Theory And Application Of Crack Propagation Considering Interfacial Effect

To study the propagation of fractures that encounter the weak inclusions lying in the rock layer and interlayer interfaces between the rock layers,is of great signifi-cance to explore the rule of underground fracture propagation and sequent influence on the mechanical properties of upper rock strata.Based on the fracture theory and actual data,in this paper,we have studied the propagation of cracks which is incident to the weak interface from the perspective of theoretical analysis,simulation and ex-perimental analysis,and then applied the conclusions to the process of hydraulic frac-turing.The main research works are as follows:?1?From the perspective of energy release rate the crack propagation criterion?F criterion?was revised,which was proved to be consistent with the original F crite-rion in predicting the direction of crack propagation in the isotropic homogeneous material.Moreover,by numerical calculation the ratio of mode II critical fracture energy to mode I critical fracture energy was found to have no influence on the dire-ction of crack propagation.Meanwhile,the critical condition for crack deflecting into the weak interface and then propagating was proposed,and the influence of different factors on the possible range of weak interface into which cracks deflect was discu-ssed,including the ratio of mode II critical fracture energy to mode I critical fracture energy,surface pressure on the crack surface and friction etc.?2?Based on the modified F criterion and the critical condition for crack deflecting into the weak interface,the propagation paths of cracks which are inclined to the weak inter-layer interface?=?-?and?=-?and have the dip angle?of 0,?/6,?/4,?/3 and?/2 were numerically analyzed.When the surface pressure P reaches a certain value,the crack whose dip anglebis 0 will propagate along the weak bedding plane?=0,which coincides with the extension line of the crack surfaces.While,the cracks with dip angle?of?/6,?/4 and?/2 tend to go through the two weak bedding and continue to propagate in the direction of?_max.For the crack whose dip angle?is?/3,it deflects into the weak bedding and propagates along the weak bedding surface?=2?/3 at first.However,with the surface pressure increasing,it turns to penetrate the weak bedding surface and propagates in the direction of?_max.?3?With the particle flow software PFC,the propagation paths of cracks that have different dip angles and subjected to the uni-axial compression or the bi-axial compression,were simulated respectively,when they encounter the horizontal inter-layer interface and circular inclusion.It was found that the mechanical properties in the interface region have a great influence on the crack deflection,and the inclinationbof the crack has little effect on the crack deflection.?4?During the preparation of similar materials,the content of gypsum powder was found to greatly affect the strength and fracture toughness.With the content of gypsum powder increasing,the strength of rock-like materials decreases,while the mode I fracture toughness K_?40?c40)c and mode II fracture toughness K_IIcIc both increase at first and then decrease gradually.Moreover,through uni-axial compression experiment,it was found that the crack would deflect into the weak interfacea?28?135^°,a?28?120^°anda?28?105^°,respectively,when its inclination angle is 45^°,60^°or 75^°.The experimental results were proved to agree well with the theoretical calculation that is based on the modified F criterion and the critical condition for crack deflection.