Shape Memory Alloy Strips Research Articles

A Wirelessly Controlled Shape‐Memory Alloy‐Based Bistable Metal Swimming Device

Shape memory Nitinol has long been used for actuation. However, utilizing Nitinol to fabricate novel devices for various applications is a challenge, but has shown incredible promise and impacts. Bistable metal strips are widely adopted for shape morphing purposes (primarily in kid's toys, e.g., snap bracelets) due to their easy and robust transformation between two states. Herein, Nitinol shape memory alloy and bistable metal strip are combined to fabricate a swimming actuator with both slow moving and fast snapping capability, akin to an octopus swimming slowly in water, but quickly moving upon encountering a threat. The actuator developed here can also swim in multiple directions, all controlled by a wireless module. Furthermore, it is demonstrated that an onboard sensor can be incorporated for potential environmental monitoring applications. The fact that the device developed here has no mechanical parts, makes this an interesting potential alternative to more expensive, and energy consuming boats. A preprint version of the article can be found at: https://www.authorea.com/doi/full/10.22541/au.164199106.60208565.

Automated shape-transformable self-solar-tracking tessellated crystalline Si solar cells using in-situ shape-memory-alloy actuation

Photovoltaic energy systems in urban situations need to achieve both high electricity production and high capacity in restricted installation areas. To maximize power output, solar-tracking systems tilt solar arrays to track the sun’s position, and typically flat modules are used to maximize the cross-sectional area. Such tracking systems are complex and expensive, and flat modules cannot utilize omnidirectional incident light. For solar systems in urban environments, we have developed two-dimensional (2D) or three-dimensional (3D) tessellated solar-cell modules that use shape transformation, and combine solar tracking and an arch structure for use in restricted areas. The modules can use scattered and omnidirectional incident light. Simply by attaching shape-memory alloy strips to the surface of the solar panels, the shape of the array can be transformed in response to heat from sunlight. Compared to a perfect solar-tracking system, our simulation results indicate that the modules present a large cross-sectional area perpendicular to the direction of sunlight and provide superior tracking performance, resulting in a 60% increase in electricity production over the course of 1 day. In addition, by using different designs for the tessellation units, dome shaped or other 3D structures are possible, for specific applications and to meet aesthetic requirements.

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.

Finite element simulation and experimental investigation on the effect of temperature on pseudoelastic behavior of perforated Ni–Ti shape memory alloy strips

In the present study, the temperature-dependent pseudoelastic behavior of shape memory alloy (SMA) sheets is studied experimentally and by finite element (FE) modeling. For this purpose, temperature-dependent mechanical properties for Ni–Ti alloy materials are first obtained by using direct tensile and three-point bending experiments at 23 °C, 50 °C, and 80 °C temperatures, respectively. The structure of these materials is examined at different temperatures using SEM images and the XRD test. Furthermore, using the FE model, the pseudoelastic behavior and the effect of temperature on the residual deflection of the prose-shape memory strips with a circular hole under three-point bending loads are studied. After validating the results of the FE model with the results of experimental tests, the effects of various parameters such as the diameter and number of holes on residual deformation and residual strains are investigated. The results show that with increasing temperature, the mechanical properties including the tensile strength, Young’s modulus, yield stress, and flexural strength of SMA strips increase significantly. For solid strips, although increasing the temperature increases the maximum flexural force, in contrast, it reduces the flexural stiffness. In solid strips, flexural stiffness decreases by 5.5% with increasing temperature from 23 °C to 80 °C.

Cyclic Behavior of Masonry Shear Walls Retrofitted with Engineered Cementitious Composite and Pseudoelastic Shape Memory Alloy.

The behavior of masonry shear walls reinforced with pseudoelastic Ni–Ti shape memory alloy (SMA) strips and engineered cementitious composite (ECC) sheets is the main focus of this paper. The walls were subjected to quasi-static cyclic in-plane loads and evaluated by using Abaqus. Eight cases of strengthening of masonry walls were investigated. Three masonry walls were strengthened with different thicknesses of ECC sheets using epoxy as adhesion, three walls were reinforced with different thicknesses of Ni–Ti strips in a cross form bonded to both the surfaces of the wall, and one was utilized as a reference wall without any reinforcing element. The final concept was a hybrid of strengthening methods in which the Ni–Ti strips were embedded in ECC sheets. The effect of mesh density on analytical outcomes is also discussed. A parameterized analysis was conducted to examine the influence of various variables such as the thickness of the Ni–Ti strips and that of ECC sheets. The results show that using the ECC sheet in combination with pseudoelastic Ni–Ti SMA strips enhances the energy absorption capacity and stiffness of masonry walls, demonstrating its efficacy as a reinforcing method.

