338 research outputs found
Perfect absorption of water waves by linear or nonlinear critical coupling
We report on experiments of perfect absorption for surface gravity waves
impinging a wall structured by a subwavelength resonator. By tuning the
geometry of the resonator, a balance is achieved between the radiation damping
and the intrinsic viscous damping, resulting in perfect absorption by critical
coupling. Besides, it is shown that the resistance of the resonator, hence the
intrinsic damping, can be controlled by the wave amplitude, which provides a
way for perfect absorption tuned by nonlinear mechanisms. The perfect absorber
that we propose, without moving parts or added material, is simple, robust and
it presents a deeply subwavelength ratio wavelength/size
Multiple scattering of elastic waves by pinned dislocation segments in a continuum
The coherent propagation of elastic waves in a solid filled with a random
distribution of pinned dislocation segments is studied to all orders in
perturbation theory. It is shown that, within the independent scattering
approximation, the perturbation series that generates the mass operator is a
geometric series that can thus be formally summed. A divergent quantity is
shown to be renormalizable to zero at low frequencies. At higher frequencies
said quantity can be expressed in terms of a cut-off with dimensions of length,
related to the dislocation length, and physical quantities can be computed in
terms of two parameters, to be determined by experiment. The approach used in
this problem is compared and contrasted with the scattering of de Broglie waves
by delta-function potentials as described by the Schr\"odinger equation
Interfacial toughness evolution under thermal cycling by laser shock and mechanical testing of an EB-PVD coating system
One of the major challenges for coatings on superalloys is to keep adherence during aging, where damage is mostly driven by thermal cycling. On the other hand, the methodology of the evaluation of the interfacial toughness should be consistent with in service loading. Recently, the use of LAser Shock Adhesion Test (LASAT) has shown its capability for both ranking different coating solutions and evaluating the evolution of a given coating as a function of aging [1-2]. The intent of this paper is to demonstrate the ability of LASAT to reproduce damage mechanisms observed under quasi-static in plane mechanical testing and to propose a general methodology to assess interfacial toughness evolution based on LASAT measurements. The material chosen in this study is a partially Y2O3 stabilized EB-PVD zirconia layer coating deposited by Electron Beam – Physical Vapor Deposition (EB-PVD) onto a first generation Ni base superalloy. Aging has been performed using thermal cycling under laboratory air. Degradation of the coating system due to ageing is quantitatively assessed by LASAT and accompanied by different microstructural analysis methods. For LASAT, if laser flux is below a threshold, no delamination occurs. When increasing laser flux above this threshold, a systematic sequence is observed: i) delamination without buckling of the ceramic layer, ii) delamination and buckling, iii) partial cracking of the ceramic layer, and iv) spallation [1-2]. These different states are also achieved in compressive quasi-static testing and assessed by means of local strain measurement using digital image correlation technique [3]. Aging is evaluated through the evolution of both the delamination and the buckling behavior induced by the LASAT method or critical strain at ceramic spallation under compressive static load.
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Short and long crack growth behavior of welded ferritic stainless steel
International audienceTo analyze the influence of welding on fatigue crack growth, several geometrical configurations have been tested using notched specimens with notch located within base metal and welded joint. This methodology has shown that fatigue crack growth rate was similar for base metal and welded part for the ferritic stainless steel F18TNb (corresponding to AISI 441 or EN 1.4509 grades) considering long crack. Whereas for short crack, the microstructure induced by the welding process was evidenced to drastically increase the resulting crack growth rate. The tested configurations have been systematically modeled by finite element analysis to obtain reliable shape function and SIF assessment for the chosen geometries. Then fatigue crack growth behavior is discussed on the basis of both SIF values and the influence of microstructure in crack path and crack growth rate
Crack morphology in a columnar thermal barrier coating system
For high temperature application, EB-PVD ceramic layers are commonly used as thermal barrier coating. During thermal transients, the thermal expansion mismatch between coating and substrate drives failure of the TBC mainly by interfacial cracking. Laser Shock Adhesion Test (LASAT) provides stresses at the ceramic/metal interface enabling controlled interfacial cracking [1-2]. For achieving a clear understanding of the influence of local morphology on interfacial toughness, this study aims at characterizing the 3D morphology of a crack at the interface between metal and an EB-PVD TBC having a columnar structure.
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Evolution de la porosité interfaciale et écaillage induit par oxydation dans un système barrière thermique
International audienceL'exposé présente les premiers développements d'un modèle de prévision de la durée de vie (DDV) d'un système « barrière thermique », sous chargement thermique isotherme. Une base de données pertinente pour la mise au point de ce modèle a été constituée en réalisant des essais d'oxydation de différentes durées. La résistance à l'écaillage a été notamment étudiée par des essais de compression. Ceci a permis la quantification du dommage interfacial qui consiste en la germination, la croissance et la coalescence de cavités entre la sous-couche et l'oxyde. Un modèle traduisant l'évolution de ces cavités a été développé : nous montrons qu'il permet de retrouver les tendances observées expérimentalement et notamment l'indépendance de la déformation à l'écaillage en fonction de la taille des défauts vis-à-vis de la géométrie. À terme, on complétera ce modèle en introduisant l'évolution des contraintes dans le système barrière thermique et en simulant le flambage des décohésions interfaciales
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