Constitutive modeling of viscoelastic-plastic strain characteristics and damage in southern China red sandstone under chemical exposure

In this investigation, we conducted graded creep cyclic loading and unloading testing to measure the viscoelastic-plastic rheological properties of red sandstone from Southern China under acidic conditions. We utilized an enhanced method to divide the strain into four components: instantaneous elastic strain, instantaneous plastic strain, viscoelastic strain, and viscoplastic strain. We analyzed the strain characteristics of the corroded samples in relation to the deformation modulus. Employing nonlinear rheological theory, we derived the constitutive equations for creep damage in rock under both one-dimensional and three-dimensional stress states. Our findings indicate that acid corrosion has a minimal impact on the resistance to elastic deformation in red sandstone, with the elastic deformation modulus remaining relatively unchanged at comparable stress levels. The relationships between instantaneous elastic strain and viscoelastic strain with deviatoric stress are nearly linear. Increased acidity enhances the plastic deformation of the sandstone, marked by a progressive increase in the instantaneous plastic modulus and a decrease in instantaneous plastic strain increments with successive loading and unloading cycles. The viscoplastic modulus decreases as stress levels rise, leading to increased viscoplastic strain. Incorporating a chemical damage variable that accounts for plastic deformation, we established a creep damage constitutive equation that separates viscoelastic-plastic strains. The validity and accuracy of our proposed model are confirmed through comparison with the traditional Nishihara model.