LAUSR

The mechanical behavior of rock under cyclic loading is quite complicated compared to monotonic loading or unloading conditions. The triaxial cyclic loading and unloading testing of rock specimens under 6 confining pressures ( σ 3 ) was carried out through the MTS 815 rock mechanics testing system, to explore the strength, deformation, and expansion characteristics of the rock specimens. The stress–strain curves of the rock specimens in the triaxial cyclic loading and unloading testing presented the hysteresis effect. Besides, as σ 3 increased, the rock specimen strength increased, while the failure form brittle to ductile. The elasticity modulus ( E l ) increased first and consequently decreased as the cycle index increased, while it increased as σ 3 increased. However, the generalized Poisson’s ratio ( μ l ) increased as the cycle index increased, whereas it decreased as the σ 3 increased. Based on the Mohr–Coulomb strength criterion and plastic shear strain ( γ p ) as the plastic parameter, the subsequent yield plane model of the loaded rock was characterized by generalized cohesion ( c´ ) and generalized internal friction angle ( φ´ ). Ultimately, the evolution rules of c´, φ´ and Ψ (dilatancy angle), with σ 3 and γ p were revealed. Moreover, the post-peak dilatancy angle models with regard to the influence of σ 3 and γ p on the volume dilatancy of the rock specimen were established, which indicated that Ψ increased first and consequently decreased along with the γ p increase, whereas it decreased as the σ 3 increased.