J.S. Wu

Synthesis of filled skutterudite compound La0.75Fe3CoSb12 by spark plasma sintering and effect of porosity on thermoelectric properties

Filled skutterudite compounds La0.75Fe3CoSb12 with different porosities were synthesized using spark plasma sintering (SPS) and the Archimedes method was used to measure the porosities of the specimens. Phase constitutions and compositions of the compounds were examined by XRD and EPMA, and fracture surfaces of the compounds were observed by SEM. The effect of porosity on the thermoelectric properties was studied based on the changes in the electrical resistivity, Seebeck coefficient and thermal conductivity as a function of porosity. Our results show that the Seebeck coefficient is independent of porosity and the increase of electrical resistivity is consistent with the reduction of thermal conductivity due to scattering of carriers and phonons by pores. In addition, the thermoelectric figure of merit ZT (where T is temperature and Z is a function of the Seebeck coefficient, electrical resistivity, and thermal conductivity) of specimens with high porosity is comparable to that of specimens with low porosity. This suggests that the ZT value of La0.75Fe3CoSb12 compound does not depend significantly on the porosity.

A study on the variations of the electrical resistance for NiTi shape memory alloy wires during the thermo-mechanical loading

Abstract In order to use NiTi wires as strain or stress sensors, the variations of electrical resistance of Ti-50.8 at%Ni wires during thermo-mechanical loading are studied in this paper. The results show that it is possible to use the NiTi wires as strain sensors, but it is difficult to use NiTi wires as stress sensors.

A study on thermomechanical deformation of elastic beam with embedded shape memory alloy wires

Abstract The bending deformation of an elastic beam with eccentrically embedded shape memory alloy (SMA) wires under the activation of electrical current has been investigated. A procedure, which accounts for the effect of transient heat transfer and gives rise to the beam bending responding to prescribed input electrical current profile is used for analysis. It is found that influences of ambient temperature, electric current intensity and prestrain of SMA on the bending deformation of beam are significant.

The nonlinear relationship between transformation strain and applied stress for nitinol

In the present paper, constant stress tests were conducted, and it was found that the relationship between transformation strain and external stress for NiTi is nonlinear because of the variation of the microstructure of martensite with external stress. Thermodynamical calculation shows that the nonlinear external stress dependence of transformation temperatures was caused by the variation of transformation strain with applied stress.

Thermo-viscoelastic bending analysis of a shape memory alloy hybrid epoxy beam

The deflection of an epoxy beam with embedded shape memory alloy (SMA) wires subjected to lateral load is predicted based on the thermo-viscoelastic finite-element analysis for the beam. The emphasis has been placed on the effect of thermo-viscoelasticity of the polymeric matrix. The SMA wires, which are located at the neutral surface of the beam, are embedded inside the sleeves and fixed externally at two ends. In this study the classical theory is applied for the beam analysis and Brinsons constitutive relation is used for the SMA. Experiments of SMA hybrid epoxy beam are conducted and the results compare well with the theory.

Thermoviscoelastic analysis for a polymeric composite plate with embedded shape memory alloy wires

The deflection of the polymeric-matrix composite plate with embedded shape memory alloy (SMA) wires and subjected to uniform lateral pressure is studied using the finite element method. The behavior of the polymeric composite plate under electrical heating of SMA is analyzed with thermoviscoelastic theory. The actuation of SMA is modeled by Brinson’s one-dimensional constitutive equation. The influences of the boundary conditions, the ply orientations and the pre-strains of SMA wires on the deflection of composite plate are discussed. It is found that the effect of thermoviscoelasticity of the polymeric matrix on the actuation of SMA is significant. 2002 Elsevier Science Ltd. All rights reserved.

Mechanical behavior and shape memory effect of an aged NiAl–Fe alloy

Abstract The mechanical behavior and shape memory effect of an aged NiAl–Fe alloy has been investigated. It was found that the first yielding stress of NiAl–Fe alloy in a compression test was decreased with the precipitation of a Ni5Al3 phase after aging at 473–673 K, and increased as the aging temperature increased higher. The one-way shape recovery of NiAl–Fe alloy increased as the aging temperature increased from 473 to 673 K, and decreased as the aging temperature increased higher than 673 K. The morphology of precipitates in the NiAl–Fe alloy aged at 473–873 K was investigated, and the mechanism of its effect on the mechanical properties is discussed.

New constitutive relationship of shape memory alloy composite

Shape Memory Alloys (SMAs) have been a major element of Smart Materials and Structures. Many applications, however, call for the embedding of SMA materials within structure components to sense and/or control the thermo-mechanical response of the structures. Therefore, the works on developing constitutive relationship of SMA composite are needed. In this paper, a one-dimensional constitutive law of Shape Memory Alloy composite is developed based on Brinsons one-dimensional SMA constitutive relationship. The thermo-mechanical behaviors of SMA composite are simulated implementing this constitutive relationship. In addition, the bending of a bias SMA composite is also investigated.

Effects of thermoviscoelastic properties of polymer on the actuation of shape memory alloy

The model of a polymeric bar, which is connected in series with a shape memory alloy (SMA) wire, is analyzed. Both the polymeric bar and SMA are exposed to a transient temperature field. The thermo-viscoelastic theory for thermoheologically simple material is applied to the polymeric bar. Brinsons one-dimensional constitutive law is employed for the analysis of SMA. The fundamental equation governing the behavior of the combined structure is obtained. Results of stress (strain)-time relation and stress (strain)-temperature relation in SMA are obtained. It is found that the results from thermo-viscoelastic analysis coincide well with those of thermo-elastic and visco-elastic analyses only within low temperature range, however, notable discrepancy occurs when temperature is close to the glass transformation temperature of the polymer. Besides, the effects of polymeric material parameters on the actuating force of SMA are also discussed.

One-dimensional Modeling of Shape Memory Alloy with Improved Kinetic Relation for Phase Transformation

Based on Brinsons one-dimensional constitutive relation for shape memory alloy (SMA), the kinetic equations for phase transformation are refined in this work in the following aspects: (1) Of both the martensite to austenite transformation and the reverse procedure, the start and finish temperatures of phase transformation for detwinned martensite is differentiated from those for twinned martensite. Different transformation parameters are introduced for the two variants of martensite. (2) The zone of the conversion from twinned martensite to detwinned martensite is extended to the austenite finish temperature Af. Numerical simulations are conducted for (1) stress- strain relation of tensile loading and unloading under constant temperature; (2) strain-temperature relation of heating and cooling under constant stress; (3) stress- temperature relation for heating and cooling under fully restrained condition. All results obtained give good descriptions of thermo-mechanical behavior of SMA.