Abstract:Mixed-mode fracture is of concern in rock mechanics. To better understand the failure behavior of rocks under mixed-mode loading, several groups of mixed-mode fracture tests were conducted on sandstone using notched deep beam (NDB) specimens. In this study, the fracture loads and corresponding fracture surfaces under different mixed-mode loadings were obtained, and the relation between mixed-mode fracture strength of the sandstone specimens and loading mixity was discussed. Using three widely accepted fracture criteria (i.e., the MTS, MSED and MERR criterions), the fracture loads were theoretically estimated, and our results show that the IR-criterion and the MERR criterion agreed well with the experimental data, whereas the MTS and MSED criterions significantly over-predicted the fracture strength of the sandstone specimens for mode-Ⅱ-dominated loadings. Finally, a surface topography measurement for each rough fracture surface was conducted using a laser profilometer, and the fracture surfaces were experimentally obtained and numerically reconstructed. Using the fractal method, the mixed-mode fracture surface topography properties of these surfaces were then theoretically investigated. In our study, no obvious relationship between the fractal dimension and the initial loading mixity was observed. Therefore, the investigated surfaces were almost statistically identical. Such results emphasized that the failure mode of rocks induced by mixed-mode loading is essentially the tensile mode.