Experimental investigation and fractional elastoplastic damage constitutive modelling of gray sandstone under loading disturbance

The mechanical behavior and deformation characteristics of gray sandstone greatly affect the stability and safety of large-scale structural engineering on the rock stratum. A series of tests for sandstone collected from Jinping hydropower stations in southwest China under complex stress paths were carried out, including hydrostatic pressure test, conventional triaxial test, cyclic loading and unloading of deviatoric stress, confining pressure and pore pressure test. Meanwhile, a fractional elastoplastic damage model was proposed. The results show that the crack closure stress, crack initiation stress, and crack damage stress under conventional triaxial compression path are 0.189~0.217, 0.475~0.615, and 0.730~0.856 of peak stress, respectively. The influence of cyclic loading and unloading on mechanical behavior and deformation of gray sandstone has a significant strengthening effect. Under deviatoric stress cyclic loading and unloading, with increasing cycle number, axial and lateral strain increment curves tend to coincide, and the volumetric strain increment decreases. Under confining and pore pressure cyclic loading and unloading, the lateral strain increment is much larger than the axial strain increment. The axial elastic modulus and lateral elastic modulus show great discretization and irregularity. Moreover, a modulus-like axial coupling parameter is analyzed and discussed. The established constitutive model can accurately reflect the hardening and plastic dilatancy behavior of gray sandstone. Meanwhile, the staggered iterative return mapping algorithm is improved to ensure the convergence of the model.