Superelastic NiTi Shape Memory Alloy Research Articles

A micromechanical constitutive model for anisotropic cyclic deformation of super-elastic NiTi shape memory alloy single crystals

Based on the experimental observations on the anisotropic cyclic deformation of super-elastic NiTi shape memory alloy single crystals done by Gall and Maier (2002), a crystal plasticity based micromechanical constitutive model is constructed to describe such anisotropic cyclic deformation. To model the internal stress caused by the unmatched inelastic deformation between the austenite and martensite phases on the plastic deformation of austenite phase, 24 induced martensite variants are assumed to be ellipsoidal inclusions with anisotropic elasticity and embedded in the austenite matrix. The homogeneous stress fields in the austenite matrix and each induced martensite variant are obtained by using the Mori–Tanaka homogenization method. Two different inelastic mechanisms, i.e., martensite transformation and transformation-induced plasticity, and their interactions are considered in the proposed model. Following the assumption of instantaneous domain growth (Cherkaoui et al., 1998), the Helmholtz free energy of a representative volume element of a NiTi shape memory single crystal is established and the thermodynamic driving forces of the internal variables are obtained from the dissipative inequalities. The capability of the proposed model to describe the anisotropic cyclic deformation of super-elastic NiTi single crystals is first verified by comparing the predicted results with the experimental ones. It is concluded that the proposed model can capture the main quantitative features observed in the experiments. And then, the proposed model is further used to predict the uniaxial and multiaxial transformation ratchetting of a NiTi single crystal.

Rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy: Thermo-mechanical coupled and physical mechanism-based constitutive model

A three-dimensional (3D) thermo-mechanical coupled and physical mechanism-based constitutive model is phenomenologically constructed to describe the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy (SMA). Two inelastic mechanisms, i.e., martensite transformation and dislocation slipping in austenite phase are included in the proposed model. The thermodynamic driving forces of two inelastic mechanisms, the internal heat production during cyclic deformation and the thermodynamic constraints on constitutive equations are obtained by the Clausius's dissipative inequality and constructed Helmholtz's free energy. The interactions between the martensite transformation and dislocation slipping in austenite phase are considered. The evolution equations of internal variables controlling the degeneration of super-elasticity are set to be dependent on the current density of dislocation. Finally, the proposed 3D constitutive model is reduced to its one-dimensional (1D) form with an assumption of uniform temperature field in the cross-section of the tested micro-tube, and its capability to describe the uniaxial rate-dependent cyclic deformation of super-elastic NiTi SMA micro-tube is verified by comparing the predicted results with the corresponding experimental ones.

Effect of martensite reorientation and reorientation-induced plasticity on multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy: New consideration in constitutive model

Experimental results show that the martensite reorientation and reorientation-induced plasticity play critical roles on the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy (SMA) (Song et al., 2014b). In this paper, the multiaxial transformation ratchetting of the NiTi shape memory alloy is described by developing a new constitutive model in the framework of crystal plasticity. Besides the well-known inelastic mechanisms of NiTi shape memory alloys, such as the martensite transformation, martensite reorientation, transformation-induced plasticity and accumulated residual martensite, a new inelastic deformation mechanism, i.e., reorientation-induced plasticity is included in the proposed model to address its effect on multiaxial transformation ratchetting. The constitutive model is constructed in the single crystal scale and extended to a polycrystalline version by adopting an explicit scale-transition rule. By comparing the predicted results with the corresponding experimental ones, it is verified that the proposed model describes the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy more reasonably by considering the martensite reorientation and reorientation-induced plasticity simultaneously.

Study on the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy based on a new crystal plasticity constitutive model

In this paper, a crystal plasticity based constitutive model (Yu et al., 2013) is extended to describe the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy by considering the internal heat production. Two sources of internal heat productions are included in the proposed model, i.e., the mechanical dissipations of inelastic deformation and the transformation latent heat in the NiTi shape memory alloy. With an assumption of uniform temperature field in the alloy specimen, a simplified evolution law of temperature field is obtained by the first law of thermodynamics and the heat boundary conditions. An explicit scale-transition rule is adopted to extend the proposed single crystal model to the polycrystalline version. The capability of the extended polycrystalline model to describe the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy is verified by comparing the predictions with the corresponding experimental ones. The comparison demonstrates that the proposed constitutive model considering the internal heat production predicts the rate-dependent cyclic deformation of super-elastic NiTi shape memory alloy fairly well.

Effect of surface modification by nitrogen ion implantation on the electrochemical and cellular behaviors of super-elastic NiTi shape memory alloy.

