Experimental Study on Dynamic Fracture Behaviors of Beishan NSCB and CCNSCB Granite Specimens under Different Loading Rates

Beishan area in Gansu province of China has been listed as a major candidate for underground constructions of high-level radioactive waste repository. Therefore, to explore the dynamic responses of Beishan granite is important to the related excavation engineering. This paper investigates the rate dependent effect on the dynamic fracture behaviors of the Beishan granite. The notched semi-circular bend (NSCB) and cracked chevron notched semi-circular bend (CCNSCB) specimens are prepared with speckles uniformly distributed on surface for digital image correlation (DIC) image processing. An ultra-high-speed camera is employed for the modified split Hopkinson pressure bar (SHPB) system to capture the complete cracking process of the rock specimen. The dimensionless stress intensity factors (SIFs) of NSCB and CCNSCB specimens are calculated numerically by the J-integral method in ABAQUS simulation software. To systematically reveal the dynamic fracture mechanisms of Beishan granite specimens, the key dynamic fracture parameters including the dynamic fracture toughness, the evolution of dynamic stress intensity factor (DSIF) during fracturing and the dynamic fracture energy are obtained for further investigations and comparisons. The results show that the dynamic fracture toughness of NSCB and CCNSCB specimens exhibits a distinct rate dependence-the fracture toughness increases with the loading rate rises. Furthermore, the toughness value of the CCNSCB specimen are mostly higher than that of the NSCB specimen due to a more severe stress concentration level near the notch tip. The evolutions of DSIFs of Beishan NSCB and CCNSCB specimens are quite similar to each other, and the corresponding variation curves are all subjected to three typical developing phases until specimens are totally fractured. The dynamic fracture energy of NSCB and CCNSCB specimens is relatively consistent and shows a positive relationship with the loading rate. However, the fracture energy absorption rate (defined as the ratio between the fracture energy and the energy generated in the incident bar) gradually decreases with the loading rate rising, indicating a lower loading rate may be more suitable to improve rock fragmentation efficiency.