Iron-based Shape Memory Alloys Research Articles

Effect of deformation rate on the axial joint strength made of Fe-SMA

Iron-based shape memory alloys (Fe-SMA) have been applied to structural members, such as pipelines and curved pipes, for underground structures due to their excellent properties. However, in the case of an earthquake or high winds, the resulting impact force causes a bending and twisting behavior in joints of the pipe. For this reason, a clear understanding of the relation between the deformation rate and joint strength is strongly needed. In this work, the specimen of a joint which a round bar made of SUS304 is tightened by a hollow cylinder made of Fe-SMA was prepared by first expanding the cylinder at a constant quasi-static rate and heating it up to the austenitization temperature. Then, a push-out test, which a compressive-shear combined force acts, was conducted at various deformation rate including the impact level to evaluate the axial joint strength. The results scientifically demonstrate that the axial strength would show the negative sensitivity with respect to deformation rate. However, when the deformation rate exceeds 8.3 mm/s, positive rate sensitivity can be found. This work may provide us with a systematic understanding of the deformation behavior of joints made of Fe-SMA and the safety design of the product.

Uniaxial compressive behavior of circular concrete columns actively confined with Fe-SMA strips

An increased interest in the strengthening and rehabilitation of deficient or deteriorated structures took place in the past few decades to maintain the sustainability and integrity of existing structures. Different strengthening techniques have been recently studied and used to retrofit or strengthen concrete elements. One of these techniques is the use of Shape Memory Alloys (SMA), taking advantage of its shape memory effect property. This property allows the application of an initial confining pressure, called active confinement, when the material is restrained. Such pressure is developed when the material undergoes phase transformations upon heating to a certain temperature. As a result of their unique properties, different types of SMA for different structural engineering applications have been studied. In this paper, an experimental program was conducted to evaluate the behavior of concrete columns confined with iron-based SMA (Fe-SMA) strips under uniaxial compressive loading. A total of 25 circular columns were tested, and the results were analyzed to identify the influences of selected parameters: 1) presence of internal reinforcement, 2) initial confining pressure, and 3) ratio of external SMA confinement. The test results indicated great improvement in the overall behavior of the confined columns, especially their axial deformation capabilities. In addition, an analytical approach was proposed to model the axial load-deformation behavior of the actively confined columns. The proposed expressions showed to adequately capture the axial behavior of the columns, and the obtained results were in good agreement with the experimental test results of columns confined with Fe-SMA strips.

Magnetic Properties of FeNiCoAlTiNb Shape Memory Alloys

The magnetic properties of the new Fe41Ni28Co17Al11.5(Ti+Nb)2.5 (at. %) shape memory alloy system were studied in this work. The magnetic properties were characterized by thermo-magnetization and a vibrating sample magnetometer (VSM). In iron-based shape memory alloys, aging heat treatment is crucial for obtaining the properties of superelasticity and shape memory. In this study, we focus on the magnetization, martensitic transformation temperatures, and microstructure of this alloy during the aging process at 600 °C. From the X-ray diffraction (XRD) results, the new peak γ’ is presented during the aging process. The intensity of this new peak (γ’) increases with the aging time, while the intensity of the FCC (111) austenite peak decreases with aging time. Transmission electron microscope (TEM) results show that the size of the precipitate increases with increasing the aging times from 24 to 72 h. Thermo-magnetization results show that: (1) phase transformation is observed when the aging time is at least 24 h, (2) the transformation temperature increases with the aging time, (3) transformation temperatures tend to increase while the magnetic field increases from 0.05 to 7 Tesla, and (4) the magnetization saturates after aging time reaches 24 h. Vibrating sample magnetometer (VSM) results show that thermal process was found to significantly affect the magnetic properties of this alloy, especially on saturated magnetic magnetization and magnetic moment reversal behavior.

