Superelastic NiTi Shape Memory Alloy Research Articles

Modelling on grain size dependent thermomechanical response of superelastic NiTi shape memory alloy

To describe the grain size dependence and rate dependence of superelastic NiTi, a constitutive model involving intrinsic material instability and thermomechanical coupling is proposed and implemented implicitly into finite element software. The material is regarded as a mixing of transformable phase and non-transformable phase. The thermodynamic driving force for martensitic transformation and the heat equilibrium equation are deduced in the framework of irreversible thermodynamics. The global thermomechanical response, i.e. stress–strain curve, and local thermomechanical response, i.e. strain and temperature profiles, are simulated. If the grain size is above 68 nm, NiTi is characterized with Lüders like deformation associated with strong thermomechanical coupling. If the grain size is 42 nm and 27 nm, Lüders like deformation does not take place because Considere criterion is not fulfilled. Strain localization is visible owing to the collective effect of thermomechanical coupling and temperature inhomogeneity. If the grain size is 10 nm, thermomechanical coupling is suppressed and NiTi deforms homogeneously.

Thermomechanical coupling in cyclic phase transition of shape memory material under periodic stressing—experiment and modeling

Experiment and modeling analysis are performed to study the evolution dynamics due to thermomechanical coupling in cyclic phase transformation responses of a superelastic NiTi shape memory alloy rod under periodic tensile stress. Synchronized acquisition of time evolutions in temperature and stress-strain curve of the rod were realized in the frequency range of 0.0004 ~ 4 Hz (average stress rate from 0.42 MPa/s to 4200 MPa/s). Significant frequency dependent oscillations, drifts and stabilizations in the strain, temperature and stress-strain curves are quantified and the roles of thermomechanical coupling in the observed evolution dynamics of the responses are revealed. It is found that the stress-strain responses are non-isothermal over the tested frequency range and that the evolution dynamics is quite distinct from that of displacement-controlled cyclic phase transition (Yin et al., JMPS, 2014) due to the non-prescribed heat sources and the implicit nature of the response. The evolution dynamics are modelled by two coupled non-linear governing equations to obtain numerical and approximate analytical expressions of the transient and steady-state stress-strain and temperature oscillations. It is shown that, for given material and ambient properties, the thermomechanical responses under cyclic-stressing can be divided into three regions and are essentially governed by the non-dimensional time scale tp¯ and stress Δσ¯.

Variable-friction self-centering energy-dissipation braces (VF-SCEDBs) with NiTi SMA cables for seismic resilience

This study introduces a Variable-Friction Self-Centering Energy-Dissipation Brace (VF-SCEDB) incorporating pre-tensioned superelastic NiTi shape memory alloy (SMA) cables and novel Variable Friction Devices (VFDs). The motivation behind the proposed member is to promote reliable energy dissipation without compromising the self-centering capability. In addition, the strength and the loading/unloading plateau stiffness of the brace can be adjusted separately by an appropriate combination of the NiTi SMA cables and the VFDs. The concept, configuration, and fundamental working principle of the VFD and VF-SCEDB are first presented. An experimental study on a proof-of-concept VF-SCEDB specimen is then conducted to understand its fundamental cyclic behavior. The results confirm that the proposed VF-SCEDB has the potential for exhibiting both excellent energy-dissipation and self-centering capabilities. Analytical models for predicting the hysteretic response of the VFD and the brace are also developed, and the prediction agrees well with the experimental results. Following the experimental program, a system-level analysis considering a series of 3-story concentrically braced steel frames employing different self-centering braces including the new VF-SCEDBs, is carried out. The results show that the frames can well control the residual deformation, but compared with other types of self-centering braces, the VF-SCEDB can more effectively reduce the peak deformation of the structures.

Functional Behavior of Pseudoelastic NiTi Alloy Under Variable Amplitude Loading

Abstract Cyclic loading of superelastic NiTi shape memory alloy (SMA) causes forward and reverse austenite–martensіte transformations, and also increases the volume of stabilized martensite. This appears in the change of stress-strain curve form, the decrease of dissipation energy, and increase of residual strain, that is, named transformation ratcheting. In real structures, the SMA components in most cases are under the action of variable amplitude loading. Therefore, it is obvious that the loading history will influence the functional fatigue. In the present work, the effect of stress ratio on the functional properties of superelastic NiTi shape memory alloy under variable amplitude loading sequence with two blocks was investigated. The studies were carried out under the uniaxial tension of cylindrical specimens under load-full unload and load-part unload. The change of residual strain, strain range, dissipation, and cumulative dissipation energy density of NiTi alloy related to load sequences are discussed. Under both stress ratios, the residual strain in NiTi alloy is increased depending on the number of loading cycles on the high loading block that is similar to the tests at constant stress or strain amplitude. An unusual effect of NiTi alloy residual strain reduction with the number cycles is found at a lower block loading. There was revealed the effect of residual strain reduction of NiTi alloy on the number of loading cycles on the lower amplitude block. The amount of decrement of the residual strain during a low loading block is approximately equal to the reversible part of the residual strain due to the stabilized martensite. The decrease of the residual strain during the low loading block is approximately equal to the reversible part of residual strain due to the stabilized martensite. A good correlation of the effective Young’s modulus for both load blocks with residual strain, which is a measure of the volume of irreversible martensite, is observed.

