Metamodel-based importance sampling for the simulation of rare events

In the field of structural reliability, the Monte-Carlo estimator is considered as the reference probability estimator. However, it is still untractable for real engineering cases since it requires a high number of runs of the model. In order to reduce the number of computer experiments, many other approaches known as reliability methods have been proposed. A certain approach consists in replacing the original experiment by a surrogate which is much faster to evaluate. Nevertheless, it is often difficult (or even impossible) to quantify the error made by this substitution. In this paper an alternative approach is developed. It takes advantage of the kriging meta-modeling and importance sampling techniques. The proposed alternative estimator is finally applied to a finite element based structural reliability analysis.Comment: 8 pages, 3 figures, 1 table. Preprint submitted to ICASP11 Mini-symposia entitled "Meta-models/surrogate models for uncertainty propagation, sensitivity and reliability analysis

Metamodel-based importance sampling for structural reliability analysis

Structural reliability methods aim at computing the probability of failure of systems with respect to some prescribed performance functions. In modern engineering such functions usually resort to running an expensive-to-evaluate computational model (e.g. a finite element model). In this respect simulation methods, which may require $10^{3-6}$ runs cannot be used directly. Surrogate models such as quadratic response surfaces, polynomial chaos expansions or kriging (which are built from a limited number of runs of the original model) are then introduced as a substitute of the original model to cope with the computational cost. In practice it is almost impossible to quantify the error made by this substitution though. In this paper we propose to use a kriging surrogate of the performance function as a means to build a quasi-optimal importance sampling density. The probability of failure is eventually obtained as the product of an augmented probability computed by substituting the meta-model for the original performance function and a correction term which ensures that there is no bias in the estimation even if the meta-model is not fully accurate. The approach is applied to analytical and finite element reliability problems and proves efficient up to 100 random variables.Comment: 20 pages, 7 figures, 2 tables. Preprint submitted to Probabilistic Engineering Mechanic

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

Sessile Legionella pneumophila is able to grow on surfaces and generate structured monospecies biofilms

Currently, models for studying Legionella pneumophila biofilm formation rely on multi-species biofilms with low reproducibility or on growth in rich medium, where planktonic growth is unavoidable. The present study describes a new medium adapted to the growth of L. pneumophila monospecies biofilms in vitro. A microplate model was used to test several media. After incubation for 6 days in a specific biofilm broth not supporting planktonic growth, biofilms consisted of 5.36 ± 0.40 log (cfu cm−2) or 5.34 ± 0.33 log (gu cm−2). The adhered population remained stable for up to 3 weeks after initial inoculation. In situ confocal microscope observations revealed a typical biofilm structure, comprising cell clusters ranging up to 300 μm in height. This model is adapted to growing monospecies L. pneumophila biofilms that are structurally different from biofilms formed in a rich medium. High reproducibility and the absence of other microbial species make this model useful for studying genes involved in biofilm formation

Émasculations cléricales. Itinéraires particuliers pour aborder l'identité du clerc émasculé (XIIe-XVe siècle)

International audienceThis study takes place after the gregorian reform of the eleventh century wich strives to make the chastity respect necessary for all the clergyman. In the acceptation of that continence, several itineraries appear to take into account the emasculated clerks. The revengeful secular castration and the religious enthusiasmic self-emasculation, help us to take a closer look at the twelth to fifteenth century clerks'sexual desire. However certain amont of this emasculations do not involve carnal torments and each clerck has to warn the papacy about this irregular situation. Those different applications for dispensation addressed to the pontifical chancellery allow us to interrogate the notion of " third gender " and to put into perspective the established rules of the Roman Church as for these exeptional applications.Au XIe siècle, la réforme grégorienne s'évertue à imposer la chasteté à tous les clercs, réguliers comme séculiers. Des itinéraires singuliers se dégagent dans l'appropriation de cette continence cléricale : ceux de clercs émasculés. Castration vengeresse des laïcs et auto-émasculation par ferveur religieuse aident à approcher les rapports au désir sexuel qu'entretiennent les clercs du XIIe au XVe siècle. Pour autant, certaines émasculations ne relèvent pas de ces préoccupations charnelles, quoi qu'il en soit, chacun d'entre eux doit alerter la papauté de sa situation irrégulière. Ces différentes demandes de dispense à la chancellerie pontificale permettent de questionner la notion de " troisième sexe " et de mettre en perspective les règles établies par l'Église de Rome quant à ces demandes extraordinaires

