The elastic coupling between plastic events is generally invoked to interpret plastic properties and failure of amorphous soft glassy materials. We report an experiment where the emergence of a self-organized plastic flow is observed well before the failure. For this we impose an homogeneous stress on a granular material, and measure local deformations for very small strain increments using a light scattering setup. We observe a non-homogeneous strain that appears as transient bands of mesoscopic size and well defined orientation, different from the angle of the macroscopic frictional shear band that appears at failure. The presence and the orientation of those micro-bands may be understood by considering how localized plastic reorganizations redistribute stresses in a surrounding continuous elastic medium. We characterize the lengthscale and persistence of the structure. The presence of plastic events and the mesostructure of the plastic flow are compared to numerical simulations. [5,6], suggest that such materials may be described using a common framework [1,7,8]. At the center of those descriptions is the hypothesis of localized reorganizations. Such events have been observed in many different studies [3,4,9,10]. Each event modifies locally the mechanical equilibrium, causing the surrounding material to deform, and creating internal stresses. These stresses may then provoke other events, leading to a succession or avalanche of events [11,12]. The coupling between events, and its relevance to an avalanche-like cascade scenario for the description of the final persistent shear-band is still an open question [12,13].Several experimental works show isolated reorganizations followed by localized flow structures, suggesting the existence of such coupling. Conclusions remain elusive in direct observation of colloidal glasses due to the dominance of thermal activity over the triggered events [9]. In athermal systems such as granular materials [10] or foams [3], the steps between accumulation of individual events and appearance of shear bands remain unclear. Very recent numerical and theoretical results suggest that reorganization events may indeed couple in order to produce bands [12,[14][15][16][17]25]. However, the bands observed numerically resulting from the interacting local events are transient and correspond to self-healing micro-cracks, of a different nature than the final persistent shear-bands. To our knowledge such transient micro-bands forming a clear intermittent structure have never been reported experimentally.We present in this letter the first direct experimental evidence showing the progressive emergence of cooperative effects during plastic deformations of an amorphous material. For this, we use a very sensitive light scattering setup to monitor the homogeneous biaxial compression of a granular material. We then show that the plastic flow at the early stage of the loading of a granular material is concentrated along self-healing micro-bands. The orientation of those transient micro-bands are clear...

International audienceWe present an experimental apparatus specifically designed to investigate the precursors of failure in granular materials. A sample of granular material is placed between a latex membrane and a glass plate. A confining effective pressure is applied by applying vacuum to the sample. Displacement-controlled compression is applied in the vertical direction, while the specimen deforms in plane strain. A Diffusing Wave Spectroscopy visualization setup gives access to the measurement of deformations near the glass plate. After describing the different parts of this experimental setup, we present a demonstration experiment where extremely small (of order $10^{-5}$) heterogeneous strains are measured during the loading process

Pattern heterogeneities and anisotropies often carry significant physical information. We provide a toolbox which: (i) cumulates analysis in terms of position, direction and scale; (ii) is as general as possible; (iii) is simple and fast to understand, implement, execute and exploit. It consists in dividing the image into analysis boxes at a chosen scale; in each box an ellipse (the inertia tensor) is fitted to the signal and thus determines the direction in which the signal is more present. This tensor can be averaged in position and/or be used to study the dependence with scale. This choice is formally linked with Leray transforms and anisotropic wavelet analysis. Such protocol is intuitively interpreted and consistent with what the eye detects: relevant scales, local variations in space, privileged directions. It is fast and parallelizable. Its several variants are adaptable to the users' data and needs. It is useful to statistically characterize anisotropies of 2D or 3D patterns in which individual objects are not easily distinguished, with only minimal pre-processing of the raw image, and more generally applies to data in higher dimensions. It is less sensitive to edge effects, and thus better adapted for a multiscale analysis down to small scale boxes, than pair correlation function or Fourier transform. Easy to understand and implement, it complements more sophisticated methods such as Hough transform or diffusion tensor imaging. We use it on various fracture patterns (sea ice cover, thin sections of granite, granular materials), to pinpoint the maximal anisotropy scales. The results are robust to noise and to users choices. This toolbox could turn also useful for granular materials, hard condensed matter, geophysics, thin films, statistical mechanics, characterization of networks, fluctuating amorphous systems, inhomogeneous and disordered systems, or medical imaging, among others.

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