

Influence of the SMA constitutive model on the longitudinal seismic response of RC bridges

Several studies indicate that the re-centring capacity presented by superelastic Shape Memory Alloys (SMA) can reduce the displacements of structures subjected to earthquakes. To quantify the errors of numerical predictions with different constitutive models, peak, relative and residual displacements of four two-framed reinforced concrete (RC) bridges, linked by SMA bars, subjected to seismic actions, were numerically assessed. Different ratios of the elastic natural period of vibration of the frames of the bridges were adopted. Lumped plasticity models were used to simulate the behaviour of the reinforced concrete columns. Five SMA uniaxial models were considered, combining linear, nonlinear, isothermal and nonisothermal conditions. The seismic action was modelled by real accelerograms. The results show that the relative displacements are more sensitive to the adopted SMA model than the peak and residual displacements. Significant relative differences, up to 70%, were obtained.

Flexural behaviour of superelastic shape memory alloy reinforced concrete beams during loading and unloading stages

Trend of using smart structures, which can adjust when exposed to severe unexpected loading, is increasing. One of the methods to achieve such structures relies on smart materials. For example, replacing conventional steel reinforcing bars in Reinforced Concrete (RC) structures with superelastic Shape Memory Alloy (SMA) bars significantly reduces the residual deformations caused by post-yielding behaviour. This paper provides in-depth understanding of the flexural behaviour of SMA RC beams. A sectional analysis method, which predicts the flexural behaviour of SMA RC beams during both loading and unloading stages, is adopted and validated using available experimental data. An extensive parametric study is then carried out to investigate the effect of different geometrical properties. Recommendations for the optimum amount and length of SMA bars are drawn based on results of this study.

Analytical investigation of the behavior of concrete beams reinforced with multiple circular superelastic shape memory alloy bars

A new analytical model is developed for concrete composite beams reinforced with multiple superelastic shape memory alloy (SMA) circular bars and subjected to bending. The model is derived based on the three-dimensional (3D) ZM model for SMAs combined with Timoshenko beam theory. Mathematic formulas are first developed to predict and describe the internal material structure of the composite taking into account solid phase transformation in the SMA bars. Analytical expressions are then obtained for the moment-curvature and shear force-shear strain relations. The present work is the first to derive nonlinear expressions for the axial stress and elastic moduli of regions experiencing phase transformation within SMA in the vertical y–coordinate and the evolution of phase transformation along the axial and transverse directions in the SMA bars during unloading. The proposed model is validated against 3D finite element analysis (FEA) results for the same beam. It is found that the superelasticity of the beam is further enhanced with temperature closer to austenitic finish temperature, higher SMA volume percentage, higher number of SMA bars, and location of SMA bars further away from the neutral plane. In contrast, the magnitude of the austenitic and martensitic moduli were found to have negligible influence on superelasticity.

Seismic retrofit of precast soft-storey building using diagonal steel-shape memory alloy bracing device: Numerical investigation

A diagonal steel-shape memory alloy bracing device is proposed for seismic retrofitting of vulnerable building frame structure. Superelastic shape memory alloy bar is put in series with steel bars in the bracing. The device makes use of the loading plateau of superelastic shape memory alloy to limit the tensile force of diagonal bracing. A precast soft-storey building frame in Australia was selected for a case study. A numerical model of the frame was developed and validated with the results from full-scale pull-down field tests. Nonlinear time history analysis was then conducted to evaluate the seismic performance of the frame with different retrofitting strategies. The result shows that the diagonal steel-shape memory alloy bracing can reduce the displacement demand on the soft-storey frame. Meanwhile, the level of tensile force of steel-shape memory alloy bracing can be controlled by the force plateau of the shape memory alloy bar, which is recommended to be 20% of the yielding force of the steel bar. The lower tensile force demand could alleviate the force demand at the associated support connections.

Seismic performance of reinforced concrete frames retrofitted using external superelastic shape memory alloy bars

Pre-1970s designed and built reinforced concrete frame structures are considered unsafe when subjected to seismic loads. Insufficient anchorage of the beam reinforcement in the beam-column joints of these structures is considered a main deficiency. Newly built frame structures are seismically designed for safety, where high inelastic deformations can occur under moderate to strong earthquakes. Minimizing these inelastic deformations makes the structure repairable. One way to minimize these residual deformations is by using smart materials such as superelastic shape memory alloys (SMAs). In this paper, the seismic performance of RC frames retrofitted using external superelastic SMA bars is investigated and compared to the behaviour of a regular steel RC frame structure. Nonlinear time history analysis is performed for a six storey RC frame structure located in a high seismic region. After performing the analysis, two retrofitted frames are assumed and analyzed at the load intensities causing failure of the steel RC frame. The performance of the retrofitted frames is compared to the steel RC frame in terms of the damage level, the Maximum Inter-Storey Drift (MID) ratio, Maximum Residual Inter-Storey Drift (MRID), Maximum Roof Drift Ratio (MRDR), Residual Roof Drift Ratio (RRDR), and the earthquake intensity at collapse. Analysis results show improved seismic performance for the two retrofitted frames as compared to the original steel RC frame. This improvement was represented by lower level of damage at the same earthquake intensity; small reduction (10–15%) in the MID and MRDR values; significant reduction (50–70%) in the MRID and RRDR; and increased seismic capacity.

