Stress intensity factors and fracture process zones of ISRM-suggested chevron notched specimens for mode I fracture toughness testing of rocks

The International Society for Rock Mechanics has suggested three chevron notched specimens (CB, SR and CCNBD) to measure mode I fracture toughness of rocks; however, the measured results are not comparable. In this study, the dimensionless stress intensity factors for the three ISRM-suggested specimens are calibrated and demonstrated to be either overestimated, reliable or underestimated. For the first time, the lengths of fracture process zones (FPZs) in these specimens are estimated and revealed to be partly responsible for the measuring discrepancy among these methods. Some controversies about the fracture toughness measurement using the CCNBD specimen are clarified.