Abstract: In order to accurately test the mode I fracture toughness(KIC) of shale, three point bending loading experiments are carried out using notched semi-circular bend(NSCB) and cracked chevron notched semi-circular bend(CCNSCB) samples under three types of prefabricated notches (splitter, arrester and divider), respectively. According to statistical results of standard deviation and coefficient of variation, discreteness of each group of KIC values is analyzed; and applicability of the two methods is valued respectively. The crack propagation path and fracture surface morphology are observed by optical microscope. Based on fracture mechanics, we give the calculation formulas of the rough undulating fracture surface of shale specimens. The reason for KIC value scattering of the tested shale specimens is analyzed; and the formation mechanism of rough fracture surface in the expansion process is explained using the principle of minimum energy dissipation. By comparing the KIC values of the two sample types, CCNSCB and NSCB methods yield KIC values of similar discreteness when notches are parallel to bedding planes. When notches are vertical to layers, KIC values from CCNSCB specimens is more consistent than that from NSCB. The ligaments of CCNSCB specimens are in favor of crack propagation, which counteracts the adverse influence caused by asymmetry loading configuration to some extent. Therefore, CCNSCB specimens are more suitable to test fracture toughness of anisotropic shale than NSCB.

Key words: shale, mode-I fracture toughness, three-point bending, CCNSCB and NSCB, fracture mechanism