Modélisation numérique de couplages multiphysiques et de la localisation des déformations

peer reviewe

Modelling an in-situ ventilation test in the Andra Underground Research Facilities

Wastes resulting from the nuclear electricity production have to be isolated from the biosphere for a very long period of time. For this purpose, deep underground repository in weak permeable geological layers is considered as a reliable solution for the nuclear waste storage. It is however well established that during excavation, the underground drilling process engenders cracks and eventually fractures [1] that deteriorate the hydro-mechanical properties of the surrounding host material in the so-called Excavation Damaged Zone (EDZ). The EDZ behaviour is a major issue because it may constitute a preferential flow path for radionuclide migration. Consequently, the characterisation of the material transport properties and of the transfer kinetics that occur around galleries still need to be investigated. The EDZ properties may be also affected by host rock-gallery air interactions. Ventilation induced drying may also provoke additional cracking, which potentially alters the transport properties of the damaged zone. Large-scale air ventilation experiments are performed in Underground Research Laboratories (URL) that have been constructed to check the feasibility of the repository. A numerical modelling of the SDZ air ventilation test (Andra URL) performed in a low permeability rock is proposed in order to both predict the development of the EDZ during excavation and study the air interaction with the host formation during maintenance phases

Modelling an in-situ ventilation test in the Andra Underground Research Facilities

Wastes resulting from the nuclear electricity production have to be isolated from the biosphere for a very long period of time. For this purpose, deep underground repository in weak permeable geological layers is considered as a reliable solution for the nuclear waste storage. It is however well established that during excavation, the underground drilling process engenders cracks and eventually fractures [1] that deteriorate the hydro-mechanical properties of the surrounding host material in the so-called Excavation Damaged Zone (EDZ). The EDZ behaviour is a major issue because it may constitute a preferential flow path for radionuclide migration. Consequently, the characterisation of the material transport properties and of the transfer kinetics that occur around galleries still need to be investigated. The EDZ properties may be also affected by host rock-gallery air interactions. Ventilation induced drying may also provoke additional cracking, which potentially alters the transport properties of the damaged zone. Large-scale air ventilation experiments are performed in Underground Research Laboratories (URL) that have been constructed to check the feasibility of the repository. A numerical modelling of the SDZ air ventilation test (Andra URL) performed in a low permeability rock is proposed in order to both predict the development of the EDZ during excavation and study the air interaction with the host formation during maintenance phases

Modelling the excavation damaged zone in claystone with strain localisation using coupled second gradient model and the influence of gallery ventilation

Drilling of galleries induces stress perturbations that trigger damage propagation in the surrounding medium. The excavation process creates then the so-called excavation damaged zone around the galleries. The prediction of the extension and of the fracture structure within this zone remains nowadays a major issue especially in the context of underground storage. Since localised deformation in shear band mode is frequently observed in experimental works, the excavation damaged zone can be modelled by considering the development of shear strain localisation bands. To correctly model this behaviour, an enhanced model with a regularisation method is required. In underground structures, air ventilation inside the galleries induces a rock-atmosphere interaction that may lead to drainage and to rock desaturation close to the gallery wall. Such desaturation may influence the damage zone structure and needs to be studied. A hydro-mechanical modelling of a gallery excavation including air ventilation is thus performed and the numerical results provide information about the damaged zone extension, the strain localisation bands pattern and the influence of rock desaturation

Effect of interfacial SiO2 thickness for low temperature O-2 plasma activated wafer bonding

It was experimentally demonstrated that bonding strength strongly depends on the total SiO2 thickness near the bonding interface for a given O-2 plasma surface activation. Systematic experiments of Si/SiO2 and SiO2/SiO2 wafer bonding are performed for analyzing the evolution of the bonding surface energy with the interfacial oxide thickness. Optimum plasma exposure time increases with the interfacial SiO2 thickness to achieve the maximum bonding strength in SiO2/SiO2 or SiO2/Si. An optimal process option for plasma activated SiO2/SiO2 wafer bonding is proposed

Routes towards novel active pressure sensors in SOI technology

In this paper, novel pressure sensors approach is proposed and described. Active devices and oscillating circuits are directly integrated on very thin dielectric membranes as pressure transducers. Involved patterning of the membrane is supposed to cause a drop of mechanical robustness. Finite elements simulations are performed in order to better understand stress/strain distribution and as an attempt to explain the early burst of patterned membranes. Smart circuit designs are reported as solutions with high sensitivity and reduced footprint on membrane

Molecular bonding aided by dissipative inter-layers

Molecular bonding plays an important role in the toolkit of micro-fabrication techniques used for the development of micro- and nano-electromechanical systems. Reliability of the bonded interfaces is essential for both structural and functional integrity. This work demonstrates that the insertion of thin dissipative inter-layers can substantially increase the resistance to fracture of interfaces obtained by molecular bonding. A multi-scale numerical modelling strategy was used to quantify the shielding contribution resulting from the presence of a plastically deforming inter-layer and to determine the parameter window in which first-order effects could be expected. The concept was applied to a system that is known to be difficult to bond: the plasma-enhanced chemical vapor deposition (PECVD) SiO2/PECVD SiO2 interface. The introduction of a 1 mu m thick aluminium inter-layer next to the interface led to an average increase of 67% in the overall interface fracture toughness. Proper combinations of inter-layer thickness and inter-layer mechanical properties can theoretically improve the interface fracture toughness by a factor of up to 10. (c) 2006 Published by Elsevier Ltd on behalf of Acta Materialia Inc