Thermomechanical and electrical response of a superelastic NiTi shape memory alloy cable

Superelastic shape memory alloys are a unique class of smart materials that can recover up to 6% strains. Due to their appealing properties such as high energy dissipation and corrosion resistance, several researchers have assessed the use of such materials in numerous applications ranging from biomedical to civil engineering. This article investigates the thermomechanical and electrical response of a NiTi shape memory alloy cable with a complex configuration subjected to cyclic loading of various strain amplitudes ranging from 3% to 7%. The cable consists of several multi-layered strands with a cumulative outer diameter of 5.5 mm. The thermomechanical results indicate a good correlation of the change in cable temperature with the applied strains and maximum stresses. The temperature history can be used to map the degradation pattern of the shape memory alloy cable. In addition, due to the complex geometry of the cable, the global electrical resistance change does not predict the strain/stress state of the cable; however, it can be used to predict the onset of phase transformations.

Phase-field theory based finite element simulation on thermo-mechanical cyclic deformation of polycrystalline super-elastic NiTi shape memory alloy

Based on the previously developed isothermal phase field model [1], the thermo-mechanical cyclic deformation of polycrystalline super-elastic NiTi shape memory alloy (SMA) was simulated by a newly proposed thermo-mechanical phase filed based finite element method. To ensure the efficiency of numerical simulation, a two-dimensional (2D) phase field based finite element method was proposed. From the simulations, it is concluded that the cyclic super-elasticity degradation and related phase transition mode of polycrystalline NiTi SMA and their dependence on the loading rate and loading level are reasonably modeled. Furthermore, correspondent physical mechanism is summarized, that is, the cyclic degradation of super-elasticity and the localization of phase transition (especially for that in the stable cycle) become more significant by increasing loading level and loading rate, but the physical mechanisms of them are different: they are attributed to the increase of residual martensite bands resulted from the expansion of relatively large plastic deformation domain in the polycrystalline NiTi SMA if increasing the loading level; while, if increasing the loading rate, they are attributed to the increase of residual martensite bands resulted from the impediment of thermo-mechanical coupled effect to the phase transition.

An investigation into the mechanical behaviour of fibre-reinforced geopolymer concrete incorporating NiTi shape memory alloy, steel and polypropylene fibres

This paper studies the use of nickel-titanium (NiTi) superelastic shape memory alloy (NiTi-SMA), steel, and polypropylene (PP) fibres as reinforcement in geopolymer concrete (GPC) to improve the overall mechanical properties. NiTi-SMA, steel and PP fibres were added to the GPC at the fibre volume contents of 1.00%, 0.75% and 0.50% for steel and NiTi-SMA and 0.20%, 0.15% and 0.10% for PP. In this study, workability, compressive strength, splitting tensile strength, modulus of elasticity, static flexural strength and cyclic flexural tests were conducted to investigate the behaviours of steel fibre-reinforced GPC (SFRGPC), NiTi-SMA fibre-reinforced GPC (NiTi-SMAFRGPC) and PP fibre-reinforced GPC (PPFRGPC). The results showed that the mechanical performance of FRGPC increased with the increment of steel and NiTi-SMA fibres, but deceased with the increment of PP fibre. SFRGPC mixes exhibited the largest compressive strength (39.39 MPa), splitting tensile strength (5.36 MPa) and flexural strength (12.53 MPa), whereas NiTi-SMAFRGPC mixes presented the superior cyclic flexural performance which showed the smallest residue deformation and largest re-centring ratios in four cycles in comparison with SFRGPC and PPFRGPC mixes.

