Search

Owing to the dissipative phase transformation and the porosity characteristics, cellular shape memory alloy (SMA) shows high damping capacity and thereby superior performance in vibration control. This paper studies the damping capacity and vibration behavior of the cellular SMA by means of the unit-cell finite element method. A material model of dense SMA is employed to simulate the thermodynamic behavior of the cellular SMA. A loss factor ω is introduced to evaluate the damping capacity of cellular SMA in a successive loading-unloading cycle. The effects of the material properties (the stress difference between the forward and reverse transformations, the saturated value of the transformation strain), the ambient temperature and the pore morphology (the porosity and the axial ratio of spheroid pore) on the damping capacity are investigated. Besides, the cellular SMA with randomly distributed pores is also considered. Simulation results well demonstrate the high damping capacity of the cellular SMA due to the local dissipative martensite phase transformation. Finally, by comparison of the free vibration behavior between the cellular SMA beam and the elastic beam, the cellular SMA exhibits superior performance in vibration control. [ABSTRACT FROM AUTHOR]

Pre-service thermomechanical training is of great significance to achieve functional stability for shape memory alloy device. This article presents a finite element simulation of the training behavior of a shape memory alloy wave spring actuator using a thermomechanically coupled and finite-strain shape memory alloy model (Wang et al., 2017a). The model is implemented into ABAQUS/Explicit by means of a user-defined material subroutine VUMAT. The introduction of a finite-Hencky-strain return-mapping integration scheme substantially improves the numerical efficiency and stability. Model predictions are validated against the experimental data. The good agreement between both demonstrates the capabilities of the model of well describing the training behavior of shape memory alloy when subjected to large cyclic thermomechanical loading. Simulation results illustrate several primary thermomechanical characteristics during training process, such as the expansion of the phase transformation zone, the accumulation of the residual deformation, and the concentration of the internal stress. The present finite element approach provides a powerful tool in design and optimization of shape memory alloy wave spring actuator, especially to improve the geometric precision and to enhance the two-way shape memory effect. [ABSTRACT FROM AUTHOR]

This paper presents a new 3D thermomechanical finite-strain constitutive model for cyclic pseudoelasticity of polycrystalline shape memory alloys (SMAs). The model considers four primary characteristics related to the cyclic behavior of SMA that have not been integrally addressed within the finite-strain framework: (i) large accumulated residual strain that results from the residual martensite and dislocations slipping during cycling; (ii) degeneration of pseudoelasticity and hysteresis loop due to the increase of dislocation density and internal stresses with the number of cycles; (iii) rate dependence that can be attributed to the thermomechanical coupling effect; (iv) evolution of the phase transformation from abrupt to smooth transition, as a consequence of the diversified crystallographic orientations of the grains, the heterogeneity of internal stresses, and the presence of non-transforming precipitates during cycling. Based on the decomposition of finite Hencky strain into elastic, transformation, residual and thermal components, the model is constructed within a thermodynamically consistent framework. Evolution equations associated with the internal variables are derived from the reduced form of energy balance, the Clausius-Duhem form of entropy inequality, and a Helmholtz free energy function that includes elastic, thermal, interaction and constraint energies. The model is used to simulate the cyclic tensile experiments on NiTi wire at different loading rates. The good agreement of the model predictions against the experimental data demonstrates the capabilities of the proposed model to well describe cyclic pseudoelasticity of polycrystalline SMAs, and to capture the aforementioned characteristics. Furthermore, in order to demonstrate the capability of the cyclic model to solve multi-axial problems, a finite elements simulation of a SMA torsion spring undergoing large strains and rotations resulting in local multi-axial non-proportional stress and strain evolutions is performed. [ABSTRACT FROM AUTHOR]

