1,501 research outputs found
Boundary integral formulation for interfacial cracks in thermodiffusive bimaterials
An original boundary integral formulation is proposed for the problem of a
semi-infinite crack at the interface between two dissimilar elastic materials
in the presence of heat flows and mass diffusion. Symmetric and skew-symmetric
weight function matrices are used together with a generalized Betti's
reciprocity theorem in order to derive a system of integral equations that
relate the applied loading, the temperature and mass concentration fields, the
heat and mass fluxes on the fracture surfaces and the resulting crack opening.
The obtained integral identities can have many relevant applications, such as
for the modelling of crack and damage processes at the interface between
different components in electrochemical energy devices characterized by
multi-layered structures (solid oxide fuel cells and lithium ions batteries).Comment: 43 pages, 9 figure
Effective elastic properties of planar SOFCs: A non-local dynamic homogenization approach
The focus of the article is on the analysis of effective elastic properties
of planar Solid Oxide Fuell Cell (SOFC) devices. An ideal periodic
multi-layered composite (SOFC-like) reproducing the overall properties of
multi-layer SOFC devices is defined. Adopting a non-local dynamic
homogenization method, explicit expressions for overall elastic moduli and
inertial terms of this material are derived in terms of micro-fluctuation
functions. These micro-fluctuation function are then obtained solving the cell
problems by means of finite element techniques. The effects of the temperature
variation on overall elastic and inertial properties of the fuel cells are
studied. Dispersion relations for acoustic waves in SOFC-like multilayered
materials are derived as functions of the overall constants, and the results
obtained by the proposed computational homogenization approach are compared
with those provided by rigorous Floquet-Boch theory. Finally, the influence of
the temperature and of the elastic properties variation on the Bloch spectrum
is investigated
Multiscale asymptotic homogenization analysis of thermo-diffusive composite materials
In this paper an asymptotic homogenization method for the analysis of
composite materials with periodic microstructure in presence of thermodiffusion
is described. Appropriate down-scaling relations correlating the microscopic
fields to the macroscopic displacements, temperature and mass concentration are
introduced. The effects of the material inhomogeneities are described by
perturbation functions derived from the solution of recursive cell problems.
Exact expressions for the overall elastic and thermodiffusive constants of the
equivalent first order thermodiffusive continuum are derived. The proposed
approach is applied to the case of a two-dimensional bi-phase orthotropic
layered material, where the effective elastic and thermodiffusive properties
can be determined analytically. Considering this illustrative example and
assuming periodic body forces, heat and mass sources acting on the medium, the
solution performed by the first order homogenization approach is compared with
the numerical results obtained by the heterogeneous model.Comment: 40 pages, 13 figure
Adaptive Regularization for Nonconvex Optimization Using Inexact Function Values and Randomly Perturbed Derivatives
A regularization algorithm allowing random noise in derivatives and inexact
function values is proposed for computing approximate local critical points of
any order for smooth unconstrained optimization problems. For an objective
function with Lipschitz continuous -th derivative and given an arbitrary
optimality order , it is shown that this algorithm will, in
expectation, compute such a point in at most
inexact evaluations of and its derivatives whenever , where
is the tolerance for th order accuracy. This bound becomes at
most
inexact evaluations if and all derivatives are Lipschitz continuous.
