Uniaxial strength and failure in sandstone containing a pre-existing 3-D surface flaw

A series of uniaxial compression experiments were performed on specimens of sandstone containing a pre-existing 3-D surface flaw in various configurations. The influence of the single flaw geometry on the stress-strain response and failure process was recorded and then analyzed in detail including real-time capture of surface cracking process by photographic monitoring. Uniaxial compressive strength drops of 25.7, 33.2 and 21.6 % were observed due to an increase in the flaw length and flaw depth but a decrease in the inclination of the flaw . The generation of three typical surface cracking patterns, namely wing cracks, anti-wing cracks and far-field cracks were identified and these depend on the geometry of the pre-existing surface flaw. Wing cracks initiate more readily from longer pre-existing flaws and those of deeper flaw depth or shallower flaw inclination while anti-wing cracks develop in the converse situation. Importantly, the stress required for crack initiation appears to decrease with an increase in flaw depth or a decrease in the flaw inclination. Finally, post-test imaging by X-ray computerized tomography defines the form of internal crack patterns including petal and wing cracks and their interaction and defines the macroscopic ultimate failure modes of the specimens and their dependency on the geometry of the pre-existing flaws. Although specimens containing a penetrating 2-D flaw normally rupture in a tensile splitting mode, those with a non-penetrating 3-D flaw generally fail in a combined shear mode which shows an increased dependency on the petal crack as flaw depth increases.