Abstract In this paper, a multiscale investigation of fatigue crack initiation in shape memory alloys (SMAs) based on Transformation Induced Plasticity (TRIP) is presented. A mechanism for fatigue crack initiation during cyclic stress-induced phase transformation along with theoretical model is proposed. To validate the TRIP-based model, quasi-static tests at different ambient temperatures, 40 ∘ C , 52 ∘ C and 65 ∘ C , and strain and stress controlled low-cycle fatigue tests at different frequencies ranging from 0.16 Hz to 5 Hz on pseudoelastic NiTi wires are carried out. The results show that, (i) TRIP appearing on phase transformation interfaces is the key factor that drives the fatigue crack initiation during cyclic stress-induced phase transformation in SMAs; (ii) maximum temperature during phase transformation is a relevant indicator to predict low-cycle fatigue of SMAs and, (iii) within the range of pseudoelasticity and below the plastic yield, low-cycle fatigue of SMAs is not directly correlated with the mechanical loads applied at macro-scale, in the sense that, if the maximum temperature reached during loading cycles is kept constant, the fatigue lifetime remains unchanged whatever the amplitude of the mechanical loading is. Based on the findings, a new criterion for pseudoelastic low-cycle fatigue of SMAs as well as fatigue-isolines diagram are proposed and validated experimentally. Highlights • A mechanism of fatigue crack initiation during cyclic loading in SMAs is modeled and experimentally validated. • TRIP appearing in A-M interfaces is the key factor that drives fatigue crack initiation during cyclic loading in SMAs. • Maximum temperature during phase transformation can be used as an indicator to predict low-cycle fatigue of SMAs. • In the pseudo-elastic domain, if the maximum temperature is kept constant the fatigue lifetime remains unchanged. • Fatigue failure almost always occurs at one of the martensite front at the end of the forward phase transformation. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the degradation of the recoverable strain during full transformation cycling in shape memory alloys. Results show that such degradation occurs due to: (i) untransformed austenite during full transformation, (ii) residual martensite after complete unloading, and (iii) plastic strain produced in the latest reverse transformation. Particularly, the plastic strain tends to zero when the stabilized state is reached. Furthermore, the degradation is frequency-dependent in the sense that a higher loading frequency results in a larger degradation of the recoverable strain. Finally, it is shown that fatigue lifetime decreases when transformation degradation increases. Graphical Abstract [ABSTRACT FROM AUTHOR]

This paper presents a mechanical training process that allows enhancing resistance to low cycle fatigue of shape memory alloys. To this end, three training stresses were tested (0–509.6 MPa, 0–637.0 MPa, 0–764.3 MPa); for each case, NiTi wires were first subjected to the corresponding load during first 20 cycles, and then tested to failure under strain-controlled fatigue loading. Results show that fatigue lifetime is training-dependent in the sense that specimens with higher training stresses present a better fatigue lifetime. Indeed, for sufficiently high training stress, fatigue lifetime can be 10 times extended. [ABSTRACT FROM AUTHOR]

The effect of cyclic loading frequency on pseudoelastic behavior and low cycle fatigue of shape memory alloys (SMAs) is investigated. To this end, strain and stress controlled tensile fatigue tests on NiTi wires are performed. The material is first trained for a few cycles to stabilize the hysteresis loop and reach a shakedown state and then submitted to fatigue tests. An infrared camera is used to measure the surface temperature of the wire. Strain-controlled tests are performed at different total strain amplitudes, and stress-controlled ones are performed at different maximum stress values. As the frequency increases, the fatigue lifetime under strain controlled loading shows a decreasing trend for all the strain amplitudes tested. The situation in stress controlled tests is however more complicated because the fatigue lifetime shows a maximum stress-dependent trend: for different maximum stresses, the trend in variation of fatigue lifetime with frequency is different. Based on some considerations related to the thermomechanical coupling in SMAs, a theoretical explanation for the evolution of the mechanical response and fatigue lifetime with loading frequency is presented. A new strain-energy based fatigue model is proposed, and is validated by experiments performed under different load ratios. [ABSTRACT FROM AUTHOR]

