Influence de la polydispersité sur la texture des milieux granulaires

La texture des matériaux granulaires est fortement influencée par le degré de polydispersité des tailles de particules. Dans cet article on présente un modèle de distribution granulométrique associé à un protocole de dépôt géométrique. Ce modèle est suffisamment efficace sur le plan numérique pour permettre une étude systématique des propriétés d'arrangement du milieu (compacité, connectivité, anisotropie) en fonction de la forme et de l'étalement de la courbe granulométrique. Un résultat remarquable de cette étude est la variation non linéaire de la compacité avec l'étalement. On observe notamment un palier où la compacité n'évolue pas avec l'étalement. Une transition a lieu vers les compacités plus élevées lorsque l'étalement permet l'insertion des plus petites particules dans les pores formés par les plus grosses. Cette transition se retrouve au niveau de l'arrangement granulaire qui est relativement régulier sur le pallier et désordonné ensuite

Particle shape dependence in 2D granular media

Particle shape is a key to the space-filling and strength properties of granular matter. We consider a shape parameter $\eta$ describing the degree of distortion from a perfectly spherical shape. Encompassing most specific shape characteristics such as elongation, angularity and nonconvexity, $\eta$ is a low-order but generic parameter that we used in a numerical benchmark test for a systematic investigation of shape-dependence in sheared granular packings composed of particles of different shapes. We find that the shear strength is an increasing function of $\eta$ with nearly the same trend for all shapes, the differences appearing thus to be of second order compared to $\eta$. We also observe a nontrivial behavior of packing fraction which, for all our simulated shapes, increases with $\eta$ from the random close packing fraction for disks, reaches a peak considerably higher than that for disks, and subsequently declines as $\eta$ is further increased. These findings suggest that a low-order description of particle shape accounts for the principal trends of packing fraction and shear strength. Hence, the effect of second-order shape parameters may be investigated by considering different shapes at the same level of $\eta$.Comment: 5 pages, 8 figure

Simulating regoliths in microgravity

Despite their very low surface gravities, the surfaces of asteroids and comets are covered by granular materials – regolith – that can range from a fine dust to a gravel-like structure of varying depths. Understanding the dynamics of granular materials is, therefore, vital for the interpretation of the surface geology of these small bodies and is also critical for the design and/or operations of any device planned to interact with their surfaces. We present the first measurements of transient weakening of granular material after shear reversal in microgravity as well as a summary of experimental results recently published in other journals, which may have important implications for small-body surfaces. Our results suggest that the force contact network within a granular material may be weaker in microgravity, although the influence of any change in the contact network is felt by the granular material over much larger distances. This could mean that small-body surfaces are even more unstable than previously imagined. However, our results also indicate that the consequences of, e.g., a meteorite impact or a spacecraft landing, may be very different depending on the impact angle and location, and depending on the prior history of the small-body surface

EPIdemiology of Surgery-Associated Acute Kidney Injury (EPIS-AKI) : Study protocol for a multicentre, observational trial

More than 300 million surgical procedures are performed each year. Acute kidney injury (AKI) is a common complication after major surgery and is associated with adverse short-term and long-term outcomes. However, there is a large variation in the incidence of reported AKI rates. The establishment of an accurate epidemiology of surgery-associated AKI is important for healthcare policy, quality initiatives, clinical trials, as well as for improving guidelines. The objective of the Epidemiology of Surgery-associated Acute Kidney Injury (EPIS-AKI) trial is to prospectively evaluate the epidemiology of AKI after major surgery using the latest Kidney Disease: Improving Global Outcomes (KDIGO) consensus definition of AKI. EPIS-AKI is an international prospective, observational, multicentre cohort study including 10 000 patients undergoing major surgery who are subsequently admitted to the ICU or a similar high dependency unit. The primary endpoint is the incidence of AKI within 72 hours after surgery according to the KDIGO criteria. Secondary endpoints include use of renal replacement therapy (RRT), mortality during ICU and hospital stay, length of ICU and hospital stay and major adverse kidney events (combined endpoint consisting of persistent renal dysfunction, RRT and mortality) at day 90. Further, we will evaluate preoperative and intraoperative risk factors affecting the incidence of postoperative AKI. In an add-on analysis, we will assess urinary biomarkers for early detection of AKI. EPIS-AKI has been approved by the leading Ethics Committee of the Medical Council North Rhine-Westphalia, of the Westphalian Wilhelms-University Münster and the corresponding Ethics Committee at each participating site. Results will be disseminated widely and published in peer-reviewed journals, presented at conferences and used to design further AKI-related trials. Trial registration number NCT04165369

Ballast wear: Insight from the discrete element-simulation of tracks and micro-deval tests

cited By 0International audienceThroughout the life of a track, ballast grains wear as a result of both the trains passing over the track and the maintenance operations (tamping). The morphology of the grains is changed: they lose angularity and fine particles are produced, causing a gradual loss of track performance. Eventually ballast substitution becomes necessary. In order to predict the evolution of this degradation, a multi-scale study is proposed. A comparison between the discrete element method simulations of the micro-deval attrition test and of the passage of trains over a sleeper is performed in order to link the microscopic loading to the production of fine particles experimentally observed. The same numerical approach is also used with the simulations of the track to detect families of contacts in order to determine characteristic loading paths at the micro-scale. Loads and displacements of both systems are then compared in order to discuss the relevance of the micro-deval test. © Civil-Comp Press, 2016

Ballast wear: Insight from the discrete element-simulation of tracks and micro-deval tests

cited By 0International audienceThroughout the life of a track, ballast grains wear as a result of both the trains passing over the track and the maintenance operations (tamping). The morphology of the grains is changed: they lose angularity and fine particles are produced, causing a gradual loss of track performance. Eventually ballast substitution becomes necessary. In order to predict the evolution of this degradation, a multi-scale study is proposed. A comparison between the discrete element method simulations of the micro-deval attrition test and of the passage of trains over a sleeper is performed in order to link the microscopic loading to the production of fine particles experimentally observed. The same numerical approach is also used with the simulations of the track to detect families of contacts in order to determine characteristic loading paths at the micro-scale. Loads and displacements of both systems are then compared in order to discuss the relevance of the micro-deval test. © Civil-Comp Press, 2016