Anisotropic shock response of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)
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The thermo-mechanical response of shock-induced pore collapse has been studied using non-reactive all-atom molecular dynamics (MD) and Eulerian continuum simulations for the molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Three crystal orientations, bracketed by the limiting cases with respect to the crystal structure anisotropy in TATB, are considered in the MD simulations, while an isotropic constitutive model is used for the continuum simulations. Simulations with three impact speeds from 0.5 km s[superscript -1] to 2.0 km s[superscript -1] are investigated. Results from MD and continuum simulations are in agreement in terms of shock wave speeds, temperature distributions, and pore-collapse mechanisms. However, differences arise for other quantities that are also important in hotspot ignition and growth, for example, the skewness of high-temperature distributions and the local temperature field around the post-collapse hotspot, indicating the urgent need to incorporate anisotropic crystal plasticity and strength models into the continuum descriptions. The deformation mechanisms of TATB crystals in the shock-induced pore collapse MD simulations were studied using Strain Functional Analysis. This new approach maps discrete quantities from atomistic simulations onto continuous fields via a Gaussian kernel, from which a unique and complete set of rotationally invariant Strain Functional Descriptors (SFD) is obtained from the high-order central moments of local configurations, expressed in a Solid Harmonics polynomial basis by SO(3) decomposition. Coupled with unsupervised machine learning techniques, the SFD successfully identifies and distinguishes the deformations presented in the MD simulations of shock-compressed TATB crystals. It enables automated detection of disordered structures in the system and can be readily applied to materials with any symmetry class.