Prestrained Shape Memory Alloy Research Articles

Design and Hysteretic Performance Analysis of a Novel Multi-Layer Self-Centering Damper with Shape Memory Alloy

This paper presented the development process of a novel multi-layer self-centering damper utilizing a NiTi shape memory alloy (SMA) with remarkable superelastic properties. The construction and operating principles of the novel damping device were introduced. A model for calculating the restoring force–displacement hysteretic curve of the novel damper was established, and based on this theoretical model, a parameter analysis of the damper’s hysteresis performance was conducted. The effect of SMA pre-strain, SMA diameter, number of layers in the damper, and number of SMA wires per layer on the damper’s stiffness, the unit cycle energy dissipation, and the equivalent viscous damping ratio were investigated, respectively. The results showed that the restoring force–displacement hysteretic curve of the novel SMA damper exhibits a full spindle shape, demonstrating the damper’s excellent energy dissipation capacity, self-centering capability, significant stroke, and unique variable stiffness characteristics (i.e., appropriate initial stiffness, minimal isolation stiffness, and significant limit stiffness). The results also indicated that the SMA pre-strain has a minor impact on the damper’s stiffness but a significant influence on unit cyclic energy dissipation and equivalent damping ratio. As the SMA pre-strain increased from 0.03 to 0.04, 0.05, and 0.06, the maximum stroke of the damper continuously decreases, while the unit cyclic energy dissipation initially increases and then decreases, with the optimal energy dissipation achieved at a pre-strain of 0.04. Increasing the SMA diameter results in a higher damper stiffness and energy dissipation capacity, with no significant change in maximum stroke and equivalent damping ratio. Increasing the number of damper layers leads to an increase in maximum stroke and unit-cycle energy dissipation, accompanied by a decrease in stiffness and almost constant equivalent damping ratio. As the number of SMA wires per layer increased from 8 to 16 and 32, the maximum stroke and equivalent damping ratio presented little variation, but the damper’s stiffness and unit cyclic energy dissipation continuously increased.

Experimental investigation of mechanical properties of NiTi superelastic shape memory alloy cables

As a self-centering element in a recoverable functional structure, shape memory alloys (SMAs) usually require a large diameter to provide sufficient bearing capacity, and pre-strain is applied to obtain good self-centering performance and initial stiffness. This study aims to explore the mechanical properties of large-diameter NiTi SMA cables and their potential applications in recoverable functional structures. Through the uniaxial tensile test of 11 SMA cables with 7 × 7 × 1.0 mm, the mechanical properties of SMA cables under different strain amplitudes, cyclic loading effects, loading rates, and different pre-strain modes were evaluated. The results indicate that the energy dissipation capacity of SMA cable increases with the increase of strain amplitude, but at the same time, it will reduce its self-centering capabilities. The equivalent viscous damping ratio decreases after the SMA cable enters the martensitic hardening stage. The residual strain of SMA cable depends on the combined effect of strain amplitude and number of cycles. The larger the strain amplitude and the number of cycles, the more significant the degradation of self-centering performance. After 20 times of constant amplitude loading and unloading, the mechanical properties of SMA cable are relatively stable. The reverse phase transformation stress of the pre-strained SMA cable gradually decreases with the increase of the tensile strain amplitude. The pre-strained SMA cable exhibits a stable restoring force strength in the sub-cycle, and the residual strain is zero. The test results support experimental data for applying large-diameter SMA cables in recoverable functional structures.

On the nonlinear analysis of pre-strained shape memory alloy beams

We present a theoretical and numerical investigation of the transverse dynamic response of monolithic Shape Memory Alloy (SMA) beams under the effect of axial pre-strains. The SMA beam is taken as benchmark structure to evaluate the effect of (axial) pre-strain on either the free or the forced dynamics. The combination of the static stress, produced by the pre-loading conditions, with the dynamic stress, associated with the vibratory response, affects the occurrence of the stress-induced SMA phase transformation hence providing a mechanism to passively tune the dynamic response. Particular attention is given to the effect that different pre-strain levels have on the overall ability to dissipate mechanical energy, that is on the effective damping. The numerical model of the SMA beam accounts for both material and geometric nonlinear behaviors. The nonlinear material behavior is due to the SMA phase-transformation and it is modeled via the one-dimensional improved Brinson’s model (including tension–compression asymmetry), while the geometric nonlinear behavior is due to large displacements occurring during the phase transformation and it is accounted for via von Kármán assumptions. The resulting model is solved numerically via the finite element method and used to evaluate both the free and the forced response of the beam. The free response analysis suggests the existence of optimal pre-strain levels to achieve maximum damping capacity, while the forced response highlights the occurrence of nonlinear dynamic features, such as bifurcation. In both cases, it is found that the level of pre-strain can significantly affect the dynamic response as well as the effective damping. The results presented in this work provide useful guidelines to understand the dynamics of continuous SMA structures under pre-strain and the ability to tune either their resulting dynamics or dissipation properties.

