The title molecule, C8H12NO5PS2, exhibits a crystallographic mirror plane that is perpendicular to the ring and bisects the sulfamoyl and thiophosphate ester groups. In the crystal, molecules are ...linked by N-H...O hydrogen-bonding interactions reminiscent of carboxylic acid hydrogen bonding pairs, forming chains parallel to the b-axis direction.
Using shock‐induced quasi‐2D pore collapse in β‐HMX as a specific case study, we address three practical questions that arise when designing and interpreting molecular dynamics (MD) simulations of ...explicit shock wave propagation in crystals. How sensitive are the overall results to sample thickness perpendicular to the (quasi) plane of the problem? For impacts on a given crystal surface, how much does the transverse orientation of the sample matter? And, for a given sample size and orientation, how much run‐to‐run variability exists among results for independent but statistically equivalent realizations of the shock? The first and second questions are interrelated but pertain individually to assessing the roles of finite‐size and crystal anisotropy effects, respectively, on the simulated collapse mechanisms and associated local thermo‐mechanical states in the sample during and after collapse. The third addresses the confidence with which the results of individual simulations can be regarded as representative. All three questions become particularly important if the MD predictions are intended to serve as “ground truth” for validation of continuum mesoscale models. Here, quasi‐2D samples of (010)‐oriented single‐crystal β‐HMX (β‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane) containing a cylindrical pore were subjected to reverse‐ballistic impacts, resulting in explicit, supported shock propagation initially along 010, for impact speeds up
=1.0 and 0.5 km s−1. The individual samples differed in either the thickness perpendicular to the sample plane or the transverse crystal orientation normal to 010 in the quasi‐2D cell. Three independent realizations of the shock were performed for a selected case to assess run‐to‐run variability. Comparisons of qualitative features of the collapse, temperature and pressure distributions in the samples, and time scales for pore collapse suggest that there is little sensitivity to sample thickness for the same crystal orientation and moderate sensitivity to transverse crystal orientation for samples of the same thickness. Run‐to‐run variability is evident to the eye in side‐by‐side system observations. However, overall mechanisms of collapse, distributions for temperature and pressure in the samples, and time scales for collapse are in near‐quantitative agreement among the realizations.
All-atom molecular dynamics (MD) simulations were used to study shock wave loading in oriented single crystals of the highly anisotropic triclinic molecular crystal ...1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The crystal structure consists of planar hydrogen-bonded sheets of individually planar TATB molecules that stack into graphitic-like layers. Shocks were studied for seven systematically prepared crystal orientations with limiting cases that correspond to shock propagation exactly perpendicular and exactly parallel to the graphitic-like layers. The simulations were performed for initially defect-free crystals using a reverse-ballistic configuration that generates explicit, supported shocks. Final longitudinal stress components are between ≈8.5 and ≈10.5 GPa for the 1.0 km s–1 impact speed studied. Orientation-dependent properties are reported including shock speeds, stresses, temperatures, compression ratios, and local material strain rates. Spatiotemporal maps of the temperature, stress tensor, material flow, and molecular orientations reveal complicated processes that arise for specific shock directions. The results indicate that TATB shock response is highly sensitive to crystal orientation, with significant qualitative differences for the time evolution of the stress tensor and temperature, elastic/inelastic compression response, defect formation and growth, critical von Mises stress, and strain rates during shock rise that span nearly an order of magnitude. A variety of inelastic deformation mechanisms are identified, ranging from crumpling of graphitic-like layers to dislocation-mediated plasticity to intense shear strain localization. To our knowledge, these are the first systematic MD simulations and analysis of explicit shock wave propagation along nontrivial crystal directions in a triclinic molecular crystal.
We present a machine learning framework to train and validate neural networks to predict the anisotropic elastic response of a monoclinic organic molecular crystal known as β$$ \beta $$‐HMX in the ...geometrical nonlinear regime. A filtered molecular dynamic (MD) simulations database is used to train neural networks with a Sobolev norm that uses the stress measure and a reference configuration to deduce the elastic stored free energy functional. To improve the accuracy of the elasticity tangent predictions originating from the learned stored free energy, a transfer learning technique is used to introduce additional tangential constraints from the data while necessary conditions (e.g., strong ellipticity, crystallographic symmetry) for the correctness of the model are either introduced as additional physical constraints or incorporated in the validation tests. Assessment of the neural networks is based on (1) the accuracy with which they reproduce the bottom‐line constitutive responses predicted by MD, (2) the robustness of the models measured by detailed examination of their stability and uniqueness, and (3) the admissibility of the predicted responses with respect to mechanics principles in the finite‐deformation regime. We compare the training efficiency of the neural networks under different Sobolev constraints and assess the accuracy and robustness of the models against MD benchmarks for β$$ \beta $$‐HMX.
