Thin NiTi Research Articles

Experimental Investigation of the Impact of Loading Conditions on the Change in Thin NiTi Wire Resistance during Cyclic Stretching.

This paper presents the results of an experimental study designed to evaluate the effect of repeated stretching cycles on the electrical resistance change in a NiTi alloy wire. In particular, tests were carried out to determine the effect of the type of loading on resistance change in the investigated wires. Wires with a diameter of 100 μm were used in the research. The experiment was carried out on a dedicated test stand designed for this purpose. During the test, the samples were subjected to 40 identical tensile cycles. The electrical resistance, sample elongation, and tensile force during successive stretching cycles were measured. The conducted research demonstrated the impact of elongation and reorientation of the structure on the resistance change in NiTi alloy thin wires. The research included a comparison of the effect of two different types of loading on the electrical resistance change in the sample. During cyclic stretching of a NiTi alloy sample with constant displacement, a decrease in electrical resistance was observed after each successive stretching cycle. Alternatively, when stretching with a constant force, the value of electrical resistance increased. In both types of loads, the greatest change in resistance value was observed at the initial cycles.

Effect of thermal treatment and strain rate on superelastic and mechanical properties of NiTi sheet

In this work, the influence of the heat treatment and the strain rate on the superelastic properties of the NiTi alloy was studied in the tensile testing. Thin NiTi sheets with a thickness of 2.0 mm and a content of 56.31 Ni and 43.69 Ti (in wt. %) were tested in different heat-treated conditions. The flat specimens were tensile tested at two different strain rates at the state as-received, after aging at 450 °C and after solution treatment at 850 °C followed by aging at 450 °C. The microstructure and phase analyses of the NiTi specimens in three heat-treated conditions were examined using optical and scanning electronic microscopies and were related to the superelastic behavior.

Light-force orthodontic treatment using the minitube appliance and thin NiTi wire in children and adolescent: Case reports

The mini-tube appliance (MTA) is comfortable and advantageous in terms of aesthetics and oral hygiene compared to conventional brackets. By using 0.012 inch Thermoactive (TA) NiTi wires, the frictional resistance is low. So treatment are possible within a short period and there are fewer side effects, such as root resorption. Light-force orthodontic treatment using MTA and thin TA NiTi wire is a suitable treatment method in pediatric dentistry, considering the increasing aesthetic needs and the nature of pediatric patients. By using the MTA and thin TA NiTi wire, the cases reported here have led to positive treatment outcomes in minor orthodontic treatment for the anterior teeth crowding and retreatment following relapse after orthodontic treatment.

Determination of Thin NiTi Wires' Mechanical Properties during Phase Transformations.

The modern industrial and consumer applications in accordance with the concepts of Industry 4.0 and the Internet of Things are characterized by autonomy and self-sufficiency. This has led to an increase in the interest for the so-called smart materials, capable of combining the functionalities of sensors, actuators and, in some applications, control systems. An important group of smart materials are shape-memory alloys, among which nickel-titanium (NiTi) alloys are the most known. In this article, the influence of phase transformation on the mechanical properties of thin NiTi alloy wires was investigated. During the test, the influence of the heating currents on the displacement and the force generated by the thin NiTi wires were analyzed. The temperature of the wires during heating was measured by a thermographic camera. This study proved the maximum value of the wires' displacement was related to the value of the heating current. During the research, the dependence of the transformation dynamics on the value of the heating currents was also proved. In addition, the influence of the surface inhomogeneity of the thin NiTi alloy wires on the accuracy of the thermographic measurements was analyzed. For the experimental research described in this article, we used the NiTi alloy whose trade name is Flexinol, produced by DYNALLOY (Inc. 2801 McGaw Ave. Irvine, CA, USA).

Dynamical structural phase transition model of the electrical behavior of NiTi

A general phenomenological model of the kinetics of thermally induced structural phase transitions in multi-phase alloys is introduced for arbitrary numbers of lattice phases and transitions. The model is based on a system of ODEs yielding the temporal evolution of lattice phase fractions caused by temperature variation described by a heat balance equation. The kinetics of each transition are modeled by temperature rates of phase fractions rather than sigmoids, where an extra multiplicative parameter describing the shape of the transition rate curves is introduced. The model is applied to the electrical behavior of thin NiTi filaments by relating its resistivity to the relative proportions of three main structural phases, namely Martensite, Austenite and an intermediate phase, known as R-phase. The model yields resistivity time-series for successive heating/cooling cycles. Computer simulations are compared to previously published resistivity measurements on filaments self-heated by time-varying currents of various frequencies and passively heated samples.