In this paper, the effect of thermo-mechanical coupling on fatigue crack propagation in a NiTi pseudo-elastic shape memory alloy (SMA) is investigated. Single Edge Crack (SEC) specimens are loaded at different frequencies to see the effect of frequency on mechanical response of the edge cracked NiTi plate. The crack length and the temperature at the crack tip are measured using an Infrared Thermography (IR) camera. In particular, the evolutions of the maximum temperature at the crack tip and the crack length with respect to number of loading cycles at different loading frequencies are plotted. The results show that the crack growth rate is significantly frequency-dependent, and the growth rate decreases with increasing frequency. [ABSTRACT FROM AUTHOR]

Transformation-induced plasticity (TRIP) in pseudoelastic polycrystalline NiTi shape memory alloys (SMAs) is investigated through tensile tests combined with in-situ infrared and digital image correlation (DIC) observations, and a new TRIP concentration phenomenon is revealed. To this end, an instability evolution model for solid–solid phase transformation is established to explain the multi-scale mechanisms of TRIP in SMAs. It is shown that, during the stress-induced transformation, the local stress at the grain scale evolves discontinuously inside austenite–martensite mixtures, presenting the maximum magnitude at the austenite side of the transformation interfaces. This local stress field triggers TRIP although the macroscopic stress still evolves "smoothly" with a relatively low magnitude. As the temperature increases, a further plasticity in austenite is required to lower the discontinuity at the austenite–martensite interfaces. Due to the accumulation of the heat generated in transformed regions, the temperature at the moving transformation interface continuously increases till reaching the maximum at the end of the transformation. Therefore, TRIP is concentrated at the final transformation band fronts, manifesting as concentration peaks in the residual strain map. The concentration of TRIP also shows a strong rate/frequency dependence: when the loading rate increases, TRIP accumulates to a higher average density throughout the transformation region while the concentration is significantly relieved. [Display omitted] • A new phenomenon of TRIP concentration is revealed. • A multi-scale instability model for stress induced transformation in SMAs is proposed. • The model illustrates why TRIP is triggered under relatively low applied stress. • The model captures the effect of temperature on TRIP. • The concentration of TRIP is directly and comprehensively validated. [ABSTRACT FROM AUTHOR]

Machined helical springs were designed and fabricated from NiTi shape memory alloy tube stock by cutting helical slots to make the coils, which provided them with high design flexibility and actuation potential. The springs were then subjected to isothermal tensile testing at temperatures of 20 ° C and 80 ° C , the experimental data showed that they could generate more than 50% actuation stroke at the applied force of 300 N. A material model was introduced to describe the thermally-induced actuation behavior of the machined NiTi spring. Finite element simulation of the springs was carried out to investigate the effects of geometries and applied loads on the actuation performance. With increasing applied load, the actuation stroke and transformation temperature increased as well, indicating more actuation potential at high loads. The actuation stroke was positively correlated to the outer diameter and the cross-sectional aspect ratio, negatively correlated to the cross-sectional area, and independent of the coil pitch. A variable-sweep wing actuated by two antagonistic machined springs was designed and numerically analyzed, the actuator could generate a steady cyclic motion up to 40 mm and allowed the swept-back angle of the wings to be continuously varied from zero to 90 degrees. [Display omitted] • Machined NiTi springs show same actuation stroke but 6 times force as wire-wound ones. • User-defined material model well describes thermally-induced actuation at varying stress. • Spring geometries and applied load show a strong impact on actuation performance. • Machined spring can provide large continuous swept-back deformations of the wing. [ABSTRACT FROM AUTHOR]

The paper presents a numerical implementation of the ZM model for shape memory alloys (Zaki and Moumni, 2007a) that fully accounts for nonproportional loading and its influence on martensite reorientation and phase transformation. Detailed derivation of the time-discrete implicit integration algorithm is provided, including an explicit closed-form expression for the continuous material jacobian. The algorithm is used for finite element simulations using Abaqus, in which the model is implemented by means of a user material subroutine. Extensive validation of the model is provided against multiple sets of experimental and numerical simulation data taken from the literature. [ABSTRACT FROM AUTHOR]

