Material independent crack arrest statistics

The propagation of (planar) cracks in a heterogeneous brittle material characterized by a random field of toughness is considered, taking into account explicitly the effect of the crack front roughness on the local stress intensity factor. In the so-called strong-pinning regime, the onset of crack propagation appears to map onto a second-order phase transition characterized by universal critical exponents which are independent of the local characteristics of the medium. Propagation over large distances can be described by using a simple one-dimensional description, with a correlation length and an effective macroscopic toughness distribution that scale in a non-trivial fashion with the crack front length. As an application of the above concepts, the arrest of indentation cracks is addressed, and the analytical expression for the statistical distribution of the crack radius at arrest is derived. The analysis of indentation crack radii on alumina is shown to obey the predicted algebraic expression for the radius distribution and its dependence on the indentation load

La simultanéité absolue de la théorie de l'Éther de Lorentz

Remettre en cause le postulat de la simultanéité absolue de la théorie de l'Éther de Lorentz ne devrait pas être un sujet polémique parce que cette théorie est sensée avoir été supplantée par la théorie de la relativité restreinte qui est testée avec succès depuis près d'un siècle. Peu importe donc d'avoir raison ou tord puisque cette remise en cause porte sur une théorie obsolète. Expérimentalement, la théorie de l'Éther de Lorentz serait équivalente à la relativité restreinte à cause de la conspiration des horloges et des règles. Privilégier la relativité restreinte ou la théorie de l'Éther de Lorentz serait un choix d'ordre ontologique. Ce statut-quo philosophique est inacceptable parce que physiquement : soit la vitesse de la lumière est constante dans tous les référentiels inertiels, soit elle ne l'est pas.Nous verrons dans cet article que la théorie de l'Éther de Lorentz est inconsistante, invalide expérimentalement et qu'elle prévoit comme la relativité restreinte une relativité de la simultanéit

Geometric Modelling and Deformation for Shape Optimization of Ship Hulls and Appendages

International audienceThe precise control of geometric models plays an important role in many domains such as computer-aided geometric design and numerical simulation. For shape optimization in computational fluid dynamics (CFD), the choice of control parameters and the way to deform a shape are critical.In this article, we describe a skeleton-based representation of shapes adapted for CFD simulation and automatic shape optimization. Instead of using the control points of a classic B-spline representation, we control the geometry in terms of architectural parameters. We assure valid shapeswith a strong shape consistency control. Deformations of the geometry are performed by solving optimization problems on the skeleton. Finally, a surface reconstruction method is proposed to evaluate the shape's performances with CFD solvers. We illustrate the approach on two problems: thefoil of an AC45 racing sail boat and the bulbous bow of a fishing trawler. For each case, we obtained a set of shape deformations and then we evaluated and analyzed the performances of the different shapes with CFD computations

Geometric model for automated multi-objective optimization of foils

This paper describes a new generic parametric modeller integrated into an auto- mated optimization loop for shape optimization. The modeller enables the generation of shapes by selecting a set of design parameters that controls a twofold parameterization: geometrical - based on a skeleton approach - and architectural - based on the experience of practitioners - to impact the system performance. The resulting forms are relevant and effective, thanks to a smoothing procedure that ensures the consistency of the shapes produced. As an application, we propose to perform a multi-objective shape optimization of a AC45 foil. The modeller is linked to the fluid solver AVANTI, coupled with Xfoil, and to the optimization toolbox FAMOSA

Reconstruction d'images dégradées par la turbulence atmosphérique en utilisant les statistiques d'ordre supérieur

Cet article présente quelques résultats concernant le problème de la reconstruction d'images lorsque celles-ci sont brouillées lors de leur observation à travers un milieu fluctuant. Nous précisons les conditions dans lesquelles les algorithmes fondés sur la théorie des statistiques d'ordre supérieur peuvent être utilisés à bon escient ou non. Ceci se traduit par une contrainte de factorisabilité sur les spectres d'ordre supérieur des signaux étudiés, contrainte exprimant la linéarité de l'effet du processus perturbateur. A la lumière de cette considération nous proposons un critère permettant de vérifier si la déformation subie par l'image est une déformation linéaire ou si au contraire elle présente des nonlinéarités significatives. Nous proposons aussi un algorithme résistant bien au bruit d'estimation des spectres dans le cas où l'hypothèse de linéarité est suffisamment bien vérifiée

Development of a CFD-based screening tool for VIV prediction

This paper presents an efficient screening tool, based on the 2D strip method and the CFD-based approach. The proposed method allows the study of the dynamics of the vortex-induced vibration (VIV) phenomenon with almost the same accuracy as a fully 3D Fluid-Structure interaction (FSI) model, but with a reduction in computational times by a factor of nearly 20 compared to the 3D approach. Such a large reduction makes the use of CFD-based analysis feasible on a workstation within a reasonable time frame and opens the possibility of CFD approaches in a subsea pipeline design process

Damages and matter ejection during HVI on brittle structures: implications for space environment

The population of space debris is suspected to grow on earth’s orbits. This might be due to self generation processes occurring when an object impacts a satellite’s surface. As brittle materials, widely used for large solar arrays, are particularly sensitive to HVI in terms of damage and matter ejection, they might play an important role in debris generation. This paper focuses on HVI on thin brittle targets like Hubble silicon solar cells. Simulated and in-situ Front back impacts on both HST-CS and simple SiO2 targets were analyzed. Ejected volume and fragments were collected and filmed to characterize potential secondary debris. A mechanical analysis of damages due to the multilayered structure of solar cells as well as Ls-Dyna SPH calculations have also been performed and are compared to experimental results to assess predictions capabilities of the SPH code

Material-independent crack arrest statistics: Application to indentation experiments

An extensive experimental study of indentation and crack arrest statistics is presented for four different brittle materials (alumina, silicon carbide, silicon nitride, glass). Evidence is given that the crack length statistics can be described by a universal (i.e. material independent) distribution. The latter directly derives from results obtained when modeling crack propagation as a depinning phenomenon. Crack arrest (or effective toughness) statistics appears to be fully characterized by two parameters, namely, an asymptotic crack length (or macroscopic toughness) value and a power law size dependent width. The experimental knowledge of the crack arrest statistics at one given scale thus gives access to its knowledge at all scales

Parametric shape modeler for hulls and appendages

International audienceThis paper describes a parametric shape modeler tool for deforming hulls and appendages, with the purpose of being integrated into an automatic shape optimization loop with a CFD solver. The modeler allows generating shapes by controlling the parameters of a twofold parameterization: geometrical – based on a skeleton approach – and architectural – based on the design practice and effects on the object's performance. The resulting forms are relevant and valid thanks to a smoothing term to ensure shape consistency control. Thanks to this approach, architects can directly use a NURBS CAD model in the modeler tool and will obtain variations of the initial design to improve performance without additional work. The methodology developed can be applied to any shape that can be described by a skeleton, e.g. hulls, foils, bulbous bows, but also wind turbines, airships, etc. The skeleton consists of a set of B-Spline curves composed of a generating curve and section curves. The deformation of the shape is performed by changing explicit parameters of the representation or implicit parameters such as architectural parameters. The new shape is obtained by minimizing a distance function between the current parameters and the target's in combination with a smoothing term to assure shape consistency control. Finally, the 3D surface wrapping the skeleton is rebuilt using surface network technics. This paper presents the general methodology and an example of application to a bulbous bow on a fishing trawler, with RANSE CFD computations to determine the best design