Experimental And Numerical Study On Propagation Mechanism Of Three-dimension Crack Of Rock Specimens

As rock contains a lot of cracks, its deformation and failure is closely related to the process of internal crack initiation, propagation and coalescence. For a long time, researchers simplified problems in rock fracture mechanics as 2D in order to avoid difficulties involved. However cracks in rocks are 3D in fact both from the geometric or mechanics characteristics. There are basic differences between 2D and 3D cracks, and it is much more complicated for crack propagation in 3D than 2D cases. Different from 2D cracks,3D cracks propagation both in the surface and internal at the same time. But it is more difficult to observation internal cracks. Previous scholars usually use the transparent homogeneous material such as organic glass in experimental study. Although this kind of material can make it easier to observe internal crack extension, but have ignored the heterogeneity and brittleness of rock, which will limit the experimental results. Only by taking into account the heterogeneity of rock can simulate the 3D crack propagation and coalescence mechanism.Based on the understanding and summary of predecessors’study on failure mechanism and regularity of crack extension,this paper simulates the propagation and coalescence process of specimens containing pre-existing surface flaws under compression.The 3D numerical model RFPA3D based on the microscopic damage mechanics is employed to simulate the failure process, which has taken nonlinearity and heterogeneity of rocks into account. In addition, specimens made of cement mortar which is similar to rock has been adopted in the physical experiment. The following problems have been mainly analyzed by combination of 3D numerical simulation and physical experiment:(1)The 3D cracks are classified according to mechanical characteristics and geometrical position. Mechanism of crack evolution is studied by comparing the propagation of cracks in 2D and 3D space. Results show that influenced by internal structure, the initiation location and evolution form of 3D crack is quite different with 2D cracks. The form of crack evolution changes a lot on section planes of different thickness. The specimen with penetrated crack is finally split into two halves by wing cracks,which is similar to that of 2D cracks. Lacking of free surface, the stress of inner and surface crack is higher than penetrated crack, and more likely to lead to brittle failure.(2) The uniaxial compression tests of rock specimens with single pre-existing surface crack are analyzed especially exploring the influence factors of crack growth. The numerical simulation is in great agreement with the experimental results. The existence of the pre-existing cracks will cause stress concentration around it. Damage units gathered in all the inner edges of the crack forming a shell crack; The crack inclination angle, crack length and relative thickness will affect the evolutionary mechanism and strength of the specimens.(3)Mechanism of cracks interaction and coalescence is studied through double cracks. Results show the interaction of cracks affect crack growth and extension in two aspects, either facilitates extension or restrains it. Which factor is in dominate is up to the location of cracks(angle of crack and rock bridge). The existence of lateral pressure limits the evolution of wing cracks. With the increase of lateral pressure, shear cracks play a dominant role in the failure mode and the compressive strength will improve.