Iron-based Shape Memory Alloys Research Articles

Nonlinear finite element modelling of the bond behavior of near-surface mounted Fe-SMA bars

Strengthening of concrete members with near-surface-mounted (NSM) iron-based shape memory alloy (Fe-SMA) bars can increase the load-carrying capacity and reduce deflections/crack widths owing to the unique self-prestressing ability of the Fe-SMA. However, the bond behavior of Fe-SMA bars is critical for the effectiveness of the NSM strengthening method, as inadequate bond strength can result in premature bond failure, rendering the strengthening technique ineffective. This study presents the results of a non-linear finite element (FE) model that was developed and validated to simulate the bond behavior and failure patterns of NSM Fe-SMA bars grouted into concrete. The variable parameters of the study included the effect of the compressive strength of the grout and the concrete block, the effect of cover depth and the activation of the Fe-SMA bar on the bond behavior. Investigations showed that the bond resistance and failure patterns of the NSM bars were significantly affected by the compressive strength of the concrete block and grout. The results showed that increasing the compressive strength of the grout by two times resulted in a 35 % increase in bond strength while increasing the compressive strength of the concrete block by three times resulted in a 30 % increase in bond strength. Additionally, the load transfer between the grout and concrete block was strongly influenced by the grout's compressive strength, where a higher strength of the grout resulted in a stiffer bond and increased contribution of the concrete block to resisting loads. Similarly, increasing the cover depth by four times from 5 mm to 20 mm increased the bond strength by more than twice and ultimately led to a change in failure mode from a splitting crack in the grout to a splitting crack in the concrete block. The results of the study also show that prestressing the short NSM bond with Fe-SMA can induce an initial bar slip in the range of 0.025 to 0.14 mm for recovery stresses of 100 MPa to 350 MPa.

Influence of post-processing methods on bond-slip behavior of nonlinear Fe-SMA lap-shear joints

Prestressed bonded strengthening for structures employing iron-based shape memory alloy (Fe-SMA) has been proven promising. Analyzing adhesively bonded joints necessitates a thorough understanding of the bond-slip behavior. However, when examining the bond-slip behavior of Fe-SMA-to-steel joints comprising nonlinear adhesives, the “forward” and “backward” post-processing methods, representing the current state-of-the-art, produce a trilinear and a trapezoidal bond-slip pattern, respectively, which is inconsistent. To address this inconsistency, the current study investigates the bond behavior of Fe-SMA-to-steel joints, with a particular focus on the bond-slip behavior. Two finite element (FE) joints, one featuring a linear adhesive and the other comprising a nonlinear adhesive, are modeled and compared against physical tests from literature. The “forward” and “backward” processing methods are used to analyze the bond behavior of the two FE joints. Eventually, the aforementioned inconsistency is resolved; a triangular and a trapezoidal bond-slip pattern are characterized for Fe-SMA-to-steel lap-shear joints with linear and nonlinear adhesives, respectively. The trilinear bond-slip behavior is concluded as a result of error accumulation and propagation during the “forward” processing. A hybrid post-processing method, which takes the advantages of both the “forward” and “backward” processing methods, is further proposed for inferring the full-range behavior; the resulting experimental behaviors closely align with simulations using a trapezoidal bond-slip model as input. A comparison against carbon fiber reinforced polymer (CFRP) lap-shear joints demonstrates similar bond-slip characteristics between Fe-SMA and CFRP lap-shear joints.

Shear strengthening of damaged reinforced concrete beams with iron-based shape memory alloy (Fe-SMA) strips: numerical and parametric analysis