A crystal plasticity-based constitutive model is developed to describe the thermomechanical behavior of pseudoelastic NiTi single crystal. The model includes, for the first time in the literature, all inelastic mechanisms influencing the fatigue behavior of NiTi SMAs in a finite strain framework: martensite transformation, deformation slip in austenite at high-temperature, deformation twinning in martensite at large strain, transformation-induced plasticity (TRIP) as well as thermomechanical coupling. Furthermore, new internal variables and evolution laws are introduced in the monocycle model (referred as basic model in the remainder of the paper) to reproduce the main features of anisotropic cyclic deformation of pseudoelastic NiTi single crystal. The numerical implementation of the constitutive model is performed in the CAST3M (2019) finite element software through a user-defined UMAT subroutine. A series of simulations were performed to verify the basic and generalized cyclic models under various conditions. Moreover, the robustness of the model is attested by comparing the simulation results with the reported data of the pseudoelastic NiTi single crystal. The effect of crystallographic orientation and anisotropic cyclic deformation behavior are revealed and shown to be quantitatively in a good agreement with experimental results. Finally, the evolution of dislocation density and stored energy is discussed from the perspective of fatigue analysis of SMAs. • A crystal plasticity-based constitutive model for pseudoelastic SMA is developed. • Monocyclic response of NiTi single crystal is studied in finite strain framework. • The model is generalized to predict the large cyclic deformation of NiTi SMAs. • The model qualitatively captures all the features related with pseudoelastic NiTi. • The model is quantitatively validated against experimental data in the literature. [ABSTRACT FROM AUTHOR]

• The drop-weight impact test reveals the thermodynamic behavior of SMA against low-velocity impact load. • The maximum temperature has a quasi-linear, positive correlation with the impact energy. • The proposed algorithmic scheme enables the use of the SMA model as computational tool. • The simulation results are well validated against the experimental data with the maximum deviation of 5.78%. The study of thermodynamic behavior of shape memory alloy (SMA) against impact is of great importance for the use of SMA as damping devices. This paper investigates the thermodynamic behavior of NiTi SMA sheet subjected to low-velocity impact loading, by means of experiment and explicit finite element (FE) analysis. First, the impact tests are carried out with different impact energies on the drop-weight impact system, the thermal response during the impact process is captured by the infrared camera. The impact force and the temperature rise achieve the maximum values of 8716 N and 46.93 ∘C at impact energy of 25 J. Then, an explicit Euler integration scheme is introduced to implement the SMA model (Wang et al., 2017, IJP) into finite element software ABAQUS by means of the explicit user-defined material subroutine VUMAT. Finally, the impact tests are simulated using the proposed numerical algorithm. Numerical results show good correlation with the experimental data, the maximum deviations between both for the impact force and the temperature are 5.78% and 0.63%, respectively. The analysis approach captures the prime thermodynamic features of SMA against low-velocity impact load such as superelastic deformation, stress-induced martensite phase transformation and temperature variation. [ABSTRACT FROM AUTHOR]

This paper presents an efficient structural optimization approach for shape memory alloys (SMAs) with respect to fatigue. In the proposed method, a nonparametric shape optimization approach based on optimality criteria is applied to change the structural configuration. The design optimization framework is accomplished by relying on commercially available software and tools. In addition, both low- and high-cycle fatigue criteria are used to compute the fatigue factor at each material point, indicating its degree of safeness with respect to fatigue. Meanwhile, a 3D constitutive model is utilized to predict, with good accuracy, the stabilized thermomechanical stress state of a SMA structure subjected to multiaxial nonproportional cyclic loading. Finally, numerical examples are tested to demonstrate the effectiveness of the proposed method. Unlabelled Image • A non-parametric shape optimization approach based on optimality criteria is applied to change the structural configuration. • Both low- and high-cycle fatigue criteria are used to compute the fatigue factor. • The design optimization framework is accomplished by relying on commercially available software and tools. [ABSTRACT FROM AUTHOR]