The Mechanisms Of Dynamic Damage Localization In Crack-Weakened Rock Mass And Its Application

The phenomena of Damage localization, which characterized as the rock mass suddenly enter into the deformation localization stage after a period of uniform deformation, is the beginning of rock failure. Damage localization is also the precursor of rock failure. Damage localization mainly yielded by the bifurcation of crack growth pattern, which means some cracks go on propagating while others remian still. So, it is significant for the stability analysis of surrounding rock mass of underground projects to study the onset condition and bifurcation pattern of damage localization in crack weakened rock mass. Supporting by the National Natural Science Foundation of China (Nos.50778184, 51078371), the Program of New-Century Talents by the Ministry of Education (NCET-07-0911) and Natural Science Foundation Project of CQ CSTC (No. CSTC, 2009BA4046), this paper mainly focuses on the growth and coalescence of cracks in the rock mass. Three main aspects, which are experimental observations, onset condition for damage localization and numerical method for simulating the evolution of damage in crack weakened rock mass, are studied in this paper. The main work is summarized as follows:①Adopting multiple kinds of testing means and data processing methods, the fracture mechanic parameters are obtained and the rule of dynamic damage evolution of rock is analysed too. Firstly, the fracture toughness of limestone is tested by using three point bending experiments, the rate dependence of fracture toughness is also analysed. Secondly, based on the stress-strain relationship of rock sample under uniaxial compression condition, the initial point of crack growth is determined by using the relationship between the volume strains induced by rock matrix material and cracks respectively. Meanwhile, the rule of damage evolution of rock under uniaxial compression is revealed. Thirdly, utilizing the image enhancement and segmentation technology, the rule of damage evolution of limestone under triaxial compression condition is discussed. Moreover, quantitative analysis of CT image, the relationship between CT image and damage evolution is discussed. Last but not least, uniaxial impact-compressive loading tests of different strain rate (15.2~124.3 s-1) were produced in the split Hopkinson pressure bar (SHPB) on Huanglong limestone specimens, the dynamic stress-strain curves are obtained. The onset condition of damage localization and the failure characteristic are determined too. ②Taking into account of the interaction of multiple cracks by pseudo traction method, the onset condition for rock mass that contain periodic distribution cracks is obtained. By analyzing the bifurcation of crack growth pattern, not only the critical length and critical stress for damage localization of crack weakened rock mass are obtained, but also the location of damage localization is determined. In addition, the dynamic stress-strain relationship is discussed in this paper. Parameters sensitivity analysis is carried out, the effects of the length of crack, friction coefficient, fracture toughness, confining stress, velocity of crack growth, inclination and spacing between lines and rows on the onset condition for damage localization and bifurcation pattern of rock are discussed.③On the basis of eXtended Finite Element Method, a multiscale numerical model for simulating the process of damage evolution in crack weakened rock mass is proposed. This model can be used to simulate the interaction, growth and coalescence of multiple cracks, and the effects of unloading is also taken into account. The coordination between the internal boundary (such as cracks and holes) and mesh is not needed in this multiscale model. By combining the displacement projecting method and displacement loading method, the connection between different sizes of mesh behave smoothly. This multiscale model indeed surmounts difficulties in meshing and it really save the storage space. Moreover, the multiscale model is applied to analyse the dependence of damage localization on the loading rate. It is shown from examples that the error of this multiscale model can be accepted in engineering application.④Based on the platform of Matlab, a program for simulating the multiscale dynamic damage evolution in crack weakened rock mass is compiled (MCWRM). This program is suit for simulating multiscale damage evolution behaviors of rock mass under different loading state, such as unloading, tensile and compression. The calling relation of subroutines and calculation procedure of this multiscale model are detailly explained in this thesis. Fortunately, the difficulties of dealing with the crack coalescence and connection between different sizes of mesh have been overcomed. Some examples are carried out to test and verify the MCWRM program. The numerical simulation results that obtained by MCWRM show good agreement with experimental and theoretical results.⑤Finally, the MCWRM program is applied to simulate the damage evolution process when the tunnels is excavating in JinpinⅡHydropower Station. Comparison between the results obtained by MCWRM, weak element method and experimental results of similar specimens are executed in this paper. It shows that the results are realistic and the MCWRM fit for simulating damage evolution behaviors in crack weakened rock mass.