Study On The Characteristics Of Type I Fracture Of Granite Under Cyclic Loading And Unloading

Rock is a kind of quasi brittle heterogeneous damage material,which widely exists in nature and is also the main material of the crust.Its interior usually contains a variety of mineral components.With the expansion of construction activities of underground space engineering,the construction environment of rock engineering is becoming more and more complex,the stress mode of rock is becoming more and more complex,and the phenomenon of rock cyclic fracture often occurs.Under the action of cyclic loading and unloading,the study of mechanical properties and damage failure of rock materials has become an important problem in the aspects of material or structure instability,disaster monitoring and prediction,engineering construction and maintenance.In this paper,the fracture process of granite under different loading modes is analyzed with the method of numerical simulation and laboratory test.In this paper,three-point bending monotonic loading,pre peak cyclic loading and unloading and post peak cyclic loading and unloading tests were carried out on granite samples with electro-hydraulic servo testing machine,and acoustic emission technology was used to monitor the acoustic emission characteristics of rock fracture during the test.Then,the macro-meso numerical model of granite is established by using ANSYS/LS-DYNA,and the heterogeneous meso model considering the Weibull distribution of rock material strength is established by using the aggregate placement algorithm in the key parts of the midspan.The similarities and differences of crack growth under different loading modes are calculated and analyzed.The development mechanism of rock fracture under cyclic loading and unloading is obtained.The results are as follows:(1)The fracture toughness of granite is affected by the loading rate.The fracture toughness values of the samples under different loading rates show obvious loading rate effect.When the loading rate increases,the tensile strength and fracture toughness values of the samples increase.(2)In the process of cyclic loading and unloading,the strength of rock shows weakening,and the deformation of rock shows memory and lag.The fracture toughness of granite under cyclic loading and unloading is lower than that under monotonic loading,but the decrease ratio is almost independent of cyclic loading rate.There is a cycle of opening and closing in the process of rock deformation.In the post peak cycle loading and unloading,a "hysteresis loop" will be formed during each cycle loading and unloading(3)Through the analysis of acoustic emission energy,it can be seen that in the first three cycles of pre peak cyclic loading and unloading test,the rock does not produce plastic damage,and the rock is in the stage of elastic deformation.At this stage,Kaiser effect is obvious.In the fourth cycle,the rock appeared plastic deformation,Kaiser effect failure,and the stress-strain memory of the material became poor;in the post peak cyclic loading and unloading test,when the specimen reached the maximum peak load,the irreversibility of the rock deformation was obvious,resulting in the worse stress-strain memory of the material,and then in each cycle,the load change corresponding to the specimen reaching the maximum strain of the previous level Small,showing obvious felicity effect.(4)Based on Holmquist-Johnson-cook(HJC)damage constitutive model,a heterogeneous granite beam model considering the random distribution of polycrystal strength is established,and the cyclic loading and unloading simulation under three-point bending is carried out.The fast aggregate generation and placement algorithm provides a new method for rock material micro modeling.(5)The micro heterogeneity of rock controls the propagation speed and the propagation path of the crack.The crack will crack at the center line of the model beam,and the polycrystalline block with larger particle size or higher strength should be avoided,and the contact surface or the polycrystalline block with lower strength should be preferentially selected for expansion,which has the effect of "around the core".