Experiment Research On The Fracture Failure Mechanism Of Pre-existing Cracks In Brittle Rock Under Cyclic Load

Rock mass often had suffered from geological process repetitiously, so it becamea highly nonlinear and anisotropic geological body cutting by faults, beddings,joints,schistosity and so on. The destruction of rock mass is mainly caused byextension of faults, beddings, joints, schistosity. Presently, research about the fracturefailure mechanism of crack-rock mass mainly had focused on a single load or a singleunload but barely had focused on cyclic load. However, rock mass which had sufferedfrom geological process repetitiously isn’t a single load path in natural conditions. Soit is very important to study the failure mechanism of crack-rock mass under cyclicloading for the failure mechanism of rock mass.In this paper, I matches brittle rock-like materials, designs different combinationsof variables pre-existing cracks which takes pre-existing crack dip, rock bridge lengthand rock bridge inclination as variable, researches and develops the "YY-L600"biaxial compression test system independently and designs new cycle loading andunloading tests.In the test, analyze mechanical property and the evolution of cracksextending. And then, deducing the failure mechanism of rock mass with two-parallelpre-existing cracks under cyclic loading by fracture mechanics and three mechanicalmodels are proposed. The main results are as follows:(1) Before destruction of specimens, the axial stress-strain curve remains lineargrowth and the axial residual deformation of each "hysteresis loop" growthequivalently. When specimensare fracturing, the axial residual deformation of the"hysteresis loop" increases significantly.(2) The value of the initial cycle load does not affect crack extension, but impacton the mechanical properties of specimens.With the increases of the value of theinitial cycle load, the peak strengthof specimens decrease,butthe ratio of crackinitiation strength and peak strength increase.(3)Combination of pre-existing cracks impact mechanical properties ofspecimens. The longer of the rock bridge, the greaterof the peak strength of specimens,and the greater of the ratio of crack initiation strength and peak strength.Meanwhile,whencrack dip is less, with the increase of the crack dip, the peakstrength of specimens increase. When crack dip is more, with the increase of the crackdip, the peak strength of specimens decrease.(4) Under the cyclic loading, the fracture propagation mode of theparallel-double cracks is same as the single cracks. And it can be divided into: tensilefracture mode and tensile-shear fracture mode. Tensile fracture mode also can bedivided into: non-overlapping tensional fracture mode, part-overlapping tensionalfracture mode and overlapping tensional fracture mode. While, tensile-shear fracturemode also can be divided into: pure-shear rock bridge fracture mode and tensile-shearrock bridge fracture mode. The fracture mode mainly is effectedby the combinationsof pre-existing cracks.(5) Under the cyclic loading, the fracture failure mechanism of brittle rock masswith two-parallel pre-existing cracks is leaded by combinations of pre-existing crackswithout the initial value of cycle load. The power of crack initiation and fracturefailure comes from slipping causes by the effective shear on shear fracturesurface.According to the mode of penetrating, failure model can be divided into:slipping-tensile failure model, slipping-shear failure model and slipping-tensile-shearfailure model.