Granular materials are widely used in industrial processes; granular flows are present in the handling of raw material in the pharmaceutical industry, the agriculture and the mining, for instance. The quantity of granular matter processed, conveyed or stored each year in the world is about ten billions of tons. Unfortunately, undesirable effects like pipes clogging and hoppers blocking are usually encountered in these processes increasing the costs of production. Even worse, due to similar effects, catastrophic-events have occurred, not long ago, in underground mining: they not only lead to enormous losses but also constitute unacceptable hazards for the people working in the mines. The unpredictability of these events is partly related to the lack of an adequate basic description of the flow thresholds and of the strong fluctuations in the flows of the granular matter. In regards to their practical importance, the behaviour of the granular materials has motivated an intense basic research activity. A rich variety of interesting phenomena have been found in this field, including surface waves, shock waves, convection, size segregation, sound propagation and stress fluctuation and propagation. Progress has been made to describe dilute granular systems through the incorporation of kinetic theories. These theories have been relatively successful in modelling rapid granular flows. However, many of the flows encountered in the industrial processes are rather slow and dense, conditions in which the theoretical requirements are usually not fulfilled. This conclusion has prompted many efforts oriented to determine laws based on continuous models inspired on the plasticity theories. The main idea is that, at the level of the grains, the grain-grain and grain-boundaries interactions generate a contact force network that transmits the external stress inside the granular materials and plays a fundamental role in most of the observed material-properties. For example, the force network is responsible for the pressure behaviour inside a grains column and also can be viewed as a regulating mechanism in hopper flows. Recent experiments have shown that the network structure strongly depends on the external force configuration: If the granular system is subjected to an isotropic compressive stress the interactions have short-range correlations in all possible directions; Conversely, for a pure shear-stress the interactions have long-range correlations in the directions of the force chains. As the behaviour of a granular system is shown to depend on the applied external force, these experiments suggest an intimate relationship between the mechanical behaviour and the internal distribution of forces inside granulate. In summary, the complex situations found in model systems and real applications suggest that a more reliable description must go beyond average quantities and that there is a need for an experimental effort to assess the relationships between the internal forces structure and the macroscopic behaviour of the material. The sound propagation inside the granular matter being very sensitive to the force network structure and a vibration being, in turn, able to trigger a catastrophic event, we propose to obtain pieces of information by means of acoustical measurements. Sound propagation has been used for many years as a tool to assess a wide variety of mechanical properties of complex materials. However, little effort has been devoted to explore its potential in the assessment of micro-structural changes in granular systems. On the one hand, we will consider the spontaneous acoustic emission associated with the rupture and the dense flows of these materials. On the other hand, we will assess the effects of mechanical disturbances on the stability of granular assemblies. In addition, we will evaluate the role played by the friction and the interaction with the surrounding in the propagation of mechanical waves along the internal force-chains that appear in such heterogeneous system under stress. The project is particularly promising as it involves 3 partners, 2 French and a Chilean one, that have a common interest in this field and a practical experience of the work in common and that shall, with efficiency, study complementary aspects of the mechanical properties of the granular matter from the microscopic to the microscopic scales.

High-altitude long-endurance (HALE) pseudo-satellites are drones powered by solar energy. Their almost unlimited endurance makes them a green satellite alternative. Most past and actual drone prototypes have experienced major aero-structural issues. Their highly flexible wings are extremely sensitive to aeroelastic instabilities, which causes their design to be a major technological challenge. The codes developed so far to simulate and predict such instabilities focus more on the calculation of the critical flutter speed than on the dynamical behavior reached after the onset of flutter, such as limit cycle oscillations (LCOs). These phenomena, due to geometric nonlinearities of the structure and dynamic stall, are notably challenging to model. In particular, no reliable analytical model for the aerodynamic behavior of flexible wings at high angle of attack in 3D is currently available to simulate LCOs. A new strategy based on data-driven approaches combining modelling and experimentation emerges as a promising alternative. In the context of safer and more robust designs of solar HALE drones, the main objective of the FlexHALE project is to develop a digital twin of highly flexible wings that will be the keystone of optimization procedures on the composite wing design. The creation of a nonlinear aeroelastic simulation tool for post-critical nonlinear analysis is the first step of the project. Results from wind tunnel experimentations on flexible composite wings under flutter conditions analyzed with medium speed cameras will be used to update, validate, and supplement the model with data-driven 3D aerodynamic force estimations. Finally, optimization algorithm will be applied for the aeroelastic tailoring of the wing box, optimizing its shape and the composite layups to influence the wing bending/twisting coupling directly involved in the flutter onset.

The general principles underlying the processing of high strength, and ductile, metallic materials have been quite well understood since the beginning of the development of dislocation theory. If ductility requires a significant mobility of dislocations, strengthening is related to the building of obstacles that restrict their propagation. One of the methods to introduce such obstacles is the reduction of the mean grain size. However, the mechanical properties of aggregates do not only depend on the mean grain size. They are also sensitive to the grain size dispersion and to their spatial arrangement. Within this framework, the present project focuses on the effect of grain size reduction and grain size distribution on improving the strength and ductility of polycrystals. To this end, due to their versatility, powder metallurgy routes such as Hot Isostatic compaction (HIP), Spark Plasma Sintering (SPS) and Soft Hydrostatic Extrusion (SHE) will be used to process a wide range of Nickel based microstructures (from ultrafine-grained regime to a well controlled distribution of different volume fraction of a coarse grained Ni phase within the ultrafine-grained matrix), whose effects on the macroscopic behavior will be investigated through a wide range of (monotonic and cyclic) mechanical tests at room temperature. The underlying deformation mechanisms (intergranular and/or intragranular, damage') will be tackled, post mortem and in situ, via TEM and XRD investigations. In parallel, since the grain size alone neither defines the microstructure nor characterizes the mechanical behavior of metallic polycrystals, a micromechanical model that is based on a generalized multi-scale self-consistent approach will be developed such as to specifically shed light on the effect of microstructural parameters such as the grain size and its statistical distribution on the macroscopic behavior.