The mechanical behavior of metallic materials under highly dynamical loading (schock) and especially their damage behavior is a topic of interest for the CEA-DAM. For tantalum, damage is ductile : by nucleation, growth and coalescence of voids within the material. Usual ductile damage models have been developed using the simplifying assumption that voids are isolated in the materials. However, recent studies by direct simulations explicitly describing a void population in the material (and experimental observations after failure) have shown the importance of void interaction for predicting ductile damage. Yet, the microscopical mechanisms of this interaction remain little known.
The objective of the PhD is to study the growth and coalescence phases of ductile damage through direct numerical simulations of a porous material undergoing dynamic loading. Hydrodynamic simulations, in which voids are explicitly meshed within a continuous matrix, will be used to study relevant scales of length and time. Monitoring the void population throughout the simulation will provide valuable information on the influence of void interaction during ductile damage. Firstly, the bulk behavior will be compared to the one predicted by usual models of isolated voids, showing the macroscopic effect of void interaction. Secondly, the evolution of the size distribution in the void population will be monitored. The last objective will be to understand microscopic void-to-void interaction. In order to take advantage of the wealth of simulation results, approaches based on artificial intelligence (neural networks on the graph associated with the pore population) will be used to learn the link between a void's neighborhood and its growth.
The doctoral student will have the opportunity to develop their skills in shock physics and mechanics, numerical simulations (with access to CEA-DAM supercomputers), and data science.
The operation of the tritium facilities at Valduc generates low-activity tritiated liquid effluents, which are stored in an adsorbed form on 4A zeolite for operational reasons. Understanding the mechanisms of self-radiolysis of this confined water is essential for optimizing storage conditions.
Several PhD projects have already investigated these mechanisms by combining experiments and modelling. Early work showed that below 13% hydration, the radiolytic gases H2 and O2 can recombine within the zeolite. Subsequent studies, based on DFT calculations and molecular dynamics, identified the adsorption sites and the mobility of the gases. They revealed a hydration threshold (13–15%) above which gas diffusion becomes very low, consistent with the experimentally observed cessation of recombination. However, these simulations rely on idealized models.
The new proposed PhD aims to shift the project back toward experimental work in order to better reflect real storage conditions. It will begin with a detailed characterization of the zeolite used industrially. Water–zeolite reservoirs will then be irradiated to simulate the effect of tritium, and analyzed by NMR and possibly by Electron Spin Resonance (ESR) to detect reactive species. The experimental results may feed into a macroscopic model (Kinetic Monte Carlo, KMC), also developed previously, to predict the evolution of the system and identify possible optimizations for storage. The work will be carried out mainly at the NIMBE laboratory (CEA-CNRS), with simulation collaboration in Besançon and regular exchanges with CEA Valduc.
Twinning is a displacive deformation mechanism characterized by a continuous deformation of the material. Although widely studied for other industrial materials such as titanium alloys, this inelastic mechanism remains poorly understood and incompletely modeled for complex crystallographic structures. However, due to the reduced number of symmetries in these structures, dislocation slip is insufficient to accommodate deformation in certain loading directions, requiring the activation of twinning. This is the case for tin, which has a tetragonal structure. In particular, twinning contributes significantly to the mechanical response of tin at high strain rates and low temperatures. At intermediate temperatures and strain rates, a competition between dislocation plasticity and twinning plasticity can occur, making it crucial to describe the coupling between these two phenomena. Proposing a better description of this coupling will shed new light on the experimental data available at CEA DAM. The objective of the thesis is to develop a multiscale approach, from molecular dynamics to continuum mechanics, validated by experiments, to converge on a model that describes the behavior of tin over a wide range of temperatures and strain rates.
Given the ever-increasing demands of numerical simulation, supercomputers
must constantly evolve to improve their performance and thus maintain a
high quality of service for users. These demands are reflected on storage
systems, which, to be performant, reliable, and capacitive, must contain
cutting-edge technologies concerning the optimization of data placement
and the scheduling of I/O accesses. The objective of this thesis is to
study these technologies such as GPU-based RAID and I/O virtualization,
to evaluate them, and to establish optimizations that can improve the
performance of HPC storage systems.
The objective of the PhD is to develop new synthesis and/or functionalization methods to obtain functionalized heterocyclic molecules. These molecules are based on 5- or 6-member azaheteroaromatic rings (diazines, triazines, triazoles, tetrazoles, etc.). The targeted structures make it possible to envisage high densities and enthalpies of formation, while maintaining low sensitivity (thermal, mechanical, etc.). They find applications in the energy field, notably propulsion, explosives and gas generators (airbags). In addition, these heterocyclic compounds as well as the intermediates are also structurally close to families of biologically active products and/or likely to exhibit fluorescence properties, as already shown in a previous PhD in the laboratory.