Shape Memory Alloy Tubes Research Articles

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.

Deformation mechanism of hot spinning of NiTi shape memory alloy tube based on FEM

As a successively and locally plastic deformation process, ball spinning is applied to manufacturing thin-walled Nickel-Titanium shape memory alloy (NiTi SMA) tube at high temperature. NiTi SMA tube blank belongs to the as-cast state which consists of a lot of dendritic grains and a few equiaxed grains. The compression tests of NiTi SMA were carried out at various strain rates at high temperature in order to obtain the constitutive model of NiTi SMA. Because NiTi SMA is sensitive to the strain rates at high temperature, rigid-viscoplastic finite element method (FEM) is used to simulate ball spinning of thin-walled NiTi SMA tube in order to analyze the deformation behavior of ball spinning of NiTi SMA tube. Stress fields, strain fields as well as velocity fields is obtained by means of rigid-viscoplastic FEM, which lays the profound foundations for studying the metal flow rule in ball spinning and forming perfect spun NiTi SMA tube.

Effect of heat treatment on the two-way recovery stress of tube-shaped NiTi

The effect of annealing temperatures on the thermomechanical behavior of NiTi in terms of transformation temperatures, mechanical properties, two-way shape memory strain, and two-way recovery stress was investigated experimentally to develop a NiTi tubular actuator with high two-way recovery force. Five tubular specimens were used to obtain the two-way shape memory strain and two-way recovery stress by varying heat treatment conditions. Annealing heat-treated temperatures were 400°C, 500°C, 600°C, 700°C, and 800°C. A two-way shape memory effect was introduced in the shape memory alloy tube by thermomechanical treatment after annealing heat treatment. According to the results, the two-way strain induced by shape memory cycling was independent of the annealing temperature. However, the two-way recovery stress generated by the two-way strain depended on the annealing temperature, even though the values of the two-way strain were similar in all specimens. Considering all heat treatment results, we concluded that the optimum annealing temperature to acquire two-way recovery stress and two-way strain for an actuator is 600°C.

On equilibrium domains in superelastic NiTi tubes — helix versus cylinder

Macroscopic cylinder- and helix-shaped deformation domains were observed in NiTi polycrystalline shape memory alloy tubes during the phase transition under uniaxial quasi-static isothermal stretching. Further experiments showed that the occurrence of cylinder or helix domain and its subsequent isothermal evolution strongly depend on the domain volume, tube wall-thickness and loading history. This paper studies the energetics of the cylindrical and helical domains using an elastic inclusion model. It is demonstrated that the total misfit strain energy of an equilibrium domain in tube essentially depends on two nondimensional length scales: the normalized effective domain length and the normalized wall-thickness. Based on such understanding, we quantify the length scale dependence in the energy of the equilibrium cylindrical and helical domains. The energetic preference of each type of domain is predicted and the critical condition for the helix→cylinder domain transition is established.

Fabrication of cylindrical micro actuator by etching of TiNiCu shape memory alloy tube

In this paper, we describe the fabrication of a cylindrical micro actuator from a TiNiCu shape memory alloy (SMA) tube. Ternary TiNiCu SMA is a favorable material for micro actuators with good flexibility and considerable force generation. We proposed a novel fabrication process based on an electrochemical etching and a chemical etching of the TiNiCu SMA tube. The electrochemical etching in LiCl–ethanol electrolyte solution was successfully used for fine patterning of the TiNiCu tube with a high etch factor (etched depth/under-cut). After the fine patterning by using the electrochemical etching, a chemical etching process was used for wall thinning and wall-through etching from the TiNiCu tube inside. The composition of the chemical etchant of mixture of hydrofluoric acid and nitric acid was optimized. Using the etching processes, the TiNiCu SMA cylindrical actuator, in which three meandering-shaped actuators are formed in the tube with an outer diameter of 1 mm and a wall thickness of 30–40 μm, was fabricated successfully. The fabricated SMA meandering actuator could be elongated to 60% without out-of-plane deformation. The single meandering actuator unit could generate a tensile force of 75 mN when it was heated at a current of 90 mA (23 mW).

