Experimental Study And Constitutive Model Of Creep For Rock Materials

As a branch of mechanics, rheology focuses on the deformation law which is related to time factor under the condition of different stress, strain, temperature and radiation. Rheology includes creep, stress relaxation of stress and elastic aftereffect etc. Creep is an important factor which can greatly influence the stability of rock mass. An obvious strain will appear in soft rock materials, e.g. soft surrounding rock of tunnel, when they are affected by a relatively low degree of stress. Shear creep strain can also appear in very hard rock materials under the condition of high stress, and then put a negative effect on the stability of structures. In order to reveal the characteristics of creep process, we should deeply study the creep behavior of rock materials.In this dissertation, laboratory experiments and theoretical analysis are conducted to obtain basic physical and mechanical parameters of rock materials. The mechanical property and creep behavior are systematically studied under condition of different stress. A new creep model is established based on traditional creep models in consideration of the theory of fractional calculus. Then the new model is verified by comparing with the creep experiments results of different kinds of rock materials.(1)A series of uniaxial and triaxial compression experiments are conducted. In order to reduce the randomness and difference of specimens, we choose three specimens to repeat for each experiment. Mechanical parameters including uniaxial compressive strength, triaxial compressive strength, Possion’s ratio etc. are obtained. Results show that the difference is small in this batch of specimens. Moreover, compressive strength and maximum strain both increase when we increase the confining pressure.(2)During the uniaxial creep experiments, four different axial stress are chosen to analyze the effect of axial stress to creep. Results show that higher axial stress can result in a higher steady creep strain rate and creep damage seems to progress quickly in such a case. In comparison with uniaxial creep experiments, there are many differences when we put on a confining pressure, e.g. decline of creep strain rate, increase of creep time span, lack of accelerated creep stage etc.(3)The creep strain rate indicates that there is a linear relationship between axial stress and steady creep strain rate. Moreover, rheological isochronous curves show that the effect of axial stress on creep expands as time grows. However, confining stress can obviously inhibit the progress of creep damage and lead to a relatively low steady creep strain rate.(4)A new creep model is established based on the theory of fractional calculus, and its constitutive relationship are extended to triaxial stress condition by using the theory of general plastic mechanics and tensor analysis. Then, parameters of the creep model are identified and the creep model is verified by comparing with creep experimental results. It is shown that this new creep model can simulate the whole creep process well and accurately coincide with creep experiments results. In order to verify the wide suitability of this creep model, creep experiments of other rock materials are chosen to make comparative analysis. Results show that this new creep model can be used to simulate the creep behavior of different kinds of rock materials.