Self-healing performance of a microcapsule-based structure reinforced with pre-strained shape memory alloy wires: 3-D FEM/XFEM modeling

Combination of microcapsules and shape memory alloys (SMAs) is one of the promising self-healing mechanisms. Although there are several parameters which affect the performance of such structures, limited studies are performed on this combined healing mechanism. In this work, we study the performance of such a composite structure using a 3-D finite element and extended finite element model consisting of matrix, glass microcapsule, healing agent, and Ni-Ti SMA wire. After examining the results, the effect of shape memory alloy wires on increasing the maximum fracture stress was observed. Moreover, the effect of radius of shape memory alloy wires, initial crack location, thickness ratio and volume fraction of microcapsules, and interface strength on ultimate fracture stress are investigated. Also, as a key parameter in self-healing performance, the crack opening distance decreased from 5 to 0.008 μm using 0.5% volume fraction of shape memory wires without pre-strain. In the case that the wires have a pre-strain of 1%, this value reaches almost zero and a compressive stress is induced between fracture surfaces which can enhance the healing process and adherence of healing agent.

Fatigue behavior of laminated composites with embedded SMA wires

Embedding pre-strained shape memory alloy (SMA) wires within laminated composites can enhance their fatigue performance due to pseudoelastic and shape memory effects. In the present research, an experimental program was conducted to investigate the fatigue behavior of carbon/epoxy laminated composites with/without embedded SMA wires. Moreover, a novel fatigue model was proposed to predict the behavior of laminated composites with embedded SMA wires. The model is an integration of three parts. The first part is the progressive fatigue damage model to simulate the fatigue behavior of composites. The second part is the self-heating model for the prediction of the self-temperature rise in laminated composites subjected to cyclic loading. The third part is the thermomechanical constitutive model of the mechanical behavior of the SMA wire. Effects of the stacking sequence, the applied stress level, and the pre-strain level of SMA on fatigue behavior of laminated composites were studied. Good agreement between theoretical and experimental results revealed the ability of the present model.

Study on a Fault Mitigation Scheme for Rub-Impact of an Aero-Engine Based on NiTi Wires.

The aim of this study was to solve the frequently occurring rotor-stator rub-impact fault in aero-engines without causing a significant reduction in efficiency. We proposed a fault mitigation scheme, using shape memory alloy (SMA) wire, whereby the tip clearance between the rotor and the stator is adjusted. In this scheme, an acoustic emission (AE) sensor is utilized to monitor the rub-impact fault. An active control actuator is designed with pre-strained two-way SMA wires, driven by an electric current via an Arduino control board, to mitigate the rub-impact fault once it occurs. In order to investigate the feasibility of the proposed scheme, a series of tests on the material properties of NiTi wires, including heating response rate, ultimate strain, free recovery rate, and restoring force, were carried out. A prototype of the actuator was designed, manufactured, and tested under various conditions. The experimental result verifies that the proposed scheme has the potential to mitigate or eliminate the rotor-stator rub-impact fault in aero-engines.

Thermal Modal Performance of Composite Laminates Embedded with Anti-Symmetric Oblique Coupling Gradient Pre-Strained SMA Wires for Suppressing Resonance

As an extension of conventional gradient, anti-symmetric oblique coupling gradient has a superior modal modification ability on composite laminates embedded with pre-strained shape memory alloys (SMA) wires, which is beneficial to suppress modal resonance of composite laminates in the thermal environment. This paper presents an anti-symmetric oblique coupling gradient model of SMA along the thickness direction. That is, the gradient model of SMA wires’ orientation and the positive and negative gradient model of SMA volume fraction. Considering the internal force of composite laminates composed of the pre-strain recovery force of SMA and the thermal expansion force of the substrate, the free vibration equation of composite laminates with additional internal forces energy is derived from first-order shear plate theory and Hamilton principle. The influence of coupling gradient parameters on the thermal modal performance of SMA composite laminates is analyzed and verified by experiments. The proposed anti-symmetric oblique coupling gradient SMA wires’ distribution form effectively exerts the recovery stress generated by SMA tensile pre-strain, i.e., effectively improves the stiffness and critical buckling temperature. Coupling gradient distribution broadens the frequency modulation range, which makes the fine regulation of the natural frequency and critical buckling temperature feasible.

