Caractérisation et modélisation du comportement mécanique de matériaux composites SiC/SiC

National audienceLes composites SiC/SiC sont envisagés comme matériaux de coeur des réacteurs nucléaires de 4e génération. Leur comportement mécanique à été étudié par le biais d'essais biaxés, ayant permis la construction et l'identification d'un modèle d'endommagement ainsi que son implantation dans un code éléments finis

Pore size distribution evolution in pellets based bentonite hydration: Comparison between experimental and numerical results

peer reviewedSeveral nuclear waste disposal concept designs take advantage of bentonite based materials to seal underground galleries and shafts. Safety assessment and long-term predictions of the material behaviour have been the main objective of a number of experimental campaigns and of constitutive models development. All these studies have underlined that the multi-porosity bentonite structure affects undeniably the strongly coupled hydro-mechanical processes taking place during water saturation. Due to this, in recent years, many classic experimental tests on unsaturated soils have been performed in conjunction with multi-scale observation techniques (for instance MIP, i.e. mercury intrusion porosimetry analises). Despite the well-known limitations of such observation methods, they provide interesting quantitative measurements in terms of pore diameters families, which differ by several orders of magnitude, and their distribution with respect to different assemblies' types (namely pellets mixtures and compacted bentonite blocks). On the other hand, very few studies have been focusing on the role of such pore size distributions with respect to the hydro-mechanical response, both from an experimental and a numerical point of view. The aim of this paper is to present the experimental campaign and the numerical modelling strategy adopted to analyse the role of different pore size distributions characterising MX-80 bentonite in different forms (i.e. 32 mm pellets mixture, 7 mm pellets mixture and compacted sample surrounded by gap) with same overall dry density during isochoric hydration tests. Taking advantage of multisensor-equipped cells and post-mortem analyses and of the finite element code LAGAMINE, the hydro-mechanical response of these bentonite assemblies is examined. Experimental and numerical outcomes result in good agreement and provide complementary information regarding the features of each assembly type

Bentonite mechanical evolution – experimental work for the support of model development and validation

The objectives of the Beacon experimental studies are to provide input data and parameters for development and validation of models and to reduce uncertainties about conditions and phenomena influencing bentonite homogenisation. Both the homogenisation of an initially inhomogeneous bentonite system and the persistence or development of inhomogeneities in the bentonite system under various mechanical and hydraulic conditions are investigated. Eight experiment teams perform tests involving different bentonite materials and different hydraulic and mechanical boundary conditions. The experiments performed within Beacon address the hydro-mechanical behaviour of macroscopically homogeneous bentonite materials, (the influence of initial granulometry and of hydro-mechanical paths on the final properties of bentonite, constant load and constant volume tests on the Czech Cerny vrch bentonite), the issue of swelling into a limited void (experiments performed in constant volume cells partially filled with bentonite, at different scales and with different emphases), binary mixtures or artificial inhomogeneities (experiments using bentonite block/pellet or pellet/powder mixtures, and the influence of the degree of saturation on the shearing behaviour at a bentonite – steel interface. This report is the first Beacon WP4 deliverable on experimental work and presents the status at Beacon mid-term

Effect of aggregate shapes on local fields in 3D mesoscale simulations of the concrete creep behavior

International audienceDescribing accurately the creep behavior of concrete is of significant importance for the evaluation of the long-term performance of structures. In this regard, a finer characterization of mesostructure effects and material non-linearity provides very insightful information. In this work, the effects of aggregate shapes on the creep response are studied using numerical simulations on 3D mesoscopic samples. The main focus is put on the assessment of the representativeness of generated samples versus real specimens obtained by tomography. Several mesostructures are generated by randomly distributing aggregates with different geometries, from simple spheres to realistic ones extracted from tomography. Creep simulations with finite element (FE) and Fast Fourier Transform (FFT) methods are then performed on different spatial refinements. Moreover, a classical linear viscoelastic (VE) and a viscoelastic-viscoplastic (VE-VP) behavior able to reproduce non-recoverable strains are adopted for describing the matrix behavior, to assess the relevance of a more accurate model. It is shown that numerical samples generated with tomographic aggregates may be regarded as a good approximation of the real specimen, while more 'isotropic' shapes, especially spherical, lead to significant differences at both local and macroscopic levels. Results obtained with FE and FFT methods are very close, indicating that while FFT is well adapted, FE remains attractive in this context. Finally, notable differences are observed between VE and VP response due to the development of residual strains in the matrix and correspondingly more limited strain redistribution, which indicates that VP-like models should be preferred to capture accurately the creep features at mesoscale

Simultaneous fluid and solid density measurements in swelling clay using X‐ray microtomography and 3D particle tracking

International audienceIn order to demonstrate the safety of engineered barriers for radwaste geological repositories, rich characterisations of bentonite hydro‐mechanical behaviour are essential. X‐ray computed tomography was used as the single input to measure both the full displacement field and water content in a heterogeneous bentonite sample resaturated in quasi‐isochoric conditions. Since large texture changes do not allow the use of digital volume correlation, this was achieved using 3D particle tracking to provide the kinematics in combination with calibrated and beam hardening‐corrected grey levels. Matching the grey level sampling with kinematics using a mesh‐based analysis, results show the development of heterogeneous swelling inside the cell and a final solid density gradient in the direction of water flow

Experimental study and modeling of the mechanical behavior of SiC/SiC composite tubes

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Observed heterogeneities after hydration of MX-80 bentonite under pellet/powder form

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Experimental and numerical analyses of the interaction of creep with mesoscale damage in cementitious materials

International audienceDelayed long-term strains of concrete caused by creep are a known problem leading to, \eg the loss of pre-stress and additional microcracking in concrete structures. In order to improve predictions of the creep strains and damage state of cementitious materials, a coupled experimental and numerical study of the creep/microcracking interactions was designed. Compressive creep tests on cement paste and mortar were carried out to analyze the influence of the material heterogeneity and stress level on the creep rate. The obtained data were used for the calibration of a creep constitutive model and as benchmark for predictions of the creep/damage interactions. The creep model was supplemented with separate damage models for the bulk matrix and matrix-aggregate interfaces. The resulting viscodamage models were applied on artificial microstructures of mortar to simulate, using the finite element method, damage effects on the effective mortar creep behavior. This numerical model was able to reproduce the compressive creep behavior of mortar at low stresses and predicted a tertiary creep stage in tension. Nonlinear creep at higher stresses could only be partially reproduced by taking into account these damage mechanisms, pointing toward nonlinear creep phenomena at the microscale

Using X-ray microtomography to study the initiation of chloride-induced reinforcement corrosion in cracked concrete

International audienceCorrosion is the main cause of premature degradation of reinforced concrete structures. In particular, the mechanisms of chloride-induced corrosion in cracked structures are not fully elucidated. The aim of this study is to investigate by X-ray tomographic imaging the initiation and propagation of chloride-induced corrosion in cracked concrete. On reinforced prismatic cracked specimens exposed to seawater spray-drying cycles, the approach consists in characterizing in a non-destructive way the crack geometry, corroded steel volume and corrosion products migrating in the cementitious matrix and crack, as a function of time. Sub-volume scans and difference images allow a study of the first stage of chloride-induced corrosion in cracked concrete with respect to the number of spraying cycles, geometry and crack opening. The volume of steel consumed measured by 3D imaging is validated against destructive measurements, and it is shown that insights about the phenomenology of pitting (depth, displacement of corrosion products, densification, crack clogging…) can be obtaine

DIC analysis of SiC/SiC composite damage mechanisms at the tow scale

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