Comments on the mode II fracture from disk-type specimens for rock-type materials

Laboratory determination of the mode II fracture toughness K-IIC for rocks and rock-type materials remains a challenge task. One of the testing approaches is the compression of disk-type specimens, which is simple and convenient. However, although the notch in disk-type specimen is under mode II loading, the detailed studies reveal the drawbacks about interpretations of the disk-type specimens: (i) mode II fracture is only assumed without careful identifications about the actual fracture mode; or (ii) critical value of associated mode II loading is considered to be the fracture toughness K-IIC regardless of the actual fracture mode. The widely accepted failure criterion for disk-type specimens is maximum tangential stress that actually is maximum tensile stress, such that the initiated fracture generally involves the opening displacements. Thus, it raises a serious doubt about the mode II fracture from the disk-type specimens for rock-type materials. In this study, a series of the disk-type specimens of the marble was tested, and digital image correlation (DIC) was used to obtain the displacements surrounding the notch where the fracture mode was identified. Experimental results show the initiated fractures from the specimens are the mode I fractures, suggesting that the assumption of the mode II fracture is not correct. In the discussion, the interpretation is provided for: (i) differences between the critical values of mode II loading from disk-type specimens without compression and from punch-through shear specimens under compression; (ii) application of the criterion of local symmetry; and (iii) characteristics for rocks or rock-type materials and other information.