The Theoretical And Experimental Investigation On Rock Fracture Due To Shear-Compression Loading

In this thesis, the mechanism of rock fracture due to compression-shear loading and a series of related key issues have been systematically investigated on the basis of micro-and macro-scopic level. The method adopted throughout the present studies is a combination of theoretical analysis, numerical calculation (including numerical simulation), and experimental investigation.The near-tip stress field caused by compression-shear loading was first analysed and calculated, the obtained maximum principal stresses were then depicted in the top figs. The strains near crack tips for different inclined angles of crack were also measured in experiments in order to further understand the compression-shear stress field. The results showed clearly that the stress field in the vicinity of crack tip induced by compression-shear loading is much more complicated than the one caused by tensile-shear loading. It is of following two important behaviors: 1. The near-tip stress field in this load case is quite different according to whether or not the crack faces are closed, and 2. An additional local tensile stress can be easily induced at the tips of unclosed crack subjected to compression-shear loading.The problems concerning crack co-planar extension, which are closely related to mode II fracture toughness of rock, were then discussed based on above stress analysis. The condition for crack co-planar extension was given in this thesis.The author proposed a new procedure for the analysis of rock fracture due to compression-shear loading, in which two specimen geometry-loading systems were used, i.e. the center-cracked Brazilian disk (CCB) and the double edge-cracked Brazilian disk (ECB) under different load cases. Semi-analytical formulae for the calculation of both mode I and mode II SIF under various load cases were derived using weight function method, and a procedure for mode II fracture toughness testing with ECB specimen was suggested. The SIF values evaluated by derived formulae show a high precision comparing with the numerical solutions. The suggested specimen geometry not only permits one to achieve pure mode I to any combination of mode I and mode II loading, it is also easy to fabricate, especially for ECB specimen.The problem of crack closure under compression-shear loading were also tackled in this thesis. A closure model for wing crack extension was proposed by introducing a Coulomb friction between crack faces. The SIF values at wing crack tips evaluated by derived close-formed formulae are very closed to numerical solutions, the precision of these values are also higher than that of similar work given in the international literatures.In the field of microscopic fracture, the author has payed special attention to the phenomenology and mechanism of microfracture, thus an acoustic emission (AE) technique has been used in experiments. It can be found that the wave spectrum parameters of AE seem more sensitive to the progress of microfracture than the load itself. In order to get further knowledge of microfracture mechanism, the author proposed a two-phase microscopic model in the study of brittle fracture of rock. In which brittle rock was modeled as a two-phase composite material consisting of harder mineral grains and weaker colloids. Stress field in the colloids area was calculated using FEM, the results were then compared with the numerical experiment solutions.Considering the important role of tensile stress in the rock fracture process, the author suggested two local strain energy density criteria, i.e. 1. Circumferential strain energy density factor criterion (CSED), and 2. Dialatational strain energy density criterion (DSED).Finally, some engineering applications of compression-shear fracture of rock were discussed at the end of the thesis.