Shape Memory Alloy Tubes Research Articles

Snap buckling of NiTi tubes

The aim of this article is to model and analyze the buckling behaviors of NiTi-based moderately thick shape memory alloy (SMA) tubes with short, intermediate and long lengths. A robust three-dimensional constitutive model is implemented so that it is capable of realistic simulations of anisotropic martensitic transformation, reorientation of martensite variants and asymmetry in tension and compression in the finite-strain regime. The governing equations of equilibrium are derived based on the total Lagrangian description and discretized in a finite element framework. They are then solved using an elastic-predictor inelastic-corrector return mapping algorithm along with iterative Newton–Raphson and Riks techniques to trace the non-linear equilibrium path. Experimental result of a uniaxial tension–compression test performed on an NiTi tube is first simulated in a Gauss point level. It is shown that the present constitutive model replicates well the main features such as martensitic phase transformation in a smooth and gradual manner, strain hysteresis width, pseudo-elasticity and asymmetry in tension and compression. Afterwards, computational assessment of mechanical response of NiTi tubes under axial edge load is carried out and comparisons with available experimental data are made. Effects of radial geometric imperfection and length as well as finite-strain modeling are investigated, and their implications on the buckling behaviors of NiTi moderately thick tubes are put into evidence and conclusions are drawn. It is shown that increasing the length tube reduces the structural resistance so that buckling–unbuckling behavior of short NiTi moderately thick tubes changes to kinking phenomenon in intermediate tubes and finally results in a smooth snap-type buckling in long tubes. Finally, it is concluded that the finite-strain model implementing a displacement/force-controlled method and kinking consideration is able to replicate well the main buckling features observed in the experiments.

On the influence of pseudoelastic material behaviour in planar shape-memory tubular continuum structures

For planar tubular continuum structures based on precurved shape memory alloy tubes a beam model with respect to the pseudoelastic material behaviour of NiTi is derived. Thereunto a constitutive material law respecting tension-compression asymmetry as well as hysteresis is used. The beam model is then employed to calculate equilibrium curvatures of concentric tube assemblies without clearance between the tubes. In a second step, the influence of clearance is approximated to account for non-concentric tube assemblies. These elastokinematic results are integrated into a purely kinematic model to describe the cannula path under the presence of material hysteresis and clearance. Finally a photogrammetric measurement system is used to track the path of an exemplary two-tube continuum structure to examine the accuracy of the proposed model. It is shown that material hysteresis leads to a hysteresis phenomena in the path of the tubular continuum structure.

A simplified model for high-rate actuation of shape memory alloy torque tubes using induction heating

Shape memory alloy actuators deliver high forces while being compact and reliable, making them ideal for consideration in aerospace applications. Induction heating of shape memory alloy actuators, specifically tubes that twist about the longitudinal axis, has recently been studied experimentally and computationally using finite element analysis. Reduced-order models for the torsional behavior of shape memory alloy tubes and induction heating of general metallic tubes exist, and thus, it is possible for these thermal and mechanical models to be combined and numerically solved. This work develops and implements an engineering model for inductively heated shape memory alloy tubes based on the reduction of the governing partial differential equations in space and time to an ordinary differential equation in time. An example solution is compared to finite element analysis results and agrees well. Finally, the ordinary differential equation is linearized and solved analytically. The linearized model agrees well with the nonlinear ordinary differential equation and finite element model.

High temperature shape memory alloy Ni50.3Ti29.7Hf20 torque tube actuators

The torsional behavior of Ni-rich Ni50.3Ti29.7Hf20 (at%) high-temperature shape memory alloy tubes was investigated under pure torsion loading. Torque tubes with varying geometry including outer diameter, wall thickness, and length were subjected to constant-torque thermal cycling at stresses ranging from 0 to 500 MPa (0–175 N m). It was found that the wall thickness had a notable effect on the transformation temperatures where thick-walled tubes transformed at lower temperatures when compared to the thin-walled form. In all tube outer diameters, shear strains were found to be in the order of 6% obtained at stresses above 300 MPa. At lower stresses, little to no effect of wall thickness was observed, but the influence increased at higher stresses where thin-walled tubes generated approximately 2% less strain when compared to the solid forms. Two-way shape memory effect was also evaluated after 20 cycles and was found to reach ∼3% strain when cycled at high stresses.

