shape memory alloys

In the seismic active areas, strong ground motions usually consist of the numerous successive shocks (Foreshock-mainshock or mainshock-aftershock), which have the significant potential to increase the structural response and cumulative damage. This phenomenon (as called seismic sequence) can affect on the behavior of structures, control the seismic performace of buildings. Multiple earthquakes which have been recorded in all parts of the world are proven that the structures located in the mentioned areas are not only experienced a single event, but also they withstand a series of shocks. Due to the high importance of consecutive earthquakes, application of buckling restrained braces (BRB) and shape memory alloy (SMA) materials as smart materials in engineering sciences in the past decades, this paper tries to evaluate the seismic performance of steel frames equipped with buckling restrained brace by determination of the optimal percentage of shape memory alloy under successive earthquakes. Because SMA has unique advantages and characteristics such as no need to replace after an earthquake, high resistance to corrosion and fatigue, the ability to absorb high energy, the ability to return to the original state by applying temperature, tolerating strain up to about 10% without leaving residual strain after an earthquake, and tolerating multiple cycles of loading and unloading, various applications can be found separately and combined in controlling the behavior of structures. It should be noted that despite the high damage potential of successive earthquakes, they are neglected in the seismic codes and design earthquake is still proposed without successive shocks. Hence, the acceptance of new methods for improving the seismic performance of structures under consecutive shocks seems necessary by the engineering community. Therefore, in this regard, 4 and 7 story steel frames with diagonal buckling restrained braces representing short and mid rise structures were designed based on Iranian codes in ETABS software and then implemented in OpenSees software. After selecting the reference model, the performance of the studied models is verified for the linear and non-linear region through comparison of periods and pushover curve of reference and implemented model. In the following, different percentages of shape memory alloys including 20, 40, 60, 80 and 100% for the 4 story steel frames and 5, 10, 15, 20 and 25% for 7 story steel structure has been considered. The studied models are analyzed with/without shape memory alloys under seismic scenarios with and without seismic sequence in Opensees software. For this purpose, critical successive shocks are selected based on effective peak acceleration (EPA) from PEER center. For compatibility aspects between the seismic analysis and seismic design, the selected records should be scaled by designing spectrum for each fundamental period of studied structure in order to have identical spectral acceleration. The results of nonlinear dynamic analysis show that with the increase in the percentage of shape memory alloy in the 4 story steel frame, the response ratio of steel frames under single and consecutive earthquakes increased, but in the 7 story steel frame, it almost decreased, and this reduction is better felt in the higher stories under the single earthquake. Finally, the optimal percentage of shape memory alloy among the selected percentages in the present study is suggested to be 20% for 4 story steel frame and 15% for 7 story steel frame.

Due to aerodynamic heating, temperature of aircrafts and rockets skin increases significantly, leading to reduced flight performance due to forced vibration. Shape Memory Alloys (SMAs) can be used to solve this problem due to their thermo-mechanical properties such as generating force and recovering large strains when exposed to heat. In this study, response of forced vibration in composite cylindrical shells reinforced with Nitinol SMA fibers is investigated. Variation in recovery stress and Young's modulus with temperature in SMA fibers is modeled using the Brinson's constitutive model. Equations of motion are derived based on the classical shell theory using the von Kármán nonlinear geometric strain-displacement relations with the Love's first approximation, applying Hamilton's principle. The governing equations are solved using the generalized differential quadrature method in the longitudinal direction. The effect of pre-strain and volume fraction of SMA fibers on the response of forced vibration in composite cylindrical shells reinforced with SMA fibers are investigated under varying temperatures and different boundary conditions. Numerical results indicate that appropriate use of pre-strained SMA fibers leads to an increase in the resonant frequency and a reduction in the amplitude of forced vibration.