Moreover these bounds are sharp in the order of the accuracy tolerances. An
extension to convexly constrained problems is also outlined.Comment: 22 page
Adaptive Regularization Algorithms with Inexact Evaluations for Nonconvex Optimization
A regularization algorithm using inexact function values and inexact
derivatives is proposed and its evaluation complexity analyzed. This algorithm
is applicable to unconstrained problems and to problems with inexpensive
constraints (that is constraints whose evaluation and enforcement has
negligible cost) under the assumption that the derivative of highest degree is
-H\"{o}lder continuous. It features a very flexible adaptive mechanism
for determining the inexactness which is allowed, at each iteration, when
computing objective function values and derivatives. The complexity analysis
covers arbitrary optimality order and arbitrary degree of available approximate
derivatives. It extends results of Cartis, Gould and Toint (2018) on the
evaluation complexity to the inexact case: if a th order minimizer is sought
using approximations to the first derivatives, it is proved that a suitable
approximate minimizer within is computed by the proposed algorithm
in at most iterations and at most
approximate
evaluations. An algorithmic variant, although more rigid in practice, can be
proved to find such an approximate minimizer in
evaluations.While
the proposed framework remains so far conceptual for high degrees and orders,
it is shown to yield simple and computationally realistic inexact methods when
specialized to the unconstrained and bound-constrained first- and second-order
cases. The deterministic complexity results are finally extended to the
stochastic context, yielding adaptive sample-size rules for subsampling methods
typical of machine learning.Comment: 32 page
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The GASMEMS network: Rationale, programme and initial results
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.GASMEMS is an Initial Training Network supported by the European Commission, which aims at training young researchers in the field of rarefied gas flows in MEMS, and at structuring research in Europe in the field of gas microflows in order to improve global fundamental knowledge and enable technological applications to an industrial and commercial level. The partners and the global objectives of this 4 year programme are detailed, and some initial results are presented. First experimental data about the flow of binary gas mixtures through rectangular microchannels are successfully compared with continuum and kinetic models, in the slip flow and early transition regimes. The behaviour of these mixtures has also been simulated in triangular microchannels, for the whole range of the Knudsen number, using a kinetic approach
and the McCormack model. Heat transfer in plane microchannels has been numerically investigated, pointing out compressibility and rarefaction effects. The effect of thermal creep has been studied comparing BGK, Smodel and ellipsoidal model with the solution from the full Boltzmann equation. A semi-analytical model of the Knudsen layer has been developed and used to simulate the problem of thermal transpiration in a
microchannel. Gaseous flows through rough microchannels have been simulated using kinetic theory and DSMC method, the wall roughness being simulated as a highly porous medium of variable thickness.This study is funded by the European Community's Seventh Framework Programme
FP7/2007-2013 under grant agreement ITN GASMEMS n° 215504
Assessment of co-creativity in the process of game design
We consider game design as a sociocultural and knowledge modelling activity, engaging participants in the design
of a scenario and a game universe based on a real or imaginary socio-historical context, where characters can introduce life narratives and interaction that display either known social realities or entirely new ones. In this research, participants of the co-creation activity are Malaysian students who were working in groups to design game-based learning resources for rural school children. After the co-creativity activity, the students were invited to answer the co-creativity scale, an adapted version of the Assessment Scale of Creative Collaboration (ASCC), combining both the co-creativity factors and learners’ experiences on their interests, and difficulties they faced during the co-creativity process. The preliminary results showed a high diversity on the participants’ attitudes towards collaboration, especially related to their preferences towards individual or collaborative work
Negative refraction for anti-plane elastic waves in canonical quasicrystalline laminates
Elastic anti-plane shear waves can be refracted negatively when they are transmitted across an interface between a homogeneous substrate and a transverse periodic laminate. To achieve pure negative refraction, the frequency of the source should be lower than the upper limit of the second transition zone of the harmonic spectrum of the laminate. An effective way to control the location of transition zones is to consider a canonical configuration for the laminate, a concept that originates from the properties of quasicrystalline sequences among which the Fibonacci one is a particular case. We give a detailed account of the classification in three families of canonical configurations and the role of canonical frequency. We exploit the knowledge of the scaling factor of the self-similar structure of the layout of transition zones for laminates of this kind to provide a quantitative tool to predict the relevant frequencies to accomplish negative refraction. We also investigate how the change of other parameters of the elementary cell may affect the values of those frequencies. The obtained results show that the features of quasicrystalline sequences may be profitably exploited for the realisation of elastic metamaterials
Dynamic energy release rate in couple-stress elasticity
This paper is concerned with energy release rate for dynamic steady state crack problems in elastic materials with microstructures. A Mode III semi-infinite crack subject to loading applied on the crack surfaces is considered. The micropolar behaviour of the material is described by the theory of couple-stress elasticity developed by Koiter. A general expression for the dynamic J-integral including both traslational and micro-rotational inertial contributions is derived, and the conservation of this integral on a path surrounding the crack tip is demonstrated
Remarks on the energy release rate for an antiplane moving crack in couple stress elasticity
This paper is concerned with the steady-state propagation of an antiplane
semi-infinite crack in couple stress elastic materials. A distributed loading
applied at the crack faces and moving with the same velocity of the crack tip
is considered, and the influence of the loading profile variations and
microstructural effects on the dynamic energy release rate is investigated. The
behaviour of both energy release rate and maximum total shear stress when the
crack tip speed approaches the critical speed (either that of the shear waves
or that of the localised surface waves) is studied. The limit case
corresponding to vanishing characteristic scale lengths is addressed both
numerically and analytically by means of a comparison with classical elasticity
results.Comment: 37 pages, 13 figure
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