Based on the equivalent viscoelasticity between the fractional Zener model and the time-varying viscosity Zener model

the significance of the relaxation time in the evolution of rock rheological viscoelasticity is highlighted. As the relaxation time approaches infinity

the evolution of viscosity during creep is equivalent to that during relaxation. Accordingly

the damage factor related to relaxation time is established to explain the physical significance of damage factor in rock rheological deformation. In specific

the function of variable order is constructed by using relaxation time and the variable-order fractional damage creep model is introduced. Furthermore

the variable-order fractional damage creep model is extended to the triaxial status. Based on the triaxial sandstone creep experimental data

the applicability and rationality of the variable-order fractional damage creep model in the triaxial state are verified. The results show that the developed variable-order fractional damage creep model and the creep experimental data are in good agreement

indicating that the damage creep model can be used to describe the nonlinear mechanical behavior of complete creep process. The effectiveness of the model fitting parameters is analyzed and verified

which encourages the applicability of the introduced model in other complex stress issues.