Iron-based Shape Memory Alloys Research Articles

Water immersion impact on the durability of Fe-SMA/steel joints used in reinforcement systems

Adhesively attaching iron-based shape memory alloy (Fe-SMA) to repair cracks in existing buildings represents a novel and effective approach to extending their service lives. The bond behavior between Fe-SMA and these parent buildings is crucial for the safety of reinforcement systems. Under hot-humid environments, the bond capacity of these reinforcement systems deteriorates, making the durability and reinforcement efficiency questionable. This study evaluates the distribution and progressive propagation of Fe-SMA strain and interface shear stress within the lap zone, as well as the failure behavior of Fe-SMA/steel single strap joints (SSJs) exposed to 55 °C running water. Based on experimental results, the bond-slip models of SSJs with various adhesive types and aging durations immersed in running water are proposed. As the aging duration extends, the maximum Fe-SMA strain and interface shear stress of SSJs decrease. The maximum shear stress of Type-AI SSJs at aging durations of 30, 60, and 90 days are 14.69, 11.89, and 9.66 MPa, respectively. Similarly, for Type-AII, the maximum shear stress at the same aging durations are 15.09, 11.26, and 8.82 MPa, respectively. When the aging duration increases from 30 to 90 days, the effective lap length of Type-AI and Type-AII SSJs improves by 13.33 % and 18.75 %, respectively. The maximum errors for cubic and bilinear bond-slip models in estimating fracture energy under harsh environments are 12 % and 16 %, respectively. After 90 days of exposure to running water, the fracture energy of Type-AI and Type-AII SSJs reduces by approximately 83 % and 88 %, respectively. This research offers guidelines for the durability of existing buildings strengthened with bonded Fe-SMA, reducing maintenance costs.

Numerical Investigation on Improving Shear Strength of RC Beams with Various Web Opening Shapes Using Pre-Stressed Fe-SMA Bars

The presence of web openings in the shear span significantly impacts the structural behavior of reinforced concrete (RC) beams, affecting both shear capacity and crack propagation. This study explores the feasibility of strengthening web openings in the shear zone of RC beams using iron-based shape memory alloy (Fe-SMA) bars through numerical analysis with ABAQUS software. The investigation considered various web opening shapes; diamond, circular, and square strengthened with pre-stressed Fe-SMA bars. Results showed that web openings notably decrease the ultimate loads of beams by 53%, 44%, and 39% for square, circular, and diamond shapes, respectively. However, pre-stressed Fe-SMA bars enhanced the shear capacity of beams with unstrengthened web openings by approximately 60%, making their behavior comparable to solid beams. The proposed strengthening technique was most effective for diamond web openings, nearly restoring both shear strength and stiffness, while circular openings recovered nearly 90% of shear capacity and square openings about 75%. Additionally, Fe-SMA bars effectively controlled cracking at the corners of the openings. This study highlights the importance of strengthening web openings in RC beams, especially in shear zones, and provides significant insights into enhancing such beams, contributing to safer structural designs. Further laboratory experiments are recommended to validate and extend these numerical findings.

Performance of hinge joints in hollow-core slab bridges reinforced with iron-based shape memory alloy U-bars

The mechanical behavior of hinge joints has an important influence on the hollow-core slab bridges. However, hinge joints are prone to cracking. This study proposed a new hinge joint reinforced by locally prestressed iron-based shape memory alloy (Fe-SMA) U-bars for the new hollow-core slab bridge. To verify the effectiveness of this hinge joint, six test specimens were prepared, encompassing two spans (10 m and 20 m), and three working conditions (nonactivation, activation before cracking, and repair and activation after cracking). The load transfer mechanism under flexural-shear load was investigated, and the validity was verified using a proposed simplified analytical model. The results indicated that the failure mode of the new hinge joint under flexural-shear load can be divided into three stages. The new hinge joint can effectively apply local prestressing by heating activation of the Fe-SMA U-bars, thus increasing the crack penetration load. In addition, the damaged new hinge joints can be repaired and further reinforced by pressure injection of adhesive and heating activation of the Fe-SMA U-bars. Load transfer is affected by the development of cracks, and the ultimate load of the new hinge joint is controlled by the splitting tensile strength of the concrete in the hinge joint.

