Narrow Results

Deformation rules and acoustic emission characteristics of granite under repeated loading-unloading are studied using the MTS Mechanics Testing System and Disp. acoustic emission instrument. It's found that the axial stiffness of granite as well as Poisson's ratio increases with the development of the plastic strain while the change trend of lateral stiffness is negative. For brittle rocks, strain is produced by three mechanisms: elastic deformation, axial micro-cracking and compaction. The peak AE rate is increasing with the plastic strain, which indicates that using the axial plastic strain as the internal damage variable to represent rock damage is reasonable.

This paper presents a numerical simulation method for the brittle rock failure process under compression, by combining the finite element method with micromechanics damage theory. When considering the rock as a homogeneous material, the initial elastic constant of each computational element is the same, but the microcrack distribution in the rock follows a statistical distribution. Consequently, in the loading process, microcrack propagation in each element is different, leading to an inhomogeneous distribution of changes in elastic constant. Under increased loading, this distribution will ultimately be reflected in the macro-failure mode of the rock. To investigate the macromechanics of the rock failure process, the damage variables and effective elastic constants are used to reflect the propagation of microcracks, thus coupling the micromechanics and macromechanics of the rock failure process. Finally, the paper demonstrates the numerical simulation method by simulating the failure of sandstone; these computational results show that the method performs well in simulating the mechanical characteristics of the brittle rock failure process.

Multi-crack propagation is investigated mainly by experimental measurement and little by theoretical prediction. The classical fracture criteria can better predict tensile fracture under arbitrary loading conditions (pure tensile, pure shear and mixed-mode), but have difficulty in predicting shear fracture. In this paper, Mode I and Mode II SIFs of branch-cracks initiated by the original cracks were calculated by the complex function and superposition method, and a new theory of multi-crack propagation was established based on the criterion of maximum tensile-shear SIF ratio. Theoretical results of two collinear cracks under uniaxial compression show that the cracks initiate more easily at $\alpha =30^{\circ }$ (the crack inclination angle) than other angles. Coalescence of branch-crack only occurs at $\alpha =45^{\circ }$ with the maximum crack propagation length. Peak stress $(P^{\mathrm{\text{un}}})$ reaches minimum when $\alpha \approx \phi$ (inner friction angle of rock), and the larger the $\alpha$, the closer to the compressive strength of rock the $P^{\mathrm{\text{un}}}$. Mechanism of the crack initiation and propagation are all Mode I under uniaxial compression. Uniaxial compressive test results of red sandstone (the rock material is assumed to be homogeneous) pre-cracked specimens agree well with predicted results of the crack initiation, stable and unstable propagation, which can prove the validity of the new multi-crack propagation prediction method.