Crossover from quasi-static to dense flow regime in compressed frictional granular media

We investigate the evolution of multi-scale mechanical properties towards the macroscopic mechanical instability in frictional granular media under multiaxial compressive loading. Spatial correlations of shear stress redistribution following nucleating contact sliding events and shear strain localization are investigated. We report growing correlation lengths associated to both shear stress and shear strain fields that diverge simultaneously as approaching the transition to a dense flow regime. This shows that the transition from quasi static to dense flow regime can be interpreted as a critical phase transition. Our results suggest that no shear band with a characteristic thickness has formed at the onset of instability

On the critical character of plasticity in metallic single crystals

Previous acoustic emission (AE) experiments on ice single crystals, as well as numerical simulations, called for the possible occurrence of self-organized criticality (SOC) in collective dislocation dynamics during plastic deformation. Here, we report AE experiments on hcp metallic single crystals. Dislocation avalanches in relation with slip and twinning are identified with the only sources of AE. Both types of processes exhibit a strong intermittent character. The AE waveforms of slip and twinning events seem to be different, but from the point of view of the AE event energy distributions, no distinction is possible. The distributions always follow a power law, even when multi-slip and forest hardening occur. The power law exponent is in perfect agreement with those previously found in ice single crystals. Along with observed time clustering and interactions between avalanches, these results are new and strong arguments in favour of a general, SOC-type, framework for crystalline plasticity.Comment: 12 pages, 10 figure

Variational algorithms to remove stationary noise. Application to microscopy imaging.

International audienceA framework and an algorithm are presented in order to remove stationary noise from images. This algorithm is called VSNR (Variational Stationary Noise Remover). It can be interpreted both as a restoration method in a Bayesian framework and as a cartoon+texture decomposition method. In numerous denoising applications the white noise assumption fails: structured patterns (e.g. stripes) appear in the images. The model described here addresses these cases. Applications are presented with images acquired using different modalities: scan- ning electron microscope, FIB-nanotomography, and an emerging fluorescence microscopy technique called SPIM (Selective Plane Illumination Microscope)

Modelling intersite dependence for regional frequency analysis of extreme marine events

The duration of observation at a site of interest is generally too low to reliably estimate marine extremes. Regional frequency analysis (RFA), by exploiting the similarity between sites, can help to reduce uncertainties inherent to local analyses. Extreme observations in a homogeneous region are especially assumed to follow a common regional distribution, up to a local index. The regional pooling method, by gathering observations from different sites into a regional sample, can be employed to estimate the regional distribution. However, such a procedure may be highly affected by intersite dependence in the regional sample. This paper derives a theoretical model of intersite dependence, dedicated to the regional pooling method in a "peaks over threshold" framework. This model expresses the tendency of sites to display a similar behavior during a storm generating extreme observations, by describing both the storm propagation in the region and the storm intensity. The proposed model allows the assessment of i) the regional effective duration of the regional sample and ii) different regional hazards, e.g., return periods of storms. An application to the estimation of extreme significant wave heights from the numerical sea-state database ANEMOC-2 is provided, where different patterns of regional dependence are highlighted

Damage-cluster distributions and size effect on strength in compressive failure

We investigate compressive failure of heterogeneous materials on the basis of a continuous progressive damage model. The model explicitely accounts for tensile and shear local damage and reproduces the main features of compressive failure of brittle materials like rocks or ice. We show that the size distribution of damage-clusters, as well as the evolution of an order parameter, the size of the largest damage-cluster, argue for a critical interpretation of fracture. The compressive failure strength follows a normal distribution with a very small size effect on the mean strength, in good agreement with experiments

The Clever Girl Painter

British sculptor Laura White creates three dimensional entanglements of forms in which interactions are paramount. The Clever Girl Painter brings together sculptural objects made by White to react with and against a long row of wood engraving blocks. The interaction here is physical, aesthetic and personal; the exhibition links two artists, separated by a generation but linked by a family relationship. The ‘clever girl painter’ referred to in the title is Joan Ellis, Laura White’s grandmother. An artist working in the 1920s and 30s, Joan Ellis attended the School of Leon Underwood in West London where Gertrude Hermes was a fellow student. Hermes remained a lifelong friend of Joan Ellis and both artists were known for their work with woodcut prints. Laura White recently acquired the entire collection of Ellis' wood engraving blocks, the majority of which were produced over a period of 8 years, 1924 – 32, before Joan Ellis married. After her marriage she never returned to such an intense level of production. White has a long term interest in the sculptural quality of images. She is drawn to the wood blocks’ materiality, the process and labour of their making apparent in their fragmented appearance, rather than the specific narratives intended for the final prints. They are made from box wood and are intricately hand crafted. There are areas of precision where detailed patterns are cut into the hard material and other areas that are much looser and roughly carved – these would be the areas left white on the paper print. For White, it is the physicality of the wood blocks and how they link to the labour of the artist that is of prime importance – the agency of what artworks are made from and how they are made. This connects to White’s interest in making and showing work that appears to be in transition, changing, becoming something else. One wall of the gallery is given to the wood blocks presented on a long shelf. Lined up across the space are White’s sculptures. Using a range of materials – ceramics, jesmonite, fabric, leather, steel, wood, expanding foam and dried bread dough – White has built up layers, staked uprights of multiple facades, with depth and material mass. These sculptures sit somewhere between object and material sampling, with every potential to realign into new combinations. Research is an integral part of White’s practice – she continually explores what it means to handle and come up against stuff, material and objects – how we use our hands, how a skill is acquired, the relationship between materials, hand and cognition, and the rich experience of understanding through direct engagement. In the recent past she has learnt meat skills, glass-blowing, sushi-making, shoe-making, basketmaking, and blacksmithing; “it is important for me to explore as the maker, and communicate to the viewer a material engagement, where the works suggest a human physical interaction, such as an object that can be used, yet is untried or unrecognizable.” The objects she makes from everyday and ready available materials, such as bread-dough, clay and expanding foam appear on one hand familiar, but equally strange and anomalous like prototypes or hybrids. White Mud, a recent installation of a hundred hand-made porcelain objects, fused everyday items such as plastic washing-up bottles squidged and squashed with precious museum collectables