To dissipate is to stabilize: Mechanical loss in methane hydrates

Natural gas hydrates (NGHs) represent a vast potential energy resource, yet their mechanical instability under geological or anthropogenic stresses threatens seafloor stability and poses climate risks. Using large-scale molecular dynamics simulations, we demonstrate how ultralow mechanical loss enables long-term stability in polycrystalline methane hydrates under cyclic shear. Our analysis reveals two distinct deformation regimes governed by grain size, temperature, and loading frequency. Under high-frequency, small grained methane hydrates undergo transgranular fracture, whereas large grained methane hydrates deform through elastic kink banding, maintaining structural coherence. Under low-frequency and elevated temperature conditions, which is characteristic of geological settings, the response becomes viscoelastic, with energy dissipation dominated by grain boundary activities including molecular diffusion, cage structure transformations, and sI ↔ sII phase transitions. These grain boundaries, enriched in metastable noncanonical cages, serve as dissipation hotspots where cage restructuring and phase transitions efficiently release strain energy. Crucially, the frequency-dependent mechanical loss follows the Cross model, exhibiting an ultralow loss tangent limit of 0.22 that underpins the exceptional stability of natural hydrate systems. This work establishes a molecular-to-macroscopic framework linking dissipation mechanisms to reservoir-scale stability, providing fundamental insights for assessing hydrate integrity under natural and anthropogenic perturbations.