Regarding the high cost of retrofitting and replacing the isolation system after an earthquake, re-centering seismic isolation systems have become one of the most popular research fields in the past decades. A new generation of seismic isolation is based on equipment with improved performance in terms of damping and re-centering capability. Since lead-rubber base isolation (LRB) is one of the most common seismic isolation systems, this paper focused on suggesting a re-centering LRB system with high damping capacity. The LRB isolation is based on an experimental study under a constant vertical load equal to 650 kN. The proposed combined system of LRB and friction yields a high damping capacity. A spherical shell is placed on the base isolation system, and the friction reaction emanates from this shell's sliding on the provided foundation. The primary purpose of this spherical shell is to increase the damping capacity of the isolation system. An adjustable friction reaction is reachable depending on the connection details of the spherical steel shell with the system's upper steel cap plate. The higher stiffness of this connection is provided greater friction force and, as a result, higher damping capacity. In short, this connection was performed as adjustable friction and can increase energy dissipation and maximum shear force, respectively, in the range of 12% to 80% and 6% to 230%. Increasing re-centering capacity is another goal of this study. It is of great interest to implement shape memory alloy (SMA) in the form of wires because of their re-centering capabilities. The axisymmetric design is implemented to reduce this hybrid system's vulnerability to earthquakes of arbitrary direction. The combined action of wires and the spherical steel shell provides the re-centering capacity of the system.The final system, which is a combination of LRB, friction mechanism, and vertical load transfer through SMA wires (MDLRB-SMA), exhibits enhanced properties such as reduced residual deformation, controllable shear reaction at each stage of the deformation, and increased damping capacity. The energy dissipation capacity of the MDLRB-SMA is shown to be increased by 50%, with a decreased residual deformation of up to 45% compared to the LRB. A set of parametric studies are also performed to investigate the influence of frequency, displacement amplitude, loading rate, the wires' configuration and properties, and the aspect ratio of the system's performance.

In the present work, the effect of smart shape-memory alloys on thermal buckling and post-buckling of monoclinic and unidirectional composite panels has been investigated. The aerodynamic pressure applied to the system was modeled using the piston theory method. The effect of thermal heating for ultrasonic flows was also estimated from the reference temperature method. The panel is modeled nonlinearly with large deformations based on Van Karmen's theory. The obtained results show that the shape-memory alloy was able to increase the critical temperature of thermal buckling. The effect of the arrangement of composite layers on increasing the thermal buckling temperature was also studied. The results show that the amount of thermal deflection is greatly reduced due to the use of this alloy. Also, in the higher temperature differences, the rate of reduction of the panel increases. In this work, the effects of thermal stress on buckling and thermal buckling in a rectangular composite panel with hinge-hinge boundary conditions were investigated. Also, the effect of shape retention alloy in controlling these two phenomena has been studied. The shape memory alloy wire was placed in martensitic mode to apply a compressive force to the panel to control the heating and aerodynamic forces after changing the phase to austenite. The governing equations of the system were extracted through the layer theory method to show the effects of in-plane displacements better. Investigations on the effect of different layers (symmetry effects and arrangement angle of composite sheets) were performed to investigate thermal buckling. According to the obtained results, the layer arrangement of plates (0.90 / 0.90), (-45.45), (30/60), and (0.90 / 90.0), respectively, had the greatest effect on raising the critical temperature of thermal buckling in dimensional ratios equal to or greater than one. This shows that the symmetry of the arrangements has a greater effect than the angle of the arrangements. Examining the buckling diagrams for the effect of a shape memory alloy, it can be concluded that by placing this alloy in the composite panel, in addition to raising the buckling temperature, this alloy has a greater effect on displacement control by increasing the temperature after the critical buckling temperature.

High-speed flying objects are subjected to temperature variation caused by aerodynamic heat, which reduces flight performance due to instability. Due to high tensile force generated by increasing temperature, Shape Memory Alloys (SMA) may be used to reinforce these structures. In this study, the axial buckling of composite cylindrical shells reinforced with SMA fibers made of Nitinol is studied. Properties of SMA fibers are determined using the Brinson's constitutive model. Governing equations are derived using classical shell theory with the Von-Karman type of geometrical non-linear in conjunction with Love's first approximate accompanied with the static mode of virtual displacement principle. To solve the governing equations generalized differential squares method are used in longitudinal direction. The effect of pre-strain, volume fraction and position of SMA fibers on the critical load of the cylindrical shells are investigated. Results show that shell reinforcement with small percentage of pre-strained SMA fibers significantly increases the critical load. Also, the use of SMA fibers in the longitudinal direction and in the layer close to the inner surface of the cylinder has a favorable result in increasing the critical load.