From the elastocaloric effect towards an efficient thermodynamic cycle

In recent years, elastocaloric cooling technology has been considered as one of the most promising alternatives to vapor compression technology. Given that elastocaloric technology is only in the early stages of development, a uniform method for evaluating the elastocaloric effect has not yet been established, and the thermodynamics of different elastocaloric cooling cycles have not yet been studied in detail. Therefore, the main goal of this work is to investigate these two important areas. Here, multiple thermodynamic cycles were studied, focusing on the parameters of the holding period of the cycle, which is essential for heat transfer between the elastocaloric material and the heat sink/source. The cycles were applied to commercially available superelastic thin-walled NiTi tubes under compressive loading and a thin NiTi wire under tensile loading. Isostress cycles with constant stress throughout the holding period, isostrain cycles with constant strain throughout the holding period and no-hold cycles (without a holding period) were studied across multiple stress/strain ranges. Based on the experimental results, a previously developed phenomenological model was applied to better understand and further evaluate the different cycles. The results revealed that the applied thermodynamic cycle significantly affects the thermomechanical response and thus the cooling/heating efficiency of the elastocaloric material. We show that by using isostress cycles and partial transformations, a Carnot-like thermodynamic cycle with improved heating/cooling efficiency can be generated. By applying the isostress cycles, an adiabatic temperature change of 30.2 K was measured, which is among the largest directly measured reproducible adiabatic temperature changes reported for any caloric material to date. Ultimately, this study intends to serve as a basis for establishing a uniform method for evaluating the elastocaloric effect in different materials that would allow for reliable and accurate one-to-one comparison of the reported results in the rapidly growing field of elastocalorics.

Experimental and numerical investigation of thermomechanical cycling of notched NiTi shape memory ribbon using SMA model accounting for plastic deformation

Shape memory alloys (SMAs) are being increasingly applied as thermally driven actuators. The commercially available SMA elements, however, frequently contain stress risers, either due to internal or surface defects or due to the required component shape. Stress risers represent a potential danger for the preliminary fatigue failure of such actuators but its actual mechanism is not very clear. This paper presents a combined experimental (2D DIC analysis of surface strains) and numerical analysis (SMA model with plastic deformation) of the stress, strain and phase fraction fields evolving in a thin NiTi shape memory ribbon with an artificial notch subjected to cyclic cooling-heating through transformation range under constant external force. It appeared that, even if only very low tensile stress is externally applied upon thermal cycling, local tensile stress at the notch tip sharply increases during the first cooling due to the forward martensitic transformation (MT) proceeding heterogeneously in space. This heterogeneity gives rise to plastic deformation at the notch-tip, which gradually accumulates upon thermal cycling and shields the notch tip from tensile overloading. Hence, fatigue performance of thermal NiTi actuator with stress riser depends very much on the plastic deformability of the alloy.

3D spatial reconstruction of macroscopic austenite–martensite transition zones in NiTi wires induced by tension and twisting using diffraction/scattering computed tomography

Macroscopic austenite–martensite transition zones in thin superelastic NiTi wires subjected to stretching, twisting and their combination were reconstructed via diffraction/scattering computed tomography. The method allows to visualize spatial distributions of both phases from a series of X-ray diffraction scans. A cone-shaped, radially symmetric transformation front was observed in stretched wire both with and without applied pre-twist. Twisting of a pre-stretched wire (with a cone-shaped transformation band front already developed) led to the formation of a multi-prong localization morphology recognizable on the surface. Transition zones in these cases are highly heterogeneous complex 3D objects exhibiting distinct phase gradients; the longitudinal size of the zones is comparable with the wire diameter. In contrast, localized martensitic strips parallel with wire axis and with a wedge-like cross-section appeared on the surface of the wire which was twisted only. We discuss the observed localization patterns with respect to current computational simulations and micromechanical models.

