Size Effect Analysis and Characterization of Quasibrittle Fracture of Sandstone Rocks

Sandstone rocks exhibit quasibrittle fracturing and deviate from linear elastic fracture mechanics (LEFM) at typical laboratory scales. The quasibrittleness is caused by the formation of a finite-sized fracture process zone (FPZ) during failure. This leads to a marked specimen size effect in the strength and fracture toughness of typical laboratory-scale specimens. So, to properly apply the laboratory-measured fracture properties of sandstone rocks to field scales, understanding and characterizing this size effect is essential. This work is aimed at such a characterization and analysis for two different Berea sandstone rocks (Birmingham buff and Amherst gray). This is done by conducting mode I fracture tests on geometrically scaled, single-edge notched bend (SENB) specimens of different sizes for both sandstones. A marked size effect in the nominal strength, as well as the LEFM fracture toughness, is observed. Subsequently, data analysis is conducted by invoking the type II Bazant size effect law on the nominal strengths, which allows a systematic extrapolation of lab measurements to much larger sizes where the FPZ size becomes negligible (making LEFM applicable). The size effect law-based analysis is used to estimate the size of the FPZ allowing the characterization of the degree of quasibrittleness of a given specimen size. Further, the analysis allows the determination of their true, size-independent fracture toughness. The two sandstones are found to considerably differ in their degree of quasibrittleness. Overall, the size effect method is found to work well for the characterization of fracture properties of both sandstones, and for their extrapolation from laboratory to field scales.