Investigation on model-I fracture toughness of sandstone with the structure of typical bedding inclination angles subjected to three-point bending

The fracture mechanics behaviors of rock with bedding plane structure have considerable influences on rock mass engineering, among which crack initiation toughness (KIini) and unstable fracture toughness (KIun) play a significant role. This work first adopts the classical equivalent elastic fracture theories from different perspectives to precisely determine KIun. Then the double-K fracture criterion (DKFC) and the double-G fracture criterion (DGFC) are employed to inversely compute KIini through cohesive fracture toughness (KIc). The experimental results indicate that the crucial strength and deformation parameters show obvious weakening trends as the bedding direction varies from horizontal to vertical for stratified sandstone under standard three-point bending with a loading rate of 0.0001 mm/s. Furthermore, there are little differences in KIun among three common calculation methods, including the ASTM standard, the Guinea proposed method, and the energy release rate. The average values of KIun are 8.20 MPa·mm1/2, 7.38 MPa·mm1/2, 4.76 MPa·mm1/2, and 4.49 MPa·mm1/2 for the tested specimens with bedded orientation angles 0°, 30°, 60°, and 90°, respectively. However, there is a remarkable difference between KIiniK evaluated by the DKFC and KIiniG evaluated by the DGFC, and two relationships are satisfied as KIiniK= 0.42KIun and KIiniG= 0.30KIun. Additionally, the applicability of the DKFC and the DGFC is validated by the acoustic emission (AE) technique. The comparisons manifest that the crack initiation toughness evaluated by the AE counts and cumulative AE counts is consistent with that of the DGFC. The leading significances for this study are to explore the application of various fracture mechanics theories to three-point bending specimens and provide fracture mechanics parameters for underground engineering and tunnel engineering.