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Dynamique de fissuration à basse vitesse des matériaux vitreux

Abstract : This thesis is focused on the small velocity crack regime (stress corrosion) in vitreous material samples. The experimental set-up allows us to track the propagation of the crack tip in pure mode I (opening mode) by means of optical and atomic force (AFM) microscopes in real time, in-situ conditions and in a carefully controlled atmosphere. Then we were able to investigate the vicinity of the crack tip at the typical length scales (sub-micrometric) characterizing internal heterogeneities of the studied materials. This work is divided in three parts:
1. Crack propagation in glass-ceramics at different structural states (from a pure glassy state to unglassy one) was studied by AFM. Deviations from linear propagation were observed and interpreted. In the most un-glassy material, the crack runs around the nanometric crystals inducing torsion and shearing stresses and consequently causing an increase in material toughness.
2. By using AFM experiments done on glasses at a high magnification level and at temperatures far below the glass transition temperature, the presence of nanoscale damage cavities ahead of the so-called stress-corrosion crack ‘tip' was revealed. Such a ductile fracture mechanism, widely observed in the case of metallic materials at the micrometer scale, might be also at the origin of the striking similarity of the morphologies of fracture surfaces of glass and metallic alloys at different length scales. This innovative result establishes a link between the crack mechanisms involved in ductile (metals) and brittle materials (glasses). Differences take place on length scales of the damage cavities, respectively micrometric and nanometric.
3. Finally, AFM studies of crack propagation in glasses containing alkali ions revealed migration, at the nanometer scale, of chemical species in the vicinity of the crack ‘tip'. The local mechanical stress field, the relative humidity as well as the residence time of the crack's head in a non-fractured zone of the material, are crucial parameters for the control of these diffusion phenomena at the nanometer scale. The implied physico-chemical processes (for instance local migration of sodium ions) were studied.
The in-situ study of crack propagation in the low speed regime in vitreous materials at the pertinent length scale is a key step for further understanding and improving mechanical properties of glasses.
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Submitted on : Thursday, December 2, 2004 - 9:35:14 AM
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Fabrice Célarié. Dynamique de fissuration à basse vitesse des matériaux vitreux. Mécanique [physics.med-ph]. Université Montpellier II - Sciences et Techniques du Languedoc, 2004. Français. ⟨tel-00007583⟩

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