A phenomenological approach, in the parent phase of Cu-Zn-Al shape memory alloy, establishes a predictable model (or mathematical
equations) relating the dependence of Ms with the temperature over a long period of time (i.e. seasonal or yearly room temperature). High-resolution resistance and
temperature measurements vs. time are used. The long time Ms tracks the external room temperature via two temperature dependent time constants. In steady state, the changes in Ms approach17 per cent of the ‘room’ temperature change. The detailed analysis shows the puzzling disappearance of the after
Authors:C. Auguet, A. Isalgué, F. Lovey, F. Martorell, and V. Torra
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.
Authors:C. Auguet, A. Isalgue, V. Torra, F. Lovey, and J. Pelegrina
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.
Authors:G. Carreras, A. Isalgue, V. Torra, F. Lovey, and H. Soul
The fatigue-life (traction-traction only) is experimentally studied mainly for pseudoelastic NiTi wire of 2.46 mm of diameter
for eventual application in damping of structures under wind or rain. Thermal effects are highly relevant in determining the
fatigue-life. The results shows that the fracture level overcomes 130000 cycles when the moving air is used for cooling. When
the number of working cycles overcomes 30000, the frictional energy decreases 40% vs. N roughly with an exponential behavior.
Authors:A. Isalgue, V. Torra, A. Yawny, and F. Lovey
The Clausius-Clapeyron coefficient (CCC or αCC) or relation of the stress to transform a thermoelastic SMA sample and temperature, is revisited for 1-dimensional stressed
SMA when polycrystalline materials are used. The experimental method is discussed in the frame of equilibrium thermodynamics
for polycrystalline materials. Suggestions for evaluation of the experimental value in polycrystalline NiTi and CuAlBe specimens
are proposed. The analysis for NiTi wire gives a CCC of αCC=6.3±0.3 MPa K−1. On the other hand CuAlBe provides a value of αCC=2.2±0.4 MPa K−1 for tensile stress.
Authors:A. Amengual, A. Isalgué, F. C. Lovey, F. Marco, and V. Torra
The use of high resolution automatised equipment (HRTA, conduction calorimetry, stress-strain-temperature) allows to study, locally and/or globally, the martensitic transformations of the alloys with memory (Cu-Zn-Al). The observations allow to establish the phenomenology of the effects of the cycling on the crystalline structure. Particularly, the formation of defects, the interactions between the interphases and the precipitates that change the relative stability of the phases, the effects of the nucleation and the growth. The experimental research of the area of coexistence of the two phases allows to set a phenomenological model coherent with the experimental observations.
Authors:C. Auguet, A. Isalgue, F. Lovey, J. Pelegrina, S. Ruiz, and V. Torra
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
Authors:A. Sepulveda, R. Muñoz, F. Lovey, C. Auguet, A. Isalgue, and V. Torra
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.
Authors:V. Torra, C. Auguet, A. Isalgue, F. Lovey, A. Sepulveda, and H. Soul
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.