Impact of fiber geometry, temperature, loading rate, and concrete mix on the pull-out resistance of iron-based shape memory alloy (Fe-SMA): Experimental investigation

An experimental campaign was conducted to determine the pull-out resistance of iron-based shape memory alloy (Fe-SMA) fibers embedded in high-performance fiber-reinforced concrete (HPFRC). Seventy-two specimens were examined using different thermal heating temperatures, loading rates, fiber end shapes, and concrete mixtures. The results indicated that end-hooked fibers had double the pull-out resistance of straight fibers. Increasing the temperature greatly reduced pull-out resistance in end-hooked fibers but had minimal impact on straight fibers. Concrete mixtures with higher compressive and flexural strengths exhibited greater pull-out resistance. Finally, the loading rate had a negligible effect; a tenfold increase induced merely a slight rise in pull-out resistance.

On the Effect of Chemical Heterogeneity on the Shape Memory Performance of Fe–Ni–Co–Al–Ti Single Crystals

The present work focuses on the formation of the γ′-phase and its impact on the functional properties of a Fe-28Ni–17Co–11.5Al–2,5Ti (at.-%) shape memory alloy (SMA) under cyclic loading conditions at different testing temperatures. The effect of aging treatments conducted in a wide range of aging temperatures and times was investigated. While specific heat treatments, namely 600 °C for 4 h, result in excellent superelastic properties with a fully reversible material response in single cycle experiments, interestingly, functional degradation is found to be more pronounced under cyclic loading compared to other derivatives of the Fe–Ni–Co–Al-based SMA systems. In addition to mechanical testing, a detailed microstructural analysis was conducted using transmission electron microscopy. The results of the present study clearly reveal that chemical inhomogeneities have to be carefully considered for the characterization of the functional performance of these iron-based SMAs. Chemical heterogeneities are not only identified as the underlying microstructural mechanism for the pronounced cyclic degradation behavior, but are also supposed to have a significant influence on the precipitation kinetics of the γ′-phase.

Strengthening, lifetime extension, and monitoring of a deficient steel–concrete composite roadway bridge using iron-based shape memory alloys

This study presents an innovative application of smart metals for the prestressed strengthening of roadway bridges. The target structure is a steel–concrete composite bridge, in which poor construction practices cause nonlinear creep, excessive deflection, and crack growth. However, the high flood water level of the creek below the bridge limits the application of conventional strengthening solutions. Therefore, an innovative strengthening method using iron-based shape memory alloy (Fe-SMA) bars for the post-tensioning of bridge members was designed and employed. The study framework encompassed the design, laboratory examination, installation, and monitoring of Fe-SMA reinforcements. A finite-element simulation was used to estimate the effect of applied prestressing on the stress distribution of the structure. High-cycle fatigue tests of Fe-SMA bars with different types of connections at room temperature and –20 °C, were conducted to select the most reliable connections. A total of approximately 825 m of Fe-SMA bars with a diameter of 18 mm, comprising 68 Fe-SMA bars, were installed and activated. A wireless sensor monitoring system consisting of strain gauges, potentiometers, linear-variable differential transformer sensors, and thermocouples was utilized to measure the changes in strain and stress of the designed system under field conditions. The results revealed a prestress loss of 8.5 % owing to relaxation after six months, which match well to the values obtained by the laboratory tests. A second static loading test was conducted approximately six months after strengthening, and the results indicated a 9 % reduction in mid-span deflection and a remarkable 106 % reduction in average stresses in the lower flange at the mid-span of the beams. The results of monitoring the bridge for a duration longer than 6 months highlighted a significant decrease in the mid-span deflection and indicated the potential of Fe-SMAs for the lifetime extension of bridges.

A feasibility study on improving the crack resistance of thin-walled UHPC members by reinforcement with Fe-SMA wires

To investigate the potential for further enhancing the cracking resistance of thin-walled UHPC by prestressing, the uniaxial tensile and three-point bending experiments for UHPC specimens reinforced with iron-based shape memory alloy (Fe-SMA) wires were conducted. The specimens were placed in an oven and heated to 150 °C, 200 °C, and 250 °C, which was intended to activate the Fe-SMA wires to produce self-prestress. The characteristics of different reinforced methods and heating temperatures were systematically compared. The cracking load and the crack development were recorded using Digital Image Correlation (DIC) techniques. The result shows that at the temperature of 150 ºC, compared to the dog-bone shaped specimens without reinforcement and the specimens with steel wires, the mean cracking load of specimens with Fe-SMA wires was enhanced by 13.4% and 9.8%, respectively. The study preliminary verified that this method can activate Fe-SMA inside UHPC.

