The conventional triaxial tests were carried out on sandstone to study the stress-strain relationship of rock during the loading-unloading process. The nonlinear characteristics of rock in the post-peak unloading process were analyzed

and the damage variable of rock was defined. Meanwhile

the elastic modulus model was established to describe the stress-strain relationship in the post-peak unloading process. The Poisson's ratio model under unloading was obtained by analyzing the relationship between the axial strain and radial strain in the loading-unloading process. The D-P plasticity model was introduced to modify the hardening function according to the plastic-hardening characteristics of sandstone

and a damage model associated with the equivalent plastic strain was established. At last

the established model was matrixed and numerically calculated. In this study

the following conclusions are obtained. During the loading process

the porous rock exhibits obviously nonlinear characteristics

and the elastic modulus of the rock increases with the increase of the body stress. During the post-peak unloading process

when the axial stress is greater than the confining pressure

the stress-strain can be described by the elastic modulus in the pre-peak elastic phase multiplying by the continuity factor. With the increase of equivalent plastic strain

Poisson's ratio increases first and then decreases until reaching a stable value. During the unloading process

the equivalent plastic strain maintains stable

and Poisson's ratio remains constant. The numerical calculation is conducted by the proposed constitutive model. Compared the numerical results with the experimental results

the proposed model can reflect the stress-strain relationship of rock in the post-peak unloading process.