Innovative local prestressing system for concrete crossties using shape memory alloys

This paper focuses on introducing and investigating a new prestressing system for railroad concrete crossties using shape memory alloys (SMAs). Concrete crossties are typically prestressed with straight high strength steel wires throughout the entire length, which could lead to splitting cracks at end regions and excessive compressive stress concentration at compressive zones. A new prestressing system with adjustable wire configuration allows prestressing force to be applied selectively at target regions in the crosstie. The proposed prestressing system can help prevent end-splitting cracks, adjust prestressing force depending on the demands at different sections, and adjust the reinforcement configuration along the length. In the proposed system, pre-strained SMA wires are placed in the forms prior to casting the concrete at the locations where prestressing is needed. After the concrete is set, the prestressing force can be applied at any time by activating the SMA wires using electrical resistivity heating. In this work, both experimental and numerical studies using SMAs (NiTiNb) are presented for proof of concept. Three small-scale concrete crosstie specimens with different SMA wire configurations are tested. Comparison between the SMA prestressed concrete crosstie model and steel prestressed model is performed using finite element analysis considering prestress losses. The results indicate that the new prestressing system can effectively apply prestress at target regions as designed.

Development and actuation analysis of shape memory alloy reinforced composite fin for aerodynamic application

In this investigation, a shape memory alloy (SMA) based aerodynamic composite Fin has been developed. In general, the missile fins play a crucial role in changing the direction of a conventional missile using motor arrangement. Therefore, SMA-based composite Fin might be a promising alternative to control the direction without using the motor. The composite Fin is made of laminated fiber polymer and SMA wire. Initially, rectangular composite structures have been developed with different wire prestraining percentages, lengths, diameters, and configurations to understand their electrical actuation behavior. Based on the experiments, optimized parameters have been derived and applied to the composite fin. Adaptive composite actuation was performed via selective joule heating on the composite fin. Under optimized conditions, SMA-based composite structure shows maximum displacement of 65 mm with a maximum angle of 60 degrees before failure, where the diameter of 5 % pre-strained SMA wire was 0.5 mm at 6 % of SMA to Composite ratio (volume of SMA/volume of composite). Maximum displacement and failure analysis of the composite structure has been analyzed at optimized parameters.

Viscoelastic behavior of epoxy resin reinforced with shape-memory-alloy wires

The effect of NiTi alloy long wires on the viscoelastic behavior of epoxy resin was investigated by utilizing the dynamic mechanical analysis (DMA) and a novel micromechanical model. The present model is capable of predicting the viscoelastic properties of the shape-memory-alloy (SMA) reinforced polymer as a function of the SMA volume fraction, initial martensite volume fraction, pre-strain level in wires, and the temperature variations. The model was verified by conducting experiments. Good agreement between the theoretical and experimental results was achieved. A parametric study was also performed to investigate the effect of SMA parameters. According to the results, by the addition of a small volume fraction of SMA, the storage modulus of the composite increases significantly, especially at higher temperatures. Moreover, applying a 4% pre-strain caused a 10% increase in the maximum value of the loss factor of the SMA reinforced epoxy in comparison with the 0% pre-strained SMA reinforced epoxy.

Sound Radiation of Orthogonal Antisymmetric Composite Laminates Embedded with Pre-Strained SMA Wires in Thermal Environment

The interest of this article lies in the sound radiation of shape memory alloy (SMA) composite laminates. Different from the traditional method of avoiding resonance sound radiation of composite laminates by means of structural parameter design, this paper explores the potential of adjusting the modal peak of the resonant acoustic radiation by using material characteristics of shape memory alloys (SMA), and provides a new idea for avoiding resonance sound radiation of composite laminates. For composite laminates embedded with pre-strained SMA, an innovation of vibration-acoustic modeling of SMA composite laminates considering pre-stain of SMA and thermal expansion force of graphite-epoxy resin is proposed. Based on the classical thin plate theory and Hamilton principle, the structural dynamic governing equation and the frequency equation of the laminates subjected to thermal environment are derived. The vibration sound radiation of composite laminates is calculated with Rayleigh integral. Effects of ambient temperature, pre-strain, SMA volume fraction, substrate ratio, and geometrical parameters on the sound radiation were analyzed. New laws of SMA material and pre-strain characteristics on sound radiation of composite laminates under temperature environment are revealed, which have theoretical and engineering functional significance for vibration and sound radiation control of SMA composite laminates.