Effect of elevated temperature on the bond behaviour of adhesive shear joints between glass substrate and iron-based shape memory alloy strip

Glass has been increasingly used as structural elements, such as glass beams or fins. Previous feasibility studies have shown increased initial and post-fracture load-bearing capacity of laminated glass beams post-tensioned with adhesively bonded iron-based shape memory alloy (Fe-SMA) strips. However, the potential elevated service temperatures were not considered, which significantly degraded the material properties of the adhesive. This study experimentally investigated the mechanical behaviour of Fe-SMA-to-glass lap-shear joints with an epoxy adhesive at different temperatures of 23 °C, 50 °C, and 80 °C, representing room temperature and typical elevated service temperatures. The results showed that, compared with the one at room temperature, the load-carrying capacity remained nearly unchanged at 50 °C and decreased by approximately 20 % at 80 °C. On the contrary, the effective bond length increased from approximately 116 mm to 250–300 mm. The failure modes, the tensile strain of the iron-based shape memory alloy, the bond-slip behaviour, and the fracture energy of the joints were also evaluated. The current study fills a significant research gap in the engineering application of strengthening glass structures by bonded pre-stressed Fe-SMA strips. Moreover, the results may also significantly contribute to the future application of the selected adhesive at elevated temperatures.

Investigation of Copper Addition on Corrosion Behavior of a Ferrous Based Shape Memory Alloy for Biomedical Implant Applications

Permanent biomedical implants pose several issues in long term scenarios like infections, inflammation, implant fracture, tissue damage, cancerous tumors formation, and skin allergies. Biodegradable biomedical implants are a new interest that function by degrading internally after achieving the implant goal. Shape memory alloys like Nitinol and Iron based shape memory alloys have applications in biomaterials due to the excellent property of super elasticity and shape memory effect respectively with the ease of small surgery requirement. To achieve biodegradability, the alloy composition is to be set while not compromising other properties such as biocompatibility, mechanical properties, shape memory properties, and magnetic properties. Slow corrosion rates of Fe-Mn alloys are reported and alloying addition, surface modifications, and novel manufacturing techniques are suggested to overcome this problem. In this study, the effect of addition of copper addition effect on the degradation behavior of Fe-30Mn-5Si is investigated. Austenite is the major phase present in both samples and small amounts of martensite are also present. For 10% copper, an additional copper rich phase is formed along the grain boundaries as it was beyond the solubility limit of iron matrix. The electrochemical corrosion test shows that 10% Cu addition resulted in 1.72 times higher corrosion rate than that of 5% Cu addition. As 5% Cu addition is within the solubility limit of iron matrix, and it forms a solid solution with iron that creates a passive layer during corrosion testing results in slower degradation.

Analytical model for RC beams with embedded prestressed Fe-SMAs

Prestressed iron-based shape memory alloy (Fe-SMA) strengthening is an effective means to enhance the performance of reinforced concrete (RC) structures. The applied prestress level significantly influences the strengthening efficiency. However, the absence of a reliable model predicting the applied prestress hinders the efficient design of prestressed Fe-SMA strengthening solutions. To fill this gap, the current study proposes an analytical model, with which a constant zone and a transfer zone are identified in strengthened RC beams. The constant zone, with zero interfacial shear stress, sustains a constant prestress level, while the transfer zone, experiencing non-zero shear stress, gradually reduces the Fe-SMA tensile stress from the prestress level to zero. Validated through experiments, the proposed model accurately predicts the applied prestress levels and prestress-induced deflections in RC beams strengthened with Fe-SMAs and more conventional carbon fiber reinforced polymers (CFRPs). Further analysis reveals that the transfer zone length ranges from 4 to 7 characteristic lengths, and it is proportional to the logarithm of the applied prestress level. Simplifying a constant prestress level within the beam span yields sufficiently accurate prestress analysis.