The aim of this investigation was to enhance the biological behavior of NiTi shape memory alloy while preserving its super-elastic behavior in order to facilitate its compatibility for application in human body. The surfaces of NiTi samples were bombarded by three different nitrogen doses. Small-angle X-ray diffraction was employed for evaluating the generated phases on the bombarded surfaces. The electrochemical behaviors of the bare and surface-modified NiTi samples were studied in simulated body fluid (SBF) using electrochemical impedance and potentio-dynamic polarization tests. Ni ion release during a 2-month period of service in the SBF environment was evaluated using atomic absorption spectrometry. The cellular behavior of nitrogen-modified samples was studied using fibroblast cells. Furthermore, the effect of surface modification on super-elasticity was investigated by tensile test. The results showed the improvement of both corrosion and biological behaviors of the modified NiTi samples. However, no significant change in the super-elasticity was observed. Samples modified at 1.4E18 ioncm(-2) showed the highest corrosion resistance and the lowest Ni ion release.

A physical mechanism based constitutive model for temperature-dependent transformation ratchetting of NiTi shape memory alloy: One-dimensional model

In this paper, the effect of test temperature on the transformation ratchetting of super-elastic NiTi shape memory alloy was first investigated in the cyclic tension-unloading tests. It is shown that all the residual strain, dissipation energy, the start stress of martensite transformation and their evolutions during the cyclic loading depend greatly upon the test temperature. Based on the experimental observations, a new one-dimensional constitutive model is constructed by considering two different inelastic deformation mechanisms (i.e., martensite transformation and transformation-induced plasticity). The proposed model employs a new evolution rule of transformation-induced plasticity which considers the physical mechanism of the plastic deformation, i.e., the dislocation slipping in the austenite phase near the austenite–martensite interfaces. Furthermore, the interaction between dislocation and martensite transformation is also taken into account in the proposed model. The capability of the proposed model to predict the uniaxial temperature-dependent transformation ratchetting of NiTi shape memory alloy is verified by comparing the predictions with the experimental data.

Anisotropic behavior of superelastic NiTi shape memory alloys; an experimental investigation and constitutive modeling

The effect of boundary condition on the mechanical behavior of superelastic NiTi shape memory alloys is investigated in this paper. Experimental tests were carried out on NiTi tubes subjected to tension and torsion with different boundary conditions including fixed and free end cases. Results revealed that anisotropy strain/stress appears in the material depending on the end boundary condition of the sample when martensite transformation occurs. This phenomenon is believed to be a result of anisotropy developed in NiTi during material processing and/or training procedures. Based on experimental findings, a new extension is considered to a 3-D phenomenological constitutive model to capture the anisotropic transformation strain/stress generation observed during different loading conditions. Numerical correlations between predicted and experimental data demonstrate the success of the modified model.

Computational study of self‐centering buckling‐restrained braced frame seismic performance

SUMMARYRecent research developed and experimentally validated a self‐centering buckling‐restrained brace (SC‐BRB) that employs a restoring mechanism created using concentric tubes held flush with pretensioned shape memory alloy rods, in conjunction with a buckling‐restrained brace (BRB) that dissipates seismic energy. The present computational study investigated how the SC‐BRB can be implemented in real buildings to improve seismic performance. First, a computational brace model was developed and calibrated against experimental data, including the definition of a new cyclic material model for superelastic NiTi shape memory alloy. A parametric study were then conducted to explore the design space for SC‐BRBs. Finally, a set of prototype buildings was designed and computationally subjected to a suite of ground motions. The effect of the lateral resistance of gravity framing on self‐centering was also examined.From the component study, the SC‐BRB was found to dissipate sufficient energy even with large self‐centering ratios (as large as 4) based on criteria found in the literature for limiting peak drifts. From the prototype building study, a SC‐BRB self‐centering ratio of 0.5 was capable of reliably limiting residual drifts to negligible values, which is consistent with a dynamic form of self‐centering discussed in the literature. Because large self‐centering ratios can create significant overstrength, the most efficient SC‐BRB frame designs had a self‐centering ratio in the range of 0.5–1.5. Ambient building resistance (e.g., gravity framing) was found to reduce peak drifts, but had a negligible effect on residual drifts. Copyright © 2014 John Wiley & Sons, Ltd.

