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La Ferté-sous-Jouarre, France

Fritsch A.,Vienna University of Technology | Hellmich C.,Vienna University of Technology | Dormieux L.,Ecole des Ponts Paris Technology
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2010

The successful design of ceramic bone biomaterials is challenged by two competing requirements: on the one hand, such materials need to be stiff and strong, which would suggest a low porosity (of pore sizes in the 10-100 mm range) to be targeted; on the other hand, bone biomaterials need to be bioactive (in particular vascularized), which suggests a high porosity of such materials. Conclusively, reliable information on how porosity drives the stiffness and strength properties of ceramic bone biomaterials (tissue engineering scaffolds) is of great interest. In this context, mathematical models are increasingly being introduced into the field. Recently, self-consistent continuum micromechanics formulations have turned out as expressedly efficient and reliable tools to predict hydroxyapatite biomaterials' stiffness and strength, as a function of the biomaterialspecific porosity, and of the 'universal' properties of the individual hydroxyapatite crystals: their stiffness, strength and shape. However, the precise crystal shape can be suitably approximated by specific ellipsoidal shapes: while it was shown earlier that spherical shapes do not lead to satisfactory results, and that acicular shapes are an appropriate choice, we here concentrate on disc-type crystal shape as, besides needles, plates are often reported in micrographs of hydroxyapatite biomaterials. Discbased model predictions of a substantial set of experimental data on stiffness and strength of hydroxyapatite biomaterials almost attain the quality of the very satisfactory needle-based models. This suggests that, as long as the crystal shape is clearly nonspherical, its precise shape is of secondary importance if stiffness and strength of hydroxyapatite biomaterials are predicted on the basis of continuum micromechanics, from their micromorphology and porosity. © 2010. Source

Nguyen M.T.,French Institute of Petroleum | Bemer E.,French Institute of Petroleum | Dormieux L.,Ecole des Ponts Paris Technology
45th US Rock Mechanics / Geomechanics Symposium | Year: 2011

CO2 geological storage is considered as one of the effective solutions to prevent CO2 release into atmosphere and to reduce consecutive greenhouse effect. CO2 injection implies geochemical reactions between the reactive brine and in situ formations, leading to modifications of their petrophysical and geomechanical properties. Acidification effects have been studied at IFP Energies nouvelles using an experimental procedure of chemical alteration, which ensures a homogeneous dissolution pattern of rock samples and especially avoids any wormholing process that could lead to erroneous measurements at the core scale. Geomechanical characterization of altered carbonate samples shows a mechanical weakening of the carbonate formation. This paper presents a first attempt to estimate the evolution of carbonate mechanical properties induced by acid alteration through a micromechanical approach. © 2011 ARMA, American Rock Mechanics Association. Source

Tang C.-S.,Nanjing University | Tang C.-S.,China University of Mining and Technology | Shi B.,Nanjing University | Cui Y.-J.,Ecole des Ponts Paris Technology
Yantu Gongcheng Xuebao/Chinese Journal of Geotechnical Engineering | Year: 2012

The shrinkage characteristics of buffer-backfilling materials play an important role in the security and stability of the high-level radioactive waste geological disposal system. COx argillite is considered as a kind of potential buffer-backfilling materials in France. In this investigation, both the initially saturated compacted COx specimens and the paste-like COx specimens are prepared and subjected to different test methods to study their volumetric shrinkage behaviours. For the compacted specimens, it is found that the volumetric shrinkage deformation is significantly influenced by the initial dry density; the shrinkage limit, shrinkage efficiency and shrinkage strain decrease with the increasing dry density; in addition, it is observed that the shrinkage direction of specimens shows obvious anisotropism. For example, at low degree of compaction, the radial shrinkage strain is higher than axial shrinkage strain, and the shrinkage geometry factor is larger than 3; however, the contrary results are obtained at high degree of compaction. For the paste-like specimens, three shrinkage stages can be distinguished: normal shrinkage, residual shrinkage and zero shrinkage; most of the volume shrinkage deformation occurs before the air-entry point while the soil is still fully saturated. A group of four general shrinkage models are employed to fit the shrinkage curve of the paste-like specimens. The results show that the G & C model can get the highest performance for the present soil. Source

Delage P.,Ecole des Ponts Paris Technology
Geomechanics from Micro to Macro - Proceedings of the TC105 ISSMGE International Symposium on Geomechanics from Micro to Macro, IS-Cambridge 2014 | Year: 2015

A series of oedometer compression tests and mercury intrusion porosimetry tests was carried out on 8 distinct samples of sensitive clays from eastern Canada. A good correspondence was observed between the compression index and the slope of the pore size distribution curves. This correspondence is explained based on the microstructure mechanism proposed by Delage & Lefebvre 1985 who showed that compression in such structured soils occurred, while increasing the compression stress, by the progressive and ordered collapse of pores from the largest ones to the smallest ones. The oedometer compression hence collapses, under a given stress increment, a well defined pore population, the total volume of which is given by the water expelled during the compression corresponding to this stress increment. © 2015 Taylor & Francis Group. Source

Yu L.,Belgian Nuclear Research Center | Weetjens E.,Belgian Nuclear Research Center | Sillen X.,Ondraf Niras | Vietor T.,National Cooperative for the Disposal of Radioactive Waste | And 4 more authors.
Rock Mechanics and Rock Engineering | Year: 2013

A proper evaluation of the perturbations of the host rock induced by the excavation and the emplacement of exothermic wastes is essential for the assessment of the long-term safety of high-level radioactive waste disposals in clay formations. The impact of the thermal transient on the evolution of the damaged zone (DZ) has been explored in the European Commission project TIMODAZ (thermal impact on the damaged zone around a radioactive waste disposal in clay host rocks, 2006-2010). This paper integrates the scientific results of the TIMODAZ project from a performance assessment (PA) point of view, showing how these results support and justify key PA assumptions and the values of PA model parameters. This paper also contextualises the significance of the thermal impact on the DZ from a safety case perspective, highlighting how the project outcomes result into an improved understanding of the thermo-hydro-mechanical behaviour of the clay host rocks. The results obtained in the TIMODAZ project strengthen the assessment basis of the safety evaluation of the current repository designs. There was no evidence throughout the TIMODAZ experimental observations of a temperature-induced additional opening of fractures nor of a significant permeability increase of the DZ. Instead, thermally induced plasticity, swelling and creep seem to be beneficial to the sealing of fractures and to the recovery of a very low permeability in the DZ, close to that of an undisturbed clay host rock. Results from the TIMODAZ project indicate that the favourable properties of the clayhost rock, which guarantee the effectiveness of the safety functions of the repository system, are expected to be maintained after the heating cooling cycle. Hence, the basic assumptions usually made in PA calculations so far are expected to remain valid, and the performance of the system should not be affected in a negative way by the thermal evolution of the DZ around a radioactive waste repository in clay host rock. Source

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