Self-Prestressing Performance of Iron-Based Shape Memory Alloy Bars for New Structures

Self-Prestressing Performance of Iron-Based Shape Memory Alloy Bars for New Structures

Cyclic behavior of iron-based shape memory alloy bars for high-performance seismic devices

As a new member of the shape memory alloy (SMA) material family, iron-based SMAs (Fe-SMAs) show great potential in seismic applications due to their favorable properties. The thermomechanical behavior of Fe-SMAs has been extensively studied over the past decades. However, the relevant research regarding the use of Fe-SMAs in the community of earthquake engineering is still in an early stage, particularly on their mechanical behavior under cyclic tension–compression loadings. This study conducted a systematic experimental investigation of the cyclic behavior of Fe-SMA bars with a buckling-restrained device, which was cyclically tested under tension–compression loadings. The cyclic properties, such as hysteretic response, recovery capability, fatigue behavior, and fracture behavior were evaluated with varying strain amplitudes and different loading protocols. Test results show that satisfactory hysteretic loops with excellent deformation capability are obtained under cyclic tension–compression loadings. Fe-SMA bars exhibit acceptably stable behavior under multistage loadings. Moreover, they possess unique inherent properties, including moderate shape memory effect, high “post-yield” stiffness, and excellent fatigue behavior, thereby providing a promising solution to develop high-performance seismic devices. These properties are conducive to limit peak drifts, mitigate residual drifts of structures, and withstand long-duration ground motions and strong repeated aftershocks.

The effect of solution treatment temperature on SIEM and shape recovery of Fe–Mn–Si shape memory alloy

Solution treatment process is stepwise process in manufacturing of iron based shape memory alloy. It is well known that the grain size of austenite in the solution treatment condition depends on the austenitizing/solutionizing temperature. Moreover, the solution treatment temperature influences the transformation temperatures of SMAs. It has been found by researchers that solution treatment temperature brings about modification in the grain size, the grain boundary angles, and the annealing twins. The major objective of the present work is therefore to assess the influence of modifications in the microstructural features by using different solution treatment temperatures. Optical microscopic studies reveal the microstructural changes brought about by changing the solution treatment temperature. DSC shows that somewhat change of transformation temperatures in regarding to the conjugated solution treatment temperature, but the martensite transformation temperatures are still in the range of ambient temperature. X-ray diffraction results refer to the enhancement of SIEM in the alloy that was solution treated at 1000 °C. TEM observations prove that uniform stacking faults have been generated at the austenite of the alloy that has been solution treated at 1000 °C. In addition, SADP shows that ε-martensite is the domain structure generated after 3.1% pre-strain of the sample that has been solution treated at 1000 °C. The results indicate that solution treatment at an optimal temperature of 1000 °C leads to increase of shape memory effect of iron base shape memory alloy.

Study on the Physical Properties of Split-type Iron Base Shape Memory Alloy Pipe Joint

Aiming at the leakage phenomenon of pipeline system due to corrosion and other reasons, a split type iron-based shape memory alloy pipe joint based on the button bond connection was designed for repair. This split-type pipe joint can be used for quick in-situ maintenance of pipelines without cutting off pipelines or stopping transmission without pressure relief. It is especially suitable for intensive pipeline maintenance and online emergency maintenance. In order to study the connection performance of the split iron-based shape memory alloy pipe joint, this paper took the Fe17Mn4Si10Cr4Ni alloy pipe joint as the research object, and carried out pressure sealing and pull-out experiments respectively on the integral pipe joint and the split pipe joint which had not been carried out and had been subjected to one heat and mechanical training. The results showed that, compared with the integral pipe joint, the pressure seal value of the split pipe joint decreased by 40% to 20MPa and the pulling force decreased by 48.9% without heat-mechanical training. However, after 1 time of thermal-mechanical training, the pressure sealing value of the split-type pipe joint decreased by 12.5% to 32MPa and the pulling force decreased by 21.2% compared with the integral pipe joint. It can be seen that thermal-mechanical training can significantly improve the joint performance of split pipe joints, and the split pipe joint can meet the requirements of medium and low pressure pipeline connection and maintenance.

