Collapse of quasi-two-dimensional wet granular columns

This paper deals with the experimental characterization of the collapse of wet granular columns in the pendular state, with the purpose of collecting data on triggering and jamming phenomena in wet granular media. The final deposit shape and the runout dynamics were studied for samples of glass beads, varying particle diameter, liquid surface tension, and liquid amount. We show how the runout distance decreases with increasing water amount (reaching a plateau for $w>1 \%$) and increases with increasing Bond number, while the top and toe angles and the final deposit height increase with increasing water amount and decrease with decreasing Bond number. Dimensional analysis allowed to discuss possible scalings for the runout length and the top and toe angles: a satisfying scaling was found, based on the combination of Bond number and liquid amount.Comment: 8 pages, 14 figure

Defect tolerance in soft materials

Abstract The ability of materials to withstand defects like cracks, notches or generic geometric discontinuities, is usually indicated as flaw tolerance, and is a crucial aspect of the safety assessment of structural components. Flaw tolerance in soft materials can be substantially different from that in traditional ones. As a matter of fact, the capacity of highly deformable materials to undergo large deformations with a significant rearrangement of the molecular network at the miscroscale in highly stressed regions can enhance such an ability, leading to an erroneous underestimation of their safety level against defect-driven failure, if traditional methods of analysis are employed. In the present research work, the mechanics of highly deformable notched plates is considered from the fail-safety point-of-view. Experimental, numerical and theoretical remarks are made in order to explain the mechanism of defect resistance in such a class of materials from a physically-based point-of-view

Texture analysis as a tool to study the kinetics of wet agglomeration processes

In this work wet granulation experiments were carried out in a planetary mixer with the aim to develop a novel analytical tool based on surface texture analysis. The evolution of a simple formulation (300 g of microcrystalline cellulose with a solid binders pre-dispersed in water) was monitored from the very beginning up to the end point and information on the kinetics of granulation as well as on the effect of liquid binder amount were collected. Agreement between texture analysis and granules particle size distribution obtained by sieving analysis was always found. The method proved to be robust enough to easily monitor the process and its use for more refined analyses on the different rate processes occurring during granulation is also suggested

Crack paths in soft thin sheets

Highly deformable materials (elastomers, gels, biological tissues, etc.) are ubiquitous in nature as well as in technology. The understanding of their flaw sensitivity is crucial to ensure a desired safety level. Fracture failure in soft materials usually occurs after the development of an uncommon crack path because of the non-classical near-tip stress field and the viscous effects. In a neo-Hookean material, the true opening stress singularity along the crack path (evaluated normal to the crack line) is of the order , while it is of the order ahead of the crack tip, promoting the appearance of a crack tip splitting leading to a tortuous crack. In the present paper, experimental tests concerning the fracture behavior of highly deformable thin sheets under tension are discussed, and the observed crack paths are interpreted according to the crack tip stress field arising for large deformations. The study reveals that higher strain rates facilitate the development of a simple Mode I crack path, while lower strain rates induce a mixed Mode in the first crack propagation stage, leading to the formation of new crack tips. The above described behavior seems to not be affected by the initial crack size

fracture toughness of highly deformable polymeric materials

Abstract: A fundamental requirement for safety design of structural components is flaw tolerance. In this field, the soft materials have a unique ability to bear external loads despite the presence of defects, due to their pronounced deformability. Unlike traditional materials, which have an enthalpic elasticity, the mechanical response of a polymer-based material is governed by the state of internal entropy of a molecular network which has a great ability to rearrange the material structure and shape so to minimize the local detrimental effect of flaws. For a correct estimation of the fracture toughness of these materials, a proper knowledge of this entropic effect is needed. In the present research, the mechanical behaviour up to failure of silicone-based cracked plates is examined by taking into account the time-dependent effects. Experimental and theoretical aspects are discussed in order to understand the defect tolerance of such materials

Collapse of wet granular columns: experiments and discrete element simulations

This work aims at investigating the effect of triggering and jamming due to the addition of a small quantity of fluid to the material. Collapse of dry and wet granular columns is studied both from the experimental and the numerical point of view. Wet samples of glass beads of different grain-sizes in the pendular state were packed in a rectangular box and then allowed to flow by removing a lateral wall. The dependence of the kinematics and the final state of the system on grain size and water content was particularly investigated. DEM numerical simulations were carried out in a 1:1 scale. A good qualitative agreement between experiments and DEM simulations was found with respect to the kinematic and the final slope profile. In particular, both the techniques highlight the strong effect of the liquid which decreases the run-out distance and time even for small liquid contents. This work demonstrates the suitability of the DEM approach also for the study of wet granular materials in static as well as in dynamic conditions, however it highlights that the water redistribution model is critical for the model outcome

Effective boundary conditions for dense granular flows

We derive an effective boundary condition for granular flow taking into account the effect of the heterogeneity of the force network on sliding friction dynamics. This yields an intermediate boundary condition which lies in the limit between no-slip and Coulomb friction; two simple functions relating wall stress, velocity, and velocity variance are found from numerical simulations. Moreover, we show that this effective boundary condition corresponds to Navier slip condition when GDR MiDi's model is assumed to be valid, and that the slip length depends on the length scale that characterises the system, \emph{viz} the particle diameter.Comment: 4 pages, 5 figure

Crack paths in soft thin sheets

Highly deformable materials (elastomers, gels, biological tissues, etc.) are ubiquitous in nature as well as in technology. The understanding of their flaw sensitivity is crucial to ensure a desired safety level. Fracture failure in soft materials usually occurs after the development of an uncommon crack path because of the non-classical near-tip stress field and the viscous effects. In a neo-Hookean material, the true opening stress singularity along the crack profile is of the order of , while it is of the order of  ahead of the crack tip, promoting the appearance of a crack tip splitting leading to a tortuous crack. In the present paper, experimental tests concerning the fracture behavior of highly deformable thin sheets under tension are discussed, and the observed crack paths are interpreted according to the crack tip stress field arising for large deformations. The study reveals that higher strain rates facilitate the development of a simple Mode-I crack path, while lower strain rates induce a mixed Mode in the first crack propagation stage, leading to the formation of new crack tips. The above described behavior seems to not be affected by the initial crack size