Cracks imaging in linear cutting tests with a PDC cutter: Characteristics and development sequence of cracks in the rock

Imaging cracks is a direct way to learn the rock breaking mechanism beneath a PDC cutter. In this study, a series of linear cutting tests and precise observations of cracks were conducted. The formation of major cracks was captured in-situ with a high speed camera in the process of cutting rock sheets. Then the cracks under the cutting groove (subsurface cracks) were studied by reproducing the cutting groove after cutting some rock blocks and further observed by means of thin-section optical microscopy. Finally, the development sequence of the cracks and adjacent crushed zone was determined by recording the cutting process on a rock-like silica glass block. The results show that the shape of major cracks is mainly curved, both upward and downward curves were observed. The major cracks mainly initiate at the tip of the cutter when cutting sandstone and granite, while they initiate more randomly in marble. The length and angle of the subsurface cracks vary with the rocks: the subsurface cracks extend the longest in granite, while propagating more vertically in sandstone. Finally, it is determined that the major cracks initiate after the formation of crushed zone. And the initiation of major cracks will enlarge the crushed zone and lead to the initiation and propagation of the subsurface cracks. The results in this paper are expected to further reveal the rock breaking mechanism beneath a PDC cutter.