Mechanical Properties of Methane Hydrate: Intrinsic Differences from Ice

Understanding fundamental mechanical behaviors of ice-like crystals is of importance in many engineering aspects. Herein, mechanical characteristics of monocrystalline methane hydrate (MMH) and hexagonal ice (Iₕ) under mechanical loads are contrasted by atomistic simulations. Effects of engineering strain rate, temperature, crystal orientation, and occupancy of guest molecules on the mechanical properties of MMH are investigated. Results show that the engineering strain rate, temperature, and occupancy of guest molecules in 5¹²6² cages greatly affect the mechanical strength and failure strain of MMH, whereas the effect of crystal orientation on the tensile response of MMH such as along the [100] and [110] directions is negligible. Particularly, the occupancy of guest molecules in 5¹²6² cages primarily governs the mechanical strength and elastic limits of MMH. For Iₕ, it is tensile stiffer than that of MMH at 263.15 K and 10 MPa, and shows unique mechanical characteristics such as tension-induced stiffening and compression-induced remarkable softening under the [0001] directional load. Both crystals demonstrate brittle fracture behavior but different plasticity with dislocation-free in MMH yet dislocation activities in Iₕ. The intrinsic differences in the mechanical properties of MMH and monocrystalline Iₕ mainly result from the host–guest molecule interactions and relative angles which tetrahedral hydrogen bonds make to the loading direction. These mechanical characteristics present microscopic insights to understand the mechanical responses of naturally occurring and artificial synthetic gas hydrates.