Rotational performance of frictional glulam beam-to-column connections with shape memory alloy strips

Resilience has becoming a favorable direction in developing structures against various hazards especially earthquakes. In order to promote structural maintainability and enhance self-centering effect of heavy timber structures, this paper proposed frictional glulam beam-to-column connections (FCs) with shape memory alloy (SMA) strips and experimentally investigated their rotational performance. Four half-scaled FCs with SMA strips, characterized by flat and pyramid frictional timber pads, were cyclically tested considering different loading rates, and compared with a conventional glulam beam-to-column connection (CC) at multi loading amplitude levels. The test results showed that FCs with SMA strips were damage-free in vulnerable connection zones, except part of SMA strips failed in tension but could be easily replaced. The hysteretic behavior of FCs with SMA strips was insensitive to loading rate, indicating a consistent and stable performance. Compared to those of CC, the initial stiffness of FCs with SMA strips did not significantly change, although the peak moments decreased 15%–30%, the energy dissipation capacity was increased roughly 100% and less residual deformation was obtained owning to friction mechanism and pesudoelasticity of SMA. Numerical study was further conducted to illustrate the benefits by applying the proposed connections in framed structures. It is found that adopting FCs with SMA strips instead of CCs would not considerably change the dynamic response of timber frames under minor earthquakes, but would significantly reduce the maximum story drift and residual drift of structures under strong earthquakes. Although the type of frictional timber pads had slight influence on the rotational performance of FCs with SMA strips, this influence was negligible in structural level.

Numerical Study of RC Beams Strengthened with Fe-Based Shape Memory Alloy Strips Using the NSM Method

This paper presents a finite element (FE) analysis for predicting the flexural behavior of reinforced concrete (RC) beams strengthened with Fe-based shape memory alloy (Fe-SMA) strips using a near surface mounted (NSM) method. Experimental results reported in the literature were used to verify the proposed FE model. FE analyses were conducted using OpenSees, a general-purpose structural FE analysis program. The RC beam specimens were modeled using a nonlinear beam-column element and a fiber element. The Concrete 02 model, Steel 01 model, and Pinching 04 model were applied to the concrete, steel reinforcement, and Fe-SMA strip in the fiber element, respectively, and the FE analysis was carried out in a displacement control method based on the Newton-Raphson method. The FE model of this study accurately predicted the initial crack load, yield load, and ultimate load. From parametric analyses, it was concluded that an increase in the compressive strength of the concrete increases the ductility of the specimen, and an increase in the level of recovery stress on the Fe-SMA strip increases the initial stiffness of the specimen.

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.

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.

The electro-mechanical control of element NiTi shape memory alloy strip while bending, based on thermal analysis evidence

This paper presents an experimental investigation of a commercial shape memory alloy strip consisting of nickel and titanium (NiTi SMA), which correlates the thermal properties (phase transitions) with the electrical and mechanical properties of the material, allowing to be used for control applications in engineering. The bending performances of the SMA strip at different values of the activation electric current, during heating process, are also investigated. By using the differential scanning calorimetry (DSC) experiments, the thermal characteristics of the SMA strip were identified. The results of this thermal analysis were the basis for determining the equilibrium temperature, entropy and force developed by the SMA strip during the phase transformation. For the study of the SMA strip, the authors have developed an experimental platform, which uses a portable reconfigurable I/O (RIO) device and LabVIEW graphical programming environment for the measurement, testing and rapid control of parameters, and a piezo-film sensor for controlling of SMA strip movement. The real-time measurements and acquired waveforms are displayed on a PC screen, and the data associated with these waveforms are stored for later use, i.e. for applications in safety engineering like the fire extinguishing control, where very short response/activation times are required.

Finite element modeling of RC beams externally strengthened with iron-based shape memory alloy (Fe-SMA) strips, including analytical stress-strain curves for Fe-SMA

Research into the application of iron-based shape memory alloys (Fe-SMA) for strengthening existing structures has boosted in recent years. The shape memory effect of this material can be used to introduce prestressing forces, improving the behavior of the retrofitted structures. However, knowledge about these alloys is still limited and more research is needed. This paper proposes analytical stress-strain curves for Fe-SMA based on the modified Ramberg-Osgood models for nonlinear materials, and taking into account the differences observed according to the material’s initial state. The degree of roundedness and the level of strain hardening vary depending on the initial state, achieving in all cases a satisfactory agreement between analytical expressions and experimental curves. Moreover, these curves have been implemented into a nonlinear finite element model developed to reproduce the response of reinforced concrete beams strengthened upon shear using external Fe-SMA strips that have been previously tested by the authors. The numerical results of the finite element analysis show a very good correlation with the experimental behavior observed during the tests.

