A non-isothermal fractional consistency two-surface viscoplasticity model for gas hydrate-bearing sediments

Gas hydrate-bearing sediments (GHBS), recognized as an emerging and highly promising unconventional energy resource, exhibit pronounced rate-, temperature-, and pore pressure-dependent mechanical behaviors that have been inadequately addressed or frequently overlooked in existing constitutive modeling frameworks. In this paper, a novel non-isothermal two-surface elasto-viscoplastic model is proposed based on the fractional consistency viscoplasticity and bounding surface theory to capture the key mechanical behaviors of GHBS under varying loading conditions. Specifically, a modified isotach viscosity formulation is first extended to account for hydrate conditions, with the creep coefficient expressed as an exponential function of hydrate saturation. Secondly, a two-surface (loading and yield surfaces) framework is formulated, integrating multifactorial viscoplastic hardening mechanisms, namely isotropic hardening, progressive hardening, and deviatoric degradation, along with a Caputo-formed non-orthogonal viscoplastic flow rule. Then, employing the consistency condition of the loading surface, an incremental constitutive relation is rigorously formulated to explicitly relate stress, strain, strain rate, temperature, pore pressure, and hydrate saturation. Finally, validation against experimental data demonstrates the model’s excellent capability to simulate mechanical behaviors under complex time-dependent stress paths. This robust, rate-dependent constitutive framework provides a fundamental basis for subsequent advancements aimed at incorporating a broader spectrum of pertinent factors, such as hydrate dissociation, extended temperature ranges, multi-component effects, and particle crushing, etc.