Abstract:  To select an appropriate notch prefabrication method and notch length for obtaining relatively accurate and reliable rock mode I fracture toughness (K_IC), as well as to improve understanding of macro fracture process and meso-fracture characteristics in the rock three-point bending test, three methods, i.e., wire cutting, saw blade cutting and water jet cutting were used to prefabricate notch in granite and marble specimens for three-point bending test. The comparison of micro fracture characteristics among three different cutting methods was conducted with the application of scanning electronic microscopic. The test results show that the effect of wire cutting method on K_IC is the smallest. In addition, three-point bending tests on granite and marble containing dimensionless notch length of 0.1, 0.2, 0.3, 0.4, and 0.5 show that K_IC tends to increase firstly and then decrease with increasing notch length. The dimensionless notch length of α = 0.3 is recommended for obtaining the representative K_IC. The macroscopic failure of rock specimen undergoes a process of initiation of localization zone, development of localization zone, crack initiation, and finally fracture propagation. The acoustic emission process of rock specimen is observed with an obvious brittle characteristic, the crack opening displacement curve is consistent with the trend of cumulative acoustic emission curve, and the evolution of crack opening displacement curve can be regarded as a macroscopic characterization of the specimen internal failure development process. 

Key words: rock three-point bending test, mode I fracture toughness, notch prefabrication method, notch length, fracture process