Preparation of Cu–Al–Ni shape memory alloy strips by spray deposition-hot rolling route

The present work describes the preparation of near-full density Cu–Al–Ni shape memory alloy (SMA) strips via a novel processing route consisting of ‘spray deposition’ of atomised liquid Cu–Al–Ni alloy with a jet of argon gas followed by hot-rolling densification of the deposited preform. The subsequent homogenisation of the hot rolled Cu–Al–Ni strips resulted in complete martensitic structure in the finished strip, consisting of self-accommodated plates of β1′ and γ1′ martensites. The characteristic transformation temperatures and shape memory effects of Cu–Al–Ni strips were studied. It has been demonstrated that the Cu–Al–Ni SMA strips, prepared in the present work, resulted in relatively finer grain size with better combination of strength and ductility compared to other techniques based on conventional casting method.

Shear strengthening of slender reinforced concrete T-shaped beams using iron-based shape memory alloy strips

Recent research works have shown the feasibility of using iron-based shape memory alloys (Fe-SMAs) in structural rehabilitation projects, especially for flexural strengthening. As for the shear strengthening of reinforced concrete (RC) beams with SMAs, most previous works have used small-scale members. This paper presents an experimental campaign involving eight tests run with real-scale RC beams. The total length of the beams was 5800 mm, and their T-shape cross-section was 550 mm deep. External strengthening was done using Fe-SMA strips with a U-configuration. Strips were characterized before being used for the strengthening application. The key results are herein presented. All the beams failed in shear. Tests showed an increase of around 30% in the shear strength of the retrofitted beams versus the reference beams, and flexural capacity was practically reached. For service loads, active strengthening was useful for delaying the appearance of cracks and reducing their widths. However, the U-configuration used for the strips implied having to anchor them to the web, which probably weakened the flange-web connection and impeded the correct transfer of the tensile stresses between the longitudinal tensile reinforcement and the compression flange, which resulted in less ductile behavior than expected.

On the design strategies for SMA-based morphing actuators: state of the art and common practices applied to a fascinating case study

The request of even more stringent restrictions, regarding efficiency and environmental impact of industrial components, determines an optimized use of primary energy but also entails the design of more lightweight, smart and flexible devices, able to adapt their operation as a function of several different inputs. In this framework, the use of a fascinating class of metallic materials, called Shape Memory Alloys (SMAs), could represent a valid support for the designers. The capability of these materials to react to an external stimulus, without continuing to supply energy to external actuators, represents, especially in the aerospace engineering field, a technological breakthrough. The present paper reports the basic ideas and summarizes the important aspects related to the development of SMA-based actuators in relation to the present state of the art. A case study of morphing blades, equipped with embedded SMA strips, for an automotive cooling fan is reported. Finally, some hints, regarding the design process of SMA-based actuators, are proposed.

Deformation behaviors of gradient nanostructured superelastic NiTi shape memory alloy

Deformation behaviors of the gradient nanostructured superelastic NiTi shape memory alloy strip with grain size ranging from 20 to 175 nm along the tensile axis are investigated. Compared with the commercial NiTi of uniform grain size, the gradient nanostructure leads to the increase of transformation stress as well as the morphology change, from localized deformation to homogeneous one. However, the behaviors in local different grain size regions are independent and maintain the grain-size dependent characteristics.

Long-Term Behavior of Reinforced Concrete Beams Strengthened with Near-Surface Mounted Fe-Based Shape Memory Alloy Strips

Long-Term Behavior of Reinforced Concrete Beams Strengthened with Near-Surface Mounted Fe-Based Shape Memory Alloy Strips

Development of nail-anchor strengthening system with iron-based shape memory alloy (Fe-SMA) strips

This work discusses the development process of a direct fastening technique for strengthening steel members using iron-based shape memory alloy (Fe-SMA) strips. First, several lap-shear tests were performed to examine the efficiency of five nail-anchor systems in terms of load-carrying capacity, slippage, and convenience of installation. To this end, 3D optical digital image correlation (DIC) measurements (using an ARAMIS system) and numerical simulation were conducted. Different failure modes including tensile rupture of the strips, shear rupture of the nails, and/or sliding of the strips were observed and discussed. The optimum nail-anchor system was then determined. This system has a very short anchorage length of approximately 50 mm and only 12 nails (i.e., 3 × 4). Second, to demonstrate the efficiency of the proposed system, a steel I-beam was strengthened with the selected nail-anchor system and activated using an infrared heating (IRH) technique. The SMA-strengthened beam was subjected to static and then high-cycle fatigue loadings with a loading ratio of R=0.2 for 2.5 million cycles without any failure or damage, indicating reliable performance of the system. Owing to the larger coefficient of thermal expansion of the Fe-SMA material compared to that of steel, it was observed that a decrease and increase in the environmental temperature resulted in an increase and decrease of the pre-stress level in the Fe-SMA strip, respectively (i.e., an inverse correlation).

