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Theses

Multiscale Modeling of the Mechanical Behavior of an Energetic Material : TATB

Résumé : The construction of mesoscopic (micrometer scale) constitutive laws in materialsscience is studied for a long time. However, the constant progress in high performance computing changes the perspectives. Indeed, constitutive laws now aim at explicitly take into account the microstructure and its underlying physics at the atomic scale, for which simulation techniques prove to be very accurate but definitely expensive. The multiscale approach is therefore perfectly adapted to such a challenge and the dialogue between scales necessary. In this thesis, the mechanical behavior of the energetic material TATB in temperature and pressure is investigated using molecular dynamics simulations in order to understand the microscopic deformation mechanisms responsible for plastic activity. The local computation of mechanical variables was developed in atomistic simulations, allowing the dialogue with continuum mechanical methods. Additionally, prescribed deformation paths were coupled with molecular dynamics, allowing to reveal the plasticity mechanism of TATB single crystal. Nucleation of complex dislocation structures with intrinsic dilatancy, twinning transition pathway and a twinning-buckling pseudo phase transition are three distinct behaviors triggered for different loading directions. Then, mesoscopic simulations inferred by atomic scale observations aim at reproducing the twinning-buckling pseudo-phase transition under tri-axial compression using a Lagrangian code. The comparison between both simulation techniques is made possible thanks to the mechanical tools that have been implemented in themolecular dynamics code. Finally, polycrystalline TATB is simulated with non linear elasticity and we demonstrate the necessity to consider an equation of state compatible with this pseudo phase transition, which has a strong influence on the polycristal behavior.
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Paul Lafourcade. Multiscale Modeling of the Mechanical Behavior of an Energetic Material : TATB. Materials. Ecole nationale supérieure d'arts et métiers - ENSAM, 2018. English. ⟨NNT : 2018ENAM0030⟩. ⟨tel-01916038⟩

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