Influencing Factors And Mechanism Research On Brittle-Ductile Transition In Marbles

As the geotechnical engineering around the word becames deeper, where the geological and stess environment has become more complex.The mechanical property and failure mechanism of rocks at depth need to be thoroughly researched.The brittle-ductile transition in rocks is the inverse process that the the mechanical property change after excavation in underground engineering.But the research about mechanism of transition from brittle to ductile in rocks at room temperature is less, so further research about transition from brittle to ductile in rocks is necessary. In sum,the mai work and conclusion including:(1)In order to study the influence of confining pressure, strain rate, loading modes and rock type on brittle-ductile transition in marbles, five groups of conventional triaxial compression experiments under different confining pressure are carried out. In order to analyze mechanical mechanism of brittle-ductile transition in marbles, the microstructure of marbles samples after experiments is observed by scanning electron microscope, and the supersonic velocity of marbles samples after experiments is measured.(2)According to the results of conventional triaxial compression experiments, with each influencing factors, confining pressure has been observed to be the main influencing factor that impacts the brittle-ductile transition in marbles. With the increasing of confining pressure, brittle feature reduced with the increased ductile feature. The yield platform of stress-strain curve corresponding to the axial strain is used to judge marble is brittle or ductile.(3)The impacts of strain rate on brittle-ductile transition and mechanical properties of marbles are studied, and the strain rate that is adopted in this paper has no effect on brittle-ductile transition in marbles in the range of confining pressure the samples have brittle behavior. The ductileity of marbles is facilitated by low strain rate in brittle-ductile states. The strain rate has no impact on mechanics characteristic of marbles, such as strength, Elasticity modulus and Poisson’s ratio. The yield platform of stress-strain curve that is controlled by radial deformation is flatter.(4)The strength, Elasticity modulus and Poisson’s ratio of microcrystalline marble is higher than macrograined marble under the same confining pressure. It is easier that macrograined marble is transformed from brittle failure to ductile flow than microcrystalline marble. The macrograined marble complete transition from brittle to ductile, but there is a transition phase during transition from brittle to ductile in microcrystalline marbles.(5)Marble samples are observed through scanning electron microscope after conventional triaxial test, and according to deformation behavior and wave velocity of marbles under different confining pressure the mechanism of brittle-ductile transition in marbles is discovered. The local failure is inhibited by confining pressure, the confining pressure makes marbles transform from local failure to integrity failure, and the marble produce ductile flow. The friction between grain boundaries is very small under low confining pressure, the microcrack occurs at the boundarys of local crystal grains, then the microcracks are expanded and the shearing surface is formed, the failure occurs in the local area.The friction between grain boundaries begins to increase under high confining pressure, fracture occur at more crystal boundary and microcracks are formed and expanded. The microcracks no longer focused on the local shear belt, the marble formed large permanent deformation.