A mean-field homogenisation scheme with CZM-based interfaces describing progressive inclusions debonding

The objective of the present study is to describe the progressive debonding of inclusions in particle or fibre reinforced composites. To do so, the mean-field homogenisation scheme of Mori-Tanaka is enriched to take into account imperfect interfaces. The interfaces are modelled by a bilinear Cohesive Zone Model (CZM) taking into account normal and tangential effects. Results obtained with this new mean-field homogenisation scheme are compared to 2D FE-based numerical simulations that are used as reference results. The effects of inclusions volume fraction and size are also observed

Adhesive thickness influence on a structural methacrylate adhesive behavior

Structural adhesive bonding importance has been growing steadily in the last decades as transport sector’s current problematic is to there products’ reduce mass. In addition, compared to riveting and welding, adhesive bonding offers better properties when joining dissimilar materials such as metals and composites which are a pair used more and more frequently. However, adhesive bonding suffers great-ly from a lack of confidence from industries as validating the bond quality need either destructive testing or long and costly nondestructive testing. Both these solutions can hardly be implemented at an industrial level. Nevertheless, with the implementation of robotics it is possible to auto-mate and control the entire bonding processes. In this con-text a collaborative project called S3PAC (Système de Supervision et de Simulation de la Production d’Assemblage par Collage) has been launched in order to offer a fully supervised and automated industrial bonding process

Fiabilité des assemblages structuraux collés pour applications spatiales

Le dimensionnement des joints collés est une préoccupation majeure du CNES pour lesapplications spatiales des futurs lanceurs. Pour dimensionner une structure collée, il est nécessaire depouvoir apprécier les caractéristiques mécaniques du joint collé.Dans cette étude, trois adhésifs structuraux ont été sélectionnés (Hysol®EA 9321, Hysol®EA9394 et Hysol EA® 9395). Après leur caractérisation massique, une étude statistique pour mettre enévidence les effets des différents paramètres (vitesse d essai, géométrie éprouvette, le degré depolymérisation ) a été entreprise.La deuxième étape a pour objectif de fiabiliser l analyse des essais de fissuration etd améliorer la compréhension des mécanismes d endommagement et de propagation de fissure dansles liaisons collées. Trois types d essai ont été utilisés, à savoir, l essai Double Cantilever Beam(DCB), pour l étude du mode I, l essai End Notched Flexure (ENF), pour le mode II, et l essai MixedMode Bending (MMB), pour les chargements en mode mixte I/II. Nous avons développé de nouvellesinstrumentations et méthodologies d analyse. Pour affiner le protocole de test standard, la techniquedite de backface strain monitoring a été utilisée. Elle consiste à positionnées des jauges dedéformation sur les surfaces de l éprouvette de façon à enregistrer l évolution du signalextensométrique durant la propagation de la fissure. Cette méthode permet une meilleure estimation dela position front de fissure ainsi que l'étude de la répartition des contraintes le long du joint de colle.La corrélation d'images numériques (DIC) a également été utilisée afin de proposer un nouveauprotocole de calibrage de la longueur de fissure et pour comparer un modèle analytique (poutre deTimoshenko sur fondation élastique) avec les résultats expérimentaux.Adhesive bonding is being strongly considered in space applications CNES as anadvantageous assembly technique for future launchers. Correct design of adhesive joints is of majorconcern. Aerospace adhesives are tough viscoelastic matrices (special epoxy resins) reinforced withnano-, or microparticles. Extended use of adhesive joints in structural applications is limited due to thedifficulties in predicting in-service performance, frequently leading to over-conservative design.Three structural adhesives (Hysol®EA 9321, Hysol®EA 9394 and Hysol®EA 9395) wereselected. After their bulk characterization, statistical studies to highlight effects of different parameterse.g. speed, test piece geometry, degree of polymerization were undertaken.In the second stage, fracture mechanics tests were effected employing: the double cantileverbeam (DCB) configuration (mode I characterisation), the three point bending end-notched flexure(ENF) (mode II) and the mixed-mode bending (MMB) (combined mode I/II loading). Crack growth inbonded joints was investigated in a novel way. To refine standard test protocol, the backface strainmonitoring technique was used. Strain gauges were used to measure the strain on the exposed skin ofthe adherends during crack onset and propagation. This method allows better estimation of the crackfront position as well as fine investigation of the stress distribution along the bondline and in the crackfront vicinity. Digital image correlation (DIC) was also used to compare analytical models, e.g.Timoshenko beam on elastic foundation model with experimental results.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Mode I cohesive zone model parameters identification and comparison of measurement techniques for robustness to the law shape evaluation