Investigation on Deformation Mechanisms of NiTi Shape Memory Alloy Tube under Radial Loading

NiTi shape memory alloy (SMA) tube was coupled with mild steel cylinder in order to investigate deformation mechanisms of NiTi SMA tubes undergoing radial loading. NiTi SMA tubes of interest deal with two kinds of nominal compositions; namely, Ni-50 at % Ti and Ni-49.1 at % Ti, where at room temperature, B19′ martensite is dominant in the former, and B2 austensite is complete in the latter. The mechanics of the NiTi SMA tube during radial loading were analyzed based on elastic mechanics and plastic yield theory, where effective stress and effective strain are determined as two important variables that investigate deformation mechanisms of the NiTi SMA tube during radial loading. As for the NiTi SMA tube with austenite structure, stress-induced martensite (SIM) transformation as well as plastic deformation of SIM occur with the continuous increase of effective stress. As for NiTi SMA tube which possesses martensite structure, reorientation and detwinning of twinned martensite as well as plastic deformation of reoriented and detwinned martensite occur with the continuous increase in the effective stress. Plastic deformation for dislocation slip has a negative impact on superelasticity and shape memory effect of NiTi SMA tube.

Seismic resilience timber connection-adoption of shape memory alloy tubes as dowels

This study investigates a novel timber dowel-type connection system using superelastic shape memory alloy (SMA) bar and tubes as dowels, in order to provide self-centering effect. Double-shear connections with SMA and mild steel dowels were tested under dynamic loadings at different displacement levels. The results showed that SMA dowel-type connections have good self-centering behaviours and can mitigate the residual deformation effectively compared with steel dowel-type connections after excessive deformation; although the steel dowel-type connections present higher strength. These tests reveal that the connection with tube dowels show higher equivalent viscous damping ratio than those use solid bar as tube would allow larger deformation to dissipate energy. To demonstrate application of the benefit of this system, an analytical model of a 3-storey timber framed structure was built for parametric study. The results showed that the structures with conventional dowel-type type connections exhibit large unrecoverable deformation after timber framed structures experience an earthquake. In comparison, those with the connections developed in this project show limited unrecoverable deformation due to the self-centering capacity of the connections.

Effect of heat treatment and mandrel material on precision tube drawing of Ni-Ti shape memory alloy

Abstract The Ni-Ti shape memory alloy (NITINOL) was known as one of difficult materials to meet the dimensional accuracy because of the complex phase transformation and the shape recovery characteristics. In this study, a tube drawing process with mandrel was numerically and experimentally investigated for manufacturing precision NITINOL tube to identify the stable phase transformation condition to remove the R-phase by differential scanning calorimeter. To study the non-linear material and shape recovery behavior of the NITINOL, tension and loading-unloading tests were conducted with three test velocities of 1, 5, and 10 mm/min. The three dimensional finite element (FE) analyses were carried out using the ABAQUS with hyperelastic and Mullins effect model. The amount of the shape recovery was checked for the drawn tubes compared to the numerical results. According to the present investigation, it was found out that the stable phase transformation which improved the ductility and reduced the effect of shape recovery was occurred under the condition of the heat treatment at 700˚C for 30 min and the tensile test velocity of 10 mm/min. The FE results showed similar tendency of the stress-strain curves obtained from the experiments. Moreover, the amount of the shape recovery of the NITINOL could be predicted by the FE analysis. From the results of the FE analysis and the tube drawing with red copper and brass mandrel, the dimensional accuracy was found to be improved by using the brass mandrel, but the drawing stress was somewhat higher for the brass mandrel than the case using the red copper mandrel. In conclusion, the shape recovery effect of the NITINOL could be predicted by the FE analysis and the process condition determined by the current experiments could be helpful for the precision tube drawing of the NITINOL shape memory alloy tube.

An experimental approach to the thermomechanical characterization of a NiTiCu shape memory alloy using strain gauges

In this work, a characterization of a NiTiCu (Ti44.6Ni5Cu (at.%)) shape memory alloy (tube specimens) has been done via tension, compression and torsion tests conditions. Torsion tests were done in a special homemade equipment, which is based on an instrumented dividing head with a specifically designed thermal chamber. This configuration is able to measure torque and twist angle with isothermal tests at different temperatures as well as to apply thermal cycles with a fixed twist angle. Moreover, tube specimens were instrumented with stacked strain gauges rosettes in order to obtain the strain tensor. Strain gauges were also used to calibrate the equipment and to identify the real stress state in torsion tests. The results have shown differences between the shear modulus measured on torsion tests and the shear modulus calculated from the measurements at tension and compression tests due to the tension/compression asymmetry and a non-constant strain ratio value. Thermal cycling tests at different values of fixed twist angles not only have led to characterize the evolution of torque as a function of the temperature but also to understand the different interacting mechanisms in torsion tests.

