State-of-the-Art and Comparative Review of Adaptive Sampling Methods for Kriging

Metamodels aim to approximate characteristics of functions or systems from the knowledge extracted on only a finite number of samples. In recent years kriging has emerged as a widely applied metamodeling technique for resource-intensive computational experiments. However its prediction quality is highly dependent on the size and distribution of the given training points. Hence, in order to build proficient kriging models with as few samples as possible adaptive sampling strategies have gained considerable attention. These techniques aim to find pertinent points in an iterative manner based on information extracted from the current metamodel. A review of adaptive schemes for kriging proposed in the literature is presented in this article. The objective is to provide the reader with an overview of the main principles of adaptive techniques, and insightful details to pertinently employ available tools depending on the application at hand. In this context commonly applied strategies are compared with regards to their characteristics and approximation capabilities. In light of these experiments, it is found that the success of a scheme depends on the features of a specific problem and the goal of the analysis. In order to facilitate the entry into adaptive sampling a guide is provided. All experiments described herein are replicable using a provided open source toolbox. © 2020, The Author(s)

A LATIN-based model reduction approach for the simulation of cycling damage

International audienceThe objective of this article is to introduce a new method including model order reduction for the life prediction of structures subjected to cycling damage. Contrary to classical incremental schemes for damage computation, a non-incremental technique, the LATIN method, is used herein as a solution framework. This approach allows to introduce a PGD model reduction technique which leads to a drastic reduction of the computational cost. The proposed framework is exemplified for structures subjected to cyclic loading, where damage is considered to be isotropic and micro-defect closure effects are taken into account. A difficulty herein for the use of the LATIN method comes from the state laws which can not be transformed into linear relations through an internal variable transformation. A specific treatment of this issue is introduced in this work

A multi-temporal scale model reduction approach for the computation of fatigue damage

International audienceOne of the challenges of fatigue simulation using continuum damage mechanics framework over the years has been reduction of numerical cost while maintaining acceptable accuracy. The extremely high numerical expense is due to the temporal part of the quantities of interest which must reflect the state of a structure that is subjected to exorbitant number of load cycles. A novel attempt here is to present a non-incremental LATIN-PGD framework incorporating temporal model order reduction. LATIN-PGD method is based on separation of spatial and temporal parts of the mechanical variables, thereby allowing for separate treatment of the temporal problem. The internal variables, especially damage, although extraneous to the variable separation, must also be treated in a tactical way to reduce numerical expense. A temporal multi-scale approach is proposed that is based on the idea that the quantities of interest show a slow evolution along the cycles and a rapid evolution within the cycles. This assumption boils down to a finite element like discretisation of the temporal domain using a set of "nodal cycles" defined on the slow time scale. Within them, the quantities of interest must satisfy the global admissibility conditions and constitutive relations with respect to the fast time scale. Thereafter, information of the "nodal cycles" can be interpolated to simulate the behaviour on the whole temporal domain. This numerical strategy is tested on different academic examples and leads to an extreme reduction in numerical expense

A Model Reduction Technique in Space and Time for Fatigue Simulation

International audienceThe simulation of mechanical responses of structures subjected to cyclic loadings for a large number of cycles remains a challenge. The goal herein is to develop an innovative computational scheme for fatigue computations involving non-linear mechanical behaviour of materials, described by internal variables. The focus is on the Large Time Increment (LATIN) method coupled with a model reduction technique, the Proper Generalized Decomposition (PGD). Moreover, a multi-time scale approach is proposed for the simulation of fatigue involving large number of cycles. The quantities of interest are calculated only at particular pre-defined cycles called the “nodal cycles” and a suitable interpolation is used to estimate their evolution at the intermediate cycles. The proposed framework is exemplified for a structure subjected to cyclic loading, where damage is considered to be isotropic and micro-defect closure effects are taken into account. The combination of these techniques reduce the numerical cost drastically and allows to create virtual S-N curves for large number of cycles

PI/PID controller stabilizing sets of uncertain nonlinear systems: an efficient surrogate model-based approach

Closed forms of stabilizing sets are generally only available for linearized systems. An innovative numerical strategy to estimate stabilizing sets of PI or PID controllers tackling (uncertain) nonlinear systems is proposed. The stability of the closed-loop system is characterized by the sign of the largest Lyapunov exponent (LLE). In this framework, the bottleneck is the computational cost associated with the solution of the system, particularly including uncertainties. To overcome this issue, an adaptive surrogate algorithm, the Monte Carlo intersite Voronoi (MiVor) scheme, is adopted to pertinently explore the domain of the controller parameters and classify it into stable/unstable regions from a low number of nonlinear estimations. The result of the random analysis is a stochastic set providing probability information regarding the capabilities of PI or PID controllers to stabilize the nonlinear system and the risk of instabilities. The minimum of the LLE is proposed as tuning rule of the controller parameters. It is expected that using a tuning rule like this results in PID controllers producing the highest closed-loop convergence rate, thus being robust against model parametric uncertainties and capable of avoiding large fluctuating behavior. The capabilities of the innovative approach are demonstrated by estimating robust stabilizing sets for the blood glucose regulation problem in type 1 diabetes patients

PI/PID controller stabilizing sets of uncertain nonlinear systems: an efficient surrogate model-based approach

