280 research outputs found
The role of electrostriction on the stability of dielectric elastomer actuators
In the field of soft dielectric elastomers, the notion electrostriction
indicates the dependency of the permittivity on strain. The present paper is
aimed at investigating the effects of electrostriction onto the stability
behaviour of homogeneous electrically activated dielectric elastomer actuators.
In particular, three objectives are pursued and achieved: i) the description of
the phenomenon within the general nonlinear theory of electroelasticity; ii)
the application of the recently proposed theory of bifurcation for
electroelastic bodies in order to determine its role on the onset of
electromechanical and diffuse-mode instabilities in prestressed or prestretched
dielectric layers; iii) the analysis of band-localization instability in
homogeneous dielectric elastomers. Results for a typical soft acrylic elastomer
show that electrostriction is responsible for an enhancement towards
diffuse-mode instability, while it represents a crucial property - necessarily
to be taken into account - in order to provide a solution to the problem of
electromechanical band-localization, that can be interpreted as a possible
reason of electric breakdown. A comparison between the buckling stresses of a
mechanical compressed slab and the electrically activated counterpart concludes
the paper
Optimal energy-harvesting cycles for load-driven dielectric generators in plane strain
The performances of energy harvesting generators based on dielectric
elastomers are investigated. The configuration is of a thin dielectric film
coated by stretchable electrodes at both sides. The film is first stretched,
then charged and subsequently, afterwards it is released, and finally the
charge is harvested at a higher electric potential. The amount of energy
extracted by this cycle is bounded by the electric breakdown and the ultimate
stretch ratio of the film as well as by structural instabilities due to loss of
tension. To identify the optimal cycle that complies with these limits we
formulate a constraint optimization problem and solve it with a dedicated
solver for two typical classes of elastic dielectrics. As anticipated, we find
that the performance of the generator depends critically on the ultimate
stretch ratio of the film. However, more surprising is our finding of a
universal limit on the dielectric strength of the film beyond which the optimal
cycle is independent of this parameter. Thus, we reveal that, regardless of how
large the dielectric strength of the material is, there is an upper bound on
the amount of harvested energy that depends only on the ultimate stretch ratio.
We conclude the work with detailed calculations of the optimal cycles for two
commercially available elastic dielectrics
Experimental investigation of the elastoplastic response of aluminum silicate spray dried powder during cold compaction
Mechanical experiments have been designed and performed to investigate the
elasto-plastic behaviour of green bodies formed from an aluminum silicate spray
dried powder used for tiles production. Experiments have been executed on
samples obtained from cold compaction into a cylindrical mould and include:
uniaxial strain, equi-biaxial flexure and high-pressure triaxial
compression/extension tests. Two types of powders have been used to realize the
green body samples, differing in the values of water content, which have been
taken equal to those usually employed in the industrial forming of traditional
ceramics. Yielding of the green body during compaction has been characterized
in terms of yield surface shape, failure envelope, and evolution of cohesion
and void ratio with the forming pressure, confirming the validity of previously
proposed constitutive models for dense materials obtained through cold
compaction of granulates.Comment: 17 pages; Journal of the European Ceramic Society, 201
On the Effect of the Volumetric Deformation in Soft Dielectric Composites with High Phase Contrast
Towards the accurate modelling of soft dielectric composites, this investigation aims at demonstrating that the incompressibility constraint customarily adopted in the literature may lead to largely inaccurate predictions. This claim is grounded on the premise that, even though in these composites each phase may individually be assumed to be incompressible, the volumetric deformation of the softest phase can provide a significant contribution to the effective behaviour if the phase contrast is high enough. To reach our goal, we determine the actuation response of two-phase dielectric laminated composites (DLCs) where the softest phase admits volumetric deformation. Our results, discussed in the light of the limit case in which the softest phase consists of vacuum, on the one hand, challenge the hypotheses usually assumed in the modelling of soft dielectric composites and, on the other hand, are expected to provide useful information for the design of high-performance hierarchical DLCs
On the role of the incompressibility constraint in soft dielectrics
In this work we demonstrate that
the incompressibility constraint customarily
adopted in literature to model soft dielectric composites may lead to incorrect predictions.
In fact, although in these composites each phase may individually be assumed to be incompressible,
for high-phase contrast in terms of elastic moduli the volumetric deformation of the softest phase can provide a
non-negligible contribution to the effective behaviour.
To reach our goal, we determine the effective electric response of
a two-phase Dielectric Laminated Composite (DLC) actuator, whose softest phase is
described by a constitutive law admitting volumetric deformation.
