Shape Memory Wires Research Articles

Effect of Environmentally Friendly Oil on Ni-Ti Stent Wire Using Ultraprecision Magnetic Abrasive Finishing

Nickel-titanium (Ni-Ti) has been widely used to make shape-memory actuator wire for numerous medical industrial applications, with the result that it frequently comes into contact with the human body. High-quality and nontoxic surfaces of this material are therefore in high demand. We used a rotating magnetic field for an ultraprecision finishing of Ni-Ti stent wire biomaterials and evaluated the finishing technique’s efficacy with different processing oils. To create nontoxic Ni-Ti stent wire, the industrial processing oils that are generally used in the surface improvement process were exchanged for oils with low environmental impacts, and processed under rotating magnetic fields at different speeds and processing times. The processing performance of the different oils was compared and verified. The results show that ultraprecision magnetic abrasive finishing that uses olive and castor oil improves surface roughness by 66.67%, and 45.83%, respectively. SEM and energy-dispersive X-ray spectroscopy (EDX) analyses of the finished components (before and after processing) showed that the material composition of the Ni-Ti stent wire was not changed. Additionally, the magnetic abrasive tool composition was not found on the surface of the finished Ni-Ti stent wire. In conclusion, the environmentally friendly oil effectively improved the diameter of the Ni-Ti stent wire, demonstrating the utility of olive and castor oil in ultraprecision finishing of Ni-Ti stent wire biomaterials.

Effect of hydrogen embrittlement towards thermal and mechanical behavior of NiTi shape memory alloy

NiTi arch wires are susceptible to hydrogen embrittlement upon contact with dental brackets in oral cavity during orthodontic treatment. This study investigated the effect of hydrogen absorption and diffusion over time towards the thermal and mechanical properties of NiTi shape memory wire. The hydrogen absorption process was carried out via electrolytic charging at constant current density for 16 hours in 1.0 weight percent (wt.%) sodium sulphate solution. The hydrogen charged wires were aged at room temperature in air for different durations to allow further inward diffusion of the hydrogen into the specimens. The results show that after hydrogen charging, the latent heat of forward and reverse martensitic phase transformation of the NiTi wire changed from 19.96 to 11.98 J/g, and 19.21 to 13.42 J/g, respectively. Further suppression and disappearance of thermal transformation peaks were observed as the charged specimen aged for 7 days. The transformation stress level on tensile deformation increased by almost 90 MPa after hydrogen charging, and exhibited non-flat stress plateau after further aging.

Novel flexible endoscope concept with swiveling camera tip

Abstract Endoscopy is an important modality in medical imaging. Thin flexible endoscopes are for example used to examine the upper airways, for gastroscopy procedures or lung inspection. With standard flexible endoscopes one can change the direction of view by bending the tip with the disadvantage of large space required due to the bending radius. With first experiences of a concept of a moveable camera head on the tip of rigid endoscopes, we now propose a novel design employing a swiveling camera for flexible endoscopes. This concept is based on the use of a shape memory wire used to control the movement of the camera, a flexible plastic flap joint for tight rotation and flexible printed circuits for the electronic connection. The prototype was realized in a first low cost setup using a 5,5 mm HD chip on the tip camera with LED light. The motion and imaging performance of the prototype allowed swiveling of the camera on the endoscope tip from straight view to 100° side view. The space needed in fully rotation was limited to 9mm with an overall diameter of the endoscope in straight view of only 5,6mm, but could even be further reduced in a more professional setup. The image quality is good, but close-up views appear blurry due to the fixed focus point of the low-cost camera. The presented steering concept of the camera is promising, as it could potentially improve imaging of narrow cavities using flexible endoscopes. Especially for “in office” examinations this principle could add value to diagnosis and patient comfort.

FeNiCoAlTaB superelastic and shape-memory wires with oligocrystalline grain structure

FeNiCoAlTaB shape-memory alloys are desirable alternatives to other smart metallic materials owing to their low base metal and processing costs. These alloys, however, require strong texture via thermomechanical treatments or directional solidification to obtain good functional properties. Here, we demonstrate the fabrication of FeNiCoAlTaB shape-memory wires by a rapid, continuous wire-casting process. These untextured wires exhibit 7% superelastic strains and 1.2 GPa tensile strength after training, and show good cyclability and low stress hysteresis at room temperature. These properties far surpass previous reports in untextured polycrystalline ferrous alloys, an improvement which we attribute to their unique oligocrystalline grain structure.