Adhesive bonding modelling is often realised using cohesive zone models (CZM). For pure mode I loading, these laws represent the cohesive stress versus the interface displacement evolution designated as traction-separation laws (TSL). They enable the description of the interface irreversible phenomena such as damage and/or plasticity, while permitting a refined evaluation of the cohesive stress along the overlap. However, these laws are usually chosen a priori. For brittle and ductile adhesives the TSL shapes usually chosen are respectively bilinear softening and elasto-plastic. But the development of direct CZM measurements has highlighted that the law shapes can be more complex. The wrong initial choice of the TSL shape can then have an impact on the simulation results reliability. In this article, several methods used to evaluate CZM parameters are compared in terms of TSL shape robustness. Synthetic noisy data generated from a trapezoidal CZM are used for the inverse identification of a bilinear softening TSL. By applying this procedure on different type of synthetic measurements (respectively Force-displacement, J-integral, backface strain and DIC) the ability of these techniques to capture parameters for a chosen CZM shape that is not the right one enables a rigorous evaluation the robustness to the law shape

Size effect in particle debonding: Comparisons between Finite Fracture Mechanics and Cohesive Zone Model

The present study aims at describing the debonding phenomenon of a particle embedded in an elastic matrix. Two types of fracture mechanics approaches are developed and compared in this context. The phenomenon is analytically described using a finite fracture mechanics approach, while numerical simulations are performed using a cohesive zone model to describe the decohesion process. Both methods rely on two mechanical parameters: the interface strength, σmax and the fracture energy, Gc, of the interface. Both modelling approaches produce results that show larger particles tend to debond before smaller ones although noticeable differences are observed, especially concerning the relationship between the critical load and the particle radius: in the framework of the FFM, the critical load is inversely proportional to the square root of the particle radius, while when using CZM, the critical load is inversely proportional to the particle radius

Thickness influence on a structural methacrylate adhesive behavior

Thickness influence on a structural methacrylate adhesive behavior

Quasi-trivial stacking sequences for the design of thick laminates

Quasi-trivial (QT) sequences have largely proven to be an extremely powerful tool in the design and optimisation of composites laminates. In this paper new interesting properties of this class of stacks are derived. These properties allow to obtain QT sequences by superposing (according to some prescribed rules) any number of QT elementary stacks. In this way, QT solutions with arbitrary large number of plies can be readily obtained, overcoming the computational issues arising in the search of QT solutions with huge number of layers. Moreover, a general version of the combinatorial algorithm to find QT stacks is proposed in this work. It is also proven that the previous estimation of the number of QT solutions, for a given number of plies and saturated groups, is not correct because a larger number of solutions has been found in this study

Strain rate effect on the mechanical properties of a glass fibre reinforced acrylic matrix laminate. An experimental approach

The aim of this study is to evaluate the effect of the loading rate on the mechanical properties and damage mechanisms of a Glass/Elium150 laminate composite. Quasi-static indentation (QS) and low energy dynamic impact (DYN) tests which simulate lifetime structural loadings (dropped tool, gravel impacts, …) are lead. A specific experimental approach is developed to compare results of both experiments. The effect of the loading rate on the structural response (stiffness, dissipated energy) of the composite is highlighted. The numerous damage mechanisms involved in the collapse of the material are observed at a microscopic scale using both optical and scanning electron microscopy (SEM). Finally an intra-laminar crack propagation mechanism is described based on post-mortem observations at ply scale to explain the formation of interlaminar crack

Microscopie photothermique, application à la caractérisation des propriétés thermoélastiques, microscopiques de composites carbone-carbone et de barrières thermiques

Two photothermal microScopy techniques are presented, which are devoted to the local material therit~o elastic properties measurement. Here, we focus on thermal barrier coatings for turbine blades and carbon/carbon composites. In the first part, photoreflectance technique is used for local thermal properties characterization of carbon fibres and matrix so as heterogeneous media (thin coating and functionally graded materials). Theoretical development required for experimental results analysis are presented so as results of numerous experimental data. The second part deals with interferometric photothermal microscopy technique. Once th e experimental set up is described, the photothermal signal origin is analysed to point out several experimental artefacts which can be partially experimental removed. This technique is used for thermal diffusivity measurement of an A1PdMn quasicristal so as the elastic anisotropy and crystallographic orientation of AMI single cristalCe mémoire. Présente deux techniques de microscopie photothermique dédiées à la mesure de propriétés thermo élastiques locales appliquée ici aux barrières thermiques pour aubes de turbine et aux composites thermo structuraux. La première partie est consacrée à la photoréflexion utilisée pour caractériser les propriétés thermiques locales de fibres et matrices de carbone ainsi que celles de milieux hétérogènes (revêtements minces et milieux à gradient de propriétés). Les développements théoriques qui permettent l'analyse des mesures réalisées sur ces matériaux sont présentés ainsi que les résultats de plusieurs campagnes de mesures. La deuxième partie, est consacrée à la microscopie photothermique interférométrique. Le dispositif expérimental étant décrit, l'analyse du signal photothermique mesuré révèle la présence d'artefacts expérimentaux dont on peut partiellement s'affranchir. Cette technique est utilisée pour mesurer la diffusivité thermique d'un quasicristal d'AlPdMn ainsi que l'anisotropie élastique et l'orientation cristallographique de monocristaux cubiques d'AMl