A new method, compression-shear test, has been developed for determining the mode 11 fracture toughness KIIC of rock. The effect of the specimen thickness B, the dimensionless notch length a/W as well as the notch inclination angle á on the determination of KIIC has been experimentally studied. The results show that the apparent mode 11 fracture toughness KC decreases as B and a/W increase and becomes constant when B > 70ram (i.e. cubic specimen) and a/W > 0.5. The KC is almost independent of á? in the range of 65°-75°. The constant KC is about 2-3 times that of mode I fracture toughness KC and can be considered as the true mode 11 fracture toughness KIIC of rock.
One of the fundamental goals of rock fracture mechanics is to predict the initiation and propagation of individual cracks in rock subjected to arbitrary loading conditions. One measure of the resistance of material to fracture is a parameter so- called fracture toughness. Suggested methods for determining the mode I fracture toughness, KIIC, of rock have been established (Ouchterlony 1988, Fowell 1995). However, preexisting cracks in rock materials or discontinuities in rock masses are seldom directly subjected to tensile loading but rather to compressive, shear or mixed mode loading in most cases. Therefore, determination of the mode 1I fracture toughness, KIIC, of rock, is of importance for rock engineering.