Featured Cover Vlassis, Nikolaos N.; Zhao, Puhan; Ma, Ran ...
International journal for numerical methods in engineering,
09/2022, Letnik:
123, Številka:
17
Journal Article
Recenzirano
Odprti dostop
The cover image is based on the Original Article Molecular dynamics inferred transfer learning models for finite‐strain hyperelasticity of monoclinic crystals: Sobolev training and validations ...against physical constraints by Nikolaos Vlassis et al., https://doi.org/10.1002/nme.6992.
The high-speed direct-drive blower is a high-efficiency, energy-saving, and environmentally friendly blower, and it is widely applied in industrial fields. The temperature changes significantly ...affect the performance of the high-speed direct-drive blower, and it is crucial to monitor its temperature. Infrared and visible light cameras are simultaneously used to measure and characterize temperature information. The measurement method can have both the temperature information of the infrared image and the contrast information of the structure and contour of the visible light image. This article establishes an image fusion representation and probabilistic generation model of infrared and visible images. Then, the information of the infrared and visible images is fused under the Bayesian framework. A hierarchical prior model using the Haar wavelet transform is proposed. The joint maximum a posteriori criterion is chosen, and an appropriate alternate optimization algorithm is designed to achieve information fusion. The proposed method is validated in industrial scenarios of high-speed direct-drive blowers. The experimental results demonstrate the robustness and effectiveness of the proposed method.
Shock‐induced collapse of an elongated pore in the energetic crystal β‐HMX (β‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazoctane) is examined using all‐atom molecular dynamics (MD) and continuum mechanics. The ...continuum simulation employs a recently proposed MD‐guided material model for β‐HMX. Collapse‐induced shear band formation and hotspot‐zone properties are calculated using both MD and continuum mechanics, for nearly identical simulation domains and identical impact conditions. The continuum model predicts shear band patterns and pore collapse behavior in good agreement with MD results; shear localization, plastic heating, and hydrodynamic impact‐generated temperature rise lead to geometrically complicated hotspot zones in the vicinity of the collapse site. This work demonstrates that—for the ≈10 GPa shock pressure studied—isotropic rate‐dependent Johnson‐Cook‐type elastoplastic models for HMX can provide physically consistent pore‐collapse dynamics and hotspot features in comparison to MD, for nontrivial pore shapes. Such physically accurate models are required for reliable predictions of detonation sensitivity and performance for shocked energetic crystals. Opportunities for further model improvement are identified.
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 to 2.0 km/s 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.
We present a machine learning framework to train and validate neural networks
to predict the anisotropic elastic response of the monoclinic organic molecular
crystal $\beta$-HMX in the geometrical ...nonlinear regime. A filtered molecular
dynamic (MD) simulations database is used to train the neural networks with a
Sobolev norm that uses the stress measure and a reference configuration to
deduce the elastic stored energy functional. To improve the accuracy of the
elasticity tangent predictions originating from the learned stored energy, a
transfer learning technique is used to introduce additional tangential
constraints from the data while necessary conditions (e.g. strong ellipticity,
crystallographic symmetry) for the correctness of the model are either
introduced as additional physical constraints or incorporated in the validation
tests. Assessment of the neural networks is based on (1) the accuracy with
which they reproduce the bottom-line constitutive responses predicted by MD,
(2) detailed examination of their stability and uniqueness, and (3)
admissibility of the predicted responses with respect to continuum mechanics
theory in the finite-deformation regime. We compare the neural networks'
training efficiency under different Sobolev constraints and assess the models'
accuracy and robustness against MD benchmarks for $\beta$-HMX.
We present a machine learning framework to train and validate neural networks to predict the anisotropic elastic response of the monoclinic organic molecular crystal \(\beta\)-HMX in the geometrical ...nonlinear regime. A filtered molecular dynamic (MD) simulations database is used to train the neural networks with a Sobolev norm that uses the stress measure and a reference configuration to deduce the elastic stored energy functional. To improve the accuracy of the elasticity tangent predictions originating from the learned stored energy, a transfer learning technique is used to introduce additional tangential constraints from the data while necessary conditions (e.g. strong ellipticity, crystallographic symmetry) for the correctness of the model are either introduced as additional physical constraints or incorporated in the validation tests. Assessment of the neural networks is based on (1) the accuracy with which they reproduce the bottom-line constitutive responses predicted by MD, (2) detailed examination of their stability and uniqueness, and (3) admissibility of the predicted responses with respect to continuum mechanics theory in the finite-deformation regime. We compare the neural networks' training efficiency under different Sobolev constraints and assess the models' accuracy and robustness against MD benchmarks for \(\beta\)-HMX.
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