La résistance des matériaux en pédagogie active

L'université Aix-Marseille AMU investit depuis 2012 au sein de sa cellule d'innovation pédagogique dans le développement de nouveaux dispositifs d'apprentissage. Le cursus Master Ingénierie est une nouvelle formation d'ingénieurs universitaires qui participe de cet esprit d'innovation. Cette formation forme des ingénieurs sur un cycle de cinq ans basé sur le modèle international du “master of engineering”. Dans ce double contexte, notre équipe pédagogique a suivi des formations proposées par l'école polytechnique de Louvain, nos collègues belges ont mis en place les apprentissages par problèmes (APP) lors de leur réforme en 2000. L'APP fait partie des pédagogies actives qui ont pour but de développer l'autonomie et la motivation des étudiants en donnant du sens à leur apprentissage. La méthode permet au travers d'un groupe d'apprentissage d'approfondir leurs compétences et d'acquérir des savoirs-êtres. Le premier module choisi pour mettre en place les APP est un module de master première année intitulé « Dimensionnement de mécanismes » (résistance des matériaux appliquée aux systèmes). Les pièces étudiées sont élancées (de type poutre), peuvent présenter des accidents de géométrie et sont soumises à des chargements mécaniques monotones, cyclés (fatigue) ou thermiques (thermo-élasticité). A la fin du module, les étudiants doivent maîtriser les outils de base pour le dimensionnement de pièces mécaniques et savoir modéliser un problème réel (conditions limites). Le dispositif complet se fixe comme but au travers de cinq problèmes et un projet d'atteindre l'ensemble des objectifs pédagogiques du module. Les problèmes sont conçus sur des situations concrètes, ludiques et proches des étudiants. Les évaluations sont soit formatives soit certificatives, des auto-évaluations sont fournies à chaque problème: cet ensemble permet aux étudiants de se situer en permanence par rapport aux objectifs d'apprentissage. Un exemple de problème est décrit dans l'article. Pour conclure, un retour des étudiants et une évaluation du dispositif sont présentés

GFRP beams by bonding simple panels: a low-cost design strategy

Although traditional materials (steel, concrete, timber and masonry) still dominate the building industry, new materials are constantly being explored by engineers and scientists. For instance, the use of the so-called FRPs (Fibre-Reinforced Polymers) is gradually spreading worldwide. FRPs can be qualified as non-corrosive, high mechanical strength and lightweight materials. They have achieved in the last few years a relevant role as a building material for applications regarding both the strengthening and the realization of full-composite structures. Examples of applications of FRPs are numerous [1],[2]. The first buildings made from FRP profiles were single-storey gable frames used in the electronics industry for Electromagnetic Interference (EMI) test laboratories. A five-storey building, named the Eyecatcher Building was erected for the Swiss Building Fair in 1998. The most cost-effective way of producing FRPs is the automated process of pultrusion. This process optimizes the production of bars and thin/thick-walled profiles with both closed and open cross-sections which are constant over the length. Because the industrial process is optimized for mass pultrusion of a limited number of shapes, it is difficult to produce complex shapes with standard cost targets. A low-cost design strategy inspired by modularity, able to exploit the immediate availability of “ready-to-use” standard components, plays a crucial role for the large-scale viability of FRP structures. The idea discussed in this paper is focused exactly on the possibility of achieving a complex FRP shape by bonding an appropriate number of simple pultruded shapes with a common epoxy glue. For example, a generic I-profile may be obtained by bonding three rectangular panels (the top/bottom flanges and the web panel), rather than via a unique pultrusion application. In addition, web-to-flange junctions may also be strengthened by bonding appropriate angle profiles. In this view, the possibility of considering composite profiles of a generic cross-section from simple rectangular panels would be an interesting constructive simplification. For this reason, the authors have recently initiated a large experimental investigation, still under development, in order to compare the flexural behaviour of pultruded FRP profiles with that of bonded FRP profiles. The results have shown the possibility of achieving a very good performance, in terms of both failure load and flexural stiffness, allowing us to consider the bonding system proposed as highly competitive in the field of construction of pultruded profiles

Shock characterization for fused silica glass direct bonding with a new experimental bench

 The fused silica glass direct bonding consists in joining two surfaces without using any adhesive. This technology is used in particular to manufacture optical systems like optical slicers or interferometers used in terrestrial optics. The final aim of this investigation consist to spatialize this technology. But the spatial environment is totally different from the terrestrial one. A satellite may undergo shocks, vibrations or thermal fatigue. It is necessary to characterize with accuracy the direct bonded interface under these solicitations to respect the European Space Agency requirements. In this context, a new test machine has been developed to characterize interface shock resistance. The new machine design is based on two principles. The first one, the Arcan assembly modified developed by Cognard, consists in loading an adhesively-bonded assembly with different loading types, tensile mode or shear mode. It is constituted by two half disks with several attachment points on their periphery. These attachment points allow installing the mount on a standard tensile testing machine. The second, the Beevers and Ellis testing machine, imposes a tension load on a specimen by the falling weight along a tube connected to the specimen. In our concept, we replaced the specimen in Beevers and Ellis machine by the Arcan assembly. A test campaign on the fused silica glass direct bonding has been performed with the new experimental device. The specimens are solicited by shocks in pure traction and shear mode. The aim consists in measuring the fracture energy. This energy is calculated in function of the mass and the drop height, and for all tests the drop height is the same to keep a constant speed. In order to complete the dynamic study, a static study is performed with the Arcan device on a standard testing machine in traction, shear and a mixed mode (I+II)