Free Vibration Analysis of Functionally Gradient Sandwich Composite Plate Embedded SMA Wires in Surface Layer

In this paper, a new type of composite gradient sandwich plate structure is proposed, which embeds the pre-strained shape memory alloy (SMA) into the surface layer and the core layer composed of epoxy resin and graphite-reinforced materials. In the core layer, graphite-reinforced material has a continuous gradient distribution along the thickness direction of the sandwich plate. Dynamic behavior of composite gradient sandwich plate in thermal environment is investigated. The equations of motion and frequency equation are derived based on the Reddy shear deformation theory and the constitutive equation for a composite sandwich plate, via the Hamilton principle. Some analytical study is depicted to provide an insight into the effects of volume fraction of material composition, gradient distribution of graphite in the core layer, and pre-strain of SMA in the surface layer on the dynamic behavior of a sandwich composite plate. This study investigates the modal performance of a sandwich composite plate with two aspects, a gradient core layer of graphite-reinforced material and surface layer-embedded SMA wires, which provide a new design idea for dynamic behavior of sandwich plates.

Heat-activated prestressing of NiTiNb shape memory alloy wires

Pre-strained shape memory alloys (SMAs) can develop significant level of recovery stress (prestress) when heated above their transformation temperature in the constrained conditions. In this study, thermo-mechanical tests were conducted to investigate heat-activated prestressing (HAP) of NiTiNb SMA wires. The tests were conducted to investigate: (1) the effect of pre-strain on reverse transformation temperatures (As and Af) of the NiTiNb SMA; (2) the effect of strain rate and the pre-strain level on the recovery stress; (3) post-HAP behaviour under monotonic and cyclic loading; and (4) reusability of the NiTiNb SMA wires for HAP applications. The digital image correlation technique was used to gain deeper insight into micro-mechanical behaviour of the SMA. The study highlights the complex material behaviour of the NiTiNb SMAs. The results from this study suggest that NiTiNb SMAs can be effectively used for HAP. Recommendations on optimal strain rate for pre-straining NiTiNb SMAs and the optimal pre-strain level for achieving maximum recovery stress in NiTiNb SMAs are given in this paper.

Innovative prestressing technique using curved shape memory alloy reinforcement

Concrete prestressing techniques have been widely used in various structural applications. However, the application of local prestressing has been limited due to complexities related to the on-site implementation of anchorage devices and mechanical jacking systems. In this work, a novel method is introduced to apply local prestressing using curved Shape Memory Alloy (SMA) reinforcement. The experimental part of the study involves casting and prestressing a mortar plate with embedded curved prestrained SMA wire. The prestressing of the SMA wire is conducted by applying temperature using electrical resistivity. Next, an experimental study is carried out to explore the feasibility of using a mortar plate with embedded SMA as a precast prestressing plate (PPP) for existing concrete. A comparison is carried out between different installation methods to ensure effective connection between the PPP and concrete. Digital Image Correlation (DIC) and strain gage measurements are adopted to monitor the value and distribution of prestress in specimens. The experimental results are used to validate a Finite Element (FE) model of the SMA-reinforced PPP. The model is used to conduct a parametric study to explore the effect of SMA geometric configuration on the prestressing level. The experimental and numerical results show that the proposed prestressing technique is valid and can be used effectively for local prestressing.

Jumping Tensegrity Robot Based on Torsionally Prestrained SMA Springs.

This paper introduces the addition of torsional prestrain into the manufacturing process of shape memory alloy (SMA) springs to form torsionally prestrained SMA springs. These springs have a better performance at the same power input for the same loads and same coil dimensions as regular SMA springs. A modified thermoconstitutive model was presented that can predict the behavior of the actuator based on the amount of torsional prestrain added into the manufacturing process, and a simple two-state model is used to predict its actuation stroke. These improved actuators were used in the development of a tensegrity robots capable of fast rolling motions and jumping both vertically and horizontally. This robot is capable of rolling at 0.14 BL/s (body length per second) and can jump up to nearly a full body length.

Analytical solution for twinning deformation effect of pre-strained shape memory effect beam-columns

In this article, an analytical solution is presented for twinning deformation effect of a prismatic shape memory alloy beam-column. To this end, a reduced one-dimensional Souza model is employed to study the bending stress of a pre-strained shape memory alloy beam-column at low temperatures. Analytical expressions for bending stress as well as polynomial approximations for deflection are obtained. Derived equations for bending problem are employed to analyze twinning deformation effect of shape memory alloy beam-columns with rectangular and circular cross sections. Furthermore, the distance of zero-stress fiber from the center line during loading is studied. The results of this work show good agreement when compared with experimental data and finite element results.