Shape memory alloys (SMAs) are metallic materials that are characterized by their ability to restore their original shape after large deformation when activated by heating. This unique property renders SMAs appealing for various civil engineering applications. Iron-based SMAs (Fe-SMAs), including alloys like Fe–Mn–Si, stand out due to their cost-effectiveness and high strength. The primary focus of this research lies in the computational modeling of Fe-SMA strips utilized to reinforce damaged concrete structures. To achieve this, details from an experimental test are leveraged for the computational simulation of real-scale reinforced concrete beams that were first loaded to some level of damage, then released and strengthened, and subsequently retested. The strengthening approach involves the application of external Fe-SMA strips wrapping around the beams. This paper presents an original computational modeling setup that incorporates a switch option for the Fe-SMA material. This feature enables one to use a single simulation platform for the whole process. The significance of this method originates from its capacity to ensure a robust analysis that includes all simulation steps-testing unstrengthened beams, installing and heating Fe-SMA strips, and testing both damaged and strengthened beams—in a single, multi-step analysis. The computational simulation results were compared with the outcomes of the experimental test, revealing an acceptable level of agreement. The findings indicate a substantial increase in both shear strength and ductility as a result of the application of Fe-SMA strips. Additionally, parametric and mesh sensitivity studies were conducted. These aimed to investigate the mesh dependency of the model and to identify the optimal mesh size. Furthermore, variations in the details of the Fe-SMA strips, including thickness, width, quantity, and effect of applied temperature were explored to compare the outcomes of different applications of these strips.

Axial compression test on strengthening concrete cylinders by Fe-SMA/FRP-HDPE tube and rubber concrete cladding layer

This paper proposed a new method to strengthen and repair concrete columns, which combined active and passive confinement. The method involved the use of iron-based shape memory alloy (Fe-SMA) strips to quickly exert active confinement to these columns, followed by the use of fiber-reinforced polymer (FRP) and high-density polyethylene (HDPE) tubes as external passive confinement to improve the load-bearing capacity, durability, and ductility of concrete columns. Rubber concrete that performs high energy consumption and certain strength was used as the cladding layer to improve the impact resistance of the concrete column. This paper explored the effect of this method on the bearing capacity performance of concrete columns through axial compression tests and set three test variables, including different net-spacing of Fe-SMA, different FRP wrapping forms, and different FRP layers. The test outcomes showed that all three test variables obviously improved the load-bearing capacity and ductility of concrete columns, peculiarly when applying active confinement. Eventually, this paper proposed a theoretical model to predict the peak compressive strength of the specimen, which can provide a reference for designing this active-passive confinement strengthening method.

Material characterization of iron-based shape memory alloys for use in self-centering columns

Iron-based shape memory alloys (FeSMAs) are emerging as a promising material for use in post-tensioning concrete structures to provide self-centering capabilities during a seismic event. Past experimental studies on FeSMA focused on strengthening or repairing existing structural components under gravity loading. In addition to the structural rehabilitation, FeSMA also have potential for use in self-centering columns subjected to seismic loads. However, the basic material properties, such as strength, ductility, recovery strain, actuation stress (i.e. prestress) stability, low-cycle fatigue resistance, and temperature dependence of FeSMA related to self-centering column applications have not been studied extensively to-date. To fill this knowledge gap and determine the feasibility of using FeSMA in self-centering columns, this study performed a comprehensive characterization and analysis of FeSMA both before and after actuation (i.e. thermal stimulation). The strength, ductility, energy dissipation, and recovery strain of FeSMA before actuation were tested at different temperatures from −40 °C to 50 °C. After actuation, the actuation stress, low-cycle fatigue resistance, and strain capacity of FeSMA were tested at different temperatures from −40 °C to 50 °C and prestrain levels from 4% to 30%, and under low-cycle fatigue loading with strain amplitudes from 0.5% to 1.0%. The results from this study demonstrated that FeSMA exhibit high ductility, cyclic actuation stress stability, and low-cycle fatigue resistance at temperatures from −40 °C to 50 °C. Furthermore, it was found that increasing the prestrain level can effectively increase the post-actuation strain amplitude at which the actuation stress reduces to zero. A prestrain level between 15% and 20% is recommended for application of FeSMA in self-centering columns. The research findings from this study demonstrated the feasibility of using FeSMA in self-centering columns subject to seismic loading.