Characterization of low-temperature creep and stress relaxation of an iron-based shape memory alloy (Fe-SMA) using in-situ synchrotron diffraction

Iron-based shape memory alloys (Fe-SMAs) are e-merging materials with extensive application in civil structures owing to their unique properties, including the shape memory effect. However, it is crucial to understand the time dependent behavior of Fe-SMAs for their effective application as pre-stressing element. In particular, behavior at individual stress, the underlying mechanism, and the transformation kinetics have not been investigated yet. To address these important fundamental research gaps, in-situ compression creep and stress relaxation experiments with high-energy X-ray diffraction (HEXRD) of a Fe-17Mn-5Si-10Cr-4Ni-1(V,C) Fe-SMAs were conducted. The time-dependent behavior of the Fe-SMA was investigated at different stress levels with respect to the yield strength (YS) at room temperature. The experimental result showed that the material exhibits a creep strain of up to 1.84 % and 56 MPa relaxed stress at test stress of 769 MPa (1.6 σYS) within one hour of holding. Stacking fault probability and phase volume fraction quantification provide an understanding of the mechanisms based on different stress levels. The transformation kinetics traced from the characteristics of HEXRD peaks offer further insights on creep depending on the contribution of {hkl} families. The paper concludes with an evaluation of the existing models for predicting creep and stress relaxation of Fe-SMA.

Perspectives on Adhesive–Bolted Hybrid Connection between Fe Shape Memory Alloys and Concrete Structures for Active Reinforcements

The prestressed active reinforcement of concrete structures using iron-based shape memory alloys (Fe-SMAs) is investigated in this experimental study through three connecting methods: adhesive–bolted hybrid connection, bolted connection, and adhesively bonded connection by activating at elevated temperatures (heating and cooling) and constraining deformation to generate prestress inside Fe-SMAs, through which compressive stress is generated in the parent concrete structures. In tests, the Fe-SMA is activated at 250 °C using a hot air gun, generating a prestress of 184.6–246 MPa. The experimental results show that local stress concentration in the concrete specimen and Fe-SMA plate around the hole is caused by the bolted connection. The adhesively bonded connection is prone to softening and slip of the structural adhesive during the activation process, thereby reducing the overall recovery force of Fe-SMAs. The adhesive–bolted hybrid connection effectively mitigates the local stress concentration problem of concrete and Fe-SMAs at anchor holes, while avoiding the prestress loss caused by the softening and slip of structural adhesive during elevated-temperature activation, achieving good reinforcement effect. This study on the connection methods of an Fe-SMA for reinforcing concrete structures provides both experimental support and practical guidance for its engineering application, offering new perspectives for future research.

Python-based numerical study on bolted joints between Fe-SMA and steel plates for structural reinforcements

In order to rehabilitate the performance of deteriorated steel structures, the active prestressed reinforcement employing new intelligent material, Iron-based shape memory alloy (Fe-SMA), has received extensive attentions. To reveal the mechanical properties of bolted joints between Fe-SMA and steel plates, a parametric method to establish finite element model via secondary development of ABAQUS based on Python scripts was proposed. It greatly reduces modelling costs and improves modeling efficiency. Besides, the proposed numerical models were verified against the experimental results by failure mode, final deformation, load-displacement characteristics, ultimate load-bearing capacity and initial stiffness. It is demonstrated that the numerical simulation results are in good agreement with the experimental results. What is more, a parametric study on thickness and geometry of steel plates, as well as the end distance, edge distance and thickness of Fe-SMA plate was conducted to verify the applicability of current leading specifications for steel structures. It is evident from the results that the thickness of Fe-SMA should be controlled to prevent failure of parent steel structures. Furthermore, similar to steel plates, the thickness of Fe-SMA plate can be considered linear and the failure of bolt can be avoided, as described in Chinese code GB 50017–2017 and Eurocode 3 Part 1–8. However, the minimum allowable edge distance in Chinese code GB 50017–2017 and the ultimate load-bearing capacity calculated by Eurocode 3 Part 1–8 is conservative for bolted joints between Fe-SMA and steel plates. Hence, it is urgent to formulate design specifications for this kind of bolted joints for the scientific and economical application of Fe-SMA in structural active reinforcements.