A theoretical and experimental investigation on the stress distribution in the interface of pre-strained SMA wire/polymer composites

The major concern in using the shape memory alloy (SMA) wires embedded in a polymer matrix is the possibility of debonding between SMA and the surrounding polymer due to the SMA phase transition recovery stress. Hence, it is important to examine the distribution of stresses in the SMA wire and the surrounding matrix. In the present study, using the principle of the minimum complementary energy a new analytical model was developed to determine the axial, radial, and shear stresses along the SMA/polymer interface. By considering the Poisson's effect and shear stress in the present analytical model, while ignored in the classical shear-lag model, more accurate results were obtained. Furthermore, by considering the effects of the SMA wire pre-strain, actuating temperature, recovery stress and thermal expansion, a novel relationship was also presented for calculating the maximum interfacial shear stress. To show the effect of the SMA wire pre-strain on the distribution of stresses throughout the SMA/epoxy interface, pull-out tests were conducted at different pre-strain levels in the SAM wire. Results showed applying the pre-strain in the SMA wire followed by a thermal activation enhanced the debonding load between the SMA wire and epoxy matrix and caused a significant variation in the stress distribution.

A three-dimensional phenomenological model for shape memory alloys including two-way shape memory effect and plasticity

The one-way and two-way shape memory effects (SMEs) as well as the thermal hysteresis represent fundamental properties when dealing with the design of detachable and thermally-stable connection systems based on shape memory alloys (SMAs). Such properties can be induced and tuned by thermo-mechanical processes that include thermal treatments and severe pre-deformation in martensitic state, causing the onset of plastic strains. In such complex conditions, material modeling is of great importance to support the design. This paper proposes a generalization of the three-dimensional phenomenological constitutive model by Souza et al. (1998), in order to describe the behavior of severely pre-strained NiTi-based SMAs. The proposed model allows to describe pseudoelasticity, one-way and two-way SMEs, as well as additional physical phenomena evidenced experimentally, such as transformation temperatures’ evolution, thermal hysteresis, phase transformations at low stresses, thermal strains, and phase-dependent elastic properties. Several numerical simulations, ranging from uniaxial tests to the finite element analysis of two case-studies, are performed. Model results are in good agreement with the results of a performed experimental campaign and allow to discuss SMA behavior under such complex loading conditions.

Thermal vibration analysis of SMA hybrid composite double curved sandwich panels

This work analyses the thermal vibration of a double curved sandwich panel (DCSP) with embedded pre-strained shape memory alloy (SMA) wires hybrid composite face sheets and soft core. The von Karman nonlinear displacement–strain relationships are here applied to handle large deflections due to a thermal loading. The equations of motion are derived by applying the Hamilton’s principle and the first order shear deformation theory (FSDT) for the composite face sheets and core layer. This last one features the displacement field of the Frostig's second model here, used to model the DCSP. The material properties of the DCSP are assumed to be both temperature-dependent (TD) and/or temperature-independent (TI). The effect of the SMA wires is captured by adding a stress recovery within the formulation. This term is determined by using a one-dimensional Brinson’s model in the constitutive equations of the SMA composite face sheets during the phase transformation of the pre-strained SMA wires. It is verified that SMAs can play a key role within DCSPs subjected to a thermal loading condition, whereby the proposed formulation is validated comparatively against the available literature. We also explore the sensitivity of the vibration response for a varying SMA activation temperature, volume fraction, and pre-strain, as well as for different curvature ratios, thickness ratios and different sequences in composite layers.

A novel self-healing composite made of thermally reversible polymer and shape memory alloy reinforcement

A novel self-healing polymer composite made of the thermally reversible polymer matrix and shape memory alloy reinforcement is introduced. The healing system is designed in such a way that by heating the structure, activation of shape recovery in shape memory alloy and chemical reversible reactions in polymer occur simultaneously. In the present healing method, the required crack closure force is provided by activating the embedded shape memory alloy wires in the polymer. Both superelastic and shape memory effects of shape memory alloy are considered on the fracture behavior of composites by investigating the passive and active reinforcement methods, respectively. Double cleavage drilled compression tests are utilized in order to study the fracture behavior and healing efficiency of composites. In the case of passive reinforcement, embedding 2% prestrained shape memory alloy wires caused 15% enhancement in the fracture toughness of composites. In this prestrain level, results of the micromechanical model are in good agreement with experiments. Promising results are also obtained for healing efficiency of composites in the case of active reinforcement. The average healing efficiency of 92% is achieved for shape memory alloy-reinforced thermally reversible epoxy composites. The excellent healing performance, without the necessity of external force and pressure, makes the present healing method as an ideal candidate for utilizing self-healing composite structures.