Durability of Fe-SMA/steel single-lap joints exposed to hygrothermal environments

Due to the adverse impact of hygrothermal environments and fatigue load, the performance degradation of existing steel structures necessitates repairs to extend their service life. Structures retrofitted by bonding iron-based shape memory alloy (Fe-SMA) have been demonstrated as an active non-destructive repair method. Nevertheless, the deterioration of the adhesives triggered by hygrothermal environments is a crucial challenge in its practical reinforcement applications. In this study, 54 Fe-SMA/steel single-lap joints (SLJs) are aged in 55°C and 98 % relative humidity environments for 30, 60 and 110 days, and tested under shear loads. The long-term performance, failure patterns, load-displacement responses and shear-resistance capacity of the SLJs are investigated, considering the essential parameters of aging time, overlap length and adhesive types. The experimental results exhibit that the failure patterns of aged Fe-SMA/steel SLJs depend highly on the adhesive types. The shear-resistance capacity of Fe-SMA/steel SLJs decreases dramatically with the prolongation of aging time. After 110 days of aging, the shear-resistance capacity of type-C, type-S and type-P SLJs decreases by about 14 %, 39 % and 49 %, respectively. When the overlap length increases from 50 mm to 150 mm, the shear-resistance capacity of type-C, type-S and type-P Fe-SMA/steel SLJs improves by 21 %, 39 % and 22 %, respectively, increasing the overlap length can safeguard the optimal functionality and long-term performance of the retrofitted steel structures. While the environments are relatively aggressive, it is recommended to use type-C adhesive to obtain the satisfactory serviceability of the retrofitted steel structures. The experimental achievements are significant for repairing deteriorated steel infrastructures by bonding Fe-SMA, guaranteeing the long-term performance and normal-use safety of structural reinforcement design, and reducing carbon emissions due to the avoidance of reconstruction.

Comparative Analysis of Concentrically Braced Frames with FeSMA and Steel BRBs Under Long-Duration Earthquakes

ABSTRACT Owing to high fatigue life, the iron-based shape memory alloy (FeSMA) buckling-restrained braces (BRBs) are emerging as a promising candidate against long-duration earthquakes. However, the corresponding analysis cannot be found. Current emphasis is paid on unveiling the seismic response differences between concentrically braced frames (CBFs) with FeSMA and steel BRBs when they are attacked by long-duration earthquakes. To directly assess the effect of earthquake duration, this paper selects a total of 90 pairs of short- and long-duration earthquake records, which are record-to-record spectrally equivalent over a wide range of fundamental periods, for nonlinear time history analysis. A benchmark six-story CBF is selected for demonstration purposes. In the numerical simulation process, the fatigue-induced damage is particularly considered. A preliminary examination is first conducted in the case study, in which a representative pair of short- and long-duration records is selected. In what followed, the statistical results from the ground motion suites are analyzed. This work not only highlights the higher failure risk of steel BRBs than FeSMA BRBs under long-duration earthquakes associated with maximum considered earthquake hazard level, but also confirms the advantages of FeSMA BRBs over steel BRBs in controlling residual deformations.

Self-centering of steel braced frames equipped with Fe-SMA TADAS dampers

This study proposes a novel triangular added damping and stiffness (TADAS) damper that uses the shape memory effect of iron-based shape memory alloy (Fe-SMA) to provide a self-centering system to recover the initial shape of a structure after significant inelastic deformation induced by nonlinear response of fused elements, without the need for difficult and expensive replacement procedures of conventional TADAS systems. Unlike most studies which consider simplified uniaxial behavior of Fe-SMAs, the present non-linear finite element simulations cover the full 3D material non-linearity of Fe-SMA component based on the SMA constitutive law to capture both flexural and shear behavior in various coupled thermomechanical loadings, including the mechanical loading/unloading, the heating, and the final cooling (as the recovering process). Simulations performed on a one-bay steel frame for different drift ratios reveal that although the dissipation energy of the new device is at most 10% less than the ordinary one, it enjoys the self-centering property to recover the initial shape of the frame before loading, showing that the proposed damper is an effective alternative to ordinary TADAS yielding dampers to achieve the self-centering characteristics.

