of ENGINEERING-HUNEDOARA, ROMANIA 65 1,2. ELECTRICAL ACTIVATION OF THE SHAPE MEMORY EFFECT FOR NiTi WIRES (original) (raw)
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Electro-thermomechanical characterization of Ti-Ni shape memory alloy thin wires
Materials Research, 2006
The use of shape memory alloys (SMA) as smart structures and other modern applications require a previous evaluation of its performance under load as well as a training procedure. In general, these requirements lead to the design and assembly of a specific test bench. In this work, an experimental setup was specially designed to perform the electro-thermomechanical characterization of SMA wires. This apparatus was used to determine the strain-temperature (ε-T) and electrical resistance-temperature (R-T) hysteretic characteristics curves of a Ti-Ni shape memory wire (90 mm in length and 150 µm in diameter) under mechanical load. The SMA wire is loaded by means of constant weights and a controlled system for injection of electrical power allows performing the heating-cooling cycles. The obtained hysteretic ε-T and R-T characteristics curves for some levels of applied loads are used to determine important shape memory parameters, like martensitic transformation temperatures, temperature hysteresis, temperature slopes and shape memory effect under load. These parameters were in accord with the ones found in literature for the studied SMA wires.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008
Evolution of the electrical resistivity during thermal and mechanical tests of NiTi wires showing R-phase transformation was investigated by experiments and micromechanical model simulations considering B2-R-B19 transformations. Since reasonable agreement between the simulated and experimental mechanical and electrical resistivity responses was achieved, the apparently curious behavior of electrical resistivity could be rationalized through the model simulations in terms of the activity of multiple transformation and deformation processes taking place in the activated NiTi wires.
INVESTIGATION OF SOME ELECTRICAL PROPERTIES OF NiTi WIRES PRESENTING THE SHAPE MEMORY EFFECT
2009
Lucrarea prezintǎ rezultatele unor experimente de mǎsurare a rezistivitǎţii electrice a douǎ tipuri de sârme de aliaj de NiTi tratate pentru a prezenta efectul de memoria formei.Probele utilizate au fost analizate micrografic şi compoziţional, precum şi prin scanare calorimetricǎ diferenţialǎ(DSC).Utilizarea tot mai frecventǎ a unor asemenea materiale în diverse domenii ale tehnogiilor de vârf impune cunoşterea cât mai în detaliu a proprietǎţilor acestor materiale cu memoria formei în vederea unor utilizǎri cât mai variate, dar şi pentru a valorifica pe deplin potenţialul lor mai ales cǎ NiTi pǎstreazǎ multe dintre proprietǎţile remarcabile ale Ti.
Journal of Intelligent Material Systems and Structures
Shape Memory Alloys (SMAs) exhibit a complex material behaviour due to thermo-mechanical coupling. At present, the understanding of their material behaviour under different thermo-mechanical loading conditions relevant to civil engineering applications is lacking even though they have been widely used in the past decade or so, for example mechanical behaviour at constant stress but variable temperature loading has not received much attention in civil engineering. In this study a comparative analysis of the mechanical behaviour of various NiTi-based SMAs is carried out to investigate: (1) stress-elongation and stress-temperature response under direct tensile loading; (2) thermo-mechanical behaviour under constant stress condition but under variable temperature loading (to investigate their ability to exhibit shape memory effect, if any, while carrying load); and (3) the ability to develop and retain recovery stress at room and sub-zero temperatures. Essentially, all these tests aim a...
Transformation fatigue and stress relaxation of shape memory alloy wires
Smart Materials and Structures, 2007
The present work deals with the stress generation capability of nickel-titanium shape memory alloys (SMAs) under constrained conditions for two well-defined loading modes: recurrent crystalline transformation (transformation fatigue) and a one-step continuous activation (generated stress relaxation). The data acquired will be very useful during the design process of an SMA Ni-Ti element as a functional part of an assembly. Differential scanning calorimetry (DSC) was employed in order to investigate the transformation characteristics of the alloy before and after the tests. Transformation fatigue tests revealed that the parameter that affects more the rate of the functional degradation is the number of crystalline transitions the wire undergoes. Thus, the service life limit of this material as a stress generator can be reduced to a few thousand working cycles. For stress relaxation, the main factor that affects the ability for stress generation is the working temperature: the higher the temperature above the austenite finish (TA f) limit the higher the relaxation effect. Thermomechanical treatment of the alloy during the tests reveals the 'hidden' transformation from the cubic structure (B2) of austenite to the rhombohedral structure of the R-phase. It is believed that the gradual loss of the stress generation capability of the material under constrained conditions must be associated to a gradual slipping relaxation mechanism. Scanning electron microscopy (SEM) observations on as-received, retrained , fatigued and stress-relaxed specimens in the martensitic state provide further support for this hypothesis.
