Rock Dynamic Fracture Toughness Tested with Holed-cracked Flattened Brazilian Discs

摘要

The size effect of rock dynamic fracture toughness was shown by tested values using two types of holed-cracked flattened Brazilian disc specimens. One type of specimens was geometrically similar with diameter of 42 mm, 80 mm, 122 mm and 155 mm respectively, the other type had identical 80 mm diameter and different crack lengths. The discs were diametrically impacted at the flat end by the split Hopkinson pressure bar of 100 mm diameter, and the representative strain wave recordings on the bars and failure patterns of the specimens were presented and illustrated. The rock dynamic fracture toughness increases with the diameter for the geometrically similar specimens; it also changes, in a more complicated manner, for the one-size specimens of identical diameter and different crack lengths, the trend is first increasing then decreasing with increasing crack length. Considering that the size effect of rock dynamic fracture toughness is mainly related to the fracture process zone length l and incubation time τ, the latter is an additional factor of time as compared with the counterpart static problem, a method to determine the unique dynamic fracture initiation toughness is proposed, which adopts in an average sense the integration of dynamic stress intensity factor distribution in the spatial-temporal domain defined by l and τ respectively.