Giardia trophozoite-secreted proteins and their effects on intestinal epithelia

Giardia is a major cause of diarrheal disease worldwide. It is a flagellated cyst-forming enteric pathogen that inhabits the lumen of the small intestine. Two genetically distinct lineages (assemblages A and B) are of public health relevance and are often associated with water-borne outbreaks. Yet, the mechanism of pathogenesis and virulence in Giardia is poorly understood. A soluble component derived from healthy, viable and human infective Giardia trophozoites was shown to be able to mediate profound changes in the physiology of human derived enteric cells, consistent with the production of secreted virulence factors by the parasite. Quantitative proteomic analysis was successfully applied to the whole parasite and supernatants derived from the parasite in order to ascertain which parasite proteins are secreted. The genome of Giardia is believed to contain open reading frames which could encode as many as 6,000 proteins although hitherto there was only direct evidence for expression of a few hundred of these. Approximately 1,600 proteins were identified from each assemblage, the vast majority of which being common to both lineages. To look for actual enrichment in the supernatant, the ratio of proteins in the supernatant was compared with the pellet. This defined a far smaller group of putatively secreted proteins enriched comprising a high proportion encoded by genes annotated to have signal peptides, known virulence factors such as the Cathepsin B cysteine proteases and Variable Surface Proteins, scavenging proteins such as an extracellular nuclease and a high proportion of hitherto hypothetical proteins and proteins of unknown function. Further analysis of the genes encoding these proteins indicated that they were highly variable and likely to be under positive selection pressure, confirming their probable role in host-pathogen interactions and their potential as markers for discriminating virulent strains. Based on the proteomic analysis, a new model of pathogenic mechanism for Giardia-induced damage to enteric epithelium in which extracellular nuclease, Cathepsin B and Tenascin may have a concerted action was proposed and may have important implications in the understanding of Giardia pathogenesis

A novel six3 mutation segregates with holoprosencephaly in a large family

Holoprosencephaly is the most common structural malformation of the forebrain in humans and has a complex etiology including chromosomal aberrations, single gene mutations and environmental components. Here we present the pertinent clinical findings among members of an unusually large kindred ascertained over 15 years ago following the evaluation and subsequent genetic work-up of a female infant with congenital anomalies. A genome-wide scan and linkage analysis showed only suggestive evidence of linkage to markers on chromosome 2 among the most likely of several pedigree interpretations. We now report that a novel missense mutation in the SIX3 holopro- sencephaly gene is the likely cause in this family. Molecular genetic analysis and/or clinical characterization now show that at least 15 members of this family are presumed SIX3 mutation gene carriers, with clinical manifestations ranging from pheno- typically normal adults (non-penetrance) to alobar holoprosen- cephaly incompatible with postnatal life. This particular family represents a seminal example of the variable manifestations of gene mutations in holoprosencephaly and difficulties encountered in their elucidation.Fil: Solomon, Benjamin D.. National Institutes of Health; Estados UnidosFil: Lacbawan, Felicitas. National Institutes of Health; Estados Unidos. State University of New York; Estados UnidosFil: Jain, Mahim. National Institutes of Health; Estados UnidosFil: Domene, Sabina. National Institutes of Health; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones en Ingeniería Genética y Biología Molecular "Dr. Héctor N. Torres"; ArgentinaFil: Roessler, Erich. National Institutes of Health; Estados UnidosFil: Moore, Cynthia. Indiana University School of Medicine; Estados UnidosFil: Dobyns, William B.. University of Chicago; Estados UnidosFil: Muenke, Maximilian. National Institutes of Health; Estados Unido

D'X,Y à X,Y,Z, de nouveaux outils pour l' étude architecturale et archéologique. Restitution 3D, lasergrammétrie et photogrammétrie : le cas de la citerne el-Nabih à Alexandrie

Actes du Colloque Virtual Retrospect 2009, Pessac (France) 18, 19 et 20 novembre 2009International audienceL’utilisation des nouvelles technologies 3D, appliquées au domaine de l’étude architecturale et archéologique, s’est généralisée au cours de ces dix dernières années. On a pu constater, du fait de la réduction des coûts d’acquisition et de la simplification de la mise en œuvre de ces techniques, que quelques responsables de chantier ont franchi le pas et troqué leur crayon gris et leur planchette pour un scanner laser 3D. Notre équipe CNRS, qui avait déjà acquis une certaine expérience dans le domaine de la restitution 3D appliquée à l’archéologie, a souhaité expérimenter l’apport de la lasergrammétrie et de la photogrammétrie sur l’un de ses chantiers en cours : la citerne el-Nabih à Alexandrie. Entamées en 2003, les campagnes de relevé et de fouilles touchent aujourd’hui à leur fin. À l’heure où démarre la phase du traitement des données de terrain, nous nous proposons de présenter succinctement les caractéristiques du chantier de la citerne el-Nabih, la méthode développée durant et après la fouille, et les premiers résultats de l’apport de l’utilisation de ces nouvelles technologies

An energy description of wear mechanisms and its applications to oscillating sliding contacts

To quantify wear rates, the Archard approach is classically applied. It relates the wear volume to the product of the sliding distance and the normal load. A wear coefficient is then extrapolated and is supposed to establish the wear resistance of the studied material. This synthesis shows that this approach does not work when the friction coefficient is not constant. It appears to be much more relevant to consider the interfacial shear work as a significant wear parameter. This approach is applied to study the wear response of different steels and then extended to different hard TiN, TiC coatings under reciprocating sliding conditions. By identifying wear energy coefficients the wear quantification can be rationalized and the wear resistance of the studied tribosystems can be classified. This also appears to be a convenient approach to interpret the different wear mechanisms. Metallic materials involving plastic strain are analyzed from FEM computations. The energy balance confirms that a minor part of the dissipated energy is consumed by plasticity, whereas the major part participates in the heat and debris flow through the interface. When a load energy approach is introduced an accumulated density of the dissipated energy variable is considered to quantify the TTS (Tribologically Transformed Structure) formation. A wear ”scenario” of metallic structures is then discussed. This energy wear approach is applied to analyze hard coating wear mechanisms focusing on abrasion and oxidation phenomena. The local wear energy analysis is transposed, thus allowing the lifetime of hard coatings to be quantified