Experimental investigation on shear behavior of I-shaped concrete beam with Fe-SMA rebars

Vertical prestress loss causes cracking in prestressed box-girder bridge webs. As a new type of prestressed material, iron-based shape-memory alloy (Fe-SMA) rebars can be used as vertically prestressed rebars to compensate for the loss of vertical prestress in box-girder webs. In this study, box-girder webs were simplified into I-shaped beams, and the shear performance of these I-shaped beams with vertical Fe-SMA rebars in the shear span was analyzed to estimate the feasibility of using Fe-SMA rebars. The effects of the activation state, layout spacing, and rebar diameter on the shear performance of the I-beams were explored. The variations in the cracking load, ultimate load, and main tensile strain were systematically investigated. The results indicate a significant transformation from brittle to ductile bending shear failure following Fe-SMA activation. Moreover, the ultimate deflection exhibited a significant increase exceeding a factor of 2. The activation of the Fe-SMA was beneficial for delaying specimen cracking and improving shear strength. Specifically, the shear cracking and ultimate loads increased by 24.21 % and 12.11 %, respectively. Simultaneously, the activation of the Fe-SMA rebar significantly reduced the primary tensile strain within the shear span segment and effectively delayed crack development. The diagonal fracture width of the specimens with the activated Fe-SMA was significantly reduced (67.24 %, 57.14 %, and 52.22 %, respectively) at three different stress settings (400, 450, and 500 kN). Furthermore, a methodology for estimating the shear cracking and bearing capacity of I-beams with vertical Fe-SMA rebars is proposed. The calculated values are in excellent agreement with the experimental data, thereby serving as a valuable reference for future research.

Fe-SMA for enhancing both structural robustness and seismic resilience: A pioneering study

Low-cycle fatigue (LCF) failure has been recognized as one of the most common failure modes of structures experiencing a major earthquake; on the other hand, progressive collapse, possibly triggered by impact, blast, or fire, is another dangerous consequence to structures when in the absence of adequate robustness design. Due to varied performance demands, addressing both issues with consistent design solution can be challenging. This paper proposes a new multi-hazard protection concept utilizing iron-based shape memory alloy (Fe-SMA) components to improve both the robustness (against progressive collapse) and seismic resilience (against earthquake-induced LCF failure) of steel frames. Fe-SMA has emerged as a promising material in the field of civil engineering due to its unique thermal-induced shape recovery, superior low cycle fatigue (LCF) resistance, and outstanding ductility. In this paper, the angle form of Fe-SMA components is selected, serving as energy dissipation elements in steel beam-to-column connections which are crucial for ensuring structural safety/integrity against various hazards. Experimental investigations were conducted to understand the basic mechanical properties of Fe-SMA angles under monotonic tensile and LCF loading, followed by comprehensive numerical studies demonstrating the use of Fe-SMA angles for steel beam-to-column connections against cyclic loading as well as ‘column loss’. The results confirmed that Fe-SMA is capable of resisting cyclic loading with much improved LCF life, and meanwhile, offers significantly enhanced chord rotation capacity as well as maximum dynamic load resistance of beam-to-column connections against progressive collapse.

Shear and bonding performances between Fe-SMA and steel for strengthening of steel structures

Iron-based shape memory alloy (Fe-SMA) has been employed in China and the Czech Republic to reinforce steel bridges due to its outstanding self-prestressing characteristics. Nevertheless, the lack of theoretical guidelines necessitates this study to align with the practical Fe-SMA-reinforced applications in civil engineering. This study experimentally investigates the shear performance and load-transfer mechanism of 45 Fe-SMA/steel adhesive joints. Specifically, the shear characteristics of the adhesive joints affected by the various parameters of bonding layer thickness, steel plate thickness, bonding length and adhesive type are explored, the failure mode and load-displacement relationship are assessed, and some suggestions for optimization design are proposed. The experimental results demonstrate that the Fe-SMA/steel adhesive joints experiencing interface failure exhibit a linear load-displacement relationship. Contrarily, the load-displacement relationship of the adhesive joints under cohesive failure modes contains a yield segment. Longer bonding length results in more considerable failure evolution distances for Sika S02 and LPdur 03 specimens with cohesive failure, indicating better ductility of these specimens. Additionally, the epoxy adhesives of Sika S02 and LPdur 03 are preferred while employing Fe-SMA plates for bonding reinforcement of steel structures, which can maximize the characteristics of Fe-SMA to obtain stable and superior reinforcement performance. The experimental achievements have remarkable potentials for improving the in-service performance of old and damaged steel structures and other infrastructures.

