Mechanics of Fiber-interLinked Granular materials – FiLiGran

The addition of fibers to a granular material is an inexpensive, sustainable and efficient way to reinforce its mechanical resistance. This strategy, which mimic the effect of plant roots on soil, is widely used for civil engineering applications to reinforce soils against erosion and to develop high-performance concretes, with the disadvantage of reducing the material 'workability'. From a physical point of view, entanglement between flexible fibers and rigid grains causes a strong coupling that increases the material yield stress much above the confining pressure, induces a substantial resistance to traction, hinders extensional flows and gives rise to shear-induced anisotropy. All these behaviors raise the question of whether the response of fiber-reinforced granular materials can be deduced from the classical mechanics of granular and fiber assemblies or if they constitute a new class of material with distinct mechanical properties? This project proposes to address this general question and to rationalize the mechanical and flowing responses of fiber-reinforced granular materials. For this purpose, we will realize a set of model experiments with grains and fibers having calibrated physical properties that can be tailored at the laboratory. In a first step, we will study the mixing conditions that permit to obtain an homogeneous sample and characterize its mechanical response in the limit of quasi-static deformations. The mixing problem will be studied by tracking the spatial distribution and deformation of fibers through transparent and index-matched grains during a typical preparation protocol. The mechanical response of the sample will be investigated in compression and traction using a universal testing machine. These measurements will determine how the yield stresses of the fiber-reinforced granular material depend on the physical parameters of this problem such as fiber elasticity, volume fraction and inter-fiber friction. In a second time, we will investigate the flowing behavior of a granular medium added with flexible fibers in different configurations. We will characterize the response of this material to a simple shear using a dedicated rheometer for which the normal pressure is imposed. The rationalization of these rheological measurements based on dimensional analysis will allow us to propose a rheological formulation for fiber-reinforced granular materials. In order to test the domain of validity of these rheological laws, we will compare their predictions in different flow configurations (inclined plane, gravitational collapse of a column, rapid impact of an object, etc.) with experimental observations. This approach will reveal the importance of different phenomenon such as the presence of a shear gradient, a free surface and non-stationarity in real flows. Finally, we will look for a theoretical model for the behavior of fiber-reinforced granular materials. This step will require to consider the coupling between grains and fibers at the mesoscopic scale and will benefit from DEM simulations that will bring valuable information on the evolution of the contact network during a characteristic deformation of the material. In the end, the project FiLiGran will provide a fundamental understanding of the mechanical and flowing properties of fiber-reinforced granular materials. This understanding will foster the development of fiber-reinforced techniques to realize sustainable infrastructures for civil engineering applications. The results of the project will also be disseminated to the general public and students, helping to raise their awareness and enthusiasm for science.