Ideal Strength of Methane Hydrate and Ice Ih from First-Principles

The ideal strength of methane hydrate and ice I-h was investigated and quantified from first-principles calculations. Using density functional theory, methane hydrate was studied under uniaxial, triaxial, and shear deformation modes, and the uniaxial deformation of ice I-h was considered for comparison. The resulting ideal strength was found, and the structural evolution in terms of bond lengths, bond angles, elastic moduli, and Poisson ratio was analyzed throughout the deformation. It was found that methane hydrate displays brittle behavior in terms of its strength and has no dominant slip system. Ice I-h exhibited a higher ideal uniaxial strength compared to the hydrate by deviating from the perfect tetrahedral arrangement of its water molecules. Under uniaxial tension, both structures maintain their transverse isotropy and fail at a critical hydrogen bond length despite the difference in their strength values. Under uniaxial compression, however, the hydrate loses its transverse isotropy unlike ice I-h which maintains it. While ice I-h and hydrates are similar in many of their physical properties, their ideal strength and structural deformation were found to be different. The presented new monocrystal mechanical properties and insights are a guide to future research in natural and synthetic polycrystalline gas hydrates.