Size effect of the mode II fracture toughness of rock

A new method, compression-shear test, has been developed for determining the mode II fracture toughness K-IIC of rock. The effect of the specimen thickness B, the dimensionless notch length a/W as well as the notch inclination angle alpha on the determination of K-IIC has been experimentally studied. The results show that the apparent mode II fracture toughness K-C decreases as B and a/W increase and becomes constant when B greater than or equal to 70mm (i.e. cubic specimen) and a/W > 0.5. The K-C is almost independent of alpha in the range of 65 degrees-75 degrees. The constant K-C is about 2-3 times that of mode I fracture,toughness K-IC and can be considered as the true mode II fracture toughness K-IIC of rock.