Iron-Based shape memory alloy for strengthening of 113-Year bridge

This study focuses on the large-scale application of a Fe-Mn-Si shape memory alloy (Fe-SMA) for strengthening a historic roadway bridge in Petrov nad Desnou (113-years), Czech Republic. To the best of the authors’ knowledge, this is the first application of an iron-based SMA (Fe-SMA) for prestressed strengthening of a bridge. In this study, the shape memory effect (SME) of the Fe-SMA was used for the prestressed strengthening of bridge girders. A mechanical anchorage system was developed to apply multiple Fe-SMA strips to the steel girders of the bridge subjected to daily passengers and heavy trucks. The SME of the Fe-SMA was activated by heating to approximately 260 °C using ceramic heating pads. The test results showed that the recovery stress of the Fe-SMA strips resulted in a compressive stress of approximately –33 MPa in the lower flange of the bridge girder. This compressive stress significantly increased the yield and fatigue capacity of the strengthened girder. Before and after the strengthening, the bridge was loaded with a 45-ton crane to assess the efficiency and performance of the system. Laboratory experiments were performed to optimize the mechanical anchors and examine the feasibility of the proposed strengthening method prior to application to the bridge. Finally, long-term monitoring of the prestressed Fe-SMA plates after installation on the bridge was conducted. The results showed that the main loss of the prestressing force caused by relaxation occurred within the first 30 days after activation and was approximately 20% of the original prestress.

عملکرد سازه ی مجهز به جداساز لرزه یی طبقه و آلیاژهای حافظه دار شکلی با پایه ی آهنی

در سال‌های اخیر، در ساختمان‌های خاص به طراحی جداسازهای لرزه‌یی در طبقات بالاتر از پایه که جداساز طبقه نامیده می‌شود، بسیار توجه شده است. با وجود این، جداسازهای لرزه‌یی با محدودیت‌هایی، از قبیل: ناپایداری در تغییرشکل‌های بزرگ، جابه‌جایی پسماند و غیره پس از زلزله‌های شدید هستند. استفاده از آلیاژهای حافظه‌دار شکلی با توجه به خواص منحصر به فردی که دارند، به عنوان راه حلی مناسب برای غلبه بر این نواقص درنظر گرفته شده‌اند. هدف از پژوهش حاضر، بررسی اثر ترکیب آلیاژهای حافظه‌دار شکلی و جداسازها در سازه و تأثیر آن‌ها در کاهش پاسخ لرزه‌یی سازه بوده است. به همین منظور، سازه‌ی چندطبقه‌ی فولادی همراه با ترکیب جداساز لرزه‌یی طبقه و آلیاژهای حافظه‌دار شکلی با پایه‌ی آهنی و با استفاده از نرم‌افزار O‌p‌e‌n‌S‌e‌e‌s مدل‌سازی و ارزیابی شده است. نتایج به‌دست آمده نشان داده است که استفاده از آلیاژهای مذکور در جداسازهای لرزه‌یی طبقه باعث کاهش جابه‌جایی در تراز جداساز می‌شود و می‌توان آن را به‌عنوان سامانه‌یی مطلوب در مهار سازه‌ها در نظر گرفت.

Thermally-activated shape memory alloys for retrofitting bridge double-angle connections

Double-angle connections are prone to fatigue cracking in old steel bridges; however, owing to their complexity, there are not many retrofitting solutions for these details. This study proposes a retrofitting system for double-angle connections using iron-based shape memory alloys (Fe-SMAs). An innovative connection test setup equipped with an optical 3D digital image correlation (DIC) measurement system was developed. The retrofitting system included two end-anchorages set on either side of the connection. Fe-SMA strips were inserted inside the anchorages, and then were thermally activated (pre-stressed) to a temperature of 260 °C. Numerical finite element studies were performed to investigate the static behavior of the system. It was found that the activated/prestressed Fe-SMA strips not only apply a reverse positive moment on the connection (versus an undesirable negative fixity moment), but also “absorb” a portion of the negative moment, owing to the added axial stiffness. The proposed SMA-based system offers a versatile and cost-effective technique (with quick and easy installation) for bridge connections, and can significantly reduce the effects from undesirable sources of fatigue cracking.

