Size Effect Analysis for the Characterization of Marcellus Shale Quasi-brittle Fracture Properties

The fracture characterization of shale rocks requires understanding the scaling of the measured properties to enable the extrapolation from small-scale laboratory tests to field applications. In this study, the fracture properties of Marcellus shale were obtained through size effect tests. Fracture tests were conducted on three-point-bending specimens with increasing size. The test results show that the nominal strength decreases with increasing specimen size and it can be fitted well by Baant's size effect law. This demonstrates that shale fracture behavior deviates from classical linear elastic fracture mechanics (LEFM), and it has quasi-brittle characteristics. This implies, in turn, that the fracture toughness (or fracture energy) computed according to LEFM is size-dependent and, in general, cannot be considered a material property. Furthermore, the size effect analysis allows one to accurately identify the quasi-brittle fracture properties, namely the initial fracture energy and the effective fracture process zone length. A significant anisotropy was observed in the fracture properties determined with three principal notch orientations.