FEATURE Experimental Characterization of Active Materials Series TIPS AND TRICKS FOR CHARACTERIZING SHAPE MEMORY ALLOY WIRE: PART 4 – THERMO-MECHANICAL COUPLING (original) (raw)
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Thermo-Mechanical Properties of Shape Memory Alloy
The Young's modulus and the Coefficient of Stress Influence are two important parameters of thermo-mechanical properties of shape memory alloys. These parameters are important in the design but there are no technical data on it. A novel method was used to determine the Young's modulus of shape memory alloys during martensite phase and austenite phase, and the Coefficient of Stress Influence during austenite-martensite and martensite-austenite by using Universal Testing Machine. The load applied on to the Flexinol wire for one cycle was at a very low speed of testing. Throughout the test, the Flexinol wire was heated up to a certain temperature level and maintained it constant by using chamber. The Young's modulus at martensite phase and austenite phase were 33.160 GPa and 69.592 GPa while the Coefficient of Stress Influence during austenite-martensite and martensite-austenite were 10.761 MPa/ 0 C and 9.082 MPa/ 0 C.
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...
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.
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.
Phase transformation temperatures for shape memory alloy wire
ENFORMATIKA, 2007
Phase transformation temperature is one of the most important parameters for the shape memory alloys (SMAs). The most popular method to determine these phase transformation temperatures is the Differential Scanning Calorimeter (DSC), but due to the limitation of the DSC testing itself, it made it difficult for the finished product which is not in the powder form. A novel method which uses the Universal Testing Machine has been conducted to determine the phase transformation temperatures. The Flexinol wire was applied with force and maintained throughout the experiment and at the same time it was heated up slowly until a temperature of approximately 100 0 C with direct current. The direct current was then slowly decreased to cool down the temperature of the Flexinol wire. All the phase transformation temperatures for Flexinol wire were obtained. The austenite start at 52.54 0 C and austenite finish at 60.90 0 C, while martensite start at 44.78 0 C and martensite finish at 32.84 0 C.
Part I. Thermomechanical characteristics of shape memory alloys
Materials Science and Engineering: A, 2004
Shape memory alloys (SMAs) are a group of alloys that exhibit a phenomenon known as the shape memory effect, (SME). This effect gives the alloys the ability to "recover" their original shape by heating above a certain transition temperature. There is also a large recovery strain, of up to 8%, associated with the transition. Because of this unique property, a large research effort is currently being undertaken, directed towards the use of SMAs in the actuation of smart structures for shape control, vibration control and for damage mitigation. SMAs also have a very high damping capacity due to a superelastic effect. This property of SMAs is extremely useful in vibration damping as well as reducing impact damage in structures. As such there has been much interest in using SMA-composites in structures. With the possibility of using SMA-composites in real structures such as in aviation, high speed transport industry and the automotive industry, there is increasing demands on knowing how the composites will react under everyday conditions. This paper details an investigation into the thermomechanical behaviour of SMA wires, looking at the recovery stresses produced and the stress and strain behaviour with respect to temperature, as well as changes in resistance of the wires with pre-strain.
Materials today communications, 2019
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 NiTibased 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 superelastic (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.
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.
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.