SMA Actuated Mechanism for an Adaptive Wing

A preliminary study of an adaptive unmanned aerial vehicle (UAV) wing actuated by shape memory alloy (SMA) devices is presented. The wing consists of a sandwich box substructure, flexible ribs, and a flexible laminated skin. The adaptation capability to the changing flight conditions is obtained via airfoil shape adjustments. Torsion SMA tubes are employed for wing camber control, while levers powered by SMA wires are employed for local shape control. A new architecture is proposed: the downward or upward actuation torque is provided by counterrotating concentric tubes connected through a clutch and a positioning piezoelectric motor to the flexible ribs. These actuator tubes are heated one at a time while the other is made free by the clutch in order to obtain any wanted shape without waiting for cooling. The capability of the wing to bear the aerodynamic loads, the power required by the actuators, and their force and torque are assessed by finite-element simulations. An improved version of a recently dev...

Energetics of macroscopic helical domain in different tube geometries and loading

Superelastic NiTi polycrystalline shape memory alloy tubes, when subject to slow quasistatic stretching, transform to a high strain phase by the formation and growth of a macroscopic helix-shaped domain as deformation progresses. This paper performed an experimental study on the effects of the external applied nominal strain and the tube geometry (tube radius R, wall-thickness h and length L) on the helical domains in isothermal stretching of the tubes. The evolution of the macroscopic domains with the applied strain in different tube geometries are quantified by in-situ optical measurement. We demonstrate that the equilibrium shape of the macroscopic helical domain and its evolution are governed by the competition between the domain front energy and the elastic-misfit bending strain energy of the tube system. The former favors a short helical domain, while the latter favors a long slim helical domain. The experimental results provided basic physical and experimental foundations for further modelling and quantification of the macroscopic domain morphology evolution in tube geometries.

Scaling relationship on macroscopic helical domains in NiTi tubes

Macroscopic helix-shaped deformation domains were observed in NiTi polycrystalline shape memory alloy tubes during the stress-induced martensitic phase transition of the material under uniaxial quasi-static isothermal stretching. Further experiments showed that the shape and size of the helical domain not only varied with the external applied nominal strain but also depended on the tube geometry. In this paper, we analyze and quantify the free energy of the tube system in the presence of the helical domain by the procedures of the elementary mechanics method. We prove that the shape of the helix is determined by the competition between the domain front energy and the elastic-misfit bending strain energy of the tube system. The former favors a short helical domain and promotes the domain merging into a cylindrical domain, while the latter favors a long slim helical domain. Based on the principle of minimization of free energy, the equilibrium shape of the helical domain is predicted and its dependence on the material properties, the tube geometry and the applied strain is expressed by a power-law scaling relationship. Clear physical understandings of the experimentally observed helical domain patterns are obtained and the results agree well quantitatively with the available experimental data.

Laser-Machined Shape Memory Alloy Actuators for Active Catheters

Medicine is undergoing a transformation toward minimally invasive surgery, and with it comes an increasing need for more precise miniature instruments to accurately execute complex procedures. In catheter-based surgeries, the physician must control the tip of the catheter from a distal point outside the body, which makes it difficult to achieve precise manipulative control. Providing actuation local to the desired point of manipulation will greatly improve the physician's ability to intervene on diseases in a minimally invasive fashion. We present a new actuator made from laser machining shape memory alloy (SMA) tubes for use in an active steerable catheter. Using finite-element analysis and experimental verification, we have designed a 1.5-mm-long SMA actuator cut from 1.27-mm-diameter NiTi tubing that exhibits good fatigue properties, and can produce forces of 12 N at 20% elongation. We use an iterative rapid design process, so that many actuator geometries can be tested quickly to achieve the desired mechanical properties without complex material models. In this paper, we describe the design and testing of the actuator, and verify its force, elongation, and fatigue properties.