Post-fire performance of iron-based shape memory alloy: Experimental study, prediction equations and constitutive models

To expand the application of iron-based shape memory alloy (Fe-SMA) for fire resilience improvement, this paper presents a systematic investigation on the mechanical properties of post-fire Fe-SMA. In tests, the specimens of China-made Fe-SMA were heated to target temperatures up to 1000 ℃, then cooled to room temperature in air or water, and finally subjected to tensile tests. The variation trends of mechanical properties with different cooling methods were analyzed, accompanied by the residual factor comparison with high strength steels Q460, S460, A36, A572 and A992, as well as with austenite stainless steel S30408. Finally, the prediction equations of residual factors and the post-fire constitutive model of Fe-SMA were proposed. Experimental results indicate that the exposed elevated temperature and cooling method significantly influence the post-fire performance of Fe-SMA. Compared with commonly used high strength steel and austenite stainless steel in civil engineering, Fe-SMA has unique post-fire performance, with a noticeable enhancement after exposure to elevated temperatures below 800 ℃. Therefore, the targeted theoretical prediction of post-fire Fe-SMA performance is significant and can be used as the investigation foundation for the fire resilience improvement application of Fe-SMA in civil engineering.

Experimental study on improving the fire performance of the concrete beams with Fe-SMA

The conventional approach to satisfying the fire resistance requirements in designing reinforced concrete (RC) structures is to ensure enough concrete cover depth for the steel reinforcements. The objective is to prevent the temperature of steel reinforcements from rising too quickly. However, excessive concrete cover depth may increase the risk of concrete spalling during a fire and lead to a larger crack width. Fire is a natural heat source that can activate the shape memory effect of iron-based shape memory alloys (Fe-SMA) to generate recovery stress. This study proposes a new active approach for improving the fire performance of RC structures based on the Fe-SMA. The main objective is to investigate the feasibility of improving the fire performance of RC structures through the recovery stress generated by Fe-SMA under fire. The fire test of nine Fe-SMA RC beams was conducted. The temperature profile in the furnace was similar to ISO834, and the maximum temperature exceeded 800 °C. The flexural behavior of the concrete beams reinforced with Fe-SMA during and after the fire was tested. The experimental results demonstrate that the Fe-SMA has great potential in improving the fire performance of RC structures. The mid-span deflection of the Fe-SMA RC beams under fire is significantly lower than the conventional RC beams. The fire resistance of Fe-SMA RC beams increases from 97 min to 133 min at load level 0.3 and from 69 min to 103 min at load level 0.5. The residual deformation of Fe-SMA RC beams reduces from 46.3 mm to 27.6 mm at load level 0.3 and from 34.7 mm to 19.8 mm at load level 0.5. In addition, after 1 h of fire exposure, the Fe-SMA can develop permanent prestress after the beam cools to room temperature, improving the mechanical properties after fire. The residual bearing capacity and residual stiffness of the Fe-SMA RC beams are 21% and 86%–121% higher than the conventional RC beams, respectively. The prestress generated by Fe-SMA after 1 h of fire exposure was estimated using the equivalent section method.

Measuring traction–separation relationships of bonded Fe-SMA double cantilever beam joints

Adhesively bonded strengthening systems using self-prestressing iron-based shape memory alloys (Fe-SMA) have demonstrated significant potential. However, the premature nonlinear behavior of Fe-SMA during debonding failure presents challenges in ensuring the integrity of the adhesive joint. A recent study indicated that this behavior may lead to premature joint failure, and a similar conclusion can be drawn concerning plasticity in thin adherends. To characterize the failure process of Fe-SMA bonded joints, a method capable of measuring traction–separation relationships of thin adherends bonded with tough adhesives is essential. An algorithmic method, supported by an analytical nonlinear beam-on-foundation model and a minimization scheme, is shown to offer a solution. To benchmark the method against the J-integral approach, experiments are conducted using two types of adherends: thick steel with thickness-limiting plasticity and Fe-SMA with material behavior that inevitably leads to nonlinear deformation. Comparison of methods shows both measurement method are valid and allow for proper predictive modeling using the finite element method. The proposed method can be readily employed to characterize various thin adherends with elasto-plastic material behavior.