Strength of Superelastic NiTi Velcro-Like Fasteners

Velcro hook-and-loop fasteners invented more than 70 years ago are examples of the mechanism inspired by the tiny hooks found on the surface of burs of a plant commonly known as burdock. Several years ago, a novel Velcro-like fastener made of two arrays of hook-shaped thin NiTi wires was developed. Unique features of such fasteners, such as high thermally-tunable strength, fair force–stroke reproducibility, forceless contact or silent release, all derive from the superelasticity of the NiTi micro-wires. Recently, it was noticed that the presented fastener design allowed for a prediction of the number of active hooks. In this continuing study, the tension strength of the fastener was simulated as a function of hook density. Based on statistics, the model showed non-linear dependency of the number of interlocked hooks, N, on the hook density, m (N = round (0.21 m + 0.0035 m2 − 6.6)), for the simple hook pairs and the given hook geometry. The dependence of detachment force on stroke was simulated based on the Gaussian distribution of unhooking of individual hook connections along the stroke. The strength of the studied NiTi hook fasteners depended on hook density approximately linearly. The highest strength per cm2 reached at room temperature was 10.5 Ncm−2 for a density of m = 240 hooks/cm2.

Numerical analysis of NiTi actuators with stress risers: The role of bias load and actuation temperature

NiTi wires and thin ribbons are used in shape memory actuators as active elements undergoing repeated thermal cycles. The NiTi actuators operate either under constant force (CF loading constraint) or they act against a variable force (VF loading constraint) from an elastic bias spring. Any stress riser represents a danger for potential actuator failure upon cycling. Although NiTi actuators always include stress risers either inherently in their design or as material flaws, their effect on actuation performance has, however, not been studied thoroughly yet. This issue is addressed in the present paper through numerical analysis of the effects of actuation load type (CF or VF loading constraint), maximum temperature, and stiffness of the bias springs (in VF loading constraint) on the local evolution of martensitic transformation (MT) and mechanical fields around the notch upon the cyclic thermomechanical loading of a thin NiTi shape memory notched ribbon. The analyses clearly show a strong amplification effect of the transformation strain on the stress concentration factor. The results reveal that the evolution of the forward MT upon cooling causes a sharp stress peak around the notch-tip the magnitude of which depends on the bias spring stiffness while upon heating, the stress at the notch-tip area relaxes due to the reverse MT of the surrounding bulk. Moreover, the simulations indicate that any overheating during the actuation is harmful to the notch-tip as the complete reverse MT at the notch-tip has stress rising effect while incomplete MT at the notch-tip has stress relaxing effect.

Evaluation Methods for Non-contact Bend and Free Recovery Tests of Thin NiTi Wires and Their Effects on Measured Transformation Temperatures

Non-contact evaluation of transformation temperatures via bend and free recovery tests requires precise optical evaluation of shape changes of NiTi components. A variety of experimental setups is documented in the literature, but the influence of the evaluation method on the transformation temperatures is rarely assessed in detail. In the present work, the reverse transformation of bent wires is evaluated comparing the tracking of the lowest wire point and the tracking of the curvature. For calculating curvatures, different approaches of fitting the wire outline were applied. Fourth degree polynomials and ellipse segment fits were found to cause high noise toward the end of the reverse transformation, second degree polynomials and circle segment fits led to increased sensitivity in that region. Accordingly, the evaluation of curvature allowed to resolve a two-stage reverse transformation, which was otherwise obscured. The reasons for this effect are discussed comparing curvatures as determined by the different evaluation methods.

Finite element analysis on the effect of martensitic transformation and plastic deformation on the stress concentration factor in a thin notched superelastic NiTi ribbon

