Research On Crack Evolution Law And Fatigue Damage Of Rocks With Prefabricated Cracks Under Cyclic Load

In recent years,with the rapid development of China's construction projects,water conservancy and hydropower projects,underground engineering and other infrastructure construction,a large number of slopes,tunnels and other engineering projects have been generated,which have caused engineering accidents such as landslides and landslides,causing huge economic losses.And bad social impact.The occurrence of these accidents is due to the lack of in-depth research on the rock mass in the area where the project is located and its mechanical response under external forces.As a kind of heterogeneous complex structure,rock mass has discontinuous structural planes such as bedding,joints,holes,faults,etc.Its stability is related to the safety of the whole project.Once the damage is unstable,it will cause inestimable losses.Therefore,there is an urgent need to study the mechanical properties,crack evolution laws and failure characteristics of defective rocks.In this paper,by prefabricating rock-like specimens with crisscross cracks,RMT-150 B rock mechanics testing machine was used to perform uniaxial compression and cyclic loading and unloading tests on rock-like specimens with different crack lengths and different crack inclinationangles to study the crack evolution law,Fatigue characteristics and fatigue damage mechanism have made some exploratory research results:(1)Using the RMT-150 B rock mechanics testing machine,uniaxial compression and cyclic loading and unloading tests were carried out on rock specimens with crisscross fractures of different fracture lengths and different fracture inclination angles.The research results show that: the compressive strength of the rock with cruciform fissures under cyclic loading and unloading is less than that of the complete specimen;the compressive strength under cyclic loading and unloading is related to the fracture dip angle and the fracture length.When it is constant,the peak intensity decreases with the increase of the length of the secondary crack,that is,the deterioration of the peak intensity increases with the increase of the length of the secondary crack.(2)The high-speed camera was used to record the crack evolution law and crack destruction characteristics of rock samples with cross cracks.The test results show that the type of crack failure of rock specimens with criss-cross cracks is related to the crack length and crack dip angle.When the inclination angle of the crack is 0 ° and 60 °,the specimen appears as a tensile crack;when the inclination angle of the crack is 30 ° and 45 °,the specimen appears as a mixed crack of tensile and shear;when the length of the main crack is longer,the crack starts to crack The location is at the tip of the main crack;when the length of the secondary crack is longer,the crack initiation position is at the tip of thesecondary crack;when the length of the primary crack is the same as the length of the secondary crack,the crack initiation position is at the tip of a crack with a larger angle from the axial stress.(3)Cyclic loading and unloading tests are carried out on rock samples with cross-shaped prefabricated cracks,and the characteristics of fatigue damage and fatigue life are analyzed.The analysis results show that the damage of pre-fractured rock specimens under cross loading and unloading is a gradual accumulation process.When the damage degree of the pre-fractured rock specimens containing cross is beyond its threshold,the sample will break;the cyclic loading and unloading There is an intersection between the stress-strain curve of the pre-fractured rock sample with cross intersection and the stress-strain curve under uniaxial compression.When the axial displacement under cyclic loading and unloading exceeds this intersection point,the degree of damage The threshold is reached,and the peak stress at the intersection point is small.When the cyclic loading and unloading is performed within the range of30% to 90% of the peak stress level of uniaxial compression,its average fatigue life is 175 cycles,and it contains cross-shaped prefabricated cracks.The axial deformation of rock-like samples shows an increasing-stable-increasing trend.(4)Through the analysis of the stress-strain curve relationship under the cyclic loading and unloading of the rock sample with cross prefabricated cracks,the morphological changes of the hysteretic loop atdifferent loading stages were obtained.When the cyclic loading and unloading and the axial strain value during uniaxial compression are basically the same,the shape of the hysteresis loop shows a sparse-dense-sparse law;the hysteresis loop formed by the cyclic loading and unloading has a hysteresis loop during the first cycle The area is larger and decreases in the second cycle and the third cycle,and then the area of the hysteresis loop begins to stabilize,and finally gradually increases until rupture occurs;in the process of cyclic loading and unloading,prefabricated fissure-like rocks are tested With the dissipation of energy,the curve path of the hysteresis loop loading and the curve path of the unloading do not coincide.