Today, there is an increasing interest in use of seismic isolation to protect the structures against earthquakes. The most common type of seismic isolation is called base isolation, in which the isolation layer is installed under foundations to separate the structures from the ground and reduce the earthquake forces. However, the use of this type of seismic isolation faces some difficulties such as construction in congested urban areas or construction of near sea structures. Furthermore, for seismic retrofitting of existing buildings, the installation of the isolation under foundations is difficult or even impossible; so, it needs to be located on the middle floors of the buildings. The method of isolation design is called floor or middle story isolation. Despite the advantages of seismic isolations, they have some limitations such as instability in large deformations, residual displacement, and the need for replacement after severe earthquakes. The use of Shape Memory Alloys (SMA) regarding their unique properties is considered as an appropriate solution to overcome the above problems. These smart materials show high strength and strain capacity, high recentering ability, and high resistance to corrosion and to fatigue. The purpose of this study is to investigate the effect of combination of middle story isolation utilized by Natural Rubber Bearing (NRB) and iron-based shape memory alloys in steel structures and to compare the performance of such structures with and without the presence of the shape memory alloy in the middle-story isolation system. Then, a three-story steel structure has been modeled and evaluated. For this purpose, a structure with floor isolation and iron-based shape memory alloy was modeled in OpenSees computer program. The structures were then subjected to seismic loading. The results were presented in the form of story drift, floor acceleration, floor shear forces, and base shear. The outcome of this research showed that the use of these alloys in the middle-story isolation reduced the overall base shear and floor shear forces. The overall story drift, floor acceleration and displacement are reduced; with the exception at the isolation level. Thus, utilizing the natural rubber bearing isolator along with the iron-based shape memory alloy can be considered as a desirable system for the seismic protective design of buildings.

Seismic loading in seismic prone countries is very important and the lack of enough attention can make a irreparable damages to structures and non-structural elements. Dissipating earthquake energy just by using elastic capacity of structure, will increase the dimension and weight of structural elements like column, beam, walls and the cost of building will increases. Incoming seismic energy should be dissipated in plastic process and in this process, structure must remain stable. Shearwall is using widely because of its suitable behavior against seismic loading and good ability of energy dissipation. Sometimes it is inevitable to avoid openings in shear walls due to architectural considerations. Providing enough ductility in these shear walls is a difficult job because of stress concentration around of voids. The other problem of using shear walls is plastic deformations that make the structures useless. One way of improving ductility and self-centering ability of shear walls is using smart materials. Shape memory alloys are one of the newest smart materials that have two important behaviors, called shape memory effect and superelasticity. Removing the residual deflection after unloading of elements made by shape memory alloys by heating called shape memory effect. Superelasticity in SMAs is returning the elements to their initial shape after unloading by them. Good corrosion resistance, good fatigue behavior and weldability are the other positive behaviors of shape memory alloys. In this paper the improvement of the shear walls ductility and self-centering ability with using shape memory alloys in superelastic phase is investigated. Shape memory alloys can withstand up to 7 percent of strain without any residual deformation. In this paper shear walls modeled by shear-flexure interaction multi-vertical-line-element-model (SFI-MVLEM). This model was implemented in the Open System for Earthquake Engineering software (OpenSees). Considering interaction between shear and flexural response in shear walls has made this element superior. Superelastic reinforcement bars were embedded in plastic hinge of boundary elements, coupled beam and walls web separately. Shear wall modeled by using SMA in boundary element, coupled beam and walls web called SMAB, SMAC and SMAW respectively. To examine the effects of these alloys on energy dissipation capacity and self-centering ability of the shear walls, structure were evaluated in cyclic analysis. Place of using SMAs on shear wall in very important and can influence widely on shear wall so most optimized place of using SMAs in shear wall should recognized. Two case of optimization considered in this analyze. In first one, best place is a place that using SMAs on it causes minimum energy dissipation reduction and maximum residual displacement removing. In second one, best place is the place that causes maximum residual displacement removing with minimum usage of SMAs. In order to find the influence of SMAs on ductility of shear wall, pushover analyse was used. Using SMAs in boundary elements of shear walls made maximum increasing in ductility of shear wall but the best place for using SMAs for optimization of usage of SMAs is walls web. Based on the results, with using shape memory alloys, ductility of shear walls was increased and its residual deformation and energy dissipation capacity was decreased. The best place of using SMAs in shear wall is coupled beam for optimization of energy dissipation and residual displacement and for optimization SMA usage is walls web.

In the present study, Cu/Zn/Al multi-layered composite was processed by accumulative roll bonding (ARB) through nine passes. Afterwards, heat treatment processes at various temperatures (750-950℃) and times (10-25min) were done on the prepared composites to fabricate CuZnAl shape memory alloys. The microstructure (composites and alloy) were investigated using scanning electron microscopy and X-ray diffraction. Tensile properties and shape memory effect of the composites and alloys were also investigated by tensile test. The microstructure investigations show that plastic instability and shear bands occurred in different layers in the composite. In addition, a composite with a uniform distribution of Zn and Al reinforcing layers was produced after nine passes. The tensile strength of the composite increased from the first cycle to the third ARB cycle and then decreased from the fifth to the ninth ARB cycle. Finally, the best UTS (about 330MPa) and elongation (about 31.52%) values were obtained on the third and first pass, respectively. The results showed that CuZnAl shape memory alloy was successfully fabricated by the accumulative roll bonding process and next heat treatment. It was also found that the alloys treated at 900°C and cooled in ice water consist of martensitic phase. Additionally, the alloy annealed at 900°C for 15 minutes exhibited a good shape memory effect and strength (about 503MPa).

