Experimental Study On Fercture Characteristics And Fatigue Damage Of Noncongruent Fractured Rocklike Specimens

The rock mass is subjected to complex loads,including static loads and cyclic loads of varying degrees.When subjected to periodic loads such as vehicles,rainstorms,earthquakes,etc.,rock in special engineering locations such as highways,railways,bridges and tunnels,its bearing capacity and stability will be reduced.Changes in rock mechanical properties and fatigue damage caused by the gradual deterioration of rock properties will affect the safety of engineering activities.Therefore,studying the energy evolution law and fatigue damage characteristics of fractured rock masses under cyclic loading has important application value to geotechnical engineering.According to the similar principle,the rock-like test block was prepared indoors with white cement,fine sand and water according to the ratio of 5:5:2.On the basis of uniaxial compression test and cyclic loading and unloading test,the non-coplanar test block was further studied.Fracture characteristics and fatigue damage mechanism of fractured rock mass.The main research results and conclusions obtained are as follows:(1)The RMT-150 B rock mechanics testing machine is used to perform uniaxial compression test and staged cyclic loading and unloading test on rock-like test blocks.The relationship between the crack inclination angle,the number of cracks and the failure strength of the specimen is obtained through the experiment.The study shows that when the main crack is horizontal,the failure strength of the sample first increases and then decreases and then increases with the increase of the inclination of the secondary crack;when the main crack is inclined,the failure strength of the sample first increases with the increase in the inclination of the secondary crack.When the inclination angle of the secondary fissure is 30°,45° and 60°,the existence of the second secondary fissure has a certain strengthening effect on the bearing capacity of the specimen.The failure strength of the three-slit specimen is greater than that of the double-slit specimen.Under the action of staged cyclic loading and unloading,the relationship between the failure strength of the specimen and the change of the inclination angle is basically the same as that under uniaxial compression.(2)A high-speed camera was used to record the crack evolution law and failure process of the rock-like sample during reloading.It is observed that when the main fissure is horizontal,the initiation position of the horizontal fissure is not at the tip of the fissure,but a certain position away from the tip.The initiation position is guided by the secondary fissure and finally overlaps with the secondary fissure.When the main fissure keeps the inclination of 45° unchanged,the secondary fissures and the main fissures have mixed tension and shear penetration.The initiation of sample cracks first appeared at the tip of the secondary crack.The specimens are basically brittle failure due to shear dislocation,and obvious shear sections can be seen.(3)By performing uniaxial graded-cycle loading and unloading tests on non-congruent fracture specimens,the test results show that the fatigue damage curves of intact and non-congruent fracture specimens intersect the uniaxial compression curve in the post-peak region,and the uniaxial compression curve contains the graded-cycle loading and unloading curve.The fatigue damage of the rock is caused by the deformation reaching the limit value,and the deformation of the fatigue damage is comparable to that corresponding to the post-peak zone under static loading.The hysteresis loop exhibits the characteristics of sparse-dense-sparse throughout the cycle.The cyclic loading and unloading curves show a more rapid stress drop at specimen damage than that at uniaxial compression,implying a greater degree of damage and significant brittle damage characteristics for rock-like specimens under cyclic loading.(4)When the lower stress limit of the cyclic load is constant,the energy dissipation of the specimen increases with the increase of the upper stress limit,but the energy dissipation is relatively stable in each cycle stage,and the increase degree of the energy dissipation decreases with the increase of the crack inclination Angle.The specimens with large changes in the shape of the hysteresis loop tend to have large fluctuations in the dynamic elastic modulus.As the loading progresses,the elastic modulus shows a downward trend.The change of the damping ratio is affected by the elastic modulus and the area of the hysteresis loop.As the loading progresses,the damping ratio tends to be stable.Based on the weibull probability distribution function,the evolution curve of the damage variable of the specimen can be obtained,which well reflects the damage trend of the rock-like specimen under various levels of cyclic loading and unloading.