In this study, straight notched disk bending (SNDB) specimens of Yeongju granite were used to determine the fracture toughness with wide range of loading velocity from 0.01 to 680,000 mm/min. The SNDB specimens were dynamically loaded with the split Hopkinson pressure bar (SHPB) system. Static fracture toughness values of the granites were also determined to compare with the dynamic fracture toughness. The measured fracture toughnesses KIc under the static and dynamic loading condition were plotted against the loading rate v on a logarithmic graph. The crack opening behavior around the tip of the notch during dynamic loading was observed by using a high speed digital camera. The fracture planes around the notch tip inside the tested samples were observed by a high-resolution microfocus X-ray CT scanner.
Fracture toughness is an indication of the amount of stress required to propagate a preexisting crack and is one of the most important properties of any material for many design applications such as rock drill bit, rock crushing and hydraulic fracturing in a wellbore. It is well known that the fracture toughness of rocks is dependent on the material properties and the surrounding environment condition such as loading rates, temperature etc.. Many researchers reported that fracture toughness increased with increasing the loading rate[2,3]. It is still a great issue that why rock materials show strong loading rate/strain rate dependency of fracture toughness and strength.
In this study, the straight notched disk bend (SNDB) method was adopted to measure static and dynamic fracture toughness of granite. The SNDB samples were loaded under different loading rates using MTS machine and split Hopkinson pressure bar (SHPB) system. High speed digital camera system was used to observe the fracture behavior at the tip of the notch of SNDB samples during dynamic impact loads. The fracture planes around the notch tip of the tested samples were observed by microfocus X-ray CT scanner.