Our results, discussed in the light of the limit case in which the softest phase consists of void,
are expected to aid the design of high-performance hierarchical DLCs
Phononic canonical quasicrystalline waveguides
The dynamic behavior of the class of periodic waveguides whose unit cells are generated through a quasicrystalline sequence can be
interpreted geometrically in terms of a trace map that embodies the recursive rule obeyed by traces of the transmission matrices. We
introduce the concept of canonical quasicrystalline waveguides, for which the orbits predicted by the trace map at specific frequencies, called
canonical frequencies, are periodic. In particular, there exist three families of canonical waveguides. The theory reveals that for those (i) the
frequency spectra are periodic and the periodicity depends on the canonical frequencies, (ii) a set of multiple periodic orbits exists at
frequencies that differ from the canonical ones, and (iii) perturbation of the periodic orbit and linearization of the trace map define a scaling
parameter, linked to the golden ratio, which governs the self-similar structure of the spectra. The periodicity of the waveguide responses is
experimentally verified on finite specimens composed of selected canonical unit cells
On generalised canonical axial waveguides
The dynamic behaviour of the class of periodic phononic waveguides whose unit cells are generated through a quasicrystalline sequence can be interpreted geometrically in terms of a trace map that embodies the recursive rule obeyed by traces of the transmission matrices. It has been recently shown [1,2] that for a canonical waveguide, the orbits predicted by the trace map at specific frequencies, called canonical frequencies, are periodic onto a surface in a 3D space associated with the invariant of the problem. In this talk, we extend the concept of canonical phononic axial waveguide to generalised Fibonacci sequences and show specific behaviours of the canonical configurations for the so-called silver-mean sequence. We explore various kind of periodic orbits for the trace map associated with different self-similar properties of the stop/pass band layout. The obtained results represent both a key to a better understanding of the dynamic properties of classical two-phase composite waveguides and an important advancement towards the realisation of composite quasicrystalline metamaterials
Transformation elastodynamics and cloaking for flexural waves
The paper addresses an important issue of cloaking transformations for fourth-order partial differential equations representing exural waves in thin elastic plates. It is shown that, in contrast with the Helmholtz equation, the general form of the partial differential equation is not invariant with respect to the cloaking transformation. The significant result of this paper is the analysis of the transformed equation and its interpretation in the framework of the linear theory of pre-stressed plates. The paper provides a formal framework for transformation elastodynamics as applied to elastic plates. Furthermore, an algorithm is proposed for designing a square cloak for exural waves, which employs a regularised push-out transformation. Illustrative numerical examples show high accuracy and efficiency of the proposed cloaking algorithm. In particular, a physical configuration involving a perturbation of an interference pattern generated by two coherent sources is presented. It is demonstrated that the perturbation produced by a cloaked defect is negligibly small even for such a delicate interference pattern.
Keywords: cloaking, transformation elastodynamics, plates, invisibilit
Seismic modeling to monitor CO2 geological storage: The Atzbach-Schwanenstadt gas field
We develop a petro-elastical numerical methodology to compute realistic synthetic seismograms and analyze the sensitivity of the seismic response when injecting carbon dioxide (CO2) in a depleted gas reservoir. The petro-elastical model describes the seismic properties of the reservoir rock saturated with CO2, methane and brine, and allows us to estimate the distribution and saturation of CO2 during the injection process. The gas properties, as a function of the in-situ pressure and temperature conditions, are computed with the Peng-Robinson equation of state, taking into account the absorption of gas by brine. Wave attenuation and velocity dispersion are based on the mesoscopic loss mechanism, which is simulated by an upscaling procedure to obtain an equivalent viscoelastic medium corresponding to partial saturation at the mesoscopic scale. Having the equivalent complex and frequency-dependent bulk (dilatational) modulus, we include shear attenuation and perform numerical simulations of wave propagation at the macroscale by solving the viscoelastic differential equations using the memory-variable approach. The pseudo-spectral modeling method allows general material variability and provides a complete and accurate characterization of the reservoir. The methodology is used to assess the sensitivity of the seismic method for monitoring the CO2 geological storage at the Atzbach-Schwanestadt depleted gas-field in Austria. The objective of monitoring is the detection of the CO2 plume in the reservoir and possible leakages of CO2. The leakages are located at different depths, where the CO2 is present as gaseous, liquid and supercritical phases. Even though the differences can be very subtle, this work shows that seismic monitoring of CO2 from the surface is possible. While the identification of shallow leakages is feasible, the detection of the plume and deep leakages, located in the caprock just above the injection formation, is more difficult, but possible by using repeatability metrics, such as the normalized RMS (NRMS) images. Considering real-data conditions, affected by random noise, a reference detection threshold for deep leakages and the CO 2 plume in the reservoir corresponds to a signal-to-noise ratio of about 10 dB.Fil: Picotti, Stefano. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; ItaliaFil: Carcione, José M.. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; ItaliaFil: Gei, Davide. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; ItaliaFil: Rossi, Giuliana. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; ItaliaFil: Santos, Juan Enrique. Consejo Nacional de Investigaciones CientÃficas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de IngenierÃa. Instituto del Gas y del Petróleo; Argentina. Universidad Nacional de La Plata; Argentina. Purdue University; Estados Unido
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