Effect of loading rate on fracture mechanics of NiTi SMA

Superelastic NiTi shape memory alloy is characterized by phase transformation under loading. In this work, the effect of loading rate on fracture mechanics of NiTi is investigated. First, to characterize the behavior of the material, tensile experiments are conducted on dog-bone specimens at different quasi-static-range loading rates and the transformation stresses–strains are calculated. An increase in forward transformation stresses–strains, and a decrease in the reverse transformation stresses–strains with increasing loading rate is observed. Surface mean temperature is measured using a thermal camera, and an increase in temperature difference is observed with increasing loading rate. Displacement field and strain maps are obtained simultaneously using Digital Image Correlation (DIC). The strain maps show strain localizations that change with increasing loading rate. At higher loading rates, localized strain bands increase in number. Fracture experiments are then conducted under Mode I loading on plane stress Compact Tension (CT) specimens, and for further confirmation of the behavior, Finite Element Analysis (FEA) is used with a user defined subroutine to account for thermomechanical coupling. Crack mouth opening displacements (CMODs) are measured and an increase with increasing loading rate is observed. Stress Intensity Factors (SIFs) are calculated using ASTM E399 equations, the displacement data obtained from DIC, and FEA. In all cases, stress intensity factors show an increasing-decreasing trend with increasing loading rate. Results are summarized in a figure to show the trend. Austenite to martensite transformation region sizes around the crack tip are evaluated experimentally, analytically and computationally, and are found to be decreasing with increasing loading rate as well. An interesting but coherent behavior in change of fracture parameters with increasing loading rate is obtained. This study is expected to stimulate new discussions on the subject.

Constitutive modelling of transformation pattern in superelastic NiTi shape memory alloy under cyclic loading

In this paper, multiple cases of cyclic loadings on superelastic NiTi are studied experimentally and then numerically by means of finite element simulations. A novel constitutive framework is proposed to account for cyclic response of superelastic NiTi shape memory alloy. Martensitic transformation and degradation of mechanical properties are taken into consideration. The mechanical instability is involved in the model to reproduce transformation pattern under primarily tensile stress states. The Helmholtz free energy of the representative volume element is derived and the evolutions of internal variables are obtained keeping to dissipative inequality. Based on backward Euler integration scheme, the model is implemented implicitly into a finite element code. By comparing the simulation results with the experimental ones, it is verified that the proposed approach captures the features of the cyclic response and the associated strain pattern reasonably.

Effects of grain size on fatigue crack growth behaviors of nanocrystalline superelastic NiTi shape memory alloys

The grain size (GS) dependence of fatigue crack growth in nanocrystalline superelastic NiTi shape memory alloys (SMAs) with the average GS of 10, 30, 60 and 235 nm is investigated. Synchronized measurements of the fatigue crack growth, load-displacement response and temperature field of the compact tension specimens are performed. It is found that the fatigue crack growth rate (da/dN) is monotonically decreased by 10 times when the GS is increased from 10 to 235 nm. The significant GS effect on the fatigue crack growth is also confirmed by the direct observation of fatigue striation spacing on the postmortem fracture surface. It is shown that the increase of the GS significantly enhances the phase transition and plasticity of the material and therefore enhances the crack-tip shielding (Ksh) which reduces the effective driving force (ΔKeff) of the fatigue crack growth. Subtracting Ksh due to phase transition and plastic deformation from the external applied Kapp, the effective crack-tip fatigue driving force ΔKeff is obtained. The da/dN-ΔKeff curves for different GS specimens then almost collapse onto a single curve which may represent an intrinsic fatigue property of the NiTi in the absence of the shielding effects. Therefore the observed apparent GS dependence of the fatigue crack growth resistance in the superelastic NiTi SMAs is mainly caused by the shielding effect from the GS dependent phase transition and plastic deformation.

Experimental Investigation on Buckling and Post-buckling Behavior of Superelastic Shape Memory Alloy Bars

This paper examines the buckling and post-buckling behavior of superelastic shape memory alloy (SMA) bars. A NiTi SMA bar with a diameter of 12 mm was used in all the experimental tests. First, the tensile and compression responses of NiTi bar were characterized under monotonic loading up to failure. A total of 15 specimens with slenderness ratios that range from 25 to 115 were tested to study the critical buckling load and post-buckling behavior of SMA bars. Digital image correlation (DIC) system was implemented to monitor full-field surface displacements. The interaction between material nonlinearity due to phase transformation and geometric nonlinearity was explored. Data obtained from the DIC measurement system were further analyzed to identify the onset of buckling and to extract experimental critical buckling loads. The effect of loading rate on buckling response of SMAs was investigated by conducting additional testing at higher loading rates on the specimens with three selected slenderness ratios. The temperature field on the surface of the specimens was recorded by an infrared camera. The analytical critical buckling loads were computed and compared with experimental results. All specimens exhibited a unique buckling behavior characterized with almost a complete shape recovery upon unloading.