Finite-element modelling of NiTi shape-memory wires for morphing aerofoils

AbstractThis paper presents the development and implementation of a user-defined material (UMAT) model for NiTi Shape-Memory Alloy (SMA) wires for use in LS-DYNA commercial explicit finite-element analysis software. The UMAT focusses on the Shape-Memory Effect (SME), which could be used for actuation of aerostructural components. The actuation of a fundamental structure consisting of an SMA wire connected in series with a linear spring was studied first. The SMA thermomechanical behaviour obtained from the finite-element simulation was compared with that obtained from the analytical solution in MATLAB. A further comparison is presented for an SMA-actuated cantilever beam, showing excellent agreement in terms of the SMA stress and strain as well as the tip deflection of the cantilever beam. A mesh sensitivity study on the SMA wire indicated that one beam element was adequate to accurately predict the SMA thermomechanical behaviour. An analysis of several key parameters showed that, to achieve a high recovery strain, the stiffness of the actuated structure should be minimised while the cross-sectional area of the SMA wire should be maximised. The actuation of an SMA wire under a constant stress/load was also analysed. The SMA material model was finally applied to the design of morphing aluminium and composite aerofoils consisting of corrugated sections, resulting in the prediction of reasonably large trailing-edge deflections (7.8–65.9 mm).

Flexural properties of sandwich composite panels with glass laminate aluminum reinforced epoxy facesheets strengthened by SMA wires

Sandwich composite panels are widely used and significant in structural applications such as aerospace, shipbuilding and transportation, etc. This is due to their specific properties such as specific stiffness, strength and energy absorption. Still, many innovations are required to develop and upgrade their mechanical properties in various loadings and conditions, specifically in bending loads. One of the methods to enhance the properties of sandwich structures is to employ various advanced materials in these structures to change their acquired properties. In the present research work, sandwich composite panels made by fiber metal laminate like glass laminate aluminum reinforced epoxy (GLARE) as the facesheets and PVC polymer foam as the core material are investigated in flexural (bending) loading condition. To change or enhance the behaviour of sandwich panel in bending loads, shape memory alloy wires are also embedded in between glass fiber reinforced epoxy composite layers in fiber metal laminate facesheets. The shape memory wires are also pre-strained in fiber reinforced epoxy composite in sandwich panels. To study the flexural properties of sandwich panels with fiber metal laminate facesheets, the effect of shape memory alloy wires and also the effect of pre-straining of the wires, three types of sandwich panels are considered and made including panels without shape memory alloy wire, two wires with 0% tensile pre-strain, and two wires with 5% tensile pre-strain for the same cross section. Due to the importance of bending properties in structural applications, the sandwich composite specimens are subjected to flexural test according to ASTM standards. The maximum of 13% increase in maximum bending load and 84% increase in breaking load for the specimens with 0% pre-strained wires are achieved. Also, the maximum displacement and the energy absorption for the specimen with 5% pre-strain was enhanced by 26.5% and 37%, respectively. The energy absorption during the flexural test is greater in case of the specimen with pre-strained wires. Moreover, the specimens with pre-strained wires show better integrity of the structure after the failure in bending. The results represent the advantage effect of shape memory alloy wires on sandwich composite panel's behaviour in bending.

Investigation on the functionality of laser-welded NiTi to NiTiCu shape memory wires

NiTi shape memory wires are widely used in several markets, mainly for medical and aerospace applications, due to their excellent properties including superelasticity and shape memory effect. The combination of NiTi alloys with other alloys can provide high flexibility in the design of various smart structures. Nevertheless, the joining of NiTi alloys is really challenging because of the effect of thermal history on the functionality of welded joints. In fact, the temperature can change the microstructure and the balance of the material composition in the welding area, and, consequently, it leads to some alterations in the transformation temperature of the welded zone. The present study reports the effect of laser parameters, namely, laser power and scan speed, on the functionality of laser-welded NiTi to NiTiCu wires. The experimental findings allowed us to identify the best operational windows of the laser parameters, stating welding by high-power diode laser as an extremely promising technique in joining of dissimilar NiTi wires.

