Liens entre comportement multiéchelle et mécanismes locaux de la déformation à haute température et pression de silicates biphasés représentatifs de la croûte terrestre inférieure

Le comportement rhéologique à haute température d’agrégats silicatés bi-phasés a été étudié expérimentalement (essais triaxiaux et torsion à haute température et pression). Des observations au microscope à transmission et à balayage ont mis en évidence des microstructures liées à l’histoire locale de déformation. Par des calculs aux éléments finis à l’échelle de quelques grains nous cherchons à comprendre et valider la séquence de mécanismes actifs et leurs liens avec le comportement global

Multi-scale viscoplastic behaviour of Halite: In-situ SEM full field measurements, a micro-mechanical approach

Halite geological formations are already extensively used for underground storage of hydrocarbons. For example, the entire USA federal reserve of petrol resides in deep (500 - 1000 m) artificial salt caverns, which are realized by controlled dissolution. In France, many such salt caverns are used for storage of natural gas by GDF. Salt caverns and carries are also intended to become nuclear waste repositories. At this point, salt caverns are also seriously envisaged for the daily storage of energy from renewable, but intermittent sources (photovoltaic, Aeolian), under the form of compressed air. Halite mechanical behaviour was extensively studied for the purpose of safe geothechnical applications. Halite is a ductile type rock. Its differed (time-dependent) mechanical response dominates by far, and therefore deep salt caverns experience convergence (closure), which may result in catastrophic subsidence of the overlaying geological layers. Hence, a particular attention was drawn to characterize salt single crystal creep properties (active slip systems and critical resolved shear stresses), and the rheology of poly-crystalline salt, at various temperatures, pressures, differential stresses and water contents (Ter Heege et al., 2007). But, most studies were concerned with macroscopically derived flow laws, corresponding to rather high differential stresses (as compared with those experienced on site), where crystal slip plasticity (CSP) dominates. But, many studies have also shown that halite is very sensitive to solution-precipitation creep (SPC) mechanisms, which may result in solution transfer accommodated grain boundary sliding (GBS). Conversely, some recent studies report that halite is able to flow at ambient conditions, and under very small loads, with strain rates much faster (four orders of magnitude) than those extrapolated from high stress experiments (Bérest et al., 2005). Though, the specific creep micro-mechanisms were not identified, Bérest et al. (2005) invoked possible SPC. Additionally, the effects on long term behaviour of cyclic loading (fatigue) are still poorly known. It is therefore still questionable weather it is really possible to safely extrapolate the laboratory data to the long term envisaged geotechnical applications. To answer we need i) additional experimental work in order to up date the deformation mechanism maps on the basis of better identified micro-physical mechanisms and quantification of their respective activity; and ii) numerical modelling at the scales of the material, and of the underground storage structures, in respect with the appropriated thermo-hygro-mechaniclal loadings. In the present work, we present our preliminary investigation of viscoplastic global and local responses of synthetic fine grained (50 - 500 m) halite by the means of full field measurements (FFM) of local strain by digital image correlation (DIC) during simple compression in-situ SEM (Doumalain et al., 2003). Figure 1 shows a typical loading curve obtained incrementally at the constant strain rate of c.a. 5x10-5 s-1. CSP evidenced by the development of slip lines on the free grain surfaces, and characterized by quasi-linear strain hardening, dominates the overall response up to several % of strain (microfracturing did not appear before 8 % strain). Yet, at the scale of the microstructure, the development of viscoplastic strain is heterogeneous, as shown by the strain maps obtained by DIC and corresponding to four incremental stages of the loading sequence. The heterogeneity of the strain field relates to the loading boundary conditions and to the local microstructure, such crystal size and orientation (which is characterized by electron back scattering diffraction, EBSD). Such micromechanical approach aims to provide the basis for the development of FE (finite element) computational CSP of polycrystalline halite

Full field investigation of salt deformation at room temperature: cooperation of crystal plasticity and grain sliding

International audienceWe observed with optical and scanning electron microscopy halite samples during uniaxial compression. Surface displacement fields were retrieved from digital images taken at different loading stages thanks to digital image correlation (DIC) techniques, on the basis of which we could 1) compute global and local strain fields, 2) identify two co-operational deformation mechanisms. The latter were 1) crystal slip plasticity (CSP), as evidenced by the occurrence of slip lines and computed discrete intracrystalline slip bands at the grain surfaces, 2) interfacial micro-cracking and grain boundary sliding (GBS), as evidenced by the computed relative interfacial displacements. The heterogeneities of the strain fields at the aggregate and at the grain scale, and the local contributions of each mechanism were clearly related to the microstructure, i.e. the relative crystallographic orientations of neighboring grains and the interfacial orientations with respect to the principal stress

