Nonisothermal fractional order two-surface viscoplastic model for stiff clays

Stiff clay exists widely in the world, but its significant time- and temperature-dependent mechanical features have not been fully modeled. In the context of fractional consistency viscoplasticity and bounding/subloading surface theory, this study proposes a novel nonisothermal fractional order two-surface viscoplastic model for stiff clays. First, by proposing a generalized plastic strain rate, the isotach viscosity is modified and extended to both over-consolidated and nonisothermal conditions that take into consideration the effects of temperature and OCR on thermal accelerated creep. Then, two strain rate and temperature-dependent yield surfaces are proposed with isotropic and progressive hardening rules to consider thermal collapse, strain rate effects, and smooth transition from elastic to viscoplastic behaviors. Next, the stress-fractional operator of the loading surface, according to the principle of fractional consistency viscoplasticity, is introduced to describe the nonassociativity of stiff clays. Finally, the predictive ability of the model is validated by simulating triaxial tests on Boom clay with various stress paths considering the temperature- and time-dependent features of stiff clays.