An experimental and theoretical assessment of semi-circular bend specimens with chevron and straight-through notches for mode I fracture toughness testing of rocks

The cracked chevron notched semi-circular bending (CCNSCB) method for measuring mode I fracture toughness of rocks has many favorable properties similar to those of the notched semi-circular bending (NSCB) method, which has been suggested by the International Society for Rock Mechanics (ISRM). In this study, the lengths of fracture process zones (FPZs) in the NSCB and CCNSCB tests with representative specimen geometries were theoretically estimated using the maximum normal stress criterion, and their effects on these fracture toughness tests were evaluated according to the effective crack model. The CCNSCB method was found to be less affected by the existence of FPZ and the measured fracture toughness was not that conservative as that using the NSCB method. Furthermore, even relatively small-size CCNSCB specimen could obtain the fracture toughness value that might require the use of a much larger sample size in the NSCB method. Subsequently, to further verify these theoretical results, laboratory tests using a total of 36 CCNSCB and NSCB specimens of three different sizes were conducted. For the same specimen size, the NSCB test always produced conservative fracture toughness value compared with the CCNSCB test. Additionally, even the toughness value measured using the NSCB specimen with a 75 mm radius was still lower than that using the 25-mm-radius CCNSCB specimen. Given the experimental and theoretical results, the CCNSCB method is demonstrated to be less influenced by the presence of FPZ than the NSCB method for mode I fracture toughness measurements.