This paper presents an experimental examination of the initiation of tensile fracture under a two-dimensional normal wedge indentor. In addition, the nondestructive technique of electronic speckle pattern interferometry (ESPI) monitored the failure process in medium strength (Berea sandstone, 50 MPa) and high strength (Sioux quartzite, 400 MPa) rock. The results show a good agreement between the cavity expansion model and the experiments in terms of indentation pressure and size of the damage zone located beneath the indentor. A localization of microcracking was identified from the high-resolution image of ESPI. It appeared that an intrinsic length, which may be interpreted as a material parameter, developed during indentation. This intrinsic flaw or defect length controlled tensile crack initiation at the interface between the damaged and intact rock. For successful fragmentation, the initiation of tensile cracks must eventually lead to progressive chipping. Therefore, the intrinsic length could be used as an index property to evaluate the cuttability of rock.
The basic mechanism of mechanical cutting of rock involves the development of fracture from the action of a rigid indentor (Cook et al. 1984). A systematic description of the failure process due to a wedgeshaped indentor was offered by Lawn and coworkers (Lawn & Evans 1977, Lawn & Marshall 1979). They observed the formation of an inelastic zone followed by various crack patterns in ceramics. Studies in rock confirmed that a damage zone preceded the formation of a tensile crack (Linquist et al. 1984), and it was suggested that tensile fracture was the dominant "chip forming" mechanism (Kutter & Sanio 1982, Sanio 1985). Recognizing the importance of crack initiation and propagation, Nelson et al. (1985) correlated performance of tunnel boring machines with fracture toughness of the rock. Key elements of the failure process were identified by Sanio (1985). A critical pressure within the damage zone was assumed as a property of the rock, and a fracture mechanics solution was used to estimate a cutting force. Kou et al. (1995) derived relations between indentation force, penetration depth, and crack length through a similarity analysis based on a large number of experiments.
Recent work at the University of Minnesota (Detoumay & Huang 2001) has focused on the
§ indentation pressure and damage zone, and
§ conditions associated with fracture.
It has been shown that the development of a damage zone beneath a wedged-shape indentor provides a mechanism by which tensile stresses are induced in the rock. Furthermore, the maximum tensile stress is reached at the boundary between the damaged rock and intact rock. This means that fracture will occur at some distance from the tip of the indentor. However, the magnitude of the tensile stress will not depend on the penetration depth, although a critical amount of indentation needs to be reached for a tensile fracture to initiate.