Abstract

At microscopic level, natural pre-existing flaws in rocks act as local stress concentrators, favoring the propagation of microcracks, resulting in a loss of structural integrity and brittle failure of rocks. Crack propagation in rocks causes a sudden release of energy as transient elastic waves known as acoustic 97 emission (AE). AE monitoring enables an insight into the cracking process without affecting the integrity of the specimen. In this work, marble and monzogranite specimens were subjected to displacement-controlled diametral compression tests. The descriptions of the loss of structural integrity and the failure process of these rock specimens on a scale ranging from microscopic to macroscopic were based on: three-dimensional localization of AE sources, AE parameters and signal processing in both the time domain and the frequency domain. Such descriptions were complemented with petrographic analyses, interpretation of the load versus displacement curves and visual examination of the specimens. 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. After the peak load, cracking extends through the thickness of the specimen all the way to the other face. The cracking process in monzogranite is progressive, brittle, explosive and localized, and involves a high release of AE energy. In contrast, marble exhibits a continuous, gradual and scattered cracking that releases low levels of AE energy in comparison with monzogranite. The analysis of AE signals in the frequency domain showed that the microcracking is characterized by wide band emissions.