Experimental study on shear strengthening of shear critical RC beams using iron-based shape memory alloy strips

Shape memory alloys (SMAs) have been introduced into structural engineering in recent years. This paper presents research results about developing a new methodology to actively strengthen reinforced concrete (RC) beams by low-cost iron-based SMA strips (Fe-SMA). These strips can transversally prestress, or confine, the cross-section of beams thanks to the shape memory effect of Fe-SMA, without having to apply prestressing force. Activation of strips is carried out by heating them up to 160 °C and then cooling them. An experimental campaign was carried out at two levels: characterization of Fe-SMA strips, and the practical application of the strengthening technique on small-scale beams without internal stirrups. The retrofitted beams with activated strips failed by bending, and the appearance of shear cracks was clearly delayed. Meanwhile, the reference beams failed due to shear.

Physical and thermophysical properties of a commercial Ni–Ti shape memory alloy strip

Some physical properties like the thermal, thermomechanical, calorimetric, magnetic, and adhesive properties of a commercial shape memory alloy (SMA) with rectangular geometry were studied. Depending on the applied forces, there were identified range of elasticity, the elasticity–viscoelasticity coexistence domain, and the domain in which a maximum force of 18 N is applied, for the SMA strip. The controlled force module, in the tension mode, was used for the determination of the SMA strip elongation at application of the stretching forces from 0 to 13 N, at 30 °C, maintaining each static force value for 3 min. By employing the multi-frequency strain–stress modulus in the tension mode, DMA cyclic heating–cooling measurements were carried out. The measured dynamic mechanical properties for SMA strip were Storage Modulus, Loss Modulus, tanδ, and Stiffness, both at heating and cooling. Thus, the characteristic temperatures of the phase transitions (As, Af, Ms, Mf), of SMA strip were identified. Also, the values of the elasticity modulus (Young’s Modulus) of the SMA strip were calculated at 30 °C. With the DSC Q2000 device, using temperature-modulated differential scanning calorimetry method, a multi-step temperature variation program was applied to the rectangular strip in two stages (heating–cooling). Through the interpretation of heat fluxes (reversible, nonreversible, and total), the phase transitions in the formation of martensite, austenite, and also of the rhombohedral phase (R phase) and moreover enabled for the R phase identification, thermomechanical analysis confirmed the results obtained by classical DSC method. The adherence of some commercial azoic dyes on the rectangular SMA strip, as well as the modification of the surface roughness of the strip after the deposition of the dye, was also studied. By magnetic measurements, it was established that the SMA strip had magnetic properties at room temperature, in which magnetization is the sum of a superparamagnetic contribution and a paramagnetic term, linear in the intensity of the magnetizing magnetic field.

Thermally activated iron-based shape memory alloy for strengthening metallic girders

The study presents a new retrofit solution for strengthening metallic I-girders. The retrofit system involves two iron-based shape memory alloy (Fe-SMA, ‘memory-steel’) strips (each with a width and thickness of 50 and 1.5 mm, respectively) that are mechanically anchored (using friction clamps) to the girders. The shape memory effect (SME) of the Fe-SMA material has been used to activate/prestress the strips by heating to a predefined temperature. The main advantage of the proposed SMA-retrofit system is that, unlike conventional systems, it can prestress itself without a need for heavy hydraulic jacks, which then results in a significant reduction of the required time, labor works and cost of prestressing process. In order to evaluate the efficiency of the proposed retrofit system, in this study, a series of static and fatigue four-point bending tests were performed on a 6.4-m SMA-retrofitted beam. Five static tests were performed on a steel beam with different SMA prestressing levels and included a reference un-strengthened test. The test results indicated that the achieved prestressing levels (i.e., recovery stresses) in the Fe-SMAs for activation temperatures of 100, 160, and 260 °C were approximately 160, 330, and 430 MPa, respectively. The induced compressive stresses in the bottom flange were in the range of 10–30 MPa. It was demonstrated that the Fe-SMAs could be re-activated for multiple times even up to higher temperatures (than the initial activation temperature), which would then result in higher prestressing levels. These features make the proposed SMA-based system a versatile and adaptable retrofit solution. Furthermore, the SMA-strengthened beam with the maximum prestressing level (activation temperature of 260 °C) was subjected to 2 million load cycles with a load ratio of R = 0.2 and a loading frequency of 4.35 Hz. The results of the high-cycle fatigue (HCF) tests showed no slippage in the anchorage system and a stable prestressing in the Fe-SMA members during the tests, which demonstrates the reliability of the proposed system under HCF loading regime.