The dynamic modeling of complex structures may require to take into account phenomena occurring at very different scales. However, a full refined modeling of this type of structure generally leads to prohibitive calculation costs. Multiscale modeling then presents an alternative solution to this problem, taking into account each phenomenon at the most appropriate scale.
We are interested here in slender structures subjected to mechanical stresses with frictional contacts between the structure and the retaining elements. The behavior of slender structures is in general represented by beam models, but accurately taking into account all the local contact/friction requires massive 3D models.
The originality of the work proposed here is to build an efficient multiscale and multimodel approach between beam and massive models which makes it possible to locally take into account the friction contact of slender structures. We are therefore moving towards the use of local multigrid (or multilevel) methods which naturally allow a non-intrusive multiscale coupling. The accuracy of these methods depends on the choice of transfer operators between scales, which must be carefully defined. It will also be necessary to take into account the incompatibility of the meshes supporting the models on the various relevant scales. Hence, the final model will consist in an enriched beam model taking into account local contact phenomena.
The developed model will be compared with experimental results obtained during test campaigns already carried out, and with reference numerical solutions, of increasing complexity, intended to finely validate the relevance of the proposed multiscale approach.
The strong potential of the targeted multiscale approaches, applied in this subject to the nuclear field, could be exploited by the candidate for other industrial issues such as those of aeronautics or the automotive industry.
This thesis will take place within the framework of the joint MISTRAL laboratory between the CEA and the LMA (Laboratoire de Mécanique et d’Acoustique) in Marseille. The PhD student will carry out the major part of his thesis within the CEA (IRESNE institut, Cadarache) in teams specialized in numerical methods and dynamic modeling of complex structures. The doctoral student will travel regularly to Marseille to discuss with the university supervisors.

Nuclear fuel is produced through a powder metallurgy process that involves several stages of the granular medium preparation (grinding, mixing, pressing and sintering). The powders used during these stages exhibit strong cohesion between the grains, making their flow behavior complex. Predicting powder behavior is a critical industrial challenge to quickly adapt to raw material changes, optimize product quality, and enhance production rates.

This thesis aims to establish a link between the properties of powders and their behavior during flow and pressing. Grain cohesion is a key factor that influences both the flow and densification of granular materials. This cohesion is governed by several interparticle forces, such as van der Waals forces, capillary interactions, and electrostatic forces. Understanding these interactions at the atomic scale is essential for accurately predicting and modeling powder behavior. The thesis seeks to address two central questions: How do the surface properties of grains at the atomic level influence the cohesive forces at the grain scale? And how can we scale up from the atomic level to the grain scale to simulate powders more realistically?

Multi-scale simulation approaches are crucial for bridging the gap between microscopic phenomena and the macroscopic behavior of granular materials. Current Discrete Element Method (DEM) simulations rarely incorporate fundamental interactions, such as van der Waals forces, electrostatic forces, and capillary effects, into their contact models. Recent research (1) (2) has explored the impact of cohesion using a simplified approach, treating it as an attractive force or cohesive energy. Simulation methods like Molecular Dynamics (MD) or Coarse-graining enable the modeling of material behavior at finer scales, based on these local structural and chemical properties. A deeper understanding of cohesion at small scales will enhance the predictive capabilities of DEM simulations and clarify the relationship between powder properties and their overall behavior.The main goal of this thesis is to better understand the relationships between atomic-scale interactions and grain-scale cohesion and to assess the consequences for simulations of powder pressing and flow.

The primary goal of this thesis is to make connections between the atomic-scale interactions and grain-scale cohesion and to simulate the powder flow and compaction processes.
One of the main challenges in this project is the development of DEM contact laws that incorporate complex atomic-scale interactions. This will require close collaboration between experts in atomic-level simulations and those working on DEM modeling. Additionally, validating these models through experimental comparisons is essential to ensure their accuracy and relevance for industrial applications.

The PhD candidate will be based at the IRESNE Institute (CEA-Cadarache) within the Laboratory of Numerical Methods and Physical Components on the PLEIADES platform, part of the Department of Fuel Studies. They will collaborate with the Fuel Behavior Modeling Laboratory and will have access to state-of-the-art modeling and simulation tools, as well as a collaborative environment with the Mechanics and Civil Engineering Laboratory at the University of Montpellier.

Bibliography
1. Sonzogni, Max. Modélisation du calandrage des électrodes Li-ion en tant que matériau granulaire cohésif : des propriétés des grains aux performances de l'électrode. s.l. : Thèse, 2023.
2. Tran, Trieu-Duy. Cohesive strength and bonding structure of agglomerates composed. 2023.