The effect of γ′-martensite on the corrosion resistance of an Fe-Mn-Al-Ni-Cr shape memory alloy in a sodium chloride solution

The martensitic phase transformation in iron-based shape memory alloys has a strong influence on the corrosion properties. To investigate the effect of different amounts of martensite, potentiodynamic polarization measurements were conducted on specimens that were subjected to varying prestrain levels prior to the corrosion experiments. While the passive behavior deteriorates with an increasing martensite amount, the observed corrosion damage changes as well. While an entirely austenitic specimen is characterized by a predominantly surface corrosion with some randomly distributed pitting corrosion, the introduction of the martensitic phase results in micropitting and selective corrosion of the martensite plates.

Effect of Heat Treatments on Microstructure and Mechanical Properties of Fe-Mn-Ni-Al-Gd Shape Memory Alloy

There are significant scientific and industrial efforts to develop and optimize Iron-based shape memory alloys (SMA) such as FeMnNiAl for cost-sensitive applications. This alloy system shows shape memory and superelastic properties across a large temperature range. However, many studies have pointed out the need for rather complex thermo-mechanical treatments for the optimization of the SMA properties. In addition, works considering the effects of alloying on the development of microstructures that are more conducive to pseudo-elasticity in this system remain limited. Hence, systematic studies aiming at the investigation of the microstructural evolution of the FeMnNiAl(Gd) system are of great interest. In this study, solution heat treatment is done to tune the microstructure for optimum mechanical properties. The effect of phase distribution on mechanical properties is investigated at different heat treatments. Whereas cyclic heat treatment induced abnormal grain growth (AGG) in all samples, so large grains were obtained. The phase variation and elemental composition are analyzed by X-ray diffraction and Energy Dispersive Spectroscopy, respectively. The microstructure and phase distribution are observed using Scanning Electron Microscope and then related to the microhardness results. The microstructure has a good correlation with mechanical properties where the fine distribution of phases results in a higher hardness number.

Experimental and theoretical study on mechanical performance of Fe-SMA/steel single lap joints

The activated iron-based shape memory alloy (Fe-SMA) can introduce considerable prestress in the damaged zone of deteriorated steel structures through bonding technologies. In China, it has been successfully applied to several kilometer-span steel bridges, such as Sutong Bridge and Hangzhou Bay Bridge. The shear and interfacial performance of steel-to-Fe-SMA plates are crucial issues in bonding Fe-SMA to reinforce deteriorated steel structures, which remain unclear. This paper investigates the shear characteristics of Fe-SMA/steel single lap joints (SLJs), considering the effects of adhesive types, adhesive layer thicknesses and overlap lengths. The stress distribution and the interfacial shear stress at the overlap zone are obtained through experiments, and the damage evolution and load-transfer mechanisms of adhesive layers are analyzed. The experimental outcomes indicate that the maximum tensile stresses of Fe-SMA plates can reach 61 % of their ultimate strength. The failure modes and ultimate load of Fe-SMA/steel SLJs are affected by adhesive types. Interface failure occurs in CH 120 and PCMdur™-15 specimens, whereas interface, cohesive and mixing failure are formed in Sika 31 specimens. Due to the adverse effects of adhesive layer defects, out-of-plane bending moment, and damage accumulation of the adhesive layers, the distribution of surface strain and interfacial shear stress of all specimens are mainly at the effective overlap zone. For an efficient engineering design, the linear and polynomial fitting models of cohesive zone models with different adhesive types and adhesive layer thicknesses are proposed based on experiments. The experimental and theoretical achievements can provide design recommendations for deteriorated steel structures rehabilitated by bonding Fe-SMA.

Bonded and prestressed fatigue crack retarders based on Fe-SMA

A bonded and prestressed retarder concept is proposed in this work. The shape memory effect of an iron-based shape memory alloy (Fe-SMA) has been exploited to realize a desirable self-prestressing mechanism, resulting in compressive stresses in the parent structure. The synergistic prestressing and added fail-safe features of bonded and prestressed Fe-SMA retarders desirably restrain the fatigue crack growth. The fatigue crack growth behaviour in metallic plates reinforced with bonded and prestressed retarders is experimentally studied in this work. A test campaign has been implemented to investigate the effects of prestressing, as well as the influence of the relative location between the Fe-SMA retarders and the initial crack tip on the crack growth behaviour and fatigue crack life extension. The experimental results show that the prestressing of retarders desirably extends the fatigue crack growth life. More extension in fatigue life can be achieved by positioning the bonded prestressed retarders from 40 to 20 mm before the initial crack tip, the desirable retardation due to beneficial compressive prestressing and added load path on the small crack is significant. The findings in this work imply that it is beneficial to proactively reinforce the fatigue-prone locations in metallic structures with bonded and prestressed Fe-SMA retarders in order to extend the service life.

