Estimation of the normal contact stiffness for frictional interface in sticking and sliding conditions

Modeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a newer approach for identifying reliable values of the normal contact stiffness between surfaces in contact, in both sliding and sticking conditions. The combination of experimental tests, on a dedicated set-up, with finite element modeling, allowed for an indirect determination of the normal contact stiffness. The stiffness was found to increase with increasing contact pressure and decreasing roughness, while the evolution of surface topography and third-body rheology affected the contact stiffness when sliding

Drawing the Line. Visual Representations of the Labouring and Lower Classes in the Mid-Victorian Illustrated Press.

International audienceAs part of their commitment to topicality, one of the fundamental premises of the Victorian illustrated press, The Graphic (1869-1932) and The Illustrated London News (1842-2003) devoted significant space to the representation of urban poverty. At a time when the discussions around the extension of the franchise provided a tribune for latent tensions between bourgeois power and working-class rise, citizenship came to be defined in terms of social and cultural identities, prompting an exclusive discourse which pervaded the whole political spectrum.Concentrating on a selection of illustrations published between the mid-1860s to the late 1870s, this article purports to examine the way in which this exclusive discourse was articulated by the Illustrated London News and the Graphic, illuminating the conflict between the newspapers’ stated aim of providing a truthful record of everyday events and the images produced. Its central assertion is that the artistic, satirical and technical filters used by the draughtsmen in these representations helped to establish a distinction between “deserving” and “undeserving” poverty. By drawing a literal line between suitable and unsuitable citizens, these periodicals articulated a reassuring visual discourse relying on a shared set of assumptions between artists, editors and readers about what the political nation should become.Engagés dans une démarche de fidélité à l’actualité, l’une des principales ambitions de la presse illustrée victorienne, The Graphic (1869-1932) et The Illustrated London News (1842-2003) ont consacré une part significative de leurs publications à la représentation de la pauvreté urbaine. A l’heure où les débats autour de l’extension du suffrage font affleurer les tensions entre pouvoir bourgeois et montée de la classe ouvrière, c’est en termes d’identités sociales et culturelles que se définit la citoyenneté britannique, objet d’un discours clivé dans l’ensemble de la classe politique.À travers une sélection d’illustrations publiées entre le milieu des années 1860 et la fin de la décennie suivante, le présent article s’attache à analyser la façon dont ces partis pris ont trouvé voix dans les pages de l’Illustrated London News et du Graphic, soulignant une forme de contradiction entre l’objectif affiché par ces journaux, celui d’un réalisme presque photographique, et les images produites. Elaborées au prisme de filtres artistiques et satiriques et reproduites en fonction de contraintes techniques lourdes, ces représentations ont contribué à établir une distinction entre pauvreté “respectable” et déchéance sociale. En traçant une ligne de partage littérale entre dignité et indignité, ces périodiques se sont fait le relais d’un discours visuel rassurant et largement partagé quant à l’avenir de la nation britannique

Overhead expense and percentage methods: a lecture on the cost of doing business, explaining how to find it and what to do with it when found. With practical examples of its use taken from actual business experiences

Mode of access: Internet

Mixed finite element formulation in large deformation frictional contact problem

International audienceThis paper presents a mixed variational framework and numerical examples to treat a bidimensional friction contact problem in large deformation. Two different contact algorithms with friction are developed using the 2D finite element code PLAST2. The first contact algorithm is the classical node-on-segment, and the second one corresponds to an extension of the mortar element method to a unilateral contact problem with friction. In this last method, the discretized normal and tangential stresses on the contact surface are expressed by using either continuous piecewise linear or piecewise constant Lagrange multipliers in the saddle-point formulation. The two algorithms based on Lagrange multipliers method are developed and compared for linear and quadratic elements

Numerical analysis of squeal instability

International audienceThe name “brake squeal” groups a large set of high frequency sound emissions from brake systems, generated during the braking phase and characterized by a periodic or harmonic spectrum. This paper presents two different numerical approaches to identify the mechanism bringing to the dynamic instability, producing squeal noise. The first approach performs a finite element modal analysis on the brake system, to identify the eigenfrequencies of the components and to relate them to the squeal frequencies. The second one uses a specific finite element program, Plast3, appropriate for non-linear dynamic analysis in the time domain and particularly addressed to contact problems with friction between deformable bodies. The use of this program allows to verify the behavior of the components and to link results obtained from the time analysis and the modal one. The study presented is carried on a simple model, composed by a disc, a small pad and a beam supporting the pad, to simplify the behavior of its components

A Metaheuristic for Amortized Search in High-Dimensional Parameter Spaces

Parameter inference for dynamical models of (bio)physical systems remains a challenging problem. Intractable gradients, high-dimensional spaces, and non-linear model functions are typically problematic without large computational budgets. A recent body of work in that area has focused on Bayesian inference methods, which consider parameters under their statistical distributions and therefore, do not derive point estimates of optimal parameter values. Here we propose a new metaheuristic that drives dimensionality reductions from feature-informed transformations (DR-FFIT) to address these bottlenecks. DR-FFIT implements an efficient sampling strategy that facilitates a gradient-free parameter search in high-dimensional spaces. We use artificial neural networks to obtain differentiable proxies for the model's features of interest. The resulting gradients enable the estimation of a local active subspace of the model within a defined sampling region. This approach enables efficient dimensionality reductions of highly non-linear search spaces at a low computational cost. Our test data show that DR-FFIT boosts the performances of random-search and simulated-annealing against well-established metaheuristics, and improves the goodness-of-fit of the model, all within contained run-time costs

Hydrophobic core but not amino-terminal charged residues are required for translocation of an integral thylakoid membrane protein in vivo

The integral membrane protein cytochrome f contains an amino-terminal signal sequence that is required for translocation into the thylakoid membrane. The signal sequence contains a hydrophobic core neighbored by an amino-terminal charged residue. Mutations that introduce charged amino acids into the hydrophobic core are inhibitory to cytochrome f translocation, and thus render cells non-photosynthetic. We have isolated both nuclear and chloroplast suppressors of these mutations by selecting for restoration of photosynthetic growth of Chlamydomonas. Here we describe the characterization of two chloroplast, second site suppressor mutations. Both suppressors remove the positively charged amino acid that borders the amino terminus of the hydrophobic core, and replace this arginine with either a cysteine or a leucine. The existence of these suppressors suggests that the hydrophobic core can be shifted in position within the signal sequence, and analysis of triple mutants in the signal confirms this hypothesis. Thus this signal that mediates translocation into the thylakoid membrane is characterized by a hydrophobic region whose exact amino acid content is not critical, and that need not be flanked on its amino terminus by a charged residue

Influence de la géométrie des premiers corps sur les instabilités de contact - cas du crissement

International audienceThe squeal is often studied because it is uncomfortable for the user and the environment of the vehicle although not harming the operation of the brake mechanism. A preceding study showed experimentally that, for a convenient value of the coefficient of friction and an adequate third body, the appearance of the squeal can be directly related to the geometry of the first bodies. The results presented here deal with the evolution of the tendencies (amplitude, frequency...) of the vibrations induced by friction according to the relative angle between the surfaces of the first bodies. This is a first stage to propose modifications of the first bodies to solve the problems of squeal