Simulation of cubic to monoclinic-II transformations in a single crystal Cu–Al–Ni tube

Motivated by recent unusual experimental results [Šittner, P., Hashimoto, K., Kato, M., Tokuda, M., 2003. Stress induced martensitic transformations in tension/torsion of CuAlNi single crystal tube. Scripta Materialia 48, 1153–1159] indicating a strong coupling between axial and torsional deformations in single crystal monoclinic-II shape memory alloy tubes, various computational aspects of numerically modelling the observed behavior in monoclinic-II materials are discussed. As a necessary first step, the shape strain computations in monoclinic-II systems are examined. It is found that ambiguities regarding the lattice parameters and proper unit cell in the existing literature have implications for the macroscopic constitutive theories of such alloys. This is demonstrated in the context of a detailed numerical investigation of the experimental problem of interest, wherein it is found that numerical simulations demonstrate proper quantitative response, a strong axial → torsional coupling given a biasing initial condition, and the observed lack of torsional → axial coupling. Implications of the findings are discussed with regard to both the further development of numerical models and future avenues for experimental research.

208 TiNi形状記憶合金の回復応力に関する実験的研究(OS9(1) 形状記憶合金の特性と応用)

The shape memory alloys have been expected from a view point of engineering applications because of its unique mechanical properties. When the shape memory alloy which was deformed at low temperature is heated in the condition with the restriction of strain, the recovery stress occurs by a reverse transformation. In this study, the recovery stress after the torsional loading condition was experimentally investigated in TiNi shape memory alloy tube manufactured by the sintering synthetic method. The obtained results are discussed in this paper.

Stress-induced phase transformation and detwinning in NiTi polycrystalline shape memory alloy tubes

Deformation behavior associated with initial austenite (A), rhombohedral (R) and martensite (M) phase structures was studied in polycrystalline NiTi shape memory alloy tubes by tensile testing at different temperatures. The nominal stress–strain curves of the tubes from room temperature (23 °C) to 70 °C were recorded. The deformation of NiTi tubes with initial structure of R-phase proceeded via R → M martensitic transformation, while the deformation of NiTi tubes with initial structure of M-phase proceeded via martensitic detwinning. It was found that the R → M martensitic type transformation was realized, at the macroscopic level, by nucleation and growth of an inclined cylindrical band, while the detwinning process of the tube was macroscopically homogeneous. Further, two-stage yielding, which is associated with austenite to rhombohedral (A → R) and R → M phase transformations, was observed in the stress–strain curves of NiTi tubes in a certain testing temperature range. With a further increase in temperature, the shape of the nucleated band remained cylindrical until 60 °C (> A f) when the shape of the initial band suddenly became helical which was well observed in the superelastic microtubing.

Pseudoelastic SMA Spring Elements for Passive Vibration Isolation: Part II – Simulations and Experimental Correlations

In Part II of this two-part study, system simulations and experimental correlations of a Shape Memory Alloy (SMA) based vibration isolation device (briefly described in Part I) has been presented. This device consists of layers of preconstrained SMA tubes undergoing pseudoelastic transformations under transverse dynamical loading. In Part II, detailed description of the prototype vibration isolation device, its experimental setup, and actual experimental test results are presented. An extensive parametric study has been conducted on a nonlinear hysteretic dynamical system, representing this vibration isolation device utilizing a physically based simplified SMA model and a Preisach model (an empirical model based on system identification) developed in Part I. Both the physically based simplified SMA model and the modified Preisach model have been utilized to perform experimental correlations with the results obtained from actual testing of the device. Based on the investigations, it has been shown that variable damping and tunable isolation response are major benefits of SMA pseudoelasticity. Correlation of numerical simulations and experimental results has shown that large amplitude displacements causing phase transformations of SMA components present in such a device are necessary for effective reduction in the transmissibility of such dynamical systems. It has also been shown that SMA-based devices can overcome performance trade-offs inherent in typical softening spring-damper vibration isolation systems. In terms of numerically predicting the experimental results, it has been shown that the Preisach model gave relatively accurate results due to better modeling of the actual SMA tube behavior. However, for a generic parametric study, the physically based simplified SMA model has been found to be more useful as it is motivated from the constitutive response of SMAs and hence, could easily incorporate different changes in system conditions.