Determining the up-down-up response through tension tests of a pre-twisted shape memory alloy tube

In this paper, the stress-induced phase transitions in a shape memory alloy (SMA) tube under torsion and pre-twisted tension are studied analytically. A constitutive model with the specific forms of Helmholtz free energy and mechanical dissipation rate is employed to formulate the governing system. Exact solution of the tube under pure torsion is first derived and the shear stress–shear strain response is determined, which reveals the hardening effect. For the pre-twisted tube under uniaxial tension, the one-dimensional asymptotic tensile stress–tensile strain relations for the austenite, the phase transition and the martensite regions are derived by using the asymptotic expansion method. By properly defining an elastic energy potential, the present system can be viewed as an elastic problem, which can be related to the problem of Ericksen's bar. The analytical formulas for the nucleation and propagation stresses in terms of the pre-shear strain (caused by the pre-twist) are obtained. Tension tests with fixed pre-twists on SMA thin-walled tube are conducted, with a focus on the stress–strain response. The measured values and the analytical formula for the propagation stress are used to determine the material parameters, which, in turn, yields the up-down-up response of a shape memory alloy tube under pure tension. The tendency and turning point of the phase transformation in the pre-twisted tube from localization to homogeneous deformation are also determined, which suggests a plausible way to avoid the instability in actuation applications.

Laser welding of NiTi shape memory alloy wires and tubes for multi-functional design applications

Welding and joining of NiTi shape memory alloys is essential for their integration into an increasing variety of applications. Almost all manufacturers and a significant number of researchers focus their investigation on welding NiTi, which can present both pseudoelasticity (PE) and shape memory effect. Integration of these materials would provide increased flexibility in terms of smart design, in particular for multi-functional systems. The current work investigates the mechanical, physical and phase transformation properties of similar (base materials (BMs) with the same composition) and dissimilar (BMs with different compositions) NiTi welded shape memory wires. The similar and dissimilar welded joints were successfully achieved by laser welding, which can reach up to 88.4% and 67.5% of the wire BM ductility. The joint break force of the similar and dissimilar joints were of 77.2% and 71.4% of the wire BM, respectively. Moreover, laser welding was found to effectively preserve the PE on the similar welded structures. The residual plastic strain variation of the dissimilar welded specimens at different temperatures during the cycling test may be helpful for design of multi-functional or flexible monolithic structures.

On interfacial energy of macroscopic domains in polycrystalline NiTi shape memory alloys

Under tension or tension/torsion combined loading, the formation and movement of macroscopic interfaces (domain fronts which separate the austenite and martensite regions) in the phase transition of a bulk fine-grained polycrystalline NiTi shape memory alloy (SMA) bar or tube specimen are well-observed phenomena. The interfacial energy of the macroscopic domain front, i.e., the energy per unit area of the front (interface), competes with the bulk energy of the system and plays an important role in understanding and modeling both equilibrium and non-equilibrium domain patterns. This paper investigates the physical origin of the interfacial energy of a planar interface and its orientation and length scale dependences in a bar structure. The energetics of a martensite domain with such a planar interface is quantified by using an elastic inclusion model. We show that the strain energy due to the presence of the martensite domain consists of two parts, the bulk energy (such as from bending) due to the constraints at the bar ends and the localized energy due to the mismatch between the spontaneous transformation strain of the domain and the surrounding austenite. The latter is stored around the domain front and is named as the interfacial energy (domain front energy). It is proved that the interface or the domain front of the bar prefers the orientation that minimizes the system total energy. This is supported by comparison with the experimental observation. The dependences of the interfacial energy on the bar geometry (bar thickness and width), the transformation strain and the interface orientation are quantified.

A Comparative Study of Ni49.9Ti50.1 and Ni50.3Ti29.7Hf20 Tube Actuators

A shape memory alloy (SMA) actuator typically has to operate for a large number of thermomechanical cycles due to its application requirements. Therefore, it is necessary to understand the cyclic behavioral response of the SMA actuation material and the devices into which they are incorporated under extended cycling conditions. The present work is focused on the nature of the cyclic, evolutionary behavior of two widely used SMA actuator material systems: (1) a commercially available Ni49.9Ti50.1, and (2) a developmental high-temperature Ni50.3Ti29.7Hf20 alloy. Using a recently developed general SMA modeling framework that utilizes multiple inelastic mechanisms, differences and similarities between the two classes of materials are studied, accounting for extended number of thermal cycles under a constant applied tensile/compressive force and under constant applied torque loading. From the detailed results of the simulations, there were significant qualitative differences in the evolution of deformation responses for the two different materials. In particular, the Ni49.9Ti50.1 tube showed significant evolution of the deformation response, whereas the Ni50.3Ti29.7Hf20 tube stabilized quickly. Moreover, there were significant differences in the tension-compression-shear asymmetry properties in the two materials. More specifically, the Ni50.3Ti29.7Hf20 tube exhibited much higher asymmetry effects, especially at low stress levels, compared to the Ni49.9Ti50.1. For both SMA tubes, the evolution of the deformation response under thermal cycling typically exhibited regions of initial transients, and subsequent evolution.

