Separation of elastoviscoplastic strains of rock and a nonlinear creep model

A series of triaxial creep tests were carried out on fractured limestone specimens under multilevel loading and unloading cycles to capture elastoviscoplastic strain components. A new data processing algorithm was proposed to analyze the experimental data, determine the instantaneous elastic and instantaneous plastic strain components and the viscoelastic and viscoplastic strain components from the total strain, and separate the viscoelastic and viscoplastic strain curves from the total creep strain curve. The instantaneous elastic strain, instantaneous plastic strain, viscoelastic strain, and viscoplastic strain versus deviatoric stress relationships are highly nonlinear. The proportion of the viscoplastic strain component in the total creep strain increases with increased deviatoric stress. On the basis of the experimental results, a nonlinear elastoviscoplastic (EVP) creep constitutive model was proposed by connecting a Hooke body, a parallel combination of Hooke and St. Venant bodies, a Kelvin body, and a generalized Bingham body. The proposed EVP creep model can describe both the loading and unloading creep behavior precisely. The creep model curves agree very well with the experimental results and give a precise description of the full stages of creep, especially the tertiary creep stage. Moreover, the variation law of the creep parameters is also supported by the experimental observations. So the validity and great practical potential of the proposed EVP creep model are shown well.