Shape Memory Alloys (SMAs) are a group of smart materials which demonstrate two particular non-linear stress-strain behavior, Shape Memory Effect and Super elasticity based on Austenite to Martensite transformation and vice versa. Effects of heat treatment on SMA wire property and the vibration of composite plate with embedded SMA wires are investigated in current study. Heat treatment effects was studied experimentally and transformation temperatures is determined by differential scanning calorimetry (DSC). Since the ABAQUS software is not capable of analysis the shape memory alloy structures, the UMAT subroutine in the software is used to implement the Boyd and Laguodas model to any shape memory alloy finite element analysis in ABAQUS. Extensive numerical results are depicted to provide an insight into the effects of volume fraction, pre-strain and shape memory alloy properties, transformation temperatures and stress- strain curve changing duo to heat treatment on pre and post-buckled composite plate are discussed.

In advanced composites , intelligent materials such as memory alloys have been used to improve the mechanical properties of various applications. In this study, the superellasticization property of the shape memory alloys has been used to improve the tensile strength of polymer - reinforced fiber composites. The effects of volumetric percent and thickness of memory alloy wire on the composite reinforced with unidirectional carbon fibers under tensile loading are investigated. For this purpose, composite specimens reinforced with unidirectional carbon fibers were constructed by placement of one, three, and five wires of memory alloy with thicknesses of 0.2 , 0.3 and 0.4 mm. The results showed that when the samples are stretched, a significant proportion of the tensile load is transferred to the wire, and due to the high strain tolerance in the shape memory alloys, the strength of this composite compared with the composite sample is increase about 7 times.

Re-centering is an exclusive characteristic of superelastic Shape Memory Alloys (SMAs) which can be used in manufacturing and retrofitting of reinforced concrete elements. Reinforced concrete beams retrofitted with SMA bars have more ductility and higher energy dissipation compared to conventional RC beams. Furthermore, these beams experience less damage in consecutive loading-unloading cycles. The current research aims to investigate the behavior of reinforced concrete beams retrofitted with SMA bars using Near-Surface Mounted (NSM) flexural retrofitting method. Eleven RC beam specimens with the cross section of 200*150 mm and length of 1150 mm were cast. Three of the specimens had no external strengthening, four of them were retrofitted with SMA bars and other four beams were retrofitted with GFRP reinforcements. The specimens were subjected to three-point bending test under either monastic or loading-unloading. Different parameters including load-carrying capacity, energy dissipation, deformation recovery and reduction capability of crack width were investigated. The results showed that RC beams retrofitted with SMA bars had more mid-span deflection and higher energy dissipation compared to other specimens under monotonic loading. Moreover, under loading-unloading, RC beams retrofitted with SMA bars method experienced less damage.

In general, materials include micro-cracks and small holes which is created during the manufacturing process. The growth of these micro-cracks leads to degradation of mechanical properties and resulting in deterioration of the materials. Continuum damage mechanics is the new field of failure criteria which survey the behavior and responses of weakened material during the complete process of deterioration of material. This method defines the damage growth with an internal variable and can be used to predict the failure behavior of many materials such as metals, composites, polymers, and so on. Shape memory alloys have unique features, such as, having memorable properties, being super elastic and being energy absorber, which led to new applications in science and engineering research. Super elastic property accompanies with a lot of energy absorption during creating a Hysteresis loop. In this research, we examine mechanical behavior of materials reinforced with smart alloy in the context of environmental damage mechanics. Simulation and experimental results were very close. The considered structure is a notched piece of aluminum which is reinforced by the smart alloy. This material is notched because when the smart alloy reaches to its maximum reversible strain, damage variable reaches to its critical value due to the stress concentration. Accordingly, in this case, the effect of existence of the smart alloy is studied to find how it reduces the growing of the damage. Simulation of the mentioned structure is performed with Finite Element Analysis, where the structure was modeled under longitudinal loading. UMAT code of Lemaiter model, was developed for behavioral properties with damaged aluminum, UMAT code of Brinson model was used for behavioral properties of shape memory alloy. Simulation results suggest that different behavioral aluminum with aluminum reinforced by SMA. Existence of smart alloy on the aluminum substrate reduces the damage evolution and the structure fails in higher loadings. Also, the simulation results showed that reinforcing materials such as aluminum with shape memory alloys, up to the failure, are suitable choices for cyclic loading.