Characterization of rock cracking patterns in diametral compression tests by acoustic emission and petrographic analysis

• Diametral compression tests with acoustic emission and petrographic analysis.
• Pre-peak microcracking in both rocks is distributed in the central loading plane.
• Microcracks initiate through quartz in monzogranite releasing high-level energy.
• Cracks follow cleavage planes in marble releasing low-level energy.

Pre-existing microscopic flaws in brittle rocks act as local stress concentrators and cause the nucleation and propagation of microcracks, resulting in a decrease of stiffness. The propagation of microcracks causes a sudden release of energy in the form of elastic waves, known as acoustic emission (AE), detected by sensors, enabling the non-destructive monitoring of cracking.Marble and monzogranite specimens were subjected to displacement-controlled diametral compression tests for the characterization of the cracking pattern according to AE parameters, three-dimensional localization of the AE sources and petrographic analysis. The tests included unload–reload cycles after peak. Microcracking in monzogranite and marble was initiated at approximately 90% and 60% of the peak load, respectively. Before peak load, both rocks showed the development of microcracks uniformly distributed on the plane containing the loading edges. At the post-peak stage, new microcracks were first concentrated on one of the faces at the center of the specimen along the loading axis and then spread through its thickness all the way to the other face. The main portion of the microcracking in marble was concentrated just after peak, while in monzogranite it extended through to the end of the test. The microcracking process in monzogranite was initiated and propagated mainly through quartz crystals, not along visible weakness planes and it released high-level energy, while in marble it followed the cleavage planes and released low-level energy.