Investigation on mixed-mode fracture behavior of sand stone using a SENDB specimen

Based on the fact that rock fractures are usually three-dimensional, the dimensionless stress intensity factors(i.e. YI and YII) of mode I and mode II and the dimensionless T-stress of the single edge notched deep beam(SENDB) specimen were calibrated using 3D numerical models, and compared with the results obtained with 2D numerical models from a reference. The 2D models usually provide the low values for SIFs and T* than the 3D models. Accordingly, the fracture toughness calculated with a 2D model was lower than the real fracture toughness value of rock, which may have an unfavorable effect in the design of fracturing-related rock engineering. The SENDB specimens made of sandstone were tested in a full range of mixed modes from pure mode I to pure mode II. The micro-crack zones(MFZ) were theoretically estimated based on the criterion of generalized maximum tangential stress (GMTS). Under the mode-I dominated loadings, the maximum MFZ was found in the middle of the specimen and the minimum MFZ on the sample surface. While for the mode-II dominated fractures, the size of the MFZ on sample surface is much larger than that in the middle of the sample. The critical radius of the MFZ for mode I fracture was much smaller than that for mode II fracture, indicating that the critical radius r0 of the MFZ in the crack propagation direction was not an invariant. It is obvious that the hypothesis of a constant r0 adopted by the GMTS criteria is questionable. © 2017, Science Press. All right reserved.