Mechanical Response of Nanocrystalline Ice-Contained Methane Hydrates
Key Role of Water Ice
Pinqiang Cao (The George Washington University, China University of Geosciences, Wuhan, Xiamen University)
Fulong Ning (Qingdao National Laboratory for Marine Science and Technology, China University of Geosciences, Wuhan)
Jianyang Wu (Xiamen University, Norwegian University of Science and Technology (NTNU))
Boxiao Cao (The George Washington University)
Tianshu Li (The George Washington University)
Henrik Andersen Sveinsson (Universitetet i Oslo)
Zhichao Liu (National Center for International Research on Deep Earth Drilling and Resource Development, China University of Geosciences, Wuhan)
Thijs J.H. Vlugt (TU Delft - Engineering Thermodynamics)
Masayuki Hyodo (Yamaguchi University)
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Abstract
Water ice and gas hydrates can coexist in the permafrost and polar regions on Earth and in the universe. However, the role of ice in the mechanical response of ice-contained methane hydrates is still unclear. Here, we conduct direct million-atom molecular simulations of ice-contained polycrystalline methane hydrates and identify a crossover in the tensile strength and average compressive flow stress due to the presence of ice. The average mechanical shear strengths of hydrate-hydrate bicrystals are about three times as large as those of hydrate-ice bicrystals. The ice content, especially below 70%, shows a significant effect on the mechanical strengths of the polycrystals, which is mainly governed by the proportions of the hydrate-hydrate grain boundaries (HHGBs), the hydrate-ice grain boundaries (HIGBs), and the ice-ice grain boundaries (IIGBs). Quantitative analysis of the microstructure of the water cages in the polycrystals reveals the dissociation and reformation of various water cages due to mechanical deformation. These findings provide molecular insights into the mechanical behavior and microscopic deformation mechanisms of ice-contained methane hydrate systems on Earth and in the universe.