This paper discusses the results of acoustic emission (AE) monitoring of two common rock mechanics testing methods: indirect tension strength (Brazilian) and uniaxial compression (UC) tests. The tests were carried out on very strong, isotropic gabbro rock specimens from North Africa. A servo–controlled uniaxial compression device with a constant displacement rate was used for loading and AE event