Three-dimensional full-field X-ray orientation microscopy

International audienceA previously introduced mathematical framework for full-field X-ray orientation microscopy is for the first time applied to experimental near-field diffraction data acquired from a polycrystalline sample. Grain by grain tomographic reconstructions using convex optimization and prior knowledge are carried out in a six-dimensional representation of position-orientation space, used for modelling the inverse problem of X-ray orientation imaging. From the 6D reconstruction output we derive 3D orientation maps, which are then assembled into a common sample volume. The obtained 3D orientation map is compared to an EBSD surface map and local misorientations, as well as remaining discrepancies in grain boundary positions are quantified. The new approach replaces the single orientation reconstruction scheme behind X-ray diffraction contrast tomography and extends the applicability of this diffraction imaging technique to material micro-structures exhibiting sub-grains and/or intra-granular orientation spreads of up to a few degrees. As demonstrated on textured sub-regions of the sample, the new framework can be extended to operate on experimental raw data, thereby bypassing the concept of orientation indexation based on diffraction spot peak positions. This new method enables fast, three-dimensional characterization with isotropic spatial resolution, suitable for time-lapse observations of grain microstructures evolving as a function of applied strain or temperature

Very slow creep tests on rock samples

International audienceTwelve years ago, creep tests at very low deviatoric stress were performed on an Etrez salt sample in the Varangéville Mine. Recently, a new testing campaign was performed on various salt samples to gain further insight on salt behavior. Creep tests are performed under a 0.1 MPa uniaxial loading on rock-salt samples from the Varangéville and Avery Island Mines and under a 0.24 MPa uniaxial loading on a crushed-salt sample. To minimize the effects of temperature variations, testing devices were placed in an underground mine room, where temperature fluctuations are of the order of one-hundredth of a degree Celsius. The me-chanical loading is provided by dead weights. The deformations were measured through special displacement sensors with a resolution of 1/80 µm. A typical steady-state strain rate reached after 6 months is -2.4 × 10-12 s-1. The influence of air hygrometry, which is approximately 74%RH in the mine, is smaller than expected

Etude expérimentale du comportement viscoplastique d'un polycristal de sel-gemme synthétique.

Nous présentons les résultats expérimentaux à différentes échelles obtenus sur un polycristal de NaCl synthétique : essais de compression uniaxiale, couplés à des mesures de champs cinématiques, obtenus par corrélation d'images numériques acquises in-situ, sous MEB. Différents mécanismes de déformation plastique sont observés, glissement intracristallin mais aussi glissement aux interfaces. Une étude 3D par tomographie RX complète les mesures de surface

Rheology of partially molten plagioclase containing wetting silica-rich anhydrous melt abbreviated title: Rheology of partially molten plagioclase

The present work explores the effects of melt chemistry on diffusion controlled creep of partially molten labradorite plagioclase (An 50) at anhydrous conditions. Using sol-gel and hot pressing techniques we produced: 1) nominally melt-free samples (Lab), with < 1 vol. % residual glass confined solely to multiple grain junctions; 2) SilLab1 and SilLab5 partially molten samples containing respectively 1 and 5 vol. % excess amorphous silica, resulting in partial melts wetting numerous grain boundaries as thin (< 10 nm) amorphous films. Energy dispersive X-ray analysis showed that the amorphous phases in Lab, SilLab1 and SilLab5 samples contained about ~ 70, ~ 85 and ~ 95 wt. % Si02, respectively. Infrared spectroscopy showed that the initial traces of water (~ 0.05 wt. %) were dried out by annealing in air above 1100°C. Uniaxial creep tests performed at 1100-1250°C and 3-60 MPa flow stresses showed dominantly linear viscous flow, with a strong grain size dependence indicating grain boundary sliding and diffusion control. Counter-intuitively strength and activation energy increased with the content of melts, but in accord with the silica content of the latter, that is with their polymerization state. Our results show that the kinetics of grain boundary diffusion controlled creep strongly depends on melt chemistry. Instead of acting as shortcut for diffusion, thin films of highly viscous amorphous phases may in turn considerably reduce grain boundary transport properties