Smart Material-Actuated Rotor Technology – SMART

Vibration, noise, and aerodynamic design compromises are primary barriers to further improvements in effectiveness of the helicopter. The MD900 light utility helicopter main rotor system is modified to include in-blade smart material actuation for active control. A piezoelectric (PE)-driven trailing edge flap is used for vibration, noise, and aerodynamic performance improvements. A shape memory alloy (SMA)-driven trailing edge trim tab is used for in-flight blade tracking. Sizing and conceptual design of an active flap and trim tab system for the MD900 helicopter were completed. Several two-dimensional airfoil and flap/tab models were wind tunnel tested and an aerodynamic database was established. Structural samples of the flap actuator mounts, flap section, tab, and full-span flap were fabricated and successfully tested. Several prototype actuators were developed and extensively tested to establish their performance and robustness in the dynamic operating environment. The flap actuator uses two biaxial PE stack columns, operating in a push pull mode, a column mid-support, and a stroke amplification mechanism. The tab actuator uses two biaxial SMA tubes for actuation/bias, an SMA-activated lock for power-off operation, and integrated microprocessor control electronics. Results to date confirm that smart material in-blade active control of a rotor is feasible and offers significant performance and cost benefits. Projected payoffs from reduced vibrations and noise as well as in-flight tracking include improved component lives, reduced maintenance and improved crew, passenger, and community acceptance.

Development of an SMA Actuator for In-flight Rotor Blade Tracking

A new and improved solution to the helicopter blade-tracking problem has been investigated. A discrete, blade-installed actuator utilizing shape memory alloy (SMA) torsion tubes has been developed to enable in-flight rotor tracking. SMA torsion tubes from three materials were tested and one was selected for its cyclic stability. Improved tube fabrication, heating, end attachment methods, and locking schemes were developed. The final prototype actuator employed two biaxial SMA tubes for actuation/bias, an SMA-activated lock for power off operation, and integrated microprocessor control electronics. Tests under static and dynamic loading showed that actuation requirements, except for bandwidth, were met. Increased bandwidth may be obtained by improved control algorithms or cooling. The developed SMA actuator enables an in-flight, real-time adjustment of a blade-tracking tab. Projected payoffs are reduced maintenance cost as well as reduced helicopter vibrations and improved performance.

Plane stress analysis of a shape memory annular plate subject to edge pressure

We present a boundary value problem for the equilibrium stress-strain fields in an annular plate composed of a shape memory material. The plate is subject to uniform inner and outer pressures. At a constant temperature above A r , the plate is everywhere in a state of Austenite without loads. An isothermal plane stress analysis is performed using a constitutive model that incorporates a single internal variable for the Martensite phase fraction. This phase fraction evolves with the effective stress as determined by a simple phase transformation kinetic. An issue is the partitioning of the plate into annular regions of pure Austenite, pure Martensite, or Austenite/Martensite mixture. Although closed form solutions are available in certain special cases, the general case requires a numerical analysis of a pair of governing ordinary differential equations. Analysis of these equations provides the pressure values that trigger an Austenite-to-Martensite transformation. This transformation initiates at the inner boundary, giving rise to an initiation front that moves toward the outer boundary as the loads are increased. Even higher loads trigger a similar completion front that also moves from the inner boundary to the outer boundary under increasing load. The phase partitioning in the plate as a function of the applied pressures is then concisely described in terms of structure maps that follow from numerical analysis of the governing equations. Such information would generally be useful for systems involving device control by means of shape memory alloy tubes.

Modeling the dynamic behavior of shape memory alloys

The paper studies the single degree of freedom vibration of a rigid mass suspended by a thin-walled shape memory alloy tube under torsional loading. The behavior is analyzed for the cases of quasiplasticity (low temperatures) and pseudoelasticity (high temperatures) on the basis of an improved version of the Müller–Achenbach model. To illustrate the strong hysteresis-induced damping capacity and the non-linear vibration characteristics, both, free and forced vibrations are considered in the first part of the paper. This is done on the basis of an isothermal version of the model, while the second part of the paper focuses on the effect of non-constant temperature caused by the rate-dependent release and absorption of latent heats.