Mécanisme d'émission électronique dans les structures mim et propagation des électrons dans les métaux

The distribution of electrons emitted by MIM structures has been measured by recording an electron image on a photographic plate. Electrons are emitted into vacuum by “formed” structures when a negative voltage is applied to the outer metal films.The escape depth of the electrons emitted by “unformed” structures does not depend on the applied voltage. We conclude that the energy distribution of the electrons injected into the outer electrode does not depend on the applied voltage.     

On the optimal scheduling of periodic tests and maintenance for reliable redundant components

Periodically, some m of the n redundant components of a dependable system may have to be taken out of service for inspection, testing or preventive maintenance. The system is then constrained to operate with lower (n−m) redundancy and thus with less reliability during these periods. However, more frequent periodic inspections decrease the probability that a component fail undetected in the time interval between successive inspections. An optimal time schedule of periodic preventive operations arises from these two conflicting factors, balancing the loss of redundancy during inspections against the reliability benefits of more frequent inspections.Considering no other factor than this decreased redundancy at inspection time, this paper demonstrates the existence of an optimal interval between inspections, which maximizes the mean time between system failures. By suitable transformations and variable identifications, an analytic closed form expression of the optimum is obtained for the general (m, n) case. The optimum is shown to be unique within the ranges of parameter values valid in practice; its expression is easy to evaluate and shown to be useful to analyze and understand the influence of these parameters.Inspections are assumed to be perfect, i.e. they cause no component failure by themselves and leave no failure undetected. In this sense, the optimum determines a lowest bound for the system failure rate that can be achieved by a system of n-redundant components, m of which require for inspection or maintenance recurrent periods of unavailability of length t.The model and its general closed form solution are believed to be new [2] and [5]. Previous work [1], [4] and [10] had computed optimal values for an estimation of a time average of system unavailability, but by numerical procedures only and with different numerical approximations, other objectives and model assumptions (one component only inspected at a time), and taking into account failures caused by testing itself, repair and demands (see in particular [6], [7] and [9]).System properties and practical implications are derived from the closed form analytical expression. Possible extensions of the model are discussed. The model has been applied to the scheduling of the periodic tests of nuclear reactor protection systems.     

Images of 16S ribosomal RNA by scanning tunnelling microscopy

We report the use of scanning tunnelling microscopy (STM) to study surface topographies of complex nucleic acid structures. From low-resolution STM images of uncoated 16S ribosomal RNA, we demonstrate the possibility of determining several objective parameters (molecular mass and radius of gyration) in order to characterize and identify the molecules observed. These parameters were compared with values obtained by other physical methods and the radius of gyration was found to be the most reliable. At high resolution, it was possible to measure the main dimensions of selected V-form particles more precisely than with electron microscopy. Images of the more compact form have been also obtained that show different domains in the macromolecular structure.     

Resin infusion-based processes simulation : coupled Stokes-Darcy flows in orthotropic preforms undergoing finite strain

The aim of this paper is to present an overall model for the study of resin infusion based processes, in particular, the impregnation of a liquid resin through dry deformable fibrous reinforcements. This model can be appliedto a wide range of activities in many fields of engineering. Here, our approach based on a monolithic formulation in a level-set framework allows to strongly couple a Stokes-Darcy flow in low permeability media undergoing finite strains. The Stokes-Darcy coupled problem is solved using a mixed velocity-pressure formulation stabilized by a multi-scale method. A key feature of our approach is the fluid-solid interaction leading to couple a fluid/porous flow to a non-linear solid mechanics formulation. The interaction phenomenon due to the resin flow in the orthotropic highly compressible preform is based on both Terzaghi’s law and on explicit relation expressing permeability as function of porosity in finite strains mechanical framework. Finally, simulations of industrial design parts are performed to illustrate the abilities of our approach and the relevance of this fluid/porous-solid mechanics coupled problem for composite material process simulations.     

Characterization of 3D morphology and microcracks in composites reinforced by multi-axial multi-ply stitched preforms

The micro-structure of polymer matrix composites reinforced by multi-axial multi-ply stitched carbon preforms and manufactured by liquid resin infusion is analyzed. The stitching induces deviations in fibre placement and creates openings which become resin-rich regions after the resin infusion. Characterization of the size and shape of the resin-rich regions of composites with different stitching yarn size and tightness and various stacking sequences has been performed by 2D metallographic micrography and X-ray microtomography. The resin-rich region volume was estimated at roughly 3.0 ± 0.5% of the material volume. The resin-rich regions constitute about 9% of the resin in the entire composite, whose fibre volume fraction is close to 65%. X-ray microtomography was successfully used to characterize the 3D microcracks created by hygrothermal fatigue.     

Semantic structures and logic properties of computer-based system dependability cases

This work addresses the issue of structuring the validation process of dependable computer based systems. It is motivated by the desire to make the licensing and certification of these systems more reliable and efficient. The paper analyses the structural, semantic and logic properties of the demonstration that an embedded computer-based system is adequately specified, designed and maintained in operation. Three basic classes of dependability claims are identified. Claims which address (i) the environment-system interface; (ii) the design; and (iii) the operational behaviour. To deal with the disparity of the real domains, models and languages in which these three classes of claims are formulated and supported, a structure is proposed to analyse the relations between classes and the convergence of arguments based on the diverse sources of evidence. The properties of models and languages required for the interpretations of the real domains and the proof obligations are identified. The implications of these properties on design criteria and mechanisms are discussed. A real dependability case is used to illustrate the concepts and discuss their applicability.     

Osteonecrosis of the humeral head treated by core decompression

Core decompression is one treatment used for symptomatic osteonecrosis of the humeral head. The purpose of this report was to examine the long term outcome of this procedure. Sixty-three shoulders in 43 patients who underwent a core decompression for humeral head osteonecrosis were followed up from 2 to 20 years (mean, 10 years). None of these patients had responded to nonoperative treatment before core decompression. Results of core decompression according to preoperative Ficat and Arlet stage revealed Stage I disease had 15 of 16 (94%) successful outcomes and Stage II had 15 of 17 (88%) successful outcomes. Stage III had 16 of 23 (70%) successful results and Stage IV had one of seven (14%) successful result. Core decompression of the shoulder is a safe procedure with few recognized complications and can be performed on an outpatient basis. The procedure has been successful for Stages I, II, and III osteonecrosis in terms of early relief of pain and increased function.