Méthodologie d'étude pour optimiser la tenue en fatigue superficielle de dépôts minces

La plupart des pièces revêtues utilisées dans le domaine de la transmission de puissance sont soumises à des sollicitations de chocs répétés. Lorsque ces pièces sont correctement dimensionnées, leur endommagement va être lié à des mécanismes de fatigue superificielle. Afin d'améliorer la durabilité de ces pièces, les dépôts de type PVD et PACVD semblent particulièrement adaptés. Malheureusement, ces dépôts ne peuvent actuellement pas être utilisés sur des composants dont le remplacement est plus délicat que les outils. Une compréhension des mécanismes d'endommagement rendra possible l'optimisation de la tenue en fatigue des revêtements. Dans le but de caractériser la tenue à la fatigue superficielle des couches minces et dures de type PVD, un banc d'essai d'impacts répétés a été mis au point. La possibilité de caractériser ces revêtements sous une sollicitation de chocs répétés est alors illustrée à travers une classification et des cartes de fatigue de dépôts de TiBN, de CrN etde DLC.The most of mechanical components used for transmission of movement are subjected to repeated impacts or cyclic loading. If these elements are well designed and materials well chosen, their endurance is linked to surface fatigue mechanisms. In order to improve the fatigue behaviour of these pieces, hard coatings, like PVD or PACVD coatings, seems to be appropriate. Unfortunately, nowadays, such hard coatings could not be used for elements whose replacement is more difficult than cutting tools. The comprehension of the failure mechanisms will make possible the optimisation of the fatigue behaviour of hard coatings. In order to study the surface fatigue behaviour of thin hard PVD coatings, a specific apparatus was developed to carry out repeated impacts with a broad range of energy. The possibility to chart the fatigue behaviour of different coatings is illustrated throw the example of TiBN, CrN, CrxNy and DLC coatings.LYON-Ecole Centrale (690812301) / SudocSudocFranceF

Identification and optimization of the constitutive model for die-formed expanded graphite for sealing applications

International audienceThe die-formed expanded graphite is used in industry for sealing applications subjected to extreme pressure and temperature. Its chemical inertness, excellent elastic recovery and adaptability to damaged flanges serves the sealing function. However, in some cases, the high load required for tightening leads to significant mechanical effects related to flanges bending or radial thrust. To address this effect and support the implementation of graphite seals investigations are performed to identify a constitutive model for that specific material and describe the interaction of these seals with their environment [1]. Instrumented axial die-compaction tests are performed to characterize the load-compression curve as well as the radial pressure from the sample [2], [3]. A methodology is then developed for the identification of the coefficients related to the constitutive model, assuming a Drucker-Prager Cap model, and especially the hardening behaviour, which drives the seals compression and unloading curves. The work is based on a parametric optimization and a numerical simulation close to the test device itself. The identified model is finally used for graphite seals simulation, to address mechanical interactions with their environment and comparison with tests performed with dedicated instrumented seals validate its relevance

Wedge indentation of elastoplastic solids -from single indentation to interaction between indenters

Performance of metallic seals used between face-turned surfaces is related to their abilities to flow plastically in order to fill up cavities between wedge-shaped asperities. Double wedge indentation is therefore a simple way to investigate what happens at such a seal-flange interface. In this paper, finite element analyses of single and double wedge indentations are conducted. A particular attention is paid to the effects of hardening parameters on the resulting hardness. First, it is observed that single wedge indentation hardness can be well-approximated by the adaptation of analytic models initially developed for cone indentation problems. Second, it is shown that interaction between indentation-strain field during double wedge indentation starts once the bearing ratio is about 25%. It leads to a significant mean contact pressure increase, which is strongly dependent upon the strain hardening exponent. Eventually, for a bearing ratio higher than 75%, a plastic locking stage occurs, which leads to an exponential increase of the mean contact pressure. Practical applications of this work to indentation and sealing research fields are discussed

Imbibition dynamics including corner film flow in a spiral-grooved channel

International audienceIn the field of sealing, mechanical assemblies commonly consist of two flanges made of hard metallic material and a seal made of soft metallic material as shown in figure 1a. These flanges have surface defects due to the manufacturing process, e.g., face-turning, which produces a characteristic spiral-grooved profile as shown in figure 1b. The residual opening defects thus constitute leak paths for the fluid to be sealed and the modeling of fluid flow in the resulting connected aperture field between the high and low pressure zones is essential in order to predict seal efficiency

A two-scale method for slip flow transmissivity computation ofa heterogeneous and anisotropic fracture

International audienceThe situation of pressure-driven gas flow between confined rough walls (i.e., in a fracture) is common in many industrial applications, ranging from fluid recovery through fractured rocks to leak rate determination of static metal-to-metal seals. In such applications, knowledge of the transport properties of the fractures represent a key issue as it can determine the success or failure of the whole system. A real rough fracture is usually characterized by a heterogeneous and multi-scale aperture field along with localized contact spots. A direct modelling of the flow in this connected topography can therefore be a very challenging task, demanding large computational capabilities. In this work, a two-scale method is developed to study the slightly compressible flow of a gas in the slip regime within a rough, multi-scale fracture. Starting from the first-order slip-corrected Reynolds, or lubrication equation at the microscopic scale (the roughness scale), a macroscopic model that operates at the scale of a local cell can be derived using the method of volume averaging [1, 2]. This macroscopic model, analogous to a two-dimensional Darcy law, is characterize by an effective transmissivity tensor that is function of the microstructure and rarefaction through the Knudsen number, Kn, defined as the ratio of the mean free path of the gas to a characteristic aperture. The fracture generally being heterogeneous at large scale (due to the presence of waviness for example), it is subdivided in a set of tiles in which a transmissivity tensor is locally computed by the method discussed above, embedding the local underlying structural and gas rarefaction information. Then, the transport property of the entire fracture is determined by computing the flow in this heterogeneous and fully anisotropic transmissivity 1tensor field. This is achieved making use of a boundary element method [3, 4]. Numerical examples will be given on synthetic fractures such that of figure 1. The ability of the two-scale model to reproduce the results of a direct simulation in slip flow conditions, in terms of global transmissivity, will be presented. The reduction of the overall computational cost achieved by the two-scale method will be discussed as well