Shear strengthening and damage control effects of CFRP and self-prestressing iron-based SMA for concrete columns

Although prestressed shape memory alloy (SMA) has shown a great performance in flexural retrofitting of slender RC columns, its shear strengthening effect for squat RC columns has been barely studied. This study explores the shear strengthening and damage control effects of iron-based SMA (Fe SMA) for squat RC columns. The heat-activated prestressing of Fe SMA was evaluated first at material level, then applied to component level structural testing of RC columns. The four circular RC columns with different retrofitting schemes were tested under quasi-static lateral cyclic loading. The specimens strengthened with carbon fiber-reinforced polymer (CFRP) sheet and Fe SMA strip exhibited satisfactory global responses without showing significant shear failure. From the visual inspection and non-destructive damage assessment, the prestressed Fe SMA was found to be superior in delaying damage accumulation in concrete compared to CFRP.

Bond-slip models on interfacial behavior between Fe-SMA and steel in hygrothermal environments

Strengthening the existing steel structures to enhance performance can prolong their service life and reduce carbon emissions. Previous studies have employed iron-based shape memory alloy (Fe-SMA) to adhesively and actively retrofit long-span steel bridges in practice, achieving satisfactory repair efficiency. Nonetheless, concerns persist regarding the long-term performance of Fe-SMA-bonded parent structures, principally stemming from the performance degradation of the adhesives in hygrothermal environments. In this study, the damage evolution and debonding mechanisms of Fe-SMA/steel single-lap joints (SLJs) in hygrothermal environments are investigated through 54 lap-shear tests. The evolutionary process of Fe-SMA strain and shear stress within the overlapping zone is assessed, varying with aging time and adhesive type under shear load. Furthermore, the bond-slip models of the Fe-SMA/steel SLJs are put forward, which change with the aforementioned influencing factors in hygrothermal environments. The lap-shear tests demonstrate that the ultimate shear stress of SLJs is transmitted approximately up to 50 mm away from the initial end, with shear stress developing swiftly towards the free end during this phase. In hygrothermal environments, the effective overlapping length of the SLJs is highly contingent on the material properties of adhesives, improving with the prolonged aging time. This study proposes bond-slip models that describe the interfacial performance between Fe-SMA and steel, accounting for the detrimental influence of temperature, humidity, plastic deformation of components and strain relaxation. The experimental outcomes can guide the analysis of the debonding characteristics of Fe-SMA-bonded structures, and the theoretical study can provide reliable predictions for the service performance of reinforcement systems in hygrothermal environments.

Shear Retrofitting Effect of Self-prestressed Iron-based Shape Memory Alloy for Reinforced Concrete Beams

Shear Retrofitting Effect of Self-prestressed Iron-based Shape Memory Alloy for Reinforced Concrete Beams

A Review of Additively Manufactured Iron-Based Shape Memory Alloys

Iron-based shape memory alloys (Fe-SMAs), traditionally manufactured, are favored in engineering applications owing to their cost-effectiveness and ease of fabrication. However, the conventional manufacturing process of Fe-SMAs is time-consuming and raw-material-wasting. In contrast, additive manufacturing (AM) technology offers a streamlined approach to the integral molding of materials, significantly reducing raw material usage and fabrication time. Despite its potential, research on AMed Fe-SMAs remains in its early stages. This review provides updated information on current AM technologies utilized for Fe-SMAs and their applications. It provides an in-depth discussion on how printing parameters, defects, and post-printing microstructure control affect the mechanical properties and shape memory effect (SME) of AMed Fe-SMAs. Furthermore, this review identifies existing challenges in the AMed Fe-SMA approach and proposes future research directions, highlighting potential areas for development. The insights presented aim to guide improvements in the material properties of AMed Fe-SMAs by optimizing printing parameters and enhancing the SME through microstructure adjustment.