Superior low-cycle fatigue performance of iron-based SMA for seismic damping application

This study reveals the superior low-cycle fatigue performance of iron-based shape memory alloy (Fe-SMA) for seismic damping application, catering to the need for more durable, resilient, and perhaps fatigue-free structural systems in seismic active regions. The study commences with material tests examining both the macroscopic and microscopic properties of Fe-SMA under monotonic and cyclic loading, followed by calibration of combined hardening parameters to facilitate numerical modelling. A Fe-SMA shear damper specimen is tested, and its behavior is compared with its mild steel counterpart. Among other findings, the study revealed good ductility of Fe-SMA with a fracture strain of up to 55% under monotonic loading. The fatigue life of Fe-SMA is from 4007 to 83 when the strain amplitude increases from ±1% to ±9%, and the values could be 10 times that of common structural steel. The cyclic strain-life relationships of Fe-SMA can be readily presented by the conventional Basquin-Coffin-Manson relationship. Both kinematic and isotropic hardening characteristics of Fe-SMA are observed, and a combined kinematic/isotropic hardening model with calibrated parameters is shown to adequately capture the hysteretic behavior of the material. The subsequent damper tests provide further evidence of its superior fatigue performance, where a fatigue life of 173 cycles is observed for the Fe-SMA damper under a constant rotational angle of 4%, in contrast to 16 cycles for its normal steel counterpart. The unique phase transformation characteristic of Fe-SMA could also affect the fatigue failure mode, where different crack patterns are observed for the dampers with the different materials.

Numerical Modeling of Unreinforced Masonry Walls Strengthened with Fe-Based Shape Memory Alloy Strips.

This study presents a new way to improve masonry wall behavior. Masonry structures comprise a significant part of the world’s structures. These structures are very vulnerable to earthquakes, and their performances need to be improved. One way to enhance the performances of such types of structures is the use of post-tensioning reinforcements. In the current study, the effects of shape memory alloy as post-tensioning reinforcements on originally unreinforced masonry walls were investigated using finite element simulations in Abaqus. The developed models were validated based on experimental results in the literature. Iron-based shape memory alloy strips were installed on masonry walls by three different configurations, namely in cross or vertical forms. Seven macroscopic masonry walls were modeled in Abaqus software and were subjected to cyclic loading protocol. Parameters such as stiffness, strength, durability, and energy dissipation of these models were then compared. According to the results, the Fe-based strips increased the strength, stiffness, and energy dissipation capacity. So that in the vertical-strip walls, the stiffness increases by 98.1%, and in the cross-strip model’s position, the stiffness increases by 127.9%. In the vertical-strip model, the maximum resistance is equal to 108 kN, while in the end cycle, this number is reduced by almost half and reaches 40 kN, in the cross-strip model, the maximum resistance is equal to 104 kN, and in the final cycle, this number decreases by only 13.5% and reaches 90 kN. The scattering of Fe-based strips plays an important role in energy dissipation. Based on the observed behaviors, the greater the scattering, the higher the energy dissipation. The increase was more visible in the walls with the configuration of the crossed Fe-based strips.

Strengthening and prestressing of bridge decks with ribbed iron-based shape memory alloy bars

Given the increasing age of our built environment, including concrete structures, stakeholders in the construction industry require innovative and effective strengthening methods. In several studies and onsite applications, iron-based shape memory alloy (Fe-SMA) reinforcements have proven to be competitive alternatives to well-established materials. The objective of this investigation was to study the effectiveness of two different prestressed strengthening methods for concrete members in flexure using recently available ribbed Fe-SMA (“memory-steel”) bars. Reinforced concrete slabs similar to the cantilever extension of box-girder cross-sections were strengthened using only cementitious anchorages and subjected to quasi-static loading. The first strengthening method comprised the removal of the concrete cover and the installation of memory-steel bars with an additional mortar layer. The second strengthening method comprised the cutting of grooves in the concrete cover, followed by the installation of memory-steel bars with mortar (near-surface-mounted). The prestress was obtained via resistive heating of the memory-steel bars. The strengthened specimens exhibited highly improved cracking, yielding, and maintained ductility until ultimate concrete compression failure. Observations utilizing a digital image correlation system indicated a damage-free prestressing process and no premature failure of the strengthening reinforcement. Different analytical approaches were used to estimate the prestress.