Deformation behaviors of gradient nanostructured superelastic NiTi shape memory alloy

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

An experimental investigation of the nucleation and the propagation of NiTi martensitic transformation front under impact loading

The paper presents an experimental investigation of the NiTi superelastic shape memory alloy (SMA) under high-speed impact loading. A Split Hopkinson tensile bar (SHTB) and special short strikers are used to reach impact speeds exceeding 50 m/s. A very high-speed camera allows for the capture of full resolution images (924 × 768 pixels) at a sampling rate of 2 million frames/s. Digital image correlation (DIC) analysis provides good measurements of the strain field as well as the particle velocity field of the whole specimen.Experimental results shows that the increase of the critical transformation stress is much higher at strain rates around 103/s instead of a moderate increase at lower strain rates under 102/s. The heterogeneous stress field due to elastic wave propagation at high impact velocity leads to a heterogeneous transformation zone inside the specimen. The loading and the distal ends are the privileged sites of the martensitic transformation nucleation because of stress wave reflections. Transformation fronts will afterwards occur and propagate, driven by the stress increase at the loading end or that due to the loss of particle velocity (kinetic energy) at the distal end respectively. The jump condition across the front is respected. Finally, the cumulated martensitic phase length is traced when multiple fronts occur. It is found that the growing speed of this cumulated martensitic phase length remains proportional to the loading speed and that it can exceed the shear wave speed in the martensitic phase of the material.

Properties of a superelastic NiTi shape memory alloy using laser powder bed fusion and adaptive scanning strategies

A NiTi shape memory alloy with the nominal composition Ni50.9Ti49.1 (at%) was processed by laser beam melting/laser powder bed fusion and the process parameters as well as the type of scanning strategy (point-like exposure) were optimized in a first step to obtain delicate lattice structures (strut diameters below 200 µm). In the second step, the lattice structures were analyzed by means of optical and electron microscopy as well as computer tomography to obtain the interrelation between the process parameters, strut diameter and the uniformity of the corresponding struts. The processing, especially the laser power and the type of point-like exposure, has a strong influence on the resulting strut diameter and, therefore, on the haptic stiffness of lattice structures and the mechanical properties (deformability, superelasticity). Unlike other approaches, our findings imply that filigree NiTi lattices with high uniformity can be manufactured on a standard industry laser powder bed fusion machine without modifying its hard- or software configuration.

Design, fabrication and modeling analysis of a spiral support structure with superelastic Ni-Ti shape memory alloy for continuum robot

In order to improve the working ability of continuum robots in confined space, a novel central support structure with the spiral cutting pattern is proposed in this paper. The spiral hollow scheme was selected through analysing different cutting patterns of the thin-walled superelastic Ni-Ti shape memory alloy (SMA) tube. Then the laser cutting process and heat treatment for fabricating the structure were analysed. The analysis results of their impact on the material characteristics proves that the laser cutting and heat treatment ensure the performance of the material. The single-segment and multi-segment prototypes both showed prospective motion ability in the flexural rigidity and continuous deformation tests respectively. Then the flexural rigidity model of the spiral structure was proposed, which was proved effective by comparing the model simulation and finite element analysis results. Furthermore, the central support structures with different flexural rigidities can be customized, which has broad application prospects in continuum robots.

Cyclic phase transformation behavior of nanocrystalline NiTi at microscale

Cuboidal micropillars of nanocrystalline superelastic NiTi shape memory alloys with an average grain size of 65 nm were fabricated by focused ion beam and then subjected to cyclic compression. It is found that the micropillars have maintained superelasticity for over 106 full-transformation cycles under a maximum compressive stress of 1.2 GPa. Functional degradation of the micropillars mainly occurs in the first 104 cycles where hysteresis loop area and forward transformation stress rapidly decrease from initial 11 MPa (MJ/m3) and 586 MPa to 6 MPa and 271 MPa. In the 104 ~ 106 cycles, stress-strain responses of the micropillars show asymptotic stabilization. Residual strain is accumulated to 3.3% and multiple ~50 nm wide extrusions are found at the surface of the micropillars after 106 cycles. SEM and TEM studies indicate that cyclic phase transformation results in formation and glide of transformation-induced dislocations that create surface steps and the extrusions. The dislocations inhibit reverse transformation and result in residual martensite and residual stresses. The dislocations and the residual martensite lead to the functional degradation. The role of the residual martensite in the functional degradation is further verified by 21% recovery of the residual strain and an increase of 278 MPa in the forward transformation stress after heating up the cyclically deformed micropillars to 100 °C. The recorded over 106 phase transformation cycles under a maximum stress of 1.2 GPa of the NiTi shape memory alloys at microscale open up new avenues for applications of the material in microscale devices and engineering.

