ABSTRACT
Extreme mechanical processes such as strong lattice distortion and bond breakage during fracture often lead to catastrophic failure of materials and structures. Understanding the nucleation and growth of cracks is challenged by their multiscale characteristics spanning from atomic-level structures at the crack tip to the structural features where the load is applied. Atomistic simulations offer 'first-principles' tools to resolve the progressive microstructural changes at crack fronts and are widely used to explore the underlying processes of mechanical energy dissipation, crack path selection, and dynamic instabilities (e.g., kinking, branching). Empirical force fields developed based on atomic-level structural descriptors based on atomic positions and the bond orders do not yield satisfying predictions of fracture, especially for the nonlinear, anisotropic stress- strain relations and the energy densities of edges. High-fidelity force fields thus should include the tensorial nature of strain and the energetics of bond-breaking and (re)formation events during fracture, which, unfortunately, have not been taken into account in either the state-of-the-art empirical or machine-learning force fields. Based on data generated by density functional theory calculations, we report a neural network-based force field for fracture (NN-F3) constructed by using the end-to-end symmetry preserving framework of deep potential - smooth edition (DeepPot-SE). The workflow combines pre-sampling of the space of strain states and active-learning techniques to explore the transition states at critical bonding distances. The capability of NN-F3 is demonstrated by studying the rupture of hexagonal boron nitride (h-BN) and twisted bilayer graphene as model problems. The simulation results elucidate the roughening physics of fracture defined by the lattice asymmetry in h-BN, explaining recent experimental findings, and predict the interaction between cross-layer cracks in twisted graphene bilayers, which leads to a toughening effect.
ABSTRACT
Objective To establish an HPLC method for simultaneous assay of macrocyclic polyphenols from Penthorum chinense Pursh, pinocembrin-7-O-[4'', 6''-(S)-hexahydroxydiphenoyl]-β-D-glucose (PHG), pinocembrin-7-O-[3''-O-galloyl-4'', 6''-(S)-hexahydroxydiphenoyl]-β-D-glucose (PGHG) and pinocembrin dihydrochalcone-7-O-[3''-O-galloyl-4'', 6''-(S)-hexahydroxydiphenoyl]-β-D-glucoside or thonningianin A (THA), and optimize the extraction process. Methods The total extraction rate of PHG, PGHG, THA was used as an investigated index to analyze the extracts from Penthorum chinense Pursh. Orthogonal design was applied to evaluate solvent amount, extraction time, solvent concentration and extraction times as the influencing factors for the optimal extraction process of macrocyclic polyphenols from Penthorum chinense Pursh. Results When this content assay method was adopted, there were good linear relationships for PHG, PGHG, THA in the linear range. The recoveries were between 100.90% to 102.04% with the RSDs below 1.5%. The optimal extraction process was involved in cutting Penthorum chinense Pursh into 3-5 cm, adding 10 times 80% ethanol aqueous solution by volume and refluxing 2 hours twice. The extraction rate of macrocyclic polyphenols was above 90% with this process. Conclusion This assay method is accurate, stable, and repeatable. The optimized extraction process is stable and feasible for further development and utilization.