The severe nonlinear behavior caused by the martensitic transformation (MT) and subsequent plastic deformation (PD) of detwinned martensite leads to a complex local stress redistribution at the location of stress risers of superelastic shape memory alloy (SMA) components. Nevertheless, in the literature, the simple linear elastic fracture mechanics (LEFM) equations are widely used in the evaluation of the fracture response of superelastic components which has resulted in obvious conflicts between the conclusions regarding the effect of MT on the fracture parameters, i.e. stress intensity factor (SIF) and material toughness. Furthermore, the linear elasticity method is frequently used in the literature to calculate the stress intensity range ([Formula: see text]) when the fatigue crack growth rate dependence on [Formula: see text] ([Formula: see text]) is being evaluated. Moreover, the PD followed by MT is poorly considered in the fracture mechanics of SMAs. This paper presents a numerical investigation on the role of both MT and PD, as well as the notch acuity, on the evolution of notch-tip stresses and strains and stress concentration factor ([Formula: see text]) upon the incremental application of the macroscopic tensile load on a thin NiTi notched superelastic ribbon, to mimic the effects of MT and PD on the SIF of superelastic parts. It is revealed that MT results in drastic deviations of the notch-tip stress, as well as the stress concentration factor ([Formula: see text]), from that obtained in LEFM. Due to the heterogeneous evolution of MT, the trend of the deviations is not regular and unique upon monotonic external loading. Accordingly, the results represent the ineffectiveness of the LEFM method in the evolution of the stress concentration factor (hence, the SIF) and toughness in monotonic loading, as well as the stress intensity range ([Formula: see text]) under fatigue loading in SMA components.

Corrosion Resistance of Nitinol Wires After Deformation

The corrosion resistance of thin NiTi wires in unstrained state and at 6% of pseudoelastic strain was determined by cyclic potentiodynamic polarization, considering a surface area of 2.3 cm2, equivalent to that of an actual minimally invasive implant. Surface finishing was carried out by mechanical polishing, electropolishing, and subsequent heat treatment in a salt bath furnace. Topography, microstructure, and composition of the respective surfaces were characterized in detail using light and electron microscopy, as well as glow discharge optical emission spectroscopy. Whereas breakdown below 1000 mV was observed for wires in mechanically polished condition in unstrained and strained state, wires in electropolished and electropolished + heat-treated condition exhibit no breakdown up to 1000 mV. Multiple testing reproduced uniform potentiodynamic polarization curves in unstrained and strained state. Accordingly, the observed formation of cracks in the surface oxide layer of wires in electropolished + heat-treated condition is concluded to be negligible in unstrained and strained state. Cause for the observed high performance of both the electropolished and the electropolished + heat-treated wires is apparently the smoothing of the initial surface that results in a homogeneous oxide layer even after heat treatment.

A Fuzzy Logic Control System for a Robotic Hand Driven by Shape Memory Alloy Wires

This paper presents a robotic hand using wires with shape memory (NiTi) as non-conventional actuators. The mechanical structure of the robot hand was first designed by means of a CAD computer program and afterwards 3D printed using ABS polymer. The robotic hand was designed according to the physiological characteristics of the human hand, with particular attention to the angles formed by the phalanges of the fingers. A mechanical system accommodates the thin NiTi wires compactly, thus forming an artificial muscle. A fuzzy logic based control system allows an accurate positioning of each phalanx. The contribution of the present work to science lies in the practical implementation of known techniques and materials.

A Fuzzy Logic Control System for a Robotic Hand Driven by Shape Memory Alloy Wires

This paper presents a robotic hand using wires with shape memory (NiTi) as non-conventional actuators. The mechanical structure of the robot hand was first designed by means of a CAD computer program and afterwards 3D printed using ABS polymer. The robotic hand was designed according to the physiological characteristics of the human hand, with particular attention to the angles formed by the phalanges of the fingers. A mechanical system accommodates the thin NiTi wires compactly, thus forming an artificial muscle. A fuzzy logic based control system allows an accurate positioning of each phalanx. The contribution of the present work to science lies in the practical implementation of known techniques and materials.

Characterizing Transformation Phenomena and Elastic Moduli of Austenite and Oriented Martensite of Superelastic Thin NiTi Wire through Isothermal Dynamic Mechanical Analysis

In this paper, superelastic behavior of Nickel Titanium thin wires is characterized using the method of dynamic mechanical analysis. Nominal dynamic storage modulus ′ is measured as function of nominal strain and stress during isothermal superelastic tensile tests at three testing temperatures above the reverse martensitic transformation finish temperature. The method brings new information on deformation mechanisms compared to the consideration of only tensile stress-strain curves. It is shown that determination of the elastic moduli E, especially at high strain, requires to calculate the true storage modulus ′. Using ′ and not ′, elastic modulus of oriented martensite is determined equal to 73 GPa of the same order than the elastic modulus of austenite equal to 70 GPa. Two models are then proposed to simulate experimental storage moduli evolution during the tests. A first model explains the ′ evolution during stress plateau by the localization phenomenon ; it leads to express ′ as function of the nominal strain. A second model describes the evolution of ′ after the stress plateau as function of true stress and test temperature. This model permits to determine the Clausius-Clapeyron coefficient of the forward transformation.