Influence of Heat Treatment Parameters on the Functional Behavior and Corrosion Performance of a Shape Memory Wire Actuator

In the present study, the effect of heat treatment parameters on the functional response, corrosion behavior and microstructural evolution of NiTi shape memory alloys were investigated. Various heat treatment regimes were utilized to study the impact of temperature and duration on the actuation behavior of wire samples under bending. Results clarified that austenite transformation temperatures As and Af increased at higher treatment temperatures. Cyclic response in the range of 0 to 15 degrees indicated that the actuation force exhibits an inverse relation with the treatment temperature. Higher treatment durations below 500°C elevated both the hardness and the sustained load. Bending force levels above 1500gf were achieved after a 90 min treatment at 400°C, whereas that over 500°C brought about a noticeable drop in strength. Investigations on the corrosion behavior of NiTi alloy was utilized in the simulated body fluid revealing that the sample heat treated at 400°C for 90 min showed the highest corrosion resistance.

Ultrahigh damping capacity achieved by modulating R phase in Ti49.2Ni50.8 shape memory alloy wires

An ultrahigh damping capacity with internal friction close to 0.35 was detected over a large temperature range of 75 °C (from -40 to 35 °C) in Ti49.2Ni50.8 shape memory wires by adjusting the B2→R phase transformation through a low-temperature aging treatment. Microstructural analysis showed that the excellent damping performance is attributed to high-density nanodomains in the R phase, which are induced by short-range Ni segregation. These results are expected to further expand capacity for designing damping microstructure.

Investigation of minimal strokes of NiTi shape memory wires using laser activation

Nickel-Titanium (NiTi) shape memory wires have established themselves in the actuator market (medicine, automotive industry, aerospace) because of their high power density and their simple activation by resistance heating. The actuator stroke, normally between 1 mm and 5 mm, can be adapted with control parameters (current, voltage, and duration). However, minimal strokes (< 100 µm) can only be achieved with a certain repeat accuracy since the spatially resolved resistance depends on material defects and also the fatigue state. In this study, actuator behavior when targeting for minimal strokes using laser activation is investigated.

Development of form-fit connection for NiTi shape memory wire actuators using laser processing

Shape memory alloy actuators (SMAA) are attractive for lightweight construction applications such as in the fields of e-mobility and aerospace due to their properties (high actuator performance, low weight, silent operation). Used force-fit and firmly bonded connections (crimping, casting) can only be released again non-destructively to a limited extent. Combined with the complex material behavior of NiTi-SMAA, the development of standardized SMAA is limited. In this paper, a novel approach based on a form-fit connection is presented.

Poisson Induced Bending Actuator for Soft Robotic Systems.

This article deals with a novel active bending soft body that uses metamaterials and combines soft behavior, integrated actuation, low complexity, and a high density of producible forces and moments. The presented concept consists of a tube-like structure with tailored, unconventional material properties, which enable the generation of a bending deformation and/or moment when circumferential stress and/or strain is induced. Circumferential actuation can be generated by a difference in pressure between the internal and external surface of the tube or, alternatively, by distributed expansion actuators that act radially or tangentially (e.g., shape memory wires). In addition to an analytical model, this article also presents a design procedure and deals with the implementation of the proposed concept in a functional prototype and its experimental characterization.

DSC and microstructure analysis of high temperature Ni-Ti-Hf, low hysteresis Ni-Ti-Cu and conventional super-elastic and shape memory Ni-Ti alloy ingots and wires

The progression of martensitic transformations in calorimetric experiments on NiTi-based shape memory alloys (SMA) depends significantly on the material composition, processing history and the thermo-mechanical treatment which is performed to prepare the material for use in application. The phase transformations in Ni-Ti alloys are first order transitions, i.e. the microstructure symmetry changes occur spontaneously and superheating and supercooling effects emerge. Thus, the thermal scanning rate in differential scanning calorimetry (DSC) affects the observation of important material parameters for the characterization of shape memory alloys like onset temperatures, hysteresis width and change in enthalpy of phase transformations. This work depicts how the transformation characteristics as observed by DSC measurements vary significantly by altering the alloy composition and processing conditions of the material. An in-depth calorimetric study on a wide range of NiTi-based alloys which were prepared as heat treated ingot and processed wire was performed. In order to evaluate also the rate effect on the transformation of distinct alloys, DSC rates from 1 to 30 °C/min were tested, highlighting the interplay between thermal scanning velocity and material parameters. Manifold results are presented on how chemistry, microstructure, DSC sample geometry and processing stresses in the materials contribute to award the alloys diverging sensitivity to the thermal scanning rate. Deviations in binary Ni-Ti composition alter the sensitivity to heating and cooling rate, e.g. martensitic transformation in annealed super-elastic (SE) wire is stronger affected by the rate than shape memory wire due to differences in the precipitation behavior. Ni-Ti-Hf high temperature shape memory alloys show a rather small enlargement of hysteresis, but a huge shift in transformation temperatures, while Cu addition to Ni-Ti causes very sensitive hysteresis changes up on modifying thermal rates which is assigned to B19 orthorhombic transformation. Transformation parameters of wires are less affected by DSC rates than ingots due to their high surface to volume ratio, while sample weight and surface finish is observed to have a minor impact. Small grains and residual strains owing to processing history of drawn wire tend to stabilize the transformation. In materials which exhibit R-phase transformation, the low heat capacity of this phase could be responsible for its minor sensitivity to variations in DSC rate.

