Experimental Study on Rock Strength and Deformation Characteristics Under Triaxial Cyclic Loading and Unloading Conditions

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 (sigma(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 sigma(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 sigma(3) increased. However, the generalized Poisson's ratio (mu(l)) increased as the cycle index increased, whereas it decreased as the sigma(3) increased. Based on the Mohr-Coulomb strength criterion and plastic shear strain (gamma(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 (phi '). Ultimately, the evolution rules of c ', phi ' and psi (dilatancy angle), with sigma(3) and gamma(p) were revealed. Moreover, the post-peak dilatancy angle models with regard to the influence of sigma(3) and gamma(p) on the volume dilatancy of the rock specimen were established, which indicated that psi increased first and consequently decreased along with the gamma(p) increase, whereas it decreased as the sigma(3) increased.

Automated summary

The study investigated the strength, deformation, and expansion characteristics of rock specimens under triaxial cyclic loading and unloading testing. With the increase of confining pressure sigma(3), the strength of rock specimens increased, while the elasticity modulus El showed an initial increase followed by a decrease. The generalized Poisson's ratio μ(l) and generalized internal friction angle φ ' showed different trends in response to the cycle index and confining pressure sigma(3).