Experimental and numerical study on the fracture process zone and fracture toughness determination for ISRM-suggested semi-circular bend rock specimen

•Observed fracture process zones by both acoustic emission monitoring and numerical modeling.•Explained the conservative measurements by semi-circular bend (SCB) method compared with other methods.•The measured fracture toughness using simulated effective crack lengths closes to the level I toughness.•Demonstrated K-resistance curves for determining the fracture toughness at the level II.

The semi-circular bend (SCB) method suggested by International Society for Rock Mechanics (ISRM) is demonstrated to yield much conservative mode I fracture toughness. A large fracture process zone observed by both acoustic emission (AE) monitoring and numerical modeling is analyzed to be responsible for that phenomenon. Using numerically simulated effective crack lengths at the peak-load stage in the experiments, the measured toughness values are close to the level I fracture toughness. In addition, numerically simulated K-resistance curves are demonstrated to have great potential for determining the fracture toughness at the level II.