Determination of dynamic fracture toughness of rocks using straight notched disk bending (SNDB) specimen

Fracture toughness is an indication of the amount of stress required to propagate a preexisting crack and one of the most important properties of any material for many design applications. In this study, fracture toughness test of rocks using straight notched disk bending (SNDB) specimen was suggested to investigate the loading rate dependency of the fracture toughness. Straight notched disk bending (SNDB) specimens of a limestone were loaded dynamically with the split Hopkinson pressure bar (SHPB) system. Static fracture toughnesses of the limestone were also determined to compare with the dynamic fracture toughness values. The fracture toughness of limestone samples were determined at a loading rate ranging from 0.001 to 679,800 mm/min. A micro-focus X-ray CT scanner was used to observe the micro structures inside the tested samples. The crack opening displacement (COD) was monitored using microfocus laser gap gauge (LGG) system. As a result, the fracture toughnesses of SNDB samples under static and dynamic loads increased with increasing loading rate, but showed the different slopes on the relationship between the fracture toughness and loading rate. The COD behaviors of the SNDB specimens were compared CT images of the specimens to discuss the loading rate dependence of the fracture toughness of the rock.