Microstructure and Thermomechanical Characterization of Fe-28Mn-6Si-5Cr Shape Memory Alloy

Iron-based shape memory alloys (SMAs) have been widely studied during the last years, producing new formulations with potential applications in civil engineering. In the present paper, the microstructure and the thermomechanical behavior of the Fe-28Mn-6Si-5Cr memory alloy has been investigated. At room temperature, the presence of ε-martensite and γ-austenite was confirmed using optical and electron microscopy techniques. The martensitic transformation temperatures (As, Af, Ms, and Mf) were determined by differential scanning calorimetry, together with an X-ray diffraction technique. The use of these techniques also confirmed that this transformation is not totally reversible, depending on the strain degree and the number of thermal cycles. From the kinetics study of the ε → γ transformation, the isoconversion curves (transformation degree versus time) were built, which provided the information required to optimize the thermal activation cycle. Tensile tests were performed to characterize the mechanical properties of the studied alloy. These kinds of tests were also performed to assess the shape memory effect, getting a recovery stress of 140 MPa, after a 7.6% pre-strain and a thermal activation up to 160 °C.

Fibre optic strain measurements for bond modelling of prestressed near-surface-mounted iron-based shape memory alloy bars

Near-surface-mounted (NSM) iron-based shape memory alloy (memory-steel) bars represent a novel type of reinforcement for prestressed strengthening of concrete structures. To examine the bond behaviour of these embedded reinforcements in detail, fibre optic (FO) methods can be used. In this study, distributed FO strain measurements were performed in small-scale bond experiments and large-scale structural experiments using prestressed and non-prestressed NSM memory-steel bars. The experiments demonstrated the FO measurement technique's feasibility on memory-steel bars, provided insights into the complex strain profiles before and after prestressing, and revealed the active bond lengths. When the memory-steel bar was subjected to an external load, a reduction in the bar's stiffness resulted in an increase in active bond length. The results furthermore highlighted the importance of rigid bar fixations to avoid prestress loss. The obtained strain distributions were also processed to calculate the distributed slip and axial bar stress, and to investigate the parameters of three different bond shear stress–slip laws (BSSLs). A linear increasing BSSL with a nonlinear bar stiffness led to a realistic calculation of the axial stress in the bar, while resulting in acceptable slip and strain. The implementation of a constant Young’s modulus consistently overestimated the axial bar stress.

Novel fatigue strengthening solution for metallic structures using adhesively bonded Fe-SMA strips: A proof of concept study

An innovative fatigue strengthening solution for metallic structures that takes advantage of the shape memory effect of an iron-based shape memory alloy (Fe-SMA) and the bridging mechanism offered by the bonding technique has been proposed and examined. Fatigue tests on cracked steel plates with bonded carbon fiber-reinforced polymer (CFRP) and nonprestressed and prestressed Fe-SMA strips were conducted. The experimental results demonstrate that the bonded prestressed Fe-SMA strips are much more effective than CFRP strips, extending the fatigue crack growth life by a factor of 3.51. The proposed innovative solution is highly effective in retarding fatigue crack growth in metallic structures.

