Tracer Dispersion in Rough Open Cracks

Tracer dispersion is studied in an open crack where the two rough crack faces have been translated with respect to each other. The different dispersion regimes encountered in rough-wall Hele-Shaw cell are first introduced, and the geometric dispersion regime in the case of self-affine crack surfaces is treated in detail through perturbation analysis. It is shown that a line of tracer is progressively wrinkled into a self-affine curve with an exponent equal to that of the crack surface.This leads to a global dispersion coefficient which depends on the distance from the tracer inlet, but which is still proportional to the mean advection velocity. Besides, the tracer front is subjected to a local dispersion (as could be revealed by point measurements or echo experiments) very different from the global one. The expression of this anomalous local dispersion coefficient is also obtained

Experimental study of the transport properties of rough self-affine fractures

An experimental study of the transport properties of fluid-saturated joints composed of two complementary rough fracture surfaces, translated with respect to each other and brought in contact, is reported. Quantitative roughness measurements on different fractured granite samples show that the surfaces have a self-affine geometry from which the dependence of the mean aperture on the relative displacement of fracture surfaces kept in contact can be predicted. Variations of the hydraulic and electrical conductances of the joint are measured as functions of its mean aperture. A simple parallel plane model accounts for the global trend of the measurements, but significant deviations are observed when a relative lateral displacement of the surfaces is introduced. A theoretical analysis of their origin shows that they are due both to the randomness of the aperture field and to a nonzero local slope of the surface near the injection hole; the corresponding conductivity fluctuation amplitudes have power law and linear variations with the lateral displacement, and are enhanced by the radial injection geometry

Numerical study of geometrical dispersion in self-affine rough fractures

We report a numerical study of passive tracer dispersion in fractures with rough walls modeled as the space between two complementary self-affine surfaces rigidly translated with respect to each other. Geometrical dispersion due to the disorder of the velocity distribution is computed using the lubrication approximation. Using a spectral perturbative scheme to solve the flow problem and a mapping coordinate method to compute dispersion, we perform extensive ensemble averaged simulations to test theoretical predictions on the dispersion dependence on simple geometrical parameters. We observe the expected quadratic dispersion coefficient dependence on both the mean aperture and the relative shift of the crack as of well as the anomalous dispersion dependence on tracer traveling distance. We also characterize the anisotropy of the dispersion front, which progressively wrinkles into a self-affine curve whose exponent is equal to that of the fracture surface

Comportement d'un assemblage de billes non frottantes dans la limite géométrique

8 PAGESNational audienceNous étudions numériquement le comportement d'un matériau granulaire modèle constitué de billes sphériques élastiques identiques non frottantes dans la limite géométrique macroscopique (à savoir la triple limite où les sollicitations extérieures sont très lentes et les grains extrêmement rigides et très nombreux). Nous montrons que les coefficients de frottement macroscopique statique et dynamique coïncident, que l'assemblage ne présente aucune dilatance, que le matériau satisfait un critère de rupture de Lade-Duncan et que ces résultats ne sont pas affectés par la nature du contrôle (en contrainte ou en volume) imposé. La résistance au cisaillement s'explique par l'anisotropie géométrique et mécanique que développe le matériau sous sollicitation

Roughness of sandstone fracture surfaces: Profilometry and shadow length investigations

The geometrical properties of fractured sandstone surfaces were studied by measuring the length distribution of the shadows appearing under grazing illumination. Three distinct domains of variation were found: at short length scales a cut-off of self-affinity is observed due to the inter-granular rupture of sandstones, at long length scales, the number of shadows falls off very rapidly because of the non-zero illumination angle and of the finite roughness amplitude. Finally, in the intermediate domain, the shadow length distribution displays a power law decrease with an exponent related to the roughness exponent measured by mechanical profilometry. Moreover, this method is found to be more sensitive to deviations from self-affinity than usual methods

Shear flow of sphere packings in the geometric limit

3 pagesInternational audienceWe investigate the behavior of a model granular material made of frictionless, nearly rigid equal-sized beads, in the quasistatic limit, by numerical simulation. In the macroscopic geometric limit (that is the macroscopic, rigid and quasistatic limits), with either volume or normal stress controlled simulations, static and dynamic macroscopic friction coefficients coincide, dilatancy vanishes and the material satisfies a Lade-Duncan failure criterion. The macroscopic shear strength stems from both contact network and force anisotropy

Ductile damage parameters identification for cold metal forming applications

International audienceDuctile damage mechanics is essential to predict failure during cold metal forming applications. Several damage models can be found in the literature. These damage models are coupled with the mechanical behavior so as to model the progressive softening of the material due to damage growth. However, the identification of damage parameters remains an issue. In this paper, an inverse analysis approach is set-up to identify ductile damage parameters, based on different kind of mechanical tests and observables. The Lemaitre damage model is used and damage is coupled with the material behavior. The Efficient Global Optimization (EGO) method is used in a parallel environment. This global algorithm based on kriging meta-model enables the identification of a set of damage parameters based on experimental observables. Global and local observables are used to identify these parameters and a special attention is paid to the computation of the cost function. Finally, an identification procedure based on displacement field measurements is presented and applied for damage parameters identification