Experimental And PFC Discrete-element Analysis On Mode â…  Fracture Of Sandstone Under Monotonic And Cyclic Loading

The instability and the failure of geotechnical engineering are often caused by the fatigue and fracture of rock and soil mass.This study is based on the National Natural Science Foundation of China（NO.51979293:Subcritical Age-effect Propagation of Rock Fatigue Cracks under Cyclic Loading and Multi-scale Analysis of Fracture Morphology）.In this paper,red sandstone is taken as the research object,and the monotonic loading and cyclic loading tests of the Cracked Chevron Notched Brazilian Disk（CCNBD）samples are carried out.The macro and micro mode I fracture mechanical behavior of red sandstone under monotonic and cyclic loading is analyzed based on the results of the tests and particle flow code（PFC3D）simulation.The main results are as follows:（1）The basic mechanical properties and mode I fracture toughness of red sandstone are determined by the uniaxial compression tests,the monotonic loading tests of CCNBD and the cyclic loading tests of CCNBD.The average mode I fracture toughness of red sandstone under monotonic loading is 0.8272 MPa·m^1/2.The deformation process of CCNBD sample is divided into compaction stage,expansion stage and post-peak stage.Under cyclic loading,the growth of the main crack at the groove tip is similar to that under monotonic loading,both of which belong to pure mode I fracture.Differently,the secondary crack is more obvious under cyclic loading.（2）According to the results of the uniaxial compression tests,the sensitivity analysis of key mesoscopic parameters of flat joint model in PFC is conducted,and a set of mesoscopic parameters that can simulate the macroscopic mechanical properties of intact rock is obtained.CCNBD model is established.For the simulation tests under different average particle radius and particle size ratio,the larger average particle radius results in the larger mode I fracture toughness,and as the particle size ratio increases,the type I fracture toughness decreases.The appropriate average particle radius and particle size ratio are determined.（3）The simulation tests of CCNBD under monotonic and cyclic loading are carried out respectively.The stress and strain in the chevron notched region during loading are monitored by measurement ball.It is found that under monotonic and cyclic loading,the internal microcracks of the sample generate on the tip of the notch,extend along the notch,and then propagate to both ends of the sample until the sample is damaged.At the central section of the notch,the crack plays a decisive role in the stress distribution.At the initial stage of loading,the tensile stress is mainly concentrated near the notch.The farther away from the notch,the smaller the tensile stress.Compressive stress is mainly distributed in the loading side of the sample due to the end effect.50 figures,10 tables,and 72 references.