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Metastable effects on martensitic transformation in SMA

Part VIII. Temperature effects on cycling

Journal of Thermal Analysis and Calorimetry
Authors: V. Torra, C. Auguet, A. Isalgue, F. Lovey, A. Sepulveda, and H. Soul

Abstract  

The use of Shape Memory Alloys (SMA) in technical applications as damping in civil engineering structures requires the characterization of the alloy for each specific application. This involves the evolution of the mechanical properties and damping capacity with the number of cycles, frequency, maximum deformation, applied stresses, and the evolution of the alloy with aging time and temperature. In particular, the temperature effects associated to self-heating need to be evaluated. In continuous cycling the effects of latent heat, the associated dissipation induced by the hysteresis, the heat flow to surroundings and the cycling frequency induce different states of temperature in the specimen, which in turn produces changes in the transformation-retransformation stresses. In this article, the temperature effects associated to cycling are outlined for different cycling frequencies. The results show that, for relatively faster frequency the temperature arrives at an oscillatory state superimposed to an exponential increase. For lower frequencies, some parts of the sample attain temperatures below room temperature. The experimental results are represented with an elementary model (the 1-body model or the Tian equation used in calorimetric representation) of heat transfer. For the higher fracture where life requirements are associated to damping in stayed cables for bridges, the results show (for the NiTi alloy) a reduction of the hysteresis width as the frequency increases for deformations up to 8%. For reduced deformation, under 2% appears an asymptotic behavior where the frictional area is practically independent of the cycling frequency (up to 20 Hz). In addition, it is shown that more than 4 million of working cycles can be attained if the maximum applied stress is kept below a threshold of about 200 MPa. Although under this condition the deformation must remain lower than 2% a reasonable damping capacity can still be obtained.

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Reliability is a critical word in industrial applications of Shape Memory Alloys. Accurate and reproducible transformation hysteresis cycles and internal loops were obtained in single crystals using a high resolution automatized equipment. From a mechanical model formulated for a single martensite plate, the shape of the hysteresis cycle is obtained by generalizing the representation toN plates. The observed time effects on the hysteresis loops related to diffusion processes were also taken into account. It allows to explain the martensite recoverable creep and the micromemory effects. Also, the room temperature effects on the parent phase (for instance, summer to winter) acting over the transformation temperature are quantified.

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Abstract

This article presents a study on obtaining Ni, Zn ferrite starting from Fe(III), Ni (II), Zn (II) nitrates and some polyols: 1,2-propane diol, 1,3-propane diol and glycerol. While heating, a redox reaction takes place between nitrate anion and polyol, with formation of carboxylate type precursors. The obtained precursors have been investigated by thermal analysis, FT-IR spectrometry and atomic absorption spectroscopy. The thermal decomposition of the synthesized precursors up to 350 °C leads to the formation of Ni, Zn ferrite as unique phase, evidenced by XRD. The average diameter of the ferrite crystallites, estimated from XRD data, takes values within the range 20–50 nm, depending on the annealing temperature. Transmission Electron Microscopy has evidenced the obtaining of spherical, agglomerated nanoparticles. The magnetic properties of the synthesized samples, measured in cvasistatic magnetic field (50 Hz) are characteristic for the Ni, Zn ferrite nanoparticles, with narrow hysteresis cycle and values of the saturation magnetization <70 emu/g.

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Abstract  

Shape memory alloys (SMA) are interesting for applications in damping of civil engineering structures. To achieve the SMA as a guaranteed material for damping of structures requires a reliability study of the static and dynamic properties adapted to each type of application. Here we present static (temperature and time effects) and dynamic actions in pseudo-elastic NiTi SMA. We concern with long time effects of temperature and time mainly in beta phase. NiTi results are presented, including measurements of electrical resistance as a function of temperature and time, DSC to −80°C (TA Instruments), non-conventional conduction calorimetry (to 80 K), and several X-ray diffraction spectra at different temperatures. Diffusion effects are present, and all the measurements show that transformation temperatures change with time of aging even at moderate temperatures (i.e., near 100°C), depending on time and temperature. It is possible to visualize the diffusion change in the R phase transformation via classical X-ray characterization. We include some results of pre-stressing effects applying the stress at different zones of the hysteresis cycle. The experimental analysis shows that coupling between stress and temperature enhances the changes. For long times (10, 20 years) and direct sunny actions, more deep analysis is required. Finally, some dynamic effects in cycling affecting the SMA creep are outlined.

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hysteresis cycle , 2nd International Conference on Knowledge-Based Engineering and Innovation (KBEI), Teheran, Iran, 5-6 November 2015 , pp. 710 ‒ 713 . [3] Bastos

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quenched. a Thermo-mechanical hysteresis under different loads; b comparison between the stress–stroke data derived from the MTS and the LVDT tensile tests From the hysteresis cycles conducted at different loads we

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