AbstractClosed forms of stabilizing sets are generally only available for linearized systems. An innovative numerical strategy to estimate stabilizing sets of PI or PID controllers tackling (uncertain) nonlinear systems is proposed. The stability of the closed-loop system is characterized by the sign of the largest Lyapunov exponent (LLE). In this framework, the bottleneck is the computational cost associated with the solution of the system, particularly including uncertainties. To overcome this issue, an adaptive surrogate algorithm, the Monte Carlo intersite Voronoi (MiVor) scheme, is adopted to pertinently explore the domain of the controller parameters and classify it into stable/unstable regions from a low number of nonlinear estimations. The result of the random analysis is a stochastic set providing probability information regarding the capabilities of PI or PID controllers to stabilize the nonlinear system and the risk of instabilities. The minimum of the LLE is proposed as tuning rule of the controller parameters. It is expected that using a tuning rule like this results in PID controllers producing the highest closed-loop convergence rate, thus being robust against model parametric uncertainties and capable of avoiding large fluctuating behavior. The capabilities of the innovative approach are demonstrated by estimating robust stabilizing sets for the blood glucose regulation problem in type 1 diabetes patients

Approche par éléments finis de structures électroniques

Since performances of experimental and numerical tools have been largely improved, mechanics of materials can explore smaller and smaller scales. Thus, a better comprehension, or even a prediction, of local phenomena associated with macroscopic deformations are hoped. This dissertation focuses on the smallest scale involved in mechanical behavior of materials, i.e. interactions between nuclei due to electrons behavior and especially to valence electrons. The originality of this work is setting up the finite element method as numerical tool to solve this problem. This approach largely used to solve structural mechanics problems provides powerful numerical tools to tackle electronic structures. The Hartree-Fock and post-Hartree-Fock models are employed, and mechanical properties of electronic structures are estimated. These estimates are based on a set of approximations of both model and numerical origins. Error estimates are proposed to analyze the accuracy of the results.Grâce à l’amélioration des performances des outils expérimentaux et numériques, la mécanique des matériaux peut explorer des échelles de plus en plus fines. Une meilleure compréhension, voire une prédiction, des phénomènes locaux mis en jeu est alors espérée. Cette thèse s’intéresse à la plus petite échelle impliquée dans le comportement mécanique des matériaux, c.-à-d. les interactions entre noyaux dues au comportement des électrons, et notamment des électrons de valence. L’originalité de ce travail est dans la mise en place des éléments finis comme outil numérique de résolution de ce problème. Cette approche largement utilisée dans le domaine de la mécanique des structures fournit de puissants outils numériques permettant de résoudre le problème électronique. Des modèles de Hartree-Fock et post-Hatree-Fock sont implémentés, et les caractéristiques mécaniques des structures électroniques sont estimées. Ces résultats reposent sur de nombreuses approximations dues aussi bien à la modélisation qu’aux approximations numériques. Des estimateurs d’erreur sont développés pour analyser les résultats

Explorations des discriminations ressenties par les patient∙e∙s de la part de professionnel∙le∙s de santé en France. Étude transversale exploratoire par autoquestionnaire en ligne réalisée auprès de 1340 personnes

CONTEXTE Les discriminations sont pourvoyeuses d’inégalités en santé en France. Chaque médecin fait le serment de faire preuve de non-discrimination à l’égard des patient∙e∙s. Le médecin généraliste est le témoin privilégié des disparités existant dans l’accès aux soins. Son rôle de prévention lui permet de tendre à réduire ces inégalités. Il est difficile de retrouver des éléments de prévalence des discriminations ressenties par les patient∙e∙s dans les données de la littérature.OBJECTIF Évaluer et décrire les discriminations ressenties par les patient∙e∙s de la part de professionnel∙le∙s de santé dans cinq thématiques : poids, origines ethniques, prise d’un médicament substitutif aux opiacés, orientation sexuelle, interruption volontaire de grossesse.MÉTHODE Étude descriptive transversale quantitative par auto-questionnaire en ligne auprès de sujets adultes volontaires. Questionnaire diffusé par la méthode boule de neige via les réseaux sociaux.RÉSULTATS Parmi les 1340 répondant∙e∙s, l’intensité du ressenti d’une discrimination de façon générale, de la part de professionnel∙le∙s de santé, avait une médiane de 30 sur 100 [7 ; 66]. Nous avons pu analyser 754 réponses de participant∙e∙s : 36,1 % avaient ressenti au moins une discrimination dans une des 5 thématiques étudiées ; 31,6 % se sont senti∙e∙s discriminé∙e∙s à cause de leur poids, 4.6 % à cause de leurs origines ethniques et 5,8 % à cause de leur orientation sexuelle. Parmi les individus ayant un IMC ≥ 30, 74,1 % ont ressenti une discrimination ; parmi les personnes ayant des origines ethniques visibles ou audibles autres que caucasiennes, 29 % et parmi les personnes ayant une orientation sexuelle autre que l’hétérosexualité et non cisgenre, 17,9 %. Les personnes ayant ressenti au moins une discrimination sont plus touchées par la précarité. Les patient∙e∙s qui se sont senti∙e∙s victimes de discrimination∙s se sont fréquemment orienté∙e∙s vers un∙e autre professionnel∙le de santé, et ont témoigné d’une baisse de l’estime de soi.DISCUSSION Notre méthode de travail et avoir exploré cinq thématiques discriminatoires dans une même étude sont des forces. Le biais de sélection peut constituer une limite. Sensibiliser les professionnel∙le∙s de santé à la problématique de la discrimination dans le parcours de soin est un enjeu fondamental