Corrosion of NiTi Wires with Cracked Oxide Layer

Corrosion behavior of superelastic NiTi shape memory alloy wires with cracked TiO2 surface oxide layers was investigated by electrochemical corrosion tests (Electrochemical Impedance Spectroscopy, Open Circuit Potential, and Potentiodynamic Polarization) on wires bent into U-shapes of various bending radii. Cracks within the oxide on the surface of the bent wires were observed by FIB–SEM and TEM methods. The density and width of the surface oxide cracks dramatically increase with decreasing bending radius. The results of electrochemical experiments consistently show that corrosion properties of NiTi wires with cracked oxide layers (static load keeps the cracks opened) are inferior compared to the corrosion properties of the straight NiTi wires covered by virgin uncracked oxides. Out of the three methods employed, the Electrochemical Impedance Spectroscopy seems to be the most appropriate test for the electrochemical characterization of the cracked oxide layers, since the impedance curves (Nyquist plot) of differently bent NiTi wires can be associated with increasing state of the surface cracking and since the NiTi wires are exposed to similar conditions as the surfaces of NiTi implants in human body. On the other hand, the potentiodynamic polarization test accelerates the corrosion processes and provides clear evidence that the corrosion resistance of bent superelastic NiTi wires degrades with oxide cracking.

Non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy: Experimental observations

Experimental observations are reported for the non-proportional multiaxial transformation ratchetting of super-elastic NiTi shape memory alloy performed under the stress-controlled cyclic tension–torsion loading conditions and at room temperature. The effect of axial mean stress on the evolutions of transformation ratchetting strain and dissipation energy per cycle during the cyclic tests is discussed firstly; and then the dependence of multiaxial transformation ratchetting on the different non-proportionally loading paths (e.g., linear, square, hourglass-typed, butterfly-typed, rhombic and octagonal paths) is investigated. The results show that the multiaxial transformation ratchetting occurs mainly in the axial direction because only the non-zero axial mean stress is used and the mean shear stress is set to be zero in the all prescribed multiaxial loading paths; and the axial peak and valley strains increase with the increasing axial mean stress and also depend significantly on the shapes of loading paths. Comparison with the corresponding uniaxial ones illustrates that the multiaxial stress states are more helpful to promote the development of transformation ratchetting, especially for the non-proportional ones.

The effect of martensite plasticity on the cyclic deformation of super-elastic NiTi shape memory alloy

Based on stress-controlled cyclic tension–unloading experiments with different peak stresses, the effect of martensite plasticity on the cyclic deformation of super-elastic NiTi shape memory alloy micro-tubes is investigated and discussed. The experimental results show that the reverse transformation from the induced martensite phase to the austenite phase is gradually restricted by the plastic deformation of the induced martensite phase caused by an applied peak stress that is sufficiently high (higher than 900 MPa), and the extent of such restriction increases with further increasing the peak stress. The residual and peak strains of super-elastic NiTi shape memory alloy accumulate progressively, i.e., transformation ratchetting occurs during the cyclic tension–unloading with peak stresses from 600 to 900 MPa, and the transformation ratchetting strain increases with the increase of the peak stress. When the peak stress is higher than 900 MPa, the peak strain becomes almost unchanged, but the residual strain accumulates and the dissipation energy per cycle decreases very quickly with the increasing number of cycles due to the restricted reverse transformation by the martensite plasticity. Furthermore, a quantitative relationship between the applied stress and the stabilized residual strain is obtained to reasonably predict the evolution of the peak strain and the residual strain.

Tests on superelastic Ni–Ti SMA bars under cyclic tension and direct-shear: towards practical recentring connections

Shape memory alloys (SMAs) are smart metals featuring either superelastic effect or shape memory effect. While current emphasis on the civil engineering application of SMA is mainly given to dampers and isolators, recent research has been directed to superelastic SMA bolts or tendons for recentring connections. However, available information on the mechanical shear response of SMA bars is still inadequate. Consequently, this knowledge barrier can cause significant uncertainties when such components are intended to resist shear action. In this respect, this paper presents an experimental study on SMA bars subject to cyclic direct-shear actions, and uniaxial tensile tests were also undertaken on the bars of similar sizes for comparison. Key results including stress–strain/force–displacement hysteretic response, recentring ability, and energy dissipation are discussed in detail. The results show sound recentring and moderate energy dissipation/damping capabilities of the bars under cyclic tension below a strain level of 3 %, but beyond this the properties start to degrade. The SMA bars with a smaller diameter seem to perform better than those with a larger diameter. On the other hand, the cyclic direct-shear performance of SMA bars is generally unsatisfactory with poor recentring, energy dissipation, and damping characteristics. Therefore, the direct use of SMA bars subject to shear loading is undesirable. For both tension and shear tests, fracture of the bars within the threaded area is the most typical failure mode, which should be minimised in future applications to ensure a ductile response of seismic resistance devices. Recognising both advantages and potential shortcomings of the SMA bars, an innovative recentring connection type is proposed, and the preliminarily findings indicate that such connections are feasible.

