Experimental study on the quasi-static loading rate dependency of mixed-mode I/II fractures for marble rocks

To explore the quasi-static loading rate effect on the mixed-mode I/II fractures of rock materials, the centrally cracked Brazilian disc (CCBD) specimens prepared from a marble rock were tested under different mixed-mode loading conditions (M-e = 0, 1/3, 2/3, and 1) and quasi-static loading rates (0.05 mm.min(-1), 0.1 mm.min(-1), 0.5 mm.min(-1), and 5 mm.min(-1)), and the generalized maximum tangential strain energy density (GMTSED) criterion was also derived to discuss the development of mixed-mode fracture toughness and crack initiation angle. Based on the experimental and analytical results, several conclusions were obtained. (1) The mixed-mode fracture toughness gradually increases with increasing the quasi-static loading rate, while the crack initiation angle and fracture geometry of the tested specimens change barely; the specimen might have a higher rate dependency of the fracture toughness under mode I dominant loading. (2) The modified criteria (GMTSED, EMTSN, and GMTS) match well with the experimental results; the theoretical predictions based on GMTSED criterion is between the predictions of the EMTSN and GMTS criteria. (3) The critical distance of the tested rock is not sensitive to the quasi-static loading rate, which means that the fracture behavior, i.e. crack initiation angle and mixed-mode fracture resistance, under quasi-static high loading rate can be well predicted by only taking into account the critical distance obtained at quasi-static low loading rate.