Experimental Study On Fracture Mechanism And Reconstruction Of Three-dimensional Cracks In Brittle Rock Material

In actual engineering,rock is considered to be a discontinuous material with a large number of damage defects inside,due to these defects,the crack propagation,penetration mechanism and mechanical properties of the rock under external interference are greatly affected,resulting in a decrease in the strength of the rock mass and stiffness decreasing,causing a series of major engineering geological disasters such as collapse,landslides and rock bursts.However,due to the limitations of specimen processing technology and acquisition device,most of these problems are simplified as two-dimensional damage or three-dimensional damage on rock-like materials.Simplifying 2D failure mode leads to loss of important 3D information and the failure between rock and rock-like materials is also fundamentally different.Therefore,the three-dimensional crack propagation and penetration mechanism research of rock materials is of great significance to practical engineering design and construction.In this paper,the uniaxial compression test was carried out on specimens with three kinds of non-continuous three-dimensional surface cracks under two depths of pre-casting of sandstone,combined with DIC method to capture surface deformation dynamically,then extracting the internal crack from the picture scanned by CT to reconstruction 3D cracks to make up for the loss of internal damage information during the experimental observation process.Through analysis,it is found that the three-dimensional failure law is significantly different from the two-dimensional,the main results are as follows:First,when the ratio d/t ? 1/3 of the pre-crack(depth d and the thickness t of the specimen),the failure mode can be approximated to two-dimensional destruction.When d/t<1/3,there is an obvious typical three-dimensional failure mode;Secondly,the uniaxial compressive strength of the test specimen is affected by the depth obvious.When the depth of the surface crack is larger,the strength of the test is smaller.When the surface crack depth is 7.5 mm,the strength is significantly larger than the surface crack depth of 10 mm;Thirdly,the law of the specimen strength by the crack inclination angle is also obvious,and the effective action area between the two angles determines the strength of the specimen.When the crack angle is 90°,the effective action area is the largest and the strength is the smallest.On the contrary,when the angle is 135°,it not form a closed action zone as the upper vertical crack,the influence on the strength of the test piece is minimized.Fourthly,the digital correlation technique is used to analyze the crack failure mode dynamically.The quantitative method is used to define the ratio of the main strain to the shear strain ? to define the dynamic failure mode of the crack.It is divided into tensile crack,shear crack,Tensile-Shear cracks and Shear-Tensile crack.Fifthly,through CT scanning and three-dimensional crack extraction,it can be found that there is a wrap-like layered crack near the pre-crack.When d/t ? 1/3,there is often a longitudinal and lateral through failure crack in the second layer,which similar to the ordinary two-dimensional damage.When d/t<1/3,the distribution area of the layered crack is larger,and there is no penetration around the pre-crack.