Abstract: Uniaxial creep experiments were conducted on sandstone samples obtained from Ningtiaota coal mine in Shaanxi using a five-connected rheological test system by which step loading and unloading were applied. In view of decaying, stable and accelerating stages within total creep, the Newton body in the Burgers constitutive model was modified by creating a new function with stress and time. In consideration of the effect of damage in the process of creep, a damage body was proposed based on the damage theory and Lemaitre's hypothesis. The Hooke body, Kelvin body, the modified Newton body, and the proposed damage body were combined to build a new nonlinear damage creep model and the constitutive model and creep function were also derived. The creep curves under various stress levels were described by the established nonlinear damage creep model. Moreover, parameters of the established damage model were fitted and the sensitivity of fitting parameters was analyzed according to the creep experimental data. Results suggest that the proposed damage model could well fit and describe the whole process of the creep of sandstone with no need of segmented functions, and hence overcoming the shortcoming of Burgers model in reflecting the accelerating creep stage.

Key words: sandstone, step loading and unloading, modified Newton body, damage creep model, total creep