Experimental study on flexural performance of prestressed concrete beams strengthened by Fe-SMA plates

Shape memory alloy (SMA) is an innovative intelligent material, capable of restoring its shape before deformation upon reaching the critical activation temperature. This property promotes its promising applications in civil engineering. This paper proposes a novel reinforcement method and aims to investigate the reinforcing effect of iron-based shape memory alloy (Fe-SMA) plates on prestressed concrete (PC) beams through four-point bending tests on one reference beam without reinforcement and three Fe-SMA reinforced beams, as well as with three beams reinforced by carbon fiber-reinforced polymer (CFRP). The results show that Fe-SMA can effectively reduce concrete cracks, and improve stiffness and characteristic loads of beams without negative effects. Under the same loading conditions, new cracks of Fe-SMA reinforced beams were relatively fewer than those of CFRP reinforced beams. Besides, Fe-SMA exhibited superior performance when utilizing the same cross-sectional reinforcement material to enhance the bending capacity of beam, and proved more advantageous for improving stiffness and characteristic loads of beam under the identical initial tensile force. Furthermore, the reinforcing effectiveness of Fe-SMA positively correlates with the cross-sectional area (thickness) and recovery stress (pre-stretch level and activation temperature) of Fe-SMA. Increasing the thickness of Fe-SMA plates by 100%, while maintaining other parameters constant, resulted in a 33% increase in cracking load. Similarly, elevating the recovery stress from 314 MPa to 340 MPa led to a 12.5% rise in cracking load. This paper investigates the reinforcement effect of Fe-SMA plates on PC beams, and provides a reference for the application of Fe-SMA in PC beam reinforcement, laying a foundation for promoting the utilization of Fe-SMA in civil engineering.

Study on the flexural properties of T-shaped concrete beams reinforced with iron-based shape memory alloy rebar

Iron-based shape memory alloys (Fe-SMAs), which are emerging as a novel class of self-prestressing materials dispensed with in situ tensioning, have been studied primarily in the field of structural strengthening. In addition, the self-prestressing characteristics of the Fe-SMA rebars are extremely promising for new concrete bridges that are seriously affected by prestress loss during their service life. Therefore, the effect of self-prestress generated by embedded Fe-SMA rebar was experimentally investigated by six concrete T-beam. The influence of factors such as the activation status, prestress value, and diameter of the Fe-SMA rebar on the flexural performance was investigated. The variation laws of the cracking load, yield load, ultimate load, and stiffness were analyzed. The results illustrated that the activation of the Fe-SMA rebar had a remarkable influence on delaying beam cracking. Under the same load (130 kN), the specimens with an activated Fe-SMA rebar exhibited a remarkable reduction exceed 30% in crack width compared to those with an unactivated Fe-SMA rebar. Following the activation, the cracking load, yield load, ultimate load, and stiffness increased to varying extents,. The Fe-SMA, characterized by its remarkable ductility, exerted a beneficial influence on the deformation capability of the beams. This enhancement became notably conspicuous following Fe-SMA activation. Analysis of the cracking load and concrete strain during the activation process indicated that the prestress of the embedded Fe-SMA rebar closely approximated the values observed in the rigid confinement. The calculated flexural capacity agreed with the experimental data, and the proposed calculation method can provide a reference for subsequent research.

