Transformation strain based method for characterization of convective heat transfer from shape memory alloy wires (original) (raw)
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Tailoring the response time of shape memory alloy wires through active cooling and pre-stress
Journal of Intelligent Material …, 2010
Application of shape memory alloy (SMA) actuators is limited to low frequencies due to slow cooling time especially in the embedded conditions where heat transfer rate is the controlling factor. In this study, we investigate various active cooling techniques and effect of pre-stress to improve the response time of two commercially available SMAs: Flexinol from Dynalloy Inc. and Biometal fiber from Toki Corporation. Flexinol and Biometal fiber of equal length and diameter were found to exhibit different actuation behavior under pre-stress. Time domain force response of SMA actuators was found to be dependent upon the applied prestress, heating rate, and amplitude of applied electrical stimulus. Compared to Biometal fibers, time domain response of Flexinol was found to decrease significantly with increasing pre-stress indicating the difference in transformation behavior. Fluid flow and heat sinking were found to be suitable methods for improving the response time by reducing the cooling cycle from 1.6 s to 0.300.45 s. This is a significant improvement in the actuation capability of SMAs.
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.
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.
Meshfree modeling of shape memory alloy wires thermomechanical behavior
academicjournals.org
In this research, a macro-scale, phenomenological constitutive model for shape memory alloy (SMA) is used in conjunction with energy balance equations to study the evolution of temperature and deformation profiles seen in SMA wires. In this way, the general fully-coupled thermomechanical formula for resistive heating of an SMA wire-initial detwined martensite, leading to strain recovery on heating, is used and numerical results are obtained with use of "Meshless" methods which are rather new computational techniques that do not require the use of any connectivity concept, such as those used in finite element method (FEM); since only a cloud of nodes is required, the element free Galerkin (EFG) method is particularly suitable for problems involving internal boundaries, geometry changes, etc. Comparisons between the results predicted by proposed EFG method and available reference solutions in the literature validate the method. A good agreement is obtained between the achieved results and the literature.
Shape memory alloy (SMA) actuators exhibit considerable hysteresis between the supply voltage (conventionally used in resistive heating) and strain characteristics of the SMA. Hence, it is not easy to control the strain of a thin-SMA wire, unless a model is developed that can match the actuator's nonlinearities for predicting the supply voltage required by the SMA system accurately. The work presented in this paper proposes the use of a black-box technique called the adaptive neurofuzzy inference system (ANFIS) to study the hysteretic behavior of SMAs. The input parameters for such an ANFIS model would be a physical variable at time t and at a time t + n, where n is a time shift. The present work studies the effect of a time shift on the actuator nonlinearities for two ANFIS models. One of the models studies the relationship between the desired displacement of an SMA and the supply voltage across the SMA, while the other model predicts the actual displacement of an SMA from the feedback temperature. A novel SMA–Constantan thermocouple records the feedback temperature.
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.
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