Microplane modeling of shape memory alloy tubes under tension, torsion, and proportional tension–torsion loading

In this study, a three-dimensional thermomechanical constitutive model based on the microplane theory is proposed to simulate the behavior of shape memory alloy tubes. The three-dimensional model is implemented in ABAQUS by employing a user material subroutine. In order to validate the model, the numerical results of this approach are compared with new experimental findings for a NiTi superelastic torque tube under tension, pure torsion, and proportional tension–torsion performed in stress- and strain-controlled manners. The numerical and experimental results are in agreement indicating the capability of the proposed microplane model in capturing the behavior of shape memory alloy tubes. This model is capable of predicting both superelasticity and shape memory effect by providing closed-form relationships for calculating the strain components in terms of the stress components.

The morphing properties of a vascular shape memory composite

This work investigates the fabrication, experimentation, testing, and modeling of shape memory composites consisting of two-way shape memory alloy (SMA) tubes embedded in a shape memory polymer (SMP) matrix. The hybrid system here investigated is thermally activated via internal transport of thermal fluids through the SMA vascular system. The resulting shape memory composite (SMC) combines the high modulus and high specific actuation force of SMAs with the strong shape fixing and variable stiffness of SMPs to create a light-weight composite capable of controllably and rapidly achieving two shape memory states. Specifically, a 25° thermally induced out-of-plane bending state is achieved with a 2% volume fraction of SMA in the composite after 2 min of being activated by an internal thermal fluid. Here, while the thermal structural design of the SMC was not optimized and the thermal cycling was significantly restricted by the low thermal conduction of the SMP, the deflection of the composite was within 20% of the expected value modeled by the thermal–mechanical finite element analysis (FEA) here performed. The close agreement between the experimental performance and the modeled composite response suggests that morphing composites based on SMAs and SMPs are promising structures for adaptive applications.

Equal channel angular extrusion of NiTi shape memory alloy tube

As a new attempt, equal channel angular extrusion (ECAE) of nickel–titanium shape memory alloy (NiTi SMA) tube was investigated by means of process experiment, finite element method (FEM) and microscopy. NiTi SMA tube with the steel core in it was inserted into the steel can during ECAE of NiTi SMA tube. Based on rigid-viscoplastic FEM, multiple coupled boundary conditions and multiple constitutive models were used for finite element simulation of ECAE of NiTi SMA tube, where the effective stress field, the effective strain field and the velocity field were obtained. Finite element simulation results are in good accordance with the experimental ones. Finite element simulation results reveal that the velocity field shows the minimum value in the corner of NiTi SMA tube, where severe shear deformation occurs. Microstructural observation results reveal that severe plastic deformation leads to a certain grain orientation as well as occurrence of substructures in the grain interior and dynamic recovery occurs during ECAE of NiTi SMA tube. ECAE of NiTi SMA tube provides a new approach to manufacturing ultrafine-grained NiTi SMA tube.

Coupled rate-dependent superelastic behavior of shape memory alloy bars induced by combined axial-torsional loading: a semi-analytic modeling

In this article, the coupled thermomechanical response of superelastic shape memory alloy bars and tubes in combined tension and torsion is studied both analytically and experimentally. Using the Gibbs free energy as the thermodynamic potential and choosing appropriate internal state variables, a three-dimensional phenomenological macroscopic constitutive model for shape memory alloys is derived. Taking into account the effect of latent heat during the forward and reverse martensitic phase transformation, the appropriate form of the energy balance relation is obtained. The three-dimensional coupled relations for the energy balance in the presence of the internal heat flux and the constitutive equations are reduced to a two-dimensional form for the tension-torsion case. An explicit finite difference approach is utilized to discretize the governing boundary conditions of bars. An empirical expression for the free heat convection from the surface of a shape memory alloy bar was proposed and experimentally validated. Several sets of experiments were then carried out to evaluate the mechanical and thermal responses of the model for a shape memory alloy tube subjected to uniaxial, pure torsion and non-proportional tension and torsion loading–unloading conditions. The approach could be used in the design of shape memory alloy devices undergoing combined loads with high strain rates or in the fatigue design of shape memory alloy devices subjected to cyclic loading.