Constitutive model and activation recovery performance of Fe-SMA: Experimental and theoretical study

In this study, the static tensile properties, activation recovery properties, and post-activation static tensile properties of the China-made iron-based shape memory alloys (Fe-SMA) were experimentally studied and theoretically analyzed. The material parameters of the Ramberg-Osgood curve are modified based on the static tensile tests of Fe-SMA plates, leading to the proposal of a static tensile constitutive model suitable for Fe-SMA. The applicability analysis of the theoretical model indicates that the calculated curve achieves an accuracy of R2 ≥ 0.9. Subsequent activation tests revealed a locally optimal combination of pre-strain and activation temperature, which is concluded to be associated with the martensite content. Hot-rolled Fe-SMA plates demonstrate local maximum utilization when subjected to a 4% pre-strain and an activation temperature of 300 °C, resulting in a recovery stress of 314 MPa. Moreover, it has been determined that the two activation modes, namely single heating and multiple-cyclic heating, do not significantly impact the final recovery stress of hot-rolled Fe-SMA plates. Therefore, the multiple-cyclic activation of Fe-SMA can be utilized to compensate for prestress loss in practical applications. Furthermore, a comprehensive constitutive model for Fe-SMA is proposed based on the results of the post-activation static tensile tests. This model provides an accurate description of the static-tensile stress-strain relationship of Fe-SMA during pre-straining and after activation. This study focuses on investigating the constitutive relationship and activation recovery performance of Fe-SMA, providing an experimental basis, data support, and theoretical analysis for the practical application and further analysis of Fe-SMA in structural reinforcements.

Theoretical investigation and design recommendations for the initial stiffness of Fe-SMA/steel bolted joints in shear

Due to the self-stressing performance and easy-to-install, iron-based shape memory alloy (Fe-SMA) has been successfully applied to the prestressed rehabilitation of deteriorated long-span steel bridges, through bolt anchoring joints. The initial stiffness of Fe-SMA/steel bolted joints affects the transfer and distribution of external load and initial preload, which is significant in investigating the mechanical behavior of Fe-SMA plates for structural reinforcements. However, the current leading design standards for the bolted joints are based on mild steels, whose reliability requires investigation when employing Fe-SMA. For this purpose, the applicability of EN 1993-1-8 in predicting the initial stiffness of Fe-SMA/steel bolted joints is evaluated by experimental and numerical results. A parameter study exploring the influencing parameters of Fe-SMA plate thickness as well as end and edge distances on the initial stiffness of Fe-SMA/steel bolted joints is conducted via the numerical models validated with experimental results. Furthermore, the theoretical formula for predicting the initial stiffness of Fe-SMA/steel bolted joints is proposed. The study indicates that when e/d rises from 1.0 to 4.0, the initial stiffness of single-hole Fe-SMA/steel bolted joints improves by 20% to 35%. The influence of edge distance on the initial stiffness of single-hole Fe-SMA/steel bolted joints is found significant but neglected by EN 1993-1-8. Compared with the recommended formula in EN 1993-1-8, the proposed theoretical formula improves the accuracy by 23% in predicting the initial stiffness of single-hole Fe-SMA/steel bolted joints. The research achievements contribute to the theoretical improvements in the reinforcement design of Fe-SMA in civil engineering.

Analysis and design recommendations for structures strengthened by prestressed bonded Fe-SMA

Previous studies have demonstrated a great potential of prestressed strengthening of structures employing iron-based shape memory alloys (Fe-SMAs). A bonded Fe-SMA strengthening solution with partial activation has been proposed. However, an analytical model for assessing the strengthening efficiency was lacking, due to the unique nature of the employed prestressing mechanism involving heating. In this study, a symmetric strengthening model and an asymmetric strengthening model are developed to analyze the prestress level in steel and glass beams and plates strengthened by bonded Fe-SMA strips. The asymmetric strengthening model is then modified to analyze reinforced concrete (RC) beams strengthened by embedded Fe-SMA rebars. Recovery stress at different activation temperatures, the influence of the activation temperature on the adhesive bond, as well as the prestress loss resulting from the deformation of substrate elements and adhesive joints are taken into account. The predicted strains and deflections in the parent structure closely approximate the experimental measurements that appear in current literature. A parametric study and a sensitivity analysis are then conducted to assess the impact of the four influential features on the final prestress level, and their impact is ranked in the following order: recovery stress ≈ Fe-SMA width > activation length > bonded anchorage length. Based on these findings, a design strategy, in line with Eurocode 0, for the bonded/embedded Fe-SMA strengthening system is proposed. Finally, some perspectives on potential areas for future research are offered.