Temperature analysis of one-dimensional NiTi shape memory alloys under different loading rates and boundary conditions

Superelastic polycrystalline NiTi shape memory alloys under tensile loading accompany the strain localization and propagation phenomena. Experiments showed that the number of moving phase fronts and the mechanical behavior are very sensitive to the loading rate due to the release/absorption of latent heat and the material’s inherent temperature sensitivity of the transformation stress. In this paper, the moving heat source method based on the heat diffusion equation is used to study the temperature evolution of one-dimensional superelastic NiTi specimen under different loading rates and boundary conditions with moving heat sources or a uniform heat source. Comparisons of temperature variations with different boundary conditions show that the heat exchange at the boundaries plays a major role in the nonuniform temperature profile that directly relates to the localized deformation. Analytical relation between the front temperature of a single phase front, the inherent Clausius–Clapeyron relation (sensitivity of the material’s transformation stress with temperature), heat transfer boundary conditions and the loading rate is established to analyze the nucleation of new phase fronts. Finally, the rate-dependent stress hysteresis is also simply discussed by using the results of temperature analyses.

Superelastic shape memory alloy cables: Part II – Subcomponent isothermal responses

This paper constitutes the second part of our experimental study of the thermo-mechanical behavior of superelastic NiTi shape memory alloy cables. Part I introduced the fundamental, room temperature, tensile responses of two cable designs (7×7 right regular lay, and 1×27 alternating lay). In Part II, each cable behavior is studied further by breaking down the response into the contributions of its hierarchical subcomponents. Selected wire strands were extracted from the two cable constructions, and their quasi-static tension responses were measured using the same experimental setup of Part I. Consistent with the shallow wire helix angles in the 7×7 construction, the force–elongation responses of the core wire, 1×7 core strand and full 7×7 cable were similar on a normalized basis, with only a slight decrease in transformation force plateaus and slight increase in plateau strains in this specimen sequence. By contrast, each successive 1×27 component (1×6 core strand, 1×15 strand, and full cable) included an additional outer layer of wires with a larger number of wires, greater helix radius, and deeper helix angle, so the normalized axial load responses became significantly more compliant. Each specimen in the sequence also exhibited progressively larger strains at failure, reaching 40% strain in the full 1×27 cable.Stress-induced phase transformations involved localized strain/temperature and front propagation in all of the tested 7×7 components but none of the 1×27 components aside from the 1×27 core wire. Stereo digital image correlation measurements revealed finer features within a global transformation front of the 1×7 core strand than the 7×7 cable, consisting of an staggered pattern of individual wire fronts that moved in lock-step during elongation. Although the 1×27 multi-layer strands exhibited temperature/strain localizations in a distributed pattern during transformations, the localizations did not propagate and their cause was traced back to contact indentations (stress concentrations) arising from the cable’s fabrication. The normalized axial torque responses of the multi-layer 1×27 components during transformation were distinctly non-monotonic and complex, due to the alternating handedness of the layers. Force and torque contributions of individual wire layers were deduced by subtracting 1×27 component responses, which helped to clarify the transformation kinetics within each layer and explain the unusual force and torque undulations seen in the 1×27 cable response of Part I.

Mechanical Behavior of Super-Elastic Home-Made NiTi Shape Memory Alloy Bar in Tension

An experimental study on mechanical behavior of home-made super-elastic NiTi Shape Memory Alloy (SMA) bar subjected to tension was made. The SMA bar specimen was 8mm in gage diameter and 50mm in gage length. The experimental tests were carried out by applying repeated cyclic loads. The tested tension strain amplitudes were 1%-7% and material phase was austenite. The mechanical characteristic parameters, such as phase transformation stress, deformation module, residual strain, energy loss and equivalent damping were studied. The effects of strain amplitude, loading rate and loading cycles on these parameters were analyzed. The considerable super-elasticity and stable cyclic behaviors of home-made SMA bar during the test showed its great application potential in aseismic devices.

Fabrication and Cyclic Behavior of Highly Ductile Superelastic Shape Memory Composites

AbstractTo address the problem of steel corrosion, fiber-reinforced polymer (FRP) bars are often used as reinforcement in concrete structures. However, conventional FRP bars are characterized by linear elastic behavior that limits their efficacy in seismic structural applications. This paper investigates the fabrication and cyclic behavior of a relatively new class of composite material known as SMA-FRP that could be used as seismic reinforcement for concrete structures. The new composite comprises a high elongation resin matrix, embedded with superelastic NiTi shape memory alloy (SMA) wires as primary reinforcement. The SMA wires are used either with or without the addition of conventional fibers. The hysteretic nonlinear pseudoelastic behavior of SMA provides the new composite with features that conventional FRP lacks, such as ductility and energy dissipation capability. The experimental program carried out in this study starts with the training of SMA wires, and the investigation of the hysteretic beha...

