Dynamic fracture toughness is an important fracture parameter for investigating the cracked rock masses under impact loading such as earthquake, rock bursts, rock blast and projectile penetration. The experimental results showed that the dynamic fracture toughness of rocks is strongly dependent on the size of the specimen. Therefore, here, a criterion is proposed for considering the size effect on the dynamic fracture toughness of rocks. The proposed method is based on the modified maximum tangential stress (MMTS) criterion which has been established recently for size effect analysis on the crack specimens under static loading. In the proposed criterion, the required parameters such as tensile strength and critical distance are obtained from specimens subjected to the dynamic loads. Some experimental data reported for holed-cracked flattened Brazilian disk (HCFBD) specimen with different sizes are used for evaluating the proposed criterion. It is shown that the proposed simple approach can predict the dynamic fracture toughness of the large specimens in a good agreement with the experimental data.
 
Dynamic fracture toughness is an important fracture parameter for investigating the cracked rock masses under impact loading such as earthquake, rock bursts, rock blast and projectile penetration. The experimental results showed that the dynamic fracture toughness of rocks is strongly dependent on the size of specimen. Therefore, in this paper, a criterion is proposed for considering the size effect on the dynamic fracture toughness of rocks. The proposed method is based on the modified maximum tangential stress (MMTS) criterion which has been established recently for size effect analysis on the crack specimens under static loading. In the proposed criterion, the required parameters such as tensile strength and critical distance are obtained from specimens subjected to the dynamic loads. Some experimental data reported for holed-cracked flattened Brazilian disk (HCFBD) specimen with different sizes are used for evaluating the proposed criterion. It is shown that the proposed simple approach can predict the dynamic fracture toughness of large specimens in good agreement with the experimental data.