Finite element simulation of ball spinning of NiTi shape memory alloy tube based on variable temperature field

As a new attempt, ball spinning was used to manufacture the nickel-titanium shape memory alloy (NiTi SMA) tube at elevated temperature. The NiTi bar with a nominal composition of Ni50.9Ti49.1 (mole fraction, %) was solution treated and was used as the original tube blank for ball spinning. Based on the variable temperature field and the constitutive equation, rigid-viscoplastic finite element method (FEM) was applied in order to simulate the ball spinning of NiTi SMA tube. The temperature field, the stress field, the strain field and the load prediction were obtained by means of FEM. FEM results reveal that there is a temperature increase of about 160 °C in the principal deformation zone of the spun part. It can be found from the stress fields and the strain fields that the outer wall of NiTi SMA tube is easier to meet the plastic yield criterion than the inner wall, and the plastic deformation zone is caused to be in a three-dimensional compressive stress state. The radial strain and the tangential strain are characterized by the compressive strain, while the axial strain belongs to the tensile strain. The variation of spinning loads with the progression of the ball is of great importance in predicting the stable flow of the spun part.

Modifying the torque–angle behavior of rotary shape memory alloy actuators through axial loading: A semi-analytical study of combined tension–torsion behavior

In this article, a semi-analytical modeling approach is presented to study the pseudoelastic response of shape memory alloy rods and tubes subjected to combined tension–torsion loading states. A three-dimensional phenomenological shape memory alloy constitutive model is used to obtain the corresponding two-dimensional constitutive relations. The rod is partitioned into a finite number of narrow annular regions, and the equilibrium equations are found in each region for both loading and unloading paths. The derived equations are then solved based on an iterative algorithm. A set of experiments is conducted on a shape memory alloy thin-walled tube, and the results are then used to evaluate the performance of the modeling approach. Several numerical examples along with a finite element analysis are finally presented to demonstrate the capabilities of the proposed method. Using this approach, it is possible to design active biomedical devices with shape memory alloy actuators under combined loading conditions.

Resistance welding of NiTi shape memory alloy tubes

Resistance welding is studied as a technique to join NiTi shape memory alloy thin-walled tubes. Properties of the welded metal are compared to those of the base metal by means of differential scanning calorimetry measurements, microstructure observations and nanoindentation tests. Transformation behaviour, microstructure and mechanical properties of the joint are significantly affected by the welding process. The effect of post-welding thermomechanical treatment is studied. After solution treatment followed by ageing, the transformation behaviour of the joint and of the initial material is almost identical. Strength of the joint is analysed by means of tensile and compression tests performed on as welded and heat treated tubes. This study proves that resistance welding is a feasible technique to create complex structures with low density and high reversible elasticity.

Tension, compression, and bending of superelastic shape memory alloy tubes

While many uniaxial tension experiments of shape memory alloys (SMAs) have been published in the literature, relatively few experimental studies address their behavior in compression or bending, despite the prevalence of this latter deformation mode in applications. In this study, superelastic NiTi tubes from a single lot of material were characterized in tension, compression, and pure bending, which allowed us to make direct comparisons between the deformation modes for the first time. Custom built fixtures were used to overcome some long-standing experimental difficulties with performing well-controlled loading and accurate measurements during uniaxial compression (avoiding buckling) and large-rotation bending. In all experiments, the isothermal, global, mechanical responses were measured, and stereo digital image correlation (DIC) was used to measure the evolution of the strain fields on the tube's outer surface.As is characteristic of textured NiTi, our tubes exhibited significant tension–compression asymmetry in their uniaxial responses. Stress-induced transformations in tension exhibited flat force plateaus accompanied by strain localization and propagation. No such localization, however, was observed in compression, and the stress “plateaus” during compression always maintained a positive tangent modulus. While our uniaxial results are similar to the observations of previous researchers, the DIC strain measurements provided details of localized strain behavior with more clarity and allowed more quantitative measurements to be made. Consistent with the tension–compression asymmetry, our bending experiments showed a significant shift of the neutral axis towards the compression side. Furthermore, the tube exhibited strain localization on the tension side, but no localization on the compression side during bending. This is a new observation that has not been explored before. Detailed analysis of the strain distribution across the tube diameter revealed that the traditional assumption of elementary beam theory, that plane sections remain plane, does not hold. Yet when the strain was averaged over a few diameters of axial length, the tensile and compressive responses input into elementary beam theory predicted the global bending response with reasonable accuracy. While it is encouraging that a simple model could predict the moment–curvature response, we recommend that beam theory be used with caution. The averaged strain field can under/over predict local strains by as much as two-fold due to the localized deformation morphology.