A Constitutive Model for Superelastic Shape Memory Alloys Considering the Influence of Strain Rate

Shape memory alloys (SMAs) are a relatively new class of functional materials, exhibiting special thermomechanical behaviors, such as shape memory effect and superelasticity, which enable their applications in seismic engineering as energy dissipation devices. This paper investigates the properties of superelastic NiTi shape memory alloys, emphasizing the influence of strain rate on superelastic behavior under various strain amplitudes by cyclic tensile tests. A novel constitutive equation based on Graesser and Cozzarelli’s model is proposed to describe the strain-rate-dependent hysteretic behavior of superelastic SMAs at different strain levels. A stress variable including the influence of strain rate is introduced into Graesser and Cozzarelli’s model. To verify the effectiveness of the proposed constitutive equation, experiments on superelastic NiTi wires with different strain rates and strain levels are conducted. Numerical simulation results based on the proposed constitutive equation and experimental results are in good agreement. The findings in this paper will assist the future design of superelastic SMA-based energy dissipation devices for seismic protection of structures.

Temperature Variation in NiTi Shape Memory Alloy During Cyclic Phase Transition

Superelastic NiTi shape memory alloy (SMA) has high recoverable strain and outstanding damping capacity, and has been used as a damping material for many applications. When subjected to displacementcontrolled cyclic deformation, the material exhibits distinctive temperature and stress oscillations due to the release of latent heat and hysteresis heat and the heat transfer with the ambient. In this paper, we establish a model to predict the temperature variation of NiTi SMA wire specimen under the cyclic phase transition by lumped heat transfer analysis. Closed-form solution on the evolution of the temperature is obtained. It is shown that, for all the test frequencies, steady-state cyclic thermal response of the specimen can be reached after a certain number of loading cycles in a transient stage, exhibiting a kind of ‘‘thermal shake down.’’ In the steady state, the temperature profile oscillates around a mean temperature plateau. We show that the temperature oscillation is mainly due to the release/absorption of latent heat during cyclic phase transition, while the mean temperature rise of the specimen is caused by the accumulation of the hysteretic heat of the phase transition. The model predictions agree well with the experimental results.

Micromechanical constitutive model considering plasticity for super-elastic NiTi shape memory alloy

Experimental results show that, for super-elastic NiTi shape memory alloy, the transformation start stress from austenitic to stress-induced martensitic phase increases with the increase of ambient temperature. When the ambient temperature increases to a critical value, a residual strain occurs after tensile test with one loading–unloading cycle due to the dislocation slip in austenitic phase. Based on the experimental observation, a single crystal constitutive model considering both transformation and plasticity is first established in the framework of thermodynamics, and interaction energy is introduced into the Helmholtz free energy in order to consider the effect of plasticity on the transformation. Then, an explicit scale-transition rule is adopted in the proposed micromechanical constitutive model to obtain the polycrystalline behavior of super-elastic NiTi alloy. Finally, the capability of the proposed micromechanical model to describe the temperature-dependent super-elasticity of NiTi alloy is verified by comparing the predictions with corresponding experiments.

Whole-life transformation ratchetting and fatigue of super-elastic NiTi Alloy under uniaxial stress-controlled cyclic loading

► Whole-life transformation ratchetting depends on load level and mode. ► Shakedown of transformation ratchetting occurs after certain cycles. ► Dissipated energy decreases quickly during the cyclic loading. ► Transformation ratchetting shortens the fatigue life apparently. ► Fatigue life decreases greatly with the increasing dissipated energy. The whole-life transformation ratchetting and fatigue failure (including functional fatigue and structural fatigue) of super-elastic NiTi shape memory alloy were observed by uniaxial stress-controlled cyclic loading tests and at room temperature. The effects of peak stress, mean stress and stress amplitude were discussed, and then the interaction of transformation ratchetting and fatigue was then investigated. It is concluded that the whole-life transformation ratchetting of super-elastic NiTi shape memory alloy depends greatly on the stress levels and loading modes, and the dissipated energy in each cycle progressively decreases with the increasing number of cycles, which represents a functional degradation. The results also show that the structural fatigue life of the NiTi alloy presented under the stress-controlled cyclic loading is dependent on the peak stress, mean stress and stress amplitude, and the transformation ratchetting shortens the fatigue life apparently. The fatigue life decreases with the increasing dissipated energy, and a balance between dissipated energy and structural fatigue life should be reasonably considered in the design of damping devices of the NiTi alloy.