Dynamic Mode II Fracture Toughness of Rocks Subjected to Confining Pressure

Abstract

The dynamic shear failure of rocks with confinements is important in underground rock engineering and it is thus necessary to quantify the dynamic Mode II fracture toughness KIIC of rocks under confining conditions. A punch-through shear (PTS) method is introduced to measure the KIIC of rocks under different confining pressures. The dynamic PTS specimen is a cylinder with two 10-mm-deep circular notches at two ends. A modified split Hopkinson pressure bar system with a Hoek cell is employed to apply the dynamic loading and confining pressure to the rock specimen. The equation to calculate the dynamic KIIC value is determined for the new dynamic PTS specimen. The validity and applicability of the dynamic PTS method with confinement are demonstrated via the measurement of the dynamic KIIC of Fangshan marble (FM) under five confining pressures. The PTS specimens are sheared into a short bar and a hollow cylinder and are soft recovered for post-mortem examination. The dynamic shear fracture pattern of the recovered FM specimen is examined through X-ray micro-CT technique. The results demonstrate that the rock PTS specimen with confinement fails with the shear dominant fracture and the values of KIIC of FM at a certain confining pressure increase as the loading rate. Another observation is that the values of KIIC of FM at a given loading rate rise with the lateral confinement.