Estimation du comportement thermo-viscoélastique effectif des pièces composites obtenues par impression 3D-FDM

peer reviewedDans le but d’estimer le comportement effectif des pièces obtenues par le procédé de fabrication FDM pour le cas des matériaux composites à fibres courtes on a visé une méthodologie permettant la mise en place d’une procédure d'homogénéisation analytique en thermo-viscoélasticité de façon analogue à celle des matériaux élastiques linéaires ; la prise en compte de la variation des paramètres qui déterminent l’état particulier des fibres dans le filament est achevé grâce à l’introduction des fonctions de distribution obtenues via l’analyse statistique de la microstructure. La procédure d'homogénéisation a été évaluée en comparant ses prédictions aux calculs basés sur la FFT en champ complet et des résultats des essais pour des échantillons traités en autoclave, pour enlever les porosités à l'échelle des couches du filament imprimé

ESTIMATING THERMOMECHANICAL RESIDUAL STRESSES IN FDM 3D PRINTED COMPOSITE PARTS

peer reviewedWe implemented a two-step methodology to estimate the residual stresses induced by the FDM manufacturing process in 3D printed composite parts. The first step consisted in an analytical thermo-viscoelastic homogenization procedure to derive the effective behavior of the filament. The second step consisted in a coupled thermomechanical structural analysis of the part. The homogenization procedure was assessed by comparing its predictions to full-field FFT-based computations. The structural analysis was assessed by comparing its predictions to experimental results

Mean-Field Approximations in Effective Thermo-viscoelastic Behavior for Composite Parts Obtained via Fused Deposition Modeling Technology

editorial reviewedAiming to estimate the effective behavior of the parts obtained by fused deposition modeling (FDM) in the case of short fiber composite materials, the Mean-field homogenization procedure, introduced in linear elasticity, is here extended to linear thermo-viscoelasticity. The variation of the parameters describing the state of the fibers inside the printing filament is represented by introducing appropriate distribution functions obtained through the statistical analysis of the microstructure. The validation of the procedure is achieved by comparing its predictions with calculations based on full-field Fast-Fourier-Transform homogenization and experiments results from samples treated in autoclave to remove layer-scale porosities from the printed filament

Experimental and numerical evaluation of hydro-thermal ageing's effects on adhesive connections in offshore structures

In offshore structures, the need to replace or strengthen existing metal components arises over time. One possible solution is the use of bonded connections. Nevertheless, the durability of the adhesive connections is highly compromised by the prolonged contact with water.In this paper, an experimental and numerical evaluation of hydro-thermal ageing's effects on polyurethane adhesive connections has been analysed. An efficient semi-analytical procedure is developed for the design or verification of existing structures reinforced by bonded steel elements. The model can evaluate the interfacial shear stress at all loading stages up to the failure of bonded joints exposed to the marine environment. The model is based on Bernoulli beam theory and satisfies the requirements of equilibrium and strain compatibility, allowing for interfacial deformations.Furthermore, the mechanical behaviour of bonded connections subject to accelerated ageing has been experimentally investigated. More in detail, an experimental program including double lap shear and end-notch flexure tests was performed to evaluate the cohesive behaviour in Mode II of a polyurethane structural adhesive under the combined effects of water and temperature during 150 days of exposure. The experimental and numerical investigation shows how the performance of the adhesive joints is influenced by the ageing conditions

Accelerated aging procedure of epoxy structural adhesive for marine offshore applications

Knowledge of the long-term performance of adhesive connections is undoubtedly of paramount importance to enable their deployment in civil, mechanical, and other engineering applications. Over time, adverse environmental conditions can strongly influence the performance of adhesive joints leading to a progressive deterioration of their initial mechanical properties. The use of adhesive connections for secondary structures in offshore applications is a technology that allows for the rapid creation of structural members that, however, cannot ignore the influence of hydrothermal effects on mechanical performance due to environmental conditions. In this context, the investigation of the hygrothermal durability of adhesive connections was undertaken through an extensive experimental programme. More specifically, 130 cylindrical steel joints bonded with a commercially epoxy resin for structural applications were tested in Mode I using an Arcan-modified device. Prior to test, the specimens were placed in climatic ovens capable of combining the effects of temperature and humidity for approximately 320 days. In addition, the glass transition temperature, Tg, was assessed by employing the differential scanning calorimeter (DSC) technique to correctly define the experimental ageing conditions. The experimental results show how ageing conditions influence the mechanical properties of the epoxy resin investigated. Finally, some predictive formulations are proposed to calculate the loss of strength of adhesive joints over time

Mechanical Performance of Adhesive Connections in Structural Applications

International audienceAdhesive bonding is an excellent candidate for realising connections for secondary structures in structural applications such as offshore wind turbines and installations, avoiding the risk and associated welding problems. The strength of the adhesive layer is an important parameter to consider in the design process it being lower than the strength capacity of the bonding material. The presence of defects in the adhesive materials undoubtedly influences the mechanical behaviour of bonded composite structures. More specifically, the reduction in strength is more pronounced as the presence of defects (voids) increases. For this reason, a correct evaluation of the presence of defects, which can be translated into damage parameters, has become essential in predicting the actual behaviour of the bonded joints under different external loading conditions. In this paper, an extensive experimental programme has been carried out on adhesively bonded connections subjected to Mode I and Mode II loading conditions in order to characterise the mechanical properties of a commercial epoxy resin and to define the damage parameters. The initial damage parameters of the adhesive layer have been identified according to the Kachanov-Sevostianov material definition, which is able to take into account the presence of diffuse initial cracking