11,585 research outputs found
Dynamic problems for metamaterials: Review of existing models and ideas for further research
Metamaterials are materials especially engineered to have a peculiar physical behaviour, to be exploited for some well-specified technological application. In this context we focus on the conception of general micro-structured continua, with particular attention to piezoelectromechanical structures, having a strong coupling between macroscopic motion and some internal degrees of freedom, which may be electric or, more generally, related to some micro-motion. An interesting class of problems in this context regards the design of wave-guides aimed to control wave propagation. The description of the state of the art is followed by some hints addressed to describe some possible research developments and in particular to design optimal design techniques for bone reconstruction or systems which may block wave propagation in some frequency ranges, in both linear and non-linear fields. (C) 2014 Elsevier Ltd. All rights reserved
A unifying perspective: the relaxed linear micromorphic continuum
We formulate a relaxed linear elastic micromorphic continuum model with
symmetric Cauchy force-stresses and curvature contribution depending only on
the micro-dislocation tensor. Our relaxed model is still able to fully describe
rotation of the microstructure and to predict non-polar size-effects. It is
intended for the homogenized description of highly heterogeneous, but non polar
materials with microstructure liable to slip and fracture. In contrast to
classical linear micromorphic models our free energy is not uniformly pointwise
positive definite in the control of the independent constitutive variables. The
new relaxed micromorphic model supports well-posedness results for the dynamic
and static case. There, decisive use is made of new coercive inequalities
recently proved by Neff, Pauly and Witsch and by Bauer, Neff, Pauly and Starke.
The new relaxed micromorphic formulation can be related to dislocation
dynamics, gradient plasticity and seismic processes of earthquakes. It unifies
and simplifies the understanding of the linear micromorphic models
Dynamic pore collapse in viscoplastic materials
Dynamic pore collapse in porous materials is studied by analyzing the finite deformation of an elastic/viscoplastic spherical shell under impulsive pressure loading. Effects of dynamic loading rate, pore size, initial porosity, strain-i-ate sensitivity, strain hardening, thermal softening, and mass density of the matrix material on the pore collapse process are examined and results are compared with those from quasistatic analyses of both rate-independent and rate-dependent matrix materials. Dynamic (inertia) effects are found to be significant or even dominant in certain shock wave consolidation conditions. An approximate method is proposed to incorporate dynamic effects into quasistatic pore-collapse relations of viscoplastic matrix materials. Implications of results of current study are discussed in terms of understanding the processes of shock wave consolidation of powders
Piezo-electromechanical smart materials with distributed arrays of piezoelectric transducers: Current and upcoming applications
This review paper intends to gather and organize a series of works which discuss the possibility of exploiting the mechanical properties of distributed arrays of piezoelectric transducers. The concept can be described as follows: on every structural member one can uniformly distribute an array of piezoelectric transducers whose electric terminals are to be connected to a suitably optimized electric waveguide. If the aim of such a modification is identified to be the suppression of mechanical vibrations then the optimal electric waveguide is identified to be the 'electric analog' of the considered structural member. The obtained electromechanical systems were called PEM (PiezoElectroMechanical) structures. The authors especially focus on the role played by Lagrange methods in the design of these analog circuits and in the study of PEM structures and we suggest some possible research developments in the conception of new devices, in their study and in their technological application. Other potential uses of PEMs, such as Structural Health Monitoring and Energy Harvesting, are described as well. PEM structures can be regarded as a particular kind of smart materials, i.e. materials especially designed and engineered to show a specific andwell-defined response to external excitations: for this reason, the authors try to find connection between PEM beams and plates and some micromorphic materials whose properties as carriers of waves have been studied recently. Finally, this paper aims to establish some links among some concepts which are used in different cultural groups, as smart structure, metamaterial and functional structural modifications, showing how appropriate would be to avoid the use of different names for similar concepts. © 2015 - IOS Press and the authors
Wave propagation in relaxed micromorphic continua: modelling metamaterials with frequency band-gaps
In this paper the relaxed micromorphic model proposed in [Patrizio Neff,
Ionel-Dumitrel Ghiba, Angela Madeo, Luca Placidi, Giuseppe Rosi. A unifying
perspective: the relaxed linear micromorphic continuum, submitted, 2013,
arXiv:1308.3219; and Ionel-Dumitrel Ghiba, Patrizio Neff, Angela Madeo, Luca
Placidi, Giuseppe Rosi. The relaxed linear micromorphic continuum: existence,
uniqueness and continuous dependence in dynamics, submitted, 2013,
arXiv:1308.3762] has been used to study wave propagation in unbounded continua
with microstructure. By studying dispersion relations for the considered
relaxed medium, we are able to disclose precise frequency ranges (band-gaps)
for which propagation of waves cannot occur. These dispersion relations are
strongly nonlinear so giving rise to a macroscopic dispersive behavior of the
considered medium. We prove that the presence of band-gaps is related to a
unique elastic coefficient, the so-called Cosserat couple modulus ,
which is also responsible for the loss of symmetry of the Cauchy force stress
tensor. This parameter can be seen as the trigger of a bifurcation phenomenon
since the fact of slightly changing its value around a given threshold
drastically changes the observed response of the material with respect to wave
propagation. We finally show that band-gaps cannot be accounted for by
classical micromorphic models as well as by Cosserat and second gradient ones.
The potential fields of application of the proposed relaxed model are manifold,
above all for what concerns the conception of new engineering materials to be
used for vibration control and stealth technology
Deep convolutional neural networks for estimating porous material parameters with ultrasound tomography
We study the feasibility of data based machine learning applied to ultrasound
tomography to estimate water-saturated porous material parameters. In this
work, the data to train the neural networks is simulated by solving wave
propagation in coupled poroviscoelastic-viscoelastic-acoustic media. As the
forward model, we consider a high-order discontinuous Galerkin method while
deep convolutional neural networks are used to solve the parameter estimation
problem. In the numerical experiment, we estimate the material porosity and
tortuosity while the remaining parameters which are of less interest are
successfully marginalized in the neural networks-based inversion. Computational
examples confirms the feasibility and accuracy of this approach
On the propagation of a normal shock wave through a layer of incompressible porous material
A novel numerical formulation of the two-phase macroscopic balance equations governing the flow field in incompressible porous media is presented. The numerical model makes use of the Weighted Average Flux (WAF) method and Total Variation Diminishing (TVD) flux limiting techniques, and results in a second-order accurate scheme. A shock tube study was carried out to examine the interaction of a normal shock wave with a thin layer of porous, incompressible cellular ceramic foam. Particular attention was paid to the transmitted and reflected flow fields. The numerical model was used to simulate the experimental test cases, and their results compared with a view to validating the numerical model. A phenomenological model is proposed to explain the behaviour of the transmitted flow field
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