A nonlinear creep model is researched and proposed on the basis of a new proposed strain softening index. Firstly

according to the triaxial experimental data of rock

a strain softening index R1 is presented to describe the relationship between the softening of the post-peak mechanical parameters and the plastic strain. Secondly

by analyzing the relationship between the tertiary creep and the general post-peak stage

a strain softening index R2 in the tertiary creep is put forward to establish a non-linear viscoplastic element. Thirdly

by connecting the nonlinear viscoplastic element with the Hook and the Kelvin viscoelastic models

a nonlinear creep model is established inserting into the ABAQUS software. Finally

the proposed model is applied to simulate the triaxial creep experiments of sandstone and mudstone. The numerical results are in good agreement with the experiments indicating that the proposed model is capable of characterizing the accelerated creep behaviors of hard and soft rocks. The parameter β can adjust the curve form of the tertiary creep

showing the function to model the brittle-ductile failure. Furthermore

the parameters in the model can be readily identified by the conventional compression failure test and the creep test.