title: Mechanical instability of monocrystalline and polycrystalline methane hydrates.
abstract: Despite observations of massive methane release and geohazards associated with gas hydrate instability in nature, as well as ductile flow accompanying hydrate dissociation in artificial polycrystalline methane hydrates in the laboratory, the destabilising mechanisms of gas hydrates under deformation and their grain-boundary structures have not yet been elucidated at the molecular level. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected crossover from strengthening to weakening in polycrystals. Upon uniaxial depressurisation, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilises the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insight not only into the metastable structures of grain boundaries, but also into unusual ductile flow with hydrate dissociation as observed during macroscopic compression experiments.
authors.affiliation: Department of Physics, Research Institute for Biomimetics and Soft Matter, Xiamen University, Xiamen 361005, China.|Faculty of Engineering Science and Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
authors.forename: Jianyang
authors.initials: J
authors.lastname: Wu
authors.affiliation: Faculty of Engineering, China University of Geosciences, Wuhan, Hubei 430074, China.
authors.forename: Fulong
authors.initials: F
authors.lastname: Ning
authors.affiliation: Faculty of Natural Sciences and Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
authors.forename: Thuat T
authors.initials: TT
authors.lastname: Trinh
authors.affiliation: Faculty of Natural Sciences and Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
authors.forename: Signe
authors.initials: S
authors.lastname: Kjelstrup
authors.affiliation: Process and Energy Laboratory, Delft University of Technology, Leeghwaterstraat 39, 2628CB Delft, The Netherlands.
authors.forename: Thijs J H
authors.initials: TJH
authors.lastname: Vlugt
authors.affiliation: Faculty of Engineering Science and Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
authors.forename: Jianying
authors.initials: J
authors.lastname: He
authors.affiliation: Faculty of Engineering Science and Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
authors.forename: Bjørn H
authors.initials: BH
authors.lastname: Skallerud
authors.affiliation: Faculty of Engineering Science and Technology, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
authors.forename: Zhiliang
authors.initials: Z
authors.lastname: Zhang