717 research outputs found
Reliability-based design optimization of shells with uncertain geometry using adaptive Kriging metamodels
Optimal design under uncertainty has gained much attention in the past ten
years due to the ever increasing need for manufacturers to build robust systems
at the lowest cost. Reliability-based design optimization (RBDO) allows the
analyst to minimize some cost function while ensuring some minimal performances
cast as admissible failure probabilities for a set of performance functions. In
order to address real-world engineering problems in which the performance is
assessed through computational models (e.g., finite element models in
structural mechanics) metamodeling techniques have been developed in the past
decade. This paper introduces adaptive Kriging surrogate models to solve the
RBDO problem. The latter is cast in an augmented space that "sums up" the range
of the design space and the aleatory uncertainty in the design parameters and
the environmental conditions. The surrogate model is used (i) for evaluating
robust estimates of the failure probabilities (and for enhancing the
computational experimental design by adaptive sampling) in order to achieve the
requested accuracy and (ii) for applying a gradient-based optimization
algorithm to get optimal values of the design parameters. The approach is
applied to the optimal design of ring-stiffened cylindrical shells used in
submarine engineering under uncertain geometric imperfections. For this
application the performance of the structure is related to buckling which is
addressed here by means of a finite element solution based on the asymptotic
numerical method
Reliability-based design optimization using kriging surrogates and subset simulation
The aim of the present paper is to develop a strategy for solving
reliability-based design optimization (RBDO) problems that remains applicable
when the performance models are expensive to evaluate. Starting with the
premise that simulation-based approaches are not affordable for such problems,
and that the most-probable-failure-point-based approaches do not permit to
quantify the error on the estimation of the failure probability, an approach
based on both metamodels and advanced simulation techniques is explored. The
kriging metamodeling technique is chosen in order to surrogate the performance
functions because it allows one to genuinely quantify the surrogate error. The
surrogate error onto the limit-state surfaces is propagated to the failure
probabilities estimates in order to provide an empirical error measure. This
error is then sequentially reduced by means of a population-based adaptive
refinement technique until the kriging surrogates are accurate enough for
reliability analysis. This original refinement strategy makes it possible to
add several observations in the design of experiments at the same time.
Reliability and reliability sensitivity analyses are performed by means of the
subset simulation technique for the sake of numerical efficiency. The adaptive
surrogate-based strategy for reliability estimation is finally involved into a
classical gradient-based optimization algorithm in order to solve the RBDO
problem. The kriging surrogates are built in a so-called augmented reliability
space thus making them reusable from one nested RBDO iteration to the other.
The strategy is compared to other approaches available in the literature on
three academic examples in the field of structural mechanics.Comment: 20 pages, 6 figures, 5 tables. Preprint submitted to Springer-Verla
Optimisation sous contrainte de fiabilité d une coque imparfaite
National audienceSee http://hal.archives-ouvertes.fr/docs/00/59/27/95/ANNEX/r_S5P52J4K.pd
Force and energy dissipation variations in non-contact atomic force spectroscopy on composite carbon nanotube systems
UHV dynamic force and energy dissipation spectroscopy in non-contact atomic
force microscopy were used to probe specific interactions with composite
systems formed by encapsulating inorganic compounds inside single-walled carbon
nanotubes. It is found that forces due to nano-scale van der Waals interaction
can be made to decrease by combining an Ag core and a carbon nanotube shell in
the Ag@SWNT system. This specific behaviour was attributed to a significantly
different effective dielectric function compared to the individual
constituents, evaluated using a simple core-shell optical model. Energy
dissipation measurements showed that by filling dissipation increases,
explained here by softening of C-C bonds resulting in a more deformable
nanotube cage. Thus, filled and unfilled nanotubes can be discriminated based
on force and dissipation measurements. These findings have two different
implications for potential applications: tuning the effective optical
properties and tuning the interaction force for molecular absorption by
appropriately choosing the filling with respect to the nanotube.Comment: 22 pages, 6 figure
Assessment of left ventricular recovery in Tako-Tsubo cardiomyopathy using 2D strain echocardiography
Contre la désertification
De la convention internationale à l'élaboration participative des programmes d'action nationaux
Comme toutes les conventions internationales, la convention sur la désertification est un texte consensuel qui n'indique pas de façon explicite comment mettre en oeuvre toutes ses dispositions. Pourtant, chaque pays affecté est tenu d'élaborer et d'appliquer des programmes d'action nationaux qui doivent être appliqués par les acteurs les plus concernés, les communautés locales.
Cet ouvrage propose une démarche pour l'élaboration participative de ces programmes d'action. Il est la synthèse des résultats et des recommandations de la Conférence sous-régionale de vulgarisation de la Convention de lutte contre la désertification et de mise en oeuvre du Plan d'action du RIOD (Réseau international des ONG sur la désertification) en mai 1996. Cette conférence s'inscrivait dans une série de quatre rencontres sous-régionales préconisées afin de faire connaître la convention à un plus grand nombre d'acteurs.
Masse Lo - Yacine Diagne - Emmanuel Seck - Série Etudes et recherches n° 190-191
ISBN : 92-9130-0187 - Enda-Editions - Boîte Postale 3370 - Dakar - SENEGAL
Tél.: (221) 22-42-29
Télécopie : (221) 23-51-57
E-mail : [email protected].
Prix : 100 FF frais de port inclus pour les pays du Nord - 2 000 FCFA (frais de port : 800 FCFA) pour les pays du SudDe la convention international
Absorption spectrum in the wings of the potassium second resonance doublet broadened by helium
We have measured the reduced absorption coefficients occurring in the wings
of the potassium 4S-5P doublet lines at 404.414 nm and at 404.720 nm broadened
by helium gas at pressures of several hundred Torr. At the experimental
temperature of 900 K, we have detected a shoulder-like broadening feature on
the blue wing of the doublet which is relatively flat between 401.8 nm and
402.8 nm and which drops off rapidly for shorter wavelengths, corresponding to
absorption from the X doublet Sigma+ state to the C doublet Sigma+ state of the
K-He quasimolecule. The accurate measurements of the line profiles in the
present work will sharply constrain future calculations of potential energy
surfaces and transition dipole moments correlating to the asymptotes He-K(5p),
He-K(5s), and He-K(3d).Comment: 2 figure
A generalization of Snoek's law to ferromagnetic films and composites
The present paper establishes characteristics of the relative magnetic
permeability spectrum (f) of magnetic materials at microwave frequencies.
The integral of the imaginary part of (f) multiplied with the frequency f
gives remarkable properties. A generalisation of Snoek's law consists in this
quantity being bounded by the square of the saturation magnetization multiplied
with a constant. While previous results have been obtained in the case of
non-conductive materials, this work is a generalization to ferromagnetic
materials and ferromagnetic-based composites with significant skin effect. The
influence of truncating the summation to finite upper frequencies is
investigated, and estimates associated to the finite summation are provided. It
is established that, in practice, the integral does not depend on the damping
model under consideration. Numerical experiments are performed in the exactly
solvable case of ferromagnetic thin films with uniform magnetization, and these
numerical experiments are found to confirm our theoretical results. Microwave
permeability measurements on soft amorphous films are reported. The relation
between the integral and the saturation magnetization is verified
experimentally, and some practical applications of the theoretical results are
introduced. The integral can be used to determine the average magnetization
orientation in materials with complex configurations of the magnetization, and
furthermore to demonstrate the accuracy of microwave measurement systems. For
certain applications, such as electromagnetic compatibility or radar absorbing
materials, the relations established herein provide useful indications for the
design of efficient materials, and simple figures of merit to compare the
properties measured on various materials
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