Numerical Investigation of Preload Process of Bolted Joint with Superelastic Shape Memory Alloy

A phenomenological constitutive model is developed to describe the uniaxial transformation ratcheting behaviors of the superelastic shape memory alloy (SMA) by employing a cosine–type phase transformation equation with the initial martensite evolution coefficient that can capture the feature of the predictive residual martensite accumulation evolution and the nonlinear hysteresis loop on a finite element (FE) analysis framework. The effect of the applied loading level on transformation ratcheting is considered in the proposed model. The evolutions of transformation ratcheting and transformation stresses are constructed as the function of the accumulated residual martensite volume fraction. The FE implementation of the proposed model is carried out for the numerical analysis of transformation ratcheting of the SMA bar element. The integration algorithm and the expression of consistent tangent modulus are deduced in a new form for the forward and reverse transformation. The numerical results are compared with those of existing models; experimental results show the validity of the proposed model and its FE implementation in transformation ratcheting. Finally, a FE modeling is established for a repeated preload analysis of SMA bolted joint.

Plastic hinge relocation in reinforced concrete beams using Cu-Al-Mn SMA bars

Self-centering earthquake resistant systems have attracted the attention of researchers because of their promising potential in controlling seismic residual drifts, and, therefore, reducing the associated repair costs. The use of Ni-Ti superelastic shape memory alloy (SMA) constitutes a considerable portion of this research. Cu-Al-Mn superelastic SMA has been recently developed to eliminate the high cost of Ni-Ti SMA, as well as, to have better machining characteristics. This paper explores the use of Cu-Al-Mn SMA bars to relocate the plastic hinge of concrete beams through an experimental–numerical study. The cyclic performance of four beams was examined. The first was reinforced with steel bars and the remaining three were reinforced with combination of SMA and steel bars. The location of the SMA bars was different for each of the examined beams. The beams were loaded such that the moment diagram is zero at midspan and maximum at the ends to simulate the expected seismic moments. Results of the experimental–numerical investigation confirmed the recentering capability of SMA RC beams. Relocating the plastic hinge, by placing Cu-Al-Mn SMA bars away from the beam ends, improved the strength, rigidity, and energy dissipation.

Cyclic tension–compression behavior of superelastic shape memory alloy bars with buckling-restrained devices

Shape memory alloys (SMAs) show great potential in seismic applications because of their appealing superelastic feature and good energy dissipation capacity. The tensile behavior of SMA wires and bars has been extensively studied over the past decades. However, little attention has been paid to their compressive properties under cyclic loading. Thus, this paper presents a systematic experimental study on the cyclic behavior of superelastic SMA bars with buckling-restrained devices (BRDs) when subjected to tension–compression cycles. The detailed design of BRD is described first. The material properties such as “yield-like” strength, peak strength, self-centering capability, and energy dissipation of typical interest in seismic applications are evaluated with varying strain amplitudes, strain rates, and loading protocols. Test results show that satisfactory and stable flag-shaped hysteretic loops without any strength degradation are obtained in multiple loading cycles under cyclic tension–compression loading. Moreover, the SMA bars exhibit remarkable self-centering capability that is nearly independent of strain rate and loading protocol.

Computational studies on the seismic response of the State Route 99 bridge in Seattle with SMA/ECC plastic hinges

This paper reports a computational study on the seismic response of a three-span highway bridge system incorporating conventional and novel substructure details for improved seismic performance. The bridge has three continuous spans supported by two single-column piers and integral abutments founded on drilled shafts. It will be the first full-scale highway bridge to use superelastic shape memory alloy bars (SMA) and engineered cementitious composite (ECC) to mitigate column plastic hinge damage and minimize residual displacements after a strong earthquake. A three-dimensional computational model capturing the nonlinear constitutive response of the novel materials and the effects of dynamic soil-structure interaction was developed to assess the seismic response of the bridge in finite-element software OpenSees. Two versions of the same bridge were analyzed and compared, one with conventional cast-in-place reinforced concrete columns, and the other with top plastic hinges incorporating Nickel-Titanium (NiTi) SMA reinforcing bars and ECC. The novel SMA/ECC plastic hinges were found to substantially reduce damage and post-earthquake residual displacements in the bridge substructure, but led to larger maximum drifts relative to the bridge with conventional reinforced concrete plastic hinges. The analysis results suggested that the novel plastic hinges could lead to improved post-earthquake serviceability of bridges after intense earthquakes.

