Microcracking behavior of two semi-circular bend specimens in mode I fracture toughness test of granite

Abstract

The semi-circular bend (SCB) specimen has been commonly used in measuring the mode I fracture toughness of rocks. However, little is known about the influence of pre-existing notch shape on its microcracking behavior. We experimentally examine the microcracking behavior of two groups of SCB medium-grained Kowloon granite specimens which possess either a straight-through notch (SNSCB specimen) or a chevron notch (CCNSCB specimen), using acoustic emission (AE). Comparing their microcracking behavior, we find that (1) a intenser development of microcracks is observed in the SNSCB specimen during the fracture process zone development phase, (2) the fully-developed fracture process zone (FD-FPZ) size of the SNSCB specimen is larger, (3) the Gutenberg-Richter b value of the AE hits detected before the specimen failure is smaller for the SNSCB specimen, indicating a larger proportion of the AE hits with relatively higher amplitude, (4) the two specimens exhibit distinct energy distributions for AE events characterizing the FD-FPZ, the AE events with the highest energy level (level 4) in the SNSCB specimen account for smaller number but larger event energy percentage of the total events in the FD-FPZ as compared with those in the CCNSCB specimen.