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Etude du vieillissement des assemblages structuraux acier/aluminium : influence de l'environnement sur le dimensionnement

Abstract : The objective of this work is to study the behaviour of a filled structural epoxy adhesive, in both its bulk state and as used to form a bonded assembly realised between two metallic substrates. This behaviour has to be characterised in the initial state but also after hydrothermal ageing, in order to dimension a bonded structure.
The different mechanical tests carried out on the bulk adhesive in its initial state on one hand (traction on dumbbell test, torsion pendulum, . . .) and on adhesive bonded joints on the other hand (torsion, mixed tests of Arcan type), led to very important disparities of results. The mineral fillers incorporated in the industrial formulation of the adhesive lead to modulus values (Young's modulus, shear modulus) which are (apparently) strongly dependent on the thickness of the joints. This phenomenon is not observed when the joints are realised with a formulation of adhesive free from mineral fillers.
Microscopic analyses underline the dominating role played by the microstructure. The fillers (of talc especially) are composed of layers with weak, cohesive, inter-layer energy. After crosslinking at high temperature, and during cooling, stresses are induced around these fillers and lead to local damage, responsible for the reduction of the elastic properties of the joint.
In situ SEM tests (traction or shear) also allowed us to understand the role of the microstructure and its behaviour during the test. The talc layers exfoliate and create a microscopic crack network in the joint. The final crack leading to failure passes preferentially through these fillers. The first consequence is that the maximum stress is directly linked to the fractional filler content. But the addition of these mineral fillers allows a cohesive failure to occur, whilst the adhesive without fillers gives an adhesive rupture.
Water diffusion into the polymer matrix follows non-Fickian kinetics.Water is present in a first phase, linked to the polar sites of the polymer, and in a second phase, without interaction with the matrix. This Langmuir type diffusion has allowed us to describe the experimental gravimetric curves, determined for various temperature and moisture conditions. These various ageing atmospheres enabled us to relate the different parameters of the diffusion model. The mechanical and physicochemical properties measured on the bulk adhesive vary proportionally with the water concentration within the adhesive. This model, transposed to the adhesive bonded joints, also allows us to describe the evolution of mechanical properties according with ageing. This assumes that the effects of diffusion near the metal / polymer interface are not different from those in the bulk. All these results gathered together in a numerical model allow us to know the state of a structure after ageing and allow us to dimension the desired industrial structure.
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  • HAL Id : tel-00273203, version 1

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Olivier Mario. Etude du vieillissement des assemblages structuraux acier/aluminium : influence de l'environnement sur le dimensionnement. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2007. Français. ⟨tel-00273203⟩

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