Evidence and modeling of mechanoluminescence in a transparent glass particulate composite

International audienceMechanoluminescence (ML) of a transparent alkali-phosphate glass composite with SrAl2O4:Eu, Dy particles is reported. Uniaxial compression experiments show the linear dependence of the mechanoluminescence intensity with the mechanical power. A theoretical model, based on the physics of delayed processes (in analogy of viscoelasticity), is proposed. This model accurately predicts the ML intensity changes induced by a complex mechanical loading and provides a convincing description of the mechanoluminescence response

Direct observation of the displacement field and microcracking in a glass by means of X-ray tomography during in situ Vickers indentation experiment

International audienceThe actual displacement field in a glass during an in-situ Vickers indentation experiment was determined by means of X-ray tomography, thanks to the addition of 4 vol % of X-ray absorbing particles, which acted as a speckle to further proceed through digital volume correlation. This displacement was found to agree well with the occurrence of densification beneath the contact area. The intensity of the densification contribution (Blister field proposed by Yoffe) was characterized and provides evidence for the significant contribution of densification to the mechanical fields. Densification accounts for 27% of the volume of the imprint for the studied glass, that is expected to be less sensitive to densification than amorphous silica or window glass. A major consequence is that indentation cracking methods for the evaluation of the fracture toughness, when they are based on volume conservation, as in the case of Hill-Eshelby plastic inclusion theory, are not suitable to glass. The onset for the formation of the subsurface lateral crack was also detected. The corresponding stress is z 14 GPa and is in agreement with the intrinsic glass strength

Techniques expérimentale pour la rhéologie en mécanique des verres

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Dynamique de fissuration à basse vitesse des matériaux vitreux

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.Cette thèse porte sur le régime de fracturation à faible vitesse (corrosion sous contraintes) d'échantillons de matériaux vitreux. Le système expérimental basé sur la microscopie à force atomique (AFM) permet de caractériser in-situ et en atmosphère contrôlée, la propagation d'une tête de fissure aux échelles caractéristiques (sub-micrométriques) des hétérogénéités de structure du matériau. Ce travail est divisé en trois parties : 1. La propagation de la fissure dans des vitrocéramiques a été étudiée par AFM en fonction de leur degré de dévitrification. Des écarts à la propagation rectiligne ont été mis en évidence. Dans le matériau le plus dévitrifié, la fissure contourne les cristaux nanométriques engendrant des contraintes de torsion et de cisaillement ce qui accroît d'autant la résistance à la rupture du matériau. 2. En utilisant la microscopie AFM, nous avons montré que la propagation de la fissure dans des matériaux vitreux –étudiés à des températures très inférieures à la température de transition vitreuse- se faisait, au devant de la tête de fissure, par un processus de nucléation, croissance et la coalescence de nano-cavités d'endommagement. La mise en évidence expérimentale de ce phénomène de ductilité du verre à l'échelle nanométrique est un résultat novateur faisant un lien entre les mécanismes de fissuration des matériaux ductiles (métaux) et fragiles (verres). La différence se situe au niveau des échelles de longueur des cavités, respectivement micrométriques et nanométriques. 3. Enfin, l'étude de la fissuration d'un verre contenant des ions alcalins révèle une migration, à l'échelle nanométrique, d'espèces chimiques au voisinage immédiat de la fissure. Cette migration s'observe sous la forme d'apparition et de croissance de nodules de dimensions nanométriques. Le champ de contrainte mécanique local, le taux d'humidité relative, ainsi que le temps de séjour de la tête de fissure dans une zone non fissurée du matériau, sont des paramètres essentiels pour le contrôle de ces phénomènes de diffusion à l'échelle nanométrique. Les processus physico-chimiques impliqués (comme la diffusion locale d'ions sodium) ont été étudiés.L'étude in-situ de la fissuration des matériaux vitreux aux échelles de longueur caractéristiques des hétérogénéités est d'une importance capitale pour comprendre et améliorer les propriétés mécaniques du verre, matériau qui est à l'heure actuelle de plus en plus utilisé dans de multiples domaines

Ecouter le chant des verres pour comprendre leur rhéologie

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RFDA as a tool for the investigation of the rheology of glasses

International audienceThe Resonance Frequencies and Damping Analysis (RFDA) has become a widely used technique to measurethe elastic properties of materials at high temperatures and especially of inorganic glasses. Usually, theresonance frequency of the first vibration mode of the sample and its damping are analyzed to deduced anelastic modulus and the internal friction respectively. We will show here that the RFDA technique does notactually allow for the measurement of elastic moduli, or only with very restrictive conditions. The resonancefrequencies allow for the measurement of a first viscoelastic modulus (“storage modulus”) and combinedwith the corresponding damping, for the measurement of a second viscoelastic modulus (“loss modulus”).We will show here how we can calculate these moduli, and, at the end, deduced the elastic modulus. By theanalysis of all the resonance frequencies of all the measured vibration modes, we will also show how we canextract individually the activation energies of the relaxation processes of a glass material. Examples will beshown of phosphate and silicate glasses

Brittle/quasi-brittle transition in dynamic fracture: An energetic signature

5 pages, 4 figures, published version, Article highlighted in Physical Review Focus (February 12th, 2010)International audienceDynamic fracture experiments were performed in PMMA over a wide range of velocities and reveal that the fracture energy exhibits an abrupt 3-folds increase from its value at crack initiation at a well-defined critical velocity, below the one associated to the onset of micro-branching instability. This transition is associated with the appearance of conics patterns on fracture surfaces that, in many materials, are the signature of damage spreading through the nucleation and growth of microcracks. A simple model allows to relate both the energetic and fractographic measurements. These results suggest that dynamic fracture at low velocities in amorphous materials is controlled by the brittle/quasi-brittle transition studied here

The Single-Edge Precracked Beam (SEPB) method to assess the fracture toughness, the fracture energy and the stress corrosion cracking of glass

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Composites particulaires à matrice verre : vers des matériaux aux performances mécaniques élevées

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In-situ observation of the formation of indentation cracks in glass by means of synchrotron X-ray tomography

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