The three processes of brittle crystalline rock creep

Abstract

Three elementary processes (elastic, plastic and dilatant deformations) in brittle rock under sustained load are considered. Though these occur independently and in parallel with each other, their contributions to the creep strain of brittle rock are not equal and vary with the stages of creep. The elastic creep governs the primary stage; whereas, visco-plastic flow dominates at the secondary stage; and the third stage; i.e. the accelarating creep, is associated with the dilatant strain caused by the process of microfracturing. Besides these, the instantaneous strain at the moment of loading plays a significant role too. The strain is a function of the magnitude of the load. When the latter approaches the failure level, the instantaneous strain is a sum of the elastic deformation and instantaneous plastic deformation as well. A rheological model of such a strain is described in the paper. The primary creep duration is estimated to be three times the relaxation time of the Kelvin substances modelling elastic creep. An analysis of experimental creep curves indicates that the relaxation time for the same rock is not constant and depends upon the magnitude of the sustained load, increasing with it, via a power function. The boundary time between the secondary and tertiary creep is found from the intensity of the process of brittle rock microfracturing. A rheological model as a massive dash-pot is proposed for visualizing the accelerating creep. A method of decomposing experimental creep curves is developed to facilitate the estimation of the essential parameters of creep of the rock under testing. This allows one to design a creep curve for the same rock at an arbitrary magnitude of sustained load up to the moment of failure.