Prestressing Effect of Shape Memory Alloy Reinforcements under Serviceability Tensile Loads

Repairing and strengthening of existing aged steel-reinforced concrete structures is a major challenge. Today, much of the repair work completed is insufficient and brittle. A promising new solution for repair and strengthening tasks is the use of iron-based shape memory alloy (Fe-SMA). The pre-strained Fe-SMA components enable the pre-stressing of existing building components due to the heat-triggered contraction of the steel. Thus, deflections can be reduced or even recovered. In addition, the cracking process can be adapted, and an improvement in the load, under which the first crack appears, is possible. In this paper, the effects of pre-stress generated by activated Fe-SMA rebars, which were centrally embedded inside of a concrete specimen, are shown. The objective of the study is to quantify the improvement in the loads of the first crack and show the influences of the pre-stressing on the load-bearing behavior and the cracking process. For this purpose, axial tensile tests were performed on concrete bars with height, width, and length of 50 mm, 70 mm, and 900 mm, respectively. These were compared to usual construction steel rebars, pre-strained but nonactivated Fe-SMA rebars, and activated Fe-SMA steel rebars. The evaluation of crack patterns and openings was done using digital image correlation (DIC). The pre-stressing of the concrete causes an increase in the first crack loads of more than 150%, which indicates a clear improvement in the state of serviceability limit.

Experimental study on Fe-SMA-to-steel adhesively bonded interfaces using DIC

This study aimed to reveal the interface behavior of iron-based shape memory alloy (Fe-SMA) strips bonded to metallic substrates in order to ensure the integrity of such joints. A series of single lap-shear tests was performed on Fe-SMA-to-steel adhesively bonded joints. The test results reveal that the bond strength can reach over 70% of the tensile strength of the Fe-SMA and the cohesive failure mode can be realized. It is identified that three energy dissipation mechanisms occurred during the debonding process, indicating that the experimentally obtained bond-slip relations represent more than the energy dissipation by the failure of the adhesive.

Experimental behavior of iron-based shape memory alloys under cyclic loading histories

The present paper investigates the behavior of iron-based shape memory alloys (Fe-SMAs) subjected to cyclic inelastic straining by means of uniaxial coupon experiments. The tests feature round bar coupons subjected to a broad range of uniaxial cyclic strain histories representative of earthquake loading. The experimental results suggest that the Fe-SMA under investigation exhibits an asymmetric stress-strain relation, with limited superelastic behavior. It was found that the post-yield/phase transformation behavior of the Fe-SMA alloy is both strain-rate and temperature-dependent. Quantitative comparisons with structural steels subjected to nominally identical cyclic strain histories indicate that, although the studied Fe-SMA has a similar energy dissipation per loading excursion with respect to conventional S355J2+N, the Fe-SMA’s hardening response is appreciably higher, leading to comparatively larger elastic strain energies being stored.

Iron based shape memory alloys as shear reinforcement for bridge girders

Considerable age of a very high number of bridges conjointly with a steadily increasing amount of traffic and changes in design philosophy (e.g. earthquake engineering) have made maintenance needed and retrofitting become more and more important over the years. Retrofitting can become necessary both for flexure or shear enhancements. Existing steel solutions for shear strengthening are very laborious and complex, and the durability of the steel construction is questionable. As an alternative to steel solutions, carbon fiber reinforced polymer (CFRP) sheets or strips are used for shear strengthening of reinforced concrete beams. But, prestressing of CFRP sheets or strips is hardly applicable. However, a prestressing of a shear strengthening has the advantages that the width of existing shear cracks can be reduced and the stresses in the internal steel stirrups are reduced.Therefore, in this study, a new iron-based shape memory alloy (‘memory-steel’) in the form of U-shaped (stirrups) ribbed bars with a nominal diameter of 12 mm were used in combination with sprayed mortar for shear strengthening of reinforced concrete (RC) structures. The memory-steel bars were activated with electric resistive heating. The activation resulted in a prestress of about 300 N/mm2 in the memory-steel reinforcement and consequently in vertical compressive stresses in the web of the RC beams. Large-scale experiments on T-beams with a height of 0.75 m and a total length of 5.2 m were performed to show the practicability and efficiency of the memory-steel shear strengthening. Promising results have shown that the new strengthening system works well in practice. The shear capacity could be increased significantly. Furthermore, at the serviceability limit state, the prestressed memory-steel stirrups reduced the overall beam deflections, the stresses in the internal steel stirrups, the number of cracks, and the crack widths.