On the preparation and characterization of thin NiTi shape memory alloy wires f.PDF
Shape memory alloy (SMA) wires are employed as actuators in small devices for consumer electronics, valves and automotive applications. Because of the continued miniaturization of all the industrial products, nowadays the tendency is to produce MEMS (micro electromechanical systems). Among the most promising functional MEMS materials, the thin SMA wires that are offering a rapid actuating response with high power/weigh ratio of the material, are attracting a world wide interest. This paper is aimed at showing the production process and the characterizations of thin NiTi shape memory wires. The activity was focused on drawing procedure and on functional and TEM characterizations of the final products. In particular, it was evaluated the performance of the SMA wires for actuators in terms of functional fatigue and thermo-mechanical properties by means of an experimental apparatus design ad hoc for these specific tests.
Deformation energy of NiTi shape memory wires
Materials & Design, 2011
Deformation energy of NiTi wires with B2 and R phases was studied by the multiple tensile testing (MTT) method. In traditional materials, the total energy required to tear specimens is assumed to be the sum of elastic, uniform plastic, and post-uniform or tearing energy components. For the shape memory alloys, however, this classification is not valid due to their unusual superelastic/shape memory characteristics. Using a modified MTT method, different energy components were calculated by plotting different combination of deformation energies divided by the specimen cross-sectional area against the gage length of the specimens. The slope of the obtained straight line demonstrates the summation of the elastic, superelastic/shape memory, second elastic, and plastic energy per unit volume and its intercept gives the value of tearing energy. It was found that the uniform plastic energy per unit volume for the R-phase wires was considerably higher than that for the B2-phase wires. This caused a marked enhancement in the total deformation energy of the R-phase wire, as compared to the B2-phase wire. The effect of strain rate on the tensile behaviour and deformation energies of these materials was also investigated. Except the plateau stress of the tensile curve which was raised for both wires, the B2-phase wires were almost strainrate-independent, whereas the R-phase wires were significantly influenced by the variation in strain rate.
Mechanics of Materials, 2020
Over the past few years, shape memory alloys (SMAs) have received increased attention in civil engineering research due to their unique features such as superelasticity, shape memory effect (SME), high ductility and good corrosion resistance. SMAs, however, have complex material behaviour which depends on many parameters such as chemical composition, thermo-mechanical treatment and ambient temperature. SMAs exhibit high strain-rate sensitivity and strong strain localisation during the stress-induced martensitic (SIM) phase transformation. Both have serious implication in civil engineering applications. Strain rate effects and the strain localisation in SMAs have been commonly investigated separately in the literature which has been mostly focused on NiTi SMA. This paper investigates the strain localisation phenomenon, the effect of strain rate and their interaction in NiTiNb SMA. The digital image correlation (DIC) technique was used to investigate the strain localisation phenomenon. The effect of strain rates is investigated within the quasi-static range and varies from 3.3×10 −5 /s to 3.3×10 −2 /s in strain-controlled tests and 2×10 −5 /s to 2×10 −1 /s in displacement-controlled tests. Significant non-uniform strain distribution is observed during the SIM phase transformation, which progresses through the nucleation and broadening of the transformation bands (TBs). The nucleation of the TBs and the unloading strain recovery within the TBs show high strain-rate sensitivity. At high strain rates, the nucleation of a TB is accompanied by a significant stress-drop.
2016
T his is the fourth paper in our series, identifyingunusual phenomena and providing recommenda-tions for the thermo-mechanical characterizationof shape memory alloy (SMA) wire. Part 11 pro-vided basic background of the martensitic transformations between austenite (A) and martensite (M) which are respon-sible for the shape memory (SM) effect and superelasticity (SE). Two typical NiTi SMA alloys (SM wire with austenite start temperature As> 20 ◦C and SE wire with austen-ite finish temperature Af < 20 ◦C) were characterized by differential scanning calorimetry (DSC) to measure trans-formation temperatures, specific heats, and latent heats of transformation. SM and SEwere demonstrated for each alloy in their respective temperature regimes. Part 22 reviewed various methods to obtain fundamental sets of isothermal mechanical responses for the two SMA wire alloys. Part 33
Computational Materials Science, 2003
This paper reports a computational study of the impact of variable material properties and environmental conditions (thermal boundary conditions and convection coefficients) on shape memory alloy wires undergoing (i) zero-stress, thermally-induced phase transformations, and (ii) stress-induced phase transformations at constant stress rates. A finite difference numerical approach has been employed, and has been validated by comparing with two analytical solutions. The results have been all given in non-dimensional form, and within the context of the range of parameters that have been studied, the following recommendations can be made for shape memory alloys (SMA) actuator design: (i) an uncertainty in the thermal boundary condition is not as important as long as the design process allows for a full transformation back to martensite at the end of a cycle of martensite–austenite–martensite thermal transformation, (ii) uncertainties in the thermal boundary condition, convection coefficient and thermal material properties are not as important when the phase transformation in a SMA is induced by stress.