Macro-micro evolution mechanism on sandstone failure in triaxial compression test based on PFC^2d

Laboratory and PFC^2d (particle flow code in 2-dimension) numerical tests were both employed to investigate into the macro-micro evolution mechanism of sandstone in Chongqing area, China.Triaxial compression tests of sandstone were carried out by RMT-150C rock mechanics test system in laboratory, with the obtained stress-strain curves, appropriate micro parameters of PFC^2d sandstone samples were calibrated by letting numerical sandstone samples show similar macro characteristics to the corresponding laboratory samples.Based on a series of biaxial compression tests of PFC^2d sandstone samples under the confining pressure of 0.5-20 MPa, the macro mechanical behaviors of sandstone were explained from micro level, i.e., the deformation distribution of samples, the cumulative number and angular distribution laws of three kinds of crack (total crack, shear crack and tension crack) during the failure process.Numerical tests results show that the cumulative numbers of three kinds of cracks all increase as S-shaped curves, and the changing points are consistent to the elastic, plastic and failure points of corresponding macro stress-strain curves, which indicates that the occurrence and growth of cracks determine the macro behaviors of rock.With the confining pressure increasing, the dominant type of crack is no longer shear crack but tension crack, and the number proportion of tension crack in the whole sandstone sample and fracture band are both increased.In the whole samples and fracture bands under all confining pressure levels, the proportion of tension crack in fracture band are slightly bigger than that of whole sample, and the principal directions of angle distribution for total cracks and shear cracks are always along the axial loading direction.However, the angle distribution of tension cracks are uniform, cracks close to the angle of fracture surface, axial loading direction and conjugate fracture surface are relatively obvious.