| Rock Mass, deeply underground, contain a large number of defects, such as 3-D flaws and pores. Due to External load disturbances, the internal three-dimensional(3-D) cracks would lead to the initiation, propagation, coalescence and even failure. The effect of flaws on rock mass makes material strength reduction and degradation of the stiffness or deformation. This phenomenon even can cause rock burst in underground spaces, collapse, landslip and deep fracture in the foundation engineering. So far, many researchers, at home and abroad, devote actively to studying rock fracture theory. But, the experimental study on the propagation mechanism of 3-D cracks is still at the exploration stage. And research achievements concentrate on the fracture pattern of single inclined flaw or several inclined flaws. For more complex flaws like cross flaws, there are few researches on propagation and coalescence now.In this paper, the fracture experiments of transparent PMMA specimens, containing pre-existing single group of 3-D cross flaws and two groups of 3-D cross flaws with different angles and distributions, were carried out under uniaxial compression loading, in order to explaining the propagation-coalescence mechanism and making a contribution to developing the rock fracture theory deeply. The main results include several contents like this:①Make a review of the latest advanced researches on fracture theory of fissured rock. Summarize the new developments about the initiation, propagation pattern of 3-D single flaw and coalescence, interaction mechanism of several flaws. Make a design of experimental program to meet requirement for research purpose according to some previous studies.②The research is based on model test using similar rock materials. The chose of similar rock materials are not only as brittle as rock, but also good for observing the whole process of cracks propagation. So, brittle PMMA material in subzero temperature is taken. Make a good choice of frozen temperature and duration for better brittle property.③Under uniaxial compression loading, make five fracture experiments of single cross flaw with different angles from 15°to 75°. Describe the information of 3D crack propagation in detail. The newly born cracks include more complex wing-cracks and Secondary crack such as Petal crack, shell cracks, cataphracted crack.④According to cracks propagation information, the initiation, propagation and failure modes of single cross crack together with the growth Parameters of the wing cracks are analyzed. Many different fracture characters for the specimens with 3D cross- cracks, which are much more complex than that of with single cracks are discovered.⑤Build the theory prediction model of the wing-crack initiation angle to calculate theory results, compared with the experimental results. Summarize the relationship between the wing-crack initiation angles and different pre-existing flaw angles,this is the larger angles of single flaw, the lower initiation angles of the wing-cracks.⑥Under uniaxial compression loading, nine fracture experiments about double cross flaws with different angles and different distribution are made. The information of 3D cracks propagation and coalescence are recorded. Summarize four modes of crack coalescence and the connection of wing cracks in middle region of flaws, such as wing-cracks coalescence mainly.⑦Make an analysis on the initiation stress and peak strength of cross flaws in detail. The cracks effect on peak stress of specimens is studied deeply.⑧In the point of fracture mechanism and mechanical property, make a comparison with echelon inclined flaws to explain the propagation and coalescence mechanism of cross flaws better. |
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