Failure Modes Of Rock With Two Precast Cracks Based On Non-contact Stress Field Under Compression Test

There are many kinds of joints and faults in the natural rock mass.The development of these weak structural planes often results in the failure of rock mass when subjected to loads.Study on the failure modes of crack rock under compression is of great significance both theoretically and practically,such as tunnel and underground engineering,slope engineering and water conservancy and hydropower engineering,it will help to reveal bank slop deep-rock deformation and fracture(deep crack)mechanism and contribute to the application of deep unloaded rock mass.Based on the similar material simulation basalt brittle material,samples with single precast crack(different crack angle)and samples with two precast cracks(different crack angle and bridge angle)were made.The loading test was conducted on "YDS-3 type rock mechanic multi-function testing machine",PIV was used to record the deformation and failure process of rock face.Integrated with particle imaging velocity(PIV)and stress-strain curves during experiment,displacement vector of sample during failure process was identified and moreover the stress on the surface of sample was calculated based on finite element(FE)method.In this dissertation,the stress characteristics of crack rock face,the deformation and failure process of sample,the influence of crack combination on rock strength,crack coalescence mode and so on were analyzed.The main results are as follows:(1)With the increasing of axial stress,the maximum compressive stress and maximum tensile stress on the precast crack tip increase firstly,then keep stability,and then fluctuate,increase dramatically lastly.This law reveals the process of stress concentration and stress release on the precast crack tip.(2)The influence of various factors on the lateral stress(σ_x)distribution of rock face is: with the increasing of crack angle,the areas of tensile stress concentration extend from precast crack tip to precast crack surface and bridge area,the areas of compressive stress concentration decrease and move to precast crack bottom.Rock bridge angle increases,the areas of compressive stress concentration around precast crack surface and rock bridge decrease,while the areas of tensile stress concentration increase gradually.On the condition of cyclic loading and unloading,the areas of tensile stress concentration around precast crack surface and rock bridge decrease.When lateral pressure increases,the areas of tensile stress concentration around precast crack surface and rock bridge decrease.(3)Under the external load,whether it is normal loading or cyclic loading and unloading,the deformation and failure of rock usually has four steps: compaction phase,elastic deformation phase,plastic deformation phase and post peak phase.The peak strength of samples with two precast cracks are smaller than samples with single precast crack.The mechanical properties of sample are weakened under cyclic loading and unloading,the peak strength and initiation stress are smaller than sample under normal loading.When lateral pressure increases,the peak strength and initiation stress increase.When crack angle is 45°or 60°,the peak strength and initiation stress are smallest.When crack laps(bridge angle>90°),the peak strength of rock is smaller.(4)At first,k-value increases and remains constant.Then k-value fluctuates and increases dramatically with the increasing of axial strain.The change regulation of k reveals the process of stress concentration and stress release during rock deformation and failure.Macro-crack distribution areas are consistent with stress-concentration areas.The characteristic points of k and axial stain curve are consistent with the stress thresholds of rock deformation and failure.So,according to the change regulation of k,we can divide the deformation and failure process of crack rock.(5)The obvious displacement jump of label point in precast cracks tip indicates the initiation of macroscopic crack.The region where the label point occurs displacement jump is related to the type of initiation crack,the displacement jump of label point 1or 3 indicates the initiation of shear crack while the displacement jump of label point 2or 4 indicates the initiation of tensile crack.(6)Three major types and nine sub-categories failure modes are observed in this test: tensile failure,shear failure and tensile-shear failure.The tensile stress and shear stress concentration are occurred in the precast crack surface or rock bridge area no matter which kind of rock bridge coalescence mode is.The shear stress concentration has a much greater density when the rock bridge coalescence is shear mode or tensile-shear mixed mode.(7)The tensile failure mode is more likely to occur with the small crack angle(0°,15°).The shear failure mode is more likely to occur with the large crack angle(75°,90°).The tensile-shear failure mode is more likely to occur with the medium crack angle(30°,45°,60°).The tensile failure mode is more likely to occur with the large rock bridge angle(120°,150°).The shear failure mode is more likely to occur with the small rock bridge angle(30°,60°).The tensile-shear failure mode is more likely to occur with the medium rock bridge angle(90°).When crack angle is close to rock bridge angle,that is two precast cracks are nearly coplanar,the shear failure mode is more likely to occur.The shear failure mode is more likely to occur under cyclic loading and unloading.The shear failure mode is more likely to occur when lateral pressure increases.