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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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Metastable effects onmartensitic transformation in SMA

Part 4. Thermomechanical properties of CuAlBe and NiTi observations for dampers in family houses

Journal of Thermal Analysis and Calorimetry
Authors: C. Auguet, A. Isalgué, F. Lovey, F. Martorell, and V. Torra

Abstract  

The behavior of shape memory alloys (SMA) allows their use as a passive smart material. In particular, the existence of a hysteretic cycle in the domain of the elementary coordinates strain-stress-temperature (σ, ε, T) suggests its application for damping in mechanical and/or in civil engineering. We are working in the application of SMA as dampers for earthquakes in small houses as family homes. For dampers installed in the inner porticos of the house, the suggested SMA is the CuAlBe and, eventually, the NiTi. At room temperature the used SMA wires induces forces situated between 2–3kN/wire. The properties related with the damping applications for CuAlBe and NiTi, i.e., the SMA creep and the self-heating will be presented, together with some other minor stress and temperature effects on NiTi modifying the hysteretic behavior.

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Abstract  

An improved accuracy is researched in flat and silicon based nano-calorimeters related to fluid reaction via drop-to-drop 'reaction' or via a steady state in continuous injection of two reactants inside a working chamber. Two types of the experimental aspects of the sensitivity changes, related to 3-D effects, are described: drop effects and volume rate effects. To increase the accuracy, well-controlled joule measurements are dissipated in the manufacturer resistance and in several resistances in carefully controlled experiments. The shape factor (SF) values reduce the manufacturer's joule sensitivity up to fifty per cent. a working methodology is proposed: the results are compared with the standard TRIS reaction. The results suggest that the systematic error can be reduced to 5%.

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Abstract  

A non-differential calorimetric analyzer was developed for an INSTRON 1123 machine (a stress-strain-temperature analyzer) with a temperature chamber INSTRON 1110. The study was performed using the Joule effect and pseudoelastic martensitic transformations in single crystals of Cu-Zn-Al alloys. The analysis of the system establishes that: the sensitivity of calorimetric measurements after a filter of two poles and two zeros is 166 mV W-1 (at 297 K), the noise is near 1.5 μV and the drift is close to 30 μV in 6 h. The reproducibility of the sensitivity working with one sample is better than 0.3%, and the change to a new sample keeps the value below 0.5%. The uncertainty in reproducibility in the martensitic transformation (including repositioning) does not overcome 1.6%. The used calorimetric sensors limit the temperature to 373 K. The furnace control originated fluctuations on the base line (near 20 μV), which by means of an auxiliary signal processing were reduced to 50% (less than 10 μV).

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Abstract  

The miniaturized calorimetric devices furnish a reduced working flat surface and permits measurements with extremely low-mass quantities. The experimental sensitivity shows relevant position dependence with x-y surface coordinates and with z-distance. The device identification is realized via a 2-D model based in Fourier general equation. Using the Marquardt method the experimental flat surface device can be identified and the fitted parameters used to simulate the behavior of the experimental system. From the model, the effects of several dissipation configurations can be evaluated. Also, via the RC-analogy, a way to 3-D experimental devices is roughly described.

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Metastable effects on martensitic transformation in SMA

Part III. Tentative temperature effects in a NiTi alloy

Journal of Thermal Analysis and Calorimetry
Authors: C. Auguet, A. Isalgue, F. Lovey, J. Pelegrina, S. Ruiz, and V. Torra

Abstract  

The applicability of shape memory alloys (SMA) for dampers in civil engineering requires guaranteed behavior with well controlled or irrelevant changes after several years of inactivity and/or under the action of daily or seasonal temperature effects. The long time analysis of the aging temperature effects on a NiTi alloy shows similar behavior to other SMA but with higher time constants at temperatures near 373 K: 1.9 days at 410 K and near 100 days at 363 K with an activation energy value near 12500 K (RT=104 kJ mol−1). At the present state of the art, the measurements show that the change of transformation temperatures under the action of the external temperature (or ‘room temperature’ change) is close to 15 K. The direct use of the as furnished material can suffer relevant changes of working properties in devices under the action of direct sunlight or high room temperature working conditions.

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Identification of micro-scale calorimetric devices

Part V. Basic properties for gas–solid reactions

Journal of Thermal Analysis and Calorimetry
Authors: C. Auguet, J. Seguin, F. Martorell, F. Moll, V. Torra, and J. Lerchner

Abstract  

Micro-calorimetric devices using Si-based sensors are very useful for the study of gas–solid reactions, in which very low mass of reactants are necessary. But in fact the consequence of using flat detectors is an increase of the uncertainty in the measured energy. In this work a calorimetric gas sensor based on Xensor chip is analysed studying the local x–y contributions of dissipation to the sensitivity related to the value in the centre. We study also the effects of the gas-flow on the sensitivity, comparing the results obtained with two Xensor type chips. Finally we carry out a deeper analysis of the x–y effects on the calorimetric detector for dissipations in the reactant shell extremely close to the detector surface to visualize the link between the power density distribution and the output signal.

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Metastable effects on martensitic transformation in SMA

Part II. The grain growth effects in Cu-Al-Be alloy

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

Abstract  

The efficiency of shape memory alloy (SMA) as damper and/or standard actuator is truly enhanced when the material can be cycled without any relevant accumulation of the permanent deformation (i.e. under 0.5% for several hundreds of cycles). The particular properties of the CuAlBe alloy permit relevant grain growth with reasonable reduction of mechanical properties (from 300–350 to 250–300 MPa at fracture). Samples prepared with an appropriate heat thermal treatment (HTT) and relevant mean diameter of grain avoids accumulative deformation for series of cycles (near 500) up to 3.5% of deformation. The analysis of different wires of CuAlBe alloy shows, in the first part of HTT, a proportionality between the grain surface and the time at 1123 K. In the last part of the HTT the grain growth shows an increased complexity related with interactions between the grain boundaries and the external surface of the samples.

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Journal of Thermal Analysis and Calorimetry
Authors: C. Auguet, J. Lerchner, P. Marinelli, F. Martorell, M. Rodriguez de Rivera, V. Torra, and G. Wolf

Abstract  

The experimental analysis of conventional conduction calorimeters shows excellent reproducibility and relevant systematic errors in comparison with thermodynamic values established via adiabatic calorimeters. Two examples: a DSC and a liquid flow device are schematically analyzed. When an increased accuracy will be obtained the positional effects on the experimental set-up and on the measurement process need to be modelled. From experimental measurements realized on the Xensor liquid nano-calorimeter representative models can be built. To evaluate the reliability of measurement routines, established from experimental basis, several different dissipation structures inside the working space can be simulated. Two experimental configurations related to drop to drop reaction and to continuous mixing are modelled via RC approach. The RC formalism is extended to evaluate the carried energy effect produced by the continuous inflow/outflow of reactants in the mixing enthalpy chamber.

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