Cyclic loading testing of repaired SMA and steel reinforced concrete shear walls

This paper presents the results of reverse cyclic load testing of a repaired slender concrete shear wall reinforced internally with superelastic Shape Memory Alloys (SMAs) in the boundary zones within the plastic hinge region. In addition, a companion-repaired wall reinforced with deformed mild steel was also tested. Provided herein is an assessment of the performance of the repaired walls against the same set of walls previously tested in their original condition. The repair strategy included removing heavily damaged concrete within the plastic hinge region, replacing ruptured and buckled reinforcing steel, and shortening of the SMA bars in the boundary zones. High-strength, self-consolidating concrete replaced the removed concrete. The concrete above the plastic hinge region remained intact given the negligible damage (hairline cracking) of the original walls in this zone. The test results demonstrated that SMA-reinforced concrete structural components are self-centering, permitting repair of damaged areas. Furthermore, the SMA bars were re-usable for the repair application due to their capacity to reset to their original state within the range of inelastic strains of up to 6%. The repaired walls were capable of restoring the yield and ultimate lateral load capacities, but sustained lower drift capacities. The repaired SMA wall was capable of recovering the imposed lateral drifts up to 2%, after which residual displacements accumulated due to rupturing of the SMA bars in the boundary zone. The repaired walls dissipated up to 7.8% more energy than their original walls for a significant portion of the loading range. The length of the SMA bars and the presence of starter bars in the original walls were influencing factors in the location of failure of the SMA- and steel-reinforced walls.

Retrofit of non-seismically designed beam-column joints by post-tensioned superelastic shape memory alloy bars

A series of tests on three full-scale substandard exterior beam-column joints were performed to investigate the efficiency of the proposed retrofit configuration, which is the use of externally applied post-tensioned shape memory alloy (SMA) bars. A major group of structural deficiencies resulting from lack of shear reinforcement in the joint, use of low strength concrete and plain round bars were taken into account in the construction of test specimens. While the reference specimen represents the as-built subassembly, the other two were retrofitted by the post-tensioned SMA and steel bars to compare the contribution of superelastic and conventional material on the response. The specimens were exposed to quasi-static cyclic loading up to 8% drift ratio to simulate an intensive level of seismic hazard. The reference specimen underwent a brittle shear failure as excessive cracks mostly concentrated in the joint panel while there was almost no damage in the rest of the RC components. A joint failure with enhanced response quantities was observed in the specimen retrofitted by post-tensioned steel bars. The specimen incorporating the retrofit solution via post-tensioned SMA bars was capable of performing an adequate performance and promoting minimization of the damage in the joint panel, which results in more ductile behavior. The hysteretic response of the SMA retrofitted specimen was validated with a refined numerical model in ATENA Science software. Experimentally observed response was also verified by an analytical model based on fracture mechanics considering the nonlinear behavior of plain concrete under tension. Due to inherent uncertainties in material constitutive laws, the analytical model was evolved to a stochastic level to propose a more advanced model for estimating the capacity of the reference and retrofitted joint. It is found that the experimental results were within the prominent range of Probability Density Functions (i.e. mean ± 1 SD) of the estimated joint tensile stress especially for the shear damaged specimens.

Seismic Response of the Three‐Span Bridge with Innovative Materials Including Fault‐Rupture Effect

The seismic performance of the SR99 Bridge with conventional and advanced details in Seattle, Washington, was studied via a nonlinear, time history analysis of a multidegree of freedom model. The bridge consists of three spans supported on two single‐column piers and will be the first built bridge in the world in which superelastic shape memory alloy (SMA) and engineered cementitious composite (ECC) are implemented to reduce damage at plastic hinges and minimize residual displacements. Existing finite‐element formulations in the finite‐element software OpenSees are used to capture the response of the advanced materials used in the bridge. The earthquake induced by strike‐slip fault was assumed to produce a surface rupture across the SR99 Bridge. The effect of the rupture was modeled by a static, differential ground displacement in the fault‐parallel direction across the rupture. The synthetic suite of scaled bidirectional near‐fault ground motions used in the analysis contains common near‐fault features including a directivity pulse in the fault‐normal direction and a fling step in the fault‐parallel direction. Comparisons are made on behavior of two different bridge types. The first is a conventional reinforced concrete bridge and the second is a bridge with Nickel‐Titanium (NiTi) SMA reinforcing bar at the plastic hinge zone and ECC in the whole column. Fault‐parallel near‐fault earthquakes typically exhibit a static permanent ground displacement caused by the relative movement of the two sides of the fault. When the fault is located between piers, the pier shows a higher demand. Fault‐normal analysis results show effectiveness of the innovative interventions on the bridges in providing excellent recentering capabilities with minimal damage to the columns. But the maximum drift computed in the SMA bridge is slightly higher than reinforced concrete (RC) bridges, contributed by comparatively low stiffness of the superelastic SMA bars compared to the steel reinforcing bars.