Molecular dynamics simulations on nanocrystalline super-elastic NiTi shape memory alloy by addressing transformation ratchetting and its atomic mechanism

The transformation ratchetting of super-elastic nanocrystalline (NC) NiTi shape memory alloys (SMAs) under cyclic tension-unloading conditions and its deformation mechanism in atomic scale were studied by molecular dynamics (MD) simulations. The effects of ambient temperature, applied peak stress and stress rate on the transformation ratchetting of NC NiTi SMAs were discussed under an isothermal condition; in addition, the effect of temperature rise derived from transformation latent heat and inelastic dissipation on the transformation ratchetting of NC NiTi SMA was further investigated in an adiabatic case by setting different stress rates. The MD simulations show that the transformation ratchetting in NC NiTi SMA occurs in atomic level and depends on different thermal boundary conditions. Under an isothermal condition, the transformation ratchetting is caused by both the plastic deformation occurred at grain boundaries and in the disordered structures within the grains and the accumulation of residual B19′ martensite phase; but, it is determined only by the plastic deformation at grain boundaries and in the disordered structures within the grains under an adiabatic condition. The MD simulations are verified further by qualitatively comparing them with corresponding experimental observations of NC NiTi SMA.

Rate-dependent transformation ratcheting-fatigue interaction of super-elastic NiTi alloy under uniaxial and torsional loadings: Experimental observation

To investigate the interaction between the rate-dependent transformation ratcheting and fatigue failure of super-elastic NiTi shape memory alloy (SMA) thin-walled tubes, uniaxial and torsional stress-controlled fatigue tests were conducted at various loading rates. The experimental results show that the uniaxial transformation ratcheting is more obvious than the torsional one and the torsional fatigue lives are significantly longer than the uniaxial ones, which imply that a more obvious transformation ratcheting leads to a shorter fatigue life. The rate-dependent transformation ratcheting and fatigue failure of super-elastic NiTi SMA can be attributed to the significant thermo-mechanical coupling effect.

Effects of Grain Size Gradient on Deformation Behavior of Superelastic NiTi Plate

The superelastic NiTi shape memory alloy with grain size distributed from 20 nm to 175 nm along the gauge length is fabricated by cold rolling and gradient heat treatment. This study investigated the effects of grain size gradient on deformation behavior of the NiTi plate under strain controlled quasi-static loading-unloading. Experimental results show that when the grain size decreased from 175 nm to 35 nm, the phase transformation stress increased from 380 MPa to 509 MPa, and the stress slope increased from 0.6GPa to 14.15GPa. With a simple phenomenological model, we have simulated the stress-strain responses and demonstrated the mechanism of the deformation behaviors of the grain size gradient superelastic NiTi plate. The study indicates that the particular behavior of the plate is due to grain size effects of coexisting grain size gradient.

Experimental and numerical study on the seismic performance of a self-centering bracing system using closed-loop dynamic (CLD) testing

This study investigates the seismic performance of a newly developed self-centering bracing system using a novel experimental technique named as closed-loop dynamic (CLD) testing. The bracing, named piston-based self-centering (PBSC) apparatus, employs Ni-Ti superelastic shape memory alloy (SMA) bars inside a sleeve-piston assembly for its self-centering mechanism. During cyclic tension-compression loading, the SMA bars are only subjected to tension avoiding buckling and leading to flag-shaped symmetric force-deformation hysteresis. Initially, a braced frame building fitted with PBSC is seismically designed and the preliminary sizing of the brace is determined. For testing, considering the lab capability, the brace is fabricated at a reduced scale. The process of “Closed-loop dynamic testing” starts with the brace test (step 1) under strain-rate loading to characterize the numerical model parameters (step 2), which are then scaled-up as per similitude law and implemented in a finite element software, S-FRAME’s PBSC brace model (step 3). Then the braced frame building is analyzed under an earthquake (step 4) and the axial force-deformation response of the brace under consideration is captured (step 5). In order to further understand and validate the actual response of the brace under earthquake type loading, the axial deformation obtained from S-FRAME is scaled-down (step 6) and used as input parameters for testing the reduced scale brace (step 7). The obtained response (step 8) is further scaled-up and used to match the S-FRAME’s PBSC model for validation (step 9). Iterations from step 3 to step 9 will be required until the experimental and numerical results converge. Convergence criteria used for this validation include both the energy dissipation capacity and initial stiffness within 10% accuracy. Reasonable agreement between the numerical and experimental results is achieved in the closed-loop dynamic testing. The PBSC brace shows excellent self-centering capability under various earthquake loadings.