Thermomechanically transforming Notched NiTi Thin ribbon: Effect of Martensitic Transformation on Stress Gradients

In this work, we used numerical simulations to investigate the effect of thermally induced forward martensitic transformation (MT) under constant load on the stress redistribution around stress risers. Finite element simulations of stress redistribution around two-sided notches in a thin NiTi ribbon subjected to constant tensile force and homogeneous cooling from austenite to martensite were performed. Changes in stress gradients around the notches were analyzed with regard to heterogeneity of thermomechanically driven MT. A parametric study was performed in order to understand the effect of transformation strains, Young’s modulus of martensite and notch radius on the magnitude of notch-tip stresses after the completion of MT. We found that forward MT critically amplifies the initial elastic stress gradients around the notch. The increase in magnitude of notch-tip stress is closely related to elastic deformation in martensite needed to accommodate the transformation strain in the surrounding notch unaffected zone. This approximation holds for notch radii being small compared to the ribbon width. For large radii MT is more homogeneous, the notch-tip is more shielded from transforming surroundings and, hence, the notch-tip stress approach the nominal stress.

Laser Annealing on the Surface Treatment of Thin Super Elastic NiTi Wire

Here the aim of this research is annealing the surface of NiTi wire for shape memory alloy, super-elastic wire by solid state laser beam. The laser surface treatment was carried out on the NiTi wire locally with fast, selective, surface heat treatment that enables precisely tune the localized material properties without any precipitation. Both as drawn (hard) and straight annealing NiTi wire were considered for laser annealing with input power 3 W, with precisely focusing the laser beam height 14.3 % of the Z-axis with a spot size of 1 mm. However, straight annealing wire is more interest due to its low temperature shape setting behavior and used by companies for stent materials. The variable parameter such as speed of the laser scanning and tensile stress on the NiTi wire were optimized to observe the effect of laser response on the sample. Superelastic, straight annealed NiTi wires (d: 0.10 mm) were held prestrained at the end of the superelastic plateau (ε: 5 ∼6.5 %) above the superelastic region by a tensile machine ( Mitter: miniature testing rig) at room temperature (RT). Simultaneously, the hardness of the wires along the cross-section was performed by nano-indentation (NI) method. The hardness of the NiTi wire corresponds to phase changes were correlated with NI test. The laser induced NiTi wire shows better fatigue performance with improved 6500 cycles.

A Study of Thin TiNi Fibers Superelasticity

Thin TiNi fibers which having superelasticity with diameter from 90 to 30 mm obtained from medical application alloy TN-10 by traction across dies with interval annealings are researched. Fine TiNi alloy fibers diameter of 90 to 30 microns having superelastic properties, obtained from TN-10 alloy for medical purposes by traction through dies with intermediate annealing. Availability superelastic properties allow thin fibers to function long in the body as the implant material. The optimum temperature superelastic properties of thin fibers with a diameter 90-30 mm was determined. It has been shown that with decreasing fibers diameter from 90 to 30 mm maximum temperature manifestations superelastic properties without residual deformation is shifted to higher temperatures, reaching values of 93 -95 °Cfor 30 mm diameter fibers.

List of Abstracts

. Production efficiency as well as the reduction of energy consumption and waste are the main drivers for the improvement of large-scale production in chemical industry. In addition, the demand for new processes – these days especially related to the chemical energy conversion as basis for the Energiewende – are additional triggers for innovation. Catalysis, well recognized by the public e.g. in form of the 3-way automobile catalyst, plays an important role for about 90% of all processes in chemical industry. The development of new and improved catalysts is a challenge for chemists – as is the search for new structural materials for mechanical engineers. Due to the often-necessary noble and thus expensive metals (e.g. palladium) in many catalysts, structural and chemical function have been developed separately in the two communities. Recent progress in catalytically active materials allows replacing expensive noble metals in selected catalytic processes by significant cheaper elements [1]. This offers the new perspective to combine structural and chemical functionality by innovative materials. Within this area, intermetallic compounds with their peculiar combination of crystal and electronic structure [2], represent an interesting class of materials to address this vision as will be shown in the contribution.