Welding strength of dissimilar laser-welded NiTi and NiTiCu shape memory wires

NiTi shape memory alloys are widely used in various medical and aerospace products because of their outstanding preparties, namely superelasticity and shape memory effect. Joining of dissimilar NiTi shape memory wires can be applied in the design of smart components made up of different functionalities or properties. However, preserving the shape memory effect of the welded regions is really challenging due to the influence of laser heat on the microstructure and consequently activation temperature of the welded samples. This paper investigates the mechanical properties of the welded NiTi to NiTiCu wires using tensile test and microhardness analysis. In fact, the experimental results allow to identify the optimum operational parameters for joining of dissimilar NiTi shape memory wires.

Morphodynamic Tissues via Integrated Programmable Shape Memory Actuators

AbstractManipulating the shape of preformed living tissues can present a novel fabrication route toward complex biological architectures. However, external manipulation of tissues can be challenging to implement robustly at multiple length scales and with high degrees of freedom, particularly in soft fibrous tissue constructs. Here, a versatile platform is developed to drive soft tissue morphodynamics using embeddable shape memory actuators that generate multiscale, repetitive, and highly customized tissue deformation on demand. To achieve this, a thermally isolating coating technique is designed and developed for programmable shape memory wires, which protects surrounding biological materials from cytotoxic heating effects during wire actuation. The coated tissue actuators (CTAs) can then be embedded in engineered tissues and activated to produce both large‐ and small‐scale tissue deformations in a highly customized and reproducible manner. Using this strategy, tissues can be forced to adopt specified shapes, with precise control over cell elongation and orientation within an encapsulating matrix. Furthermore, the system can produce predictable, highly localized, and customizable strains within fibrous matrices, capable of elongating cells and biasing their orientation within degrees of a desired direction. This strategy may hence have broad applicability in both applied tissue biofabrication and for fundamental studies of cell–matrix interactions.

Shape Memory Activated Self-Deployable Solar Sails: Small-Scale Prototypes Manufacturing and Planarity Analysis by 3D Laser Scanner

Solar sails are propellantless systems where the propulsive force is given by the momentum exchange of reflecting photons. Thanks to the use of shape memory alloys for the self-actuation of the system, complexity of the structure itself has decreased and so has the weight of the whole structure. Four self-deploying systems based on the NiTi shape memory wires have been designed and manufactured in different configurations (wires disposal and folding number). The deployed solar sails surfaces have been acquired by a Nextengine 3D Laser Scanner based on the Multistripe Triangulation. 3D maps have been pre-processed through Geomagic Studio and then elaborated in the Wolfram Mathematica environment. The planarity degree has been evaluated as level curves from the regression plane highlighting marked differences between the four configurations and locating the vertices as the most critical zones. These results are useful in the optimization of the best folding solution both in the weight/surface reduction and in the planarity degree of the solar sail.

Semi-exact solution for nonlinear dynamic analysis of graded carbon nanotube-reinforced beam with graded shape memory wires

In this study, the nonlinear dynamical response of a graded carbon nanotube-reinforced beam with graded shape memory wire has been investigated regarding the instantaneous variations of martensite volume fraction and therefore material nonlinearity along the beam. The boundary conditions of the beam are considered as the clamped-clamped. Carbon nanotubes and shape memory wires have been arranged in various distributions of UD, and FG – A along the thickness of the beam. The governing differential equation of the beam is obtained by the use of Brinson’s relation to describe the behavior of the shape memory wire, considering Von Karman nonlinear strain and utilizing Hamilton's principle. The obtained nonlinear equation is solved using the semi-exact solution. For this purpose, Galerkin’s method was first used to isolate the spatial derivatives from the time derivatives and then the obtained equation was solved using Runge-Kutta method. A comparative study has been conducted between the present study and the reference paper to confirm the presented model. This comparison study shows an appropriate agreement between the results of this research and the previous studies. In this research, the influence of the certain parameters such as the considering the nonlinear effects, the volume fraction of carbon nanotube, the type of carbon nanotube distribution, the volume fraction of shape memory wire, and the type of its arrangement on the vibrational behavior of the beam and its loss factor is investigated. Some results show that, in case of the vibration of the beam by the initial excitement, the higher density of carbon nanotubes in the mid-layers and the higher density of the shape memory wires in the outer layers of the beam lead to the higher amount of loss factor. Also, the percentage of the reduction in the loss factor which occurs due to the reduction of the volume fraction of the shape memory wire is almost the same in the beams with different distribution and arrangement of carbon nanotubes and shape memory wire.

Bistable morphing panels through SMA actuation

This work introduces a numerical solution scheme for the prediction of the morphing mechanism in smart composite structures. Morphing relies on bistable plates actuated through nitinol shape memory wires. Bistable panels are obtained by exploiting the anisotropic thermal expansion of anti-symmetric composite laminates, which directly influences the post-cured shape. The numerical model accounts for the thermal distortions introduced upon cooling of the laminate as a result of the production process, and for the shape memory behavior of nitinol wires. The stress-strain behavior of the nitinol wire has been evaluated experimentally and implemented in the numerical model. A bistable composite plate has been manufactured for the purpose and results about its post-cured curvature have been utilized to tune the numerical model. The verified numerical scheme has been finally utilized to investigate the possibility to morph the panels by exploiting the actuation capabilities of Nitinol wires through thermal activation. Particular interest has been devoted to the analysis of prestrain, dimension, position, and the number of wires to cyclically actuate the panel. The numerical model can serve as tool to highlight the role of plate dimension, material properties, lamination sequence, and thermal cycle on the post-cured stable configurations, and for the identification of number and location of wires to best perform the morphing cycle.

Dynamical response identification of a class of nonlinear hysteretic systems

The experimental dynamical response of three types of nonlinear hysteretic systems is identified employing phenomenological models togheter with the Differential Evolutionary algorithm. The mass–spring–damper system is characterized by hysteretic restoring forces provided by assemblies of shape memory and steel wire ropes subject to flexure or coupled states of tension and flexure. The energy dissipation due to phase transformations and inter-wire friction and the stretching-induced geometric nonlinearities give rise to different shapes of hysteresis cycles. The mechanical device subject to strong seismic excitations is investigated in its ultimate limit state whereby inelastic strains are induced in the steel wires together with a global nonsymmetric response of the system. The targeted dynamical characterization of the hysteretic oscillator up to its ultimate limit state has a special meaning when the device is employed in the field of vibration control. The dynamical response is identified exploiting the measurements of the oscillating mass relative displacement and inertia force that must be balanced, at each time instant, by the overall restoring forces provided by the mechanism. The restoring force is assumed to be the sum of different contributions such as a cubic nonsymmetric elastic force and a nonsymmetric hysteretic force modeled according to a modified version of the Bouc–Wen model. The parameters are identified minimizing the difference between the numerical and the experimental restoring force histories. High levels of accuracy are achieved in the identification with mean square errors lower than 2%.

Modeling and simulation of a compact push–pull rubber actuator energized by an outer coil of shape memory wire

ABSTRACTThis paper introduces the concept and develops the theory for a push–pull actuator made by winding a thin shape memory wire on a solid rubber cylinder. The incompressibility of the rubber converts the thermomechanical contraction of the heated wire in twice as much axial strain available in the rubber core. The intrinsic elastic backup provided by the core allows push–pull action of the device. Based on the assumption of both material and geometric linearity, a simple theoretical model is developed, which culminates in a simple closed-form equation for the output stroke of the actuator. The theoretical predictions closely agree with refined finite element simulations, anticipating a net stroke of about 5–6% of the overall actuator length, depending on the outer load applied. The working principle of the actuator and the accuracy of the model are validated by tests on a proof-of-concept prototype.