| Natural fissures inevitably exist inside the rock,and its distribution position directly affects the crack initiation and propagation characteristics and the ultimate failure mode of the rock.Studying the loading failure law of fractured rocks has important theoretical significance for revealing the failure mechanism of flawed rocks.On the GCTS rock mechanics testing machine,stepwise loading tests were performed on different sizes of a parallel prefabricated double cracks disc specimens of which crack width is 0.25 mm and crack angles,crack distances are different.Combined with acoustic emission and digital image correlation methods,the load-displacement curve,the three-dimensional positioning information inside the sample,and the displacement and strain field changing on the sample surface were obtained.The fractured specimens were observed with an optical microscope to obtain the crack initiation position and propagation path.The crack propagation process was simulated by PFC discrete element,and the tests were mutually verified and supplemented.The main research conclusions obtained are as follows:With the increase of crack angle,the failure load of the sample shows a trend of decreasing first and then increasing.With the increase of the prefabricated crack distance,the angle of the crack to obtain the minimum failure load gradually becomes smaller,and the effect of the change of the crack angle on the failure load becomes smaller.For a certain range of samples,the damage load increases with the increase of the sample size.Monitoring and analysis of the loading process of the sample through acoustic emission and high-speed camera.When the elastic energy accumulated inside the rock reaches and exceeds the energy required for the stable propagation of cracks,the specimen ruptures and becomes unstable,accompanied by a large number of acoustic emission signals.The horizontal displacement field distribution on the specimen surface is shifted due to the influence of prefabricated cracks.As the load increases,the displacement contours converge towards the prefabricated fracture.At the peak time,the distribution of the displacement contour at the crack tip reflects the crack initiation direction and propagation path.According to the test results,the ratio of the crack initiation angle?to the ratio?of the tangential stress component?xy and the axial stress component?y is linear.According to the value of arbitrary crack tip position?xy/?y of the disc specimen,combined with the fitted linear relationship formula,the corresponding crack initiation angle can be predicted.The prefabricated crack angle has an effect on the crack initiation position.The larger the prefabricated crack angle,the closer the crack initiation position is to the preformed crack tip.The crack propagation path is affected by mineral particles and cement.Under axial load,the shear slide of the prefabricated crack tip of the specimen occurs,causing the mineral particles and cement to detach and promotes crack initiation and expansion.As the angle of the prefabricated crack increases,the failure mode of the specimen transitions from tension failure to tension-shear composite failure,and eventually to tension failure.The particle flow software PFC2D was used to perform numerical simulations on parallel double cracks disc samples.The crack propagation law and its bearing characteristics are analyzed and compared.The simulation results show that the effect of crack angle and distance on the failure morphology of the specimen is consistent with the results obtained in laboratory tests.When the rock bridge angle is 0°,the specimen undergoes two stages of failure.When the crack distance is 30 mm,the change of the crack angle has the smallest effect on the peak load.The change of the crack distance has the largest effect on the peak load of the specimen with a crack angle of 15°,and has the smallest effect on the peak load of the specimen with a crack angle of 45°. |
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