Quantification of pre-peak brittle damage: Correlation between acoustic emission and observed micro-fracturing

Acoustic emission (AE) is a useful non-destructive technique to determine whether damage in rock material has occurred, even though the interpretation of AE can be complex. In this paper, two laboratory situations are studied, to investigate the correlation between micro-fractures and AE. In the first case, damage is induced by pure macro-compressive stresses and in the second case by macro-tensile stress (in one direction). The specific evolution of the recorded cumulative AE energy as a function of the applied load leads, in both cases, to a subdivision of the loading in different phases. Thresholds for the transition of these phases are defined. A thorough petrographical analysis of thin slices of samples, damaged to these different thresholds, allows a physical interpretation of damage phases. This methodology leads to a damage evolution model both for macro-compressive and macro-tensile stresses. This paper demonstrates the advantages of the combined use of AE and detailed petrographic study of thin rock slices. Furthermore, insight is provided on the pre-peak damage evolution caused by macro-compressive and macro-tensile stresses in rock material.