Non-stationary Nishihara model in the particle flow code

In order to accurately simulate the whole process of rock creep and explore the generation and development of mircofractures from the meso-level,a visco-elastoplastic Nishihara rheological constitutive model is developed in the two-dimensional particle flow code(PFC^2D). Then a non-stationary Nishihara model including two non-stationary elements is put forward in this article. A constitutive equation and a creep equation are derived based on the non-stationary Nishihara model. The user-defined Nishihara constitutive model is called in PFC^2D,and a numerical sample whose strength properties are the same as those of a real rock specimen is acquired by parameter testing. Using the data of uniaxial creep tests in laboratory,the non-stationary parameters are back analyzed in Matlab. At last,uniaxial creep tests are simulated using the stationary and non-stationary models,and micro-fractures are monitored.A comparison of these results show that the stationary model can only be used to describe the decay and steady stages,while the nonstationary model is also applicable to the accelerated stage,and thus can simulate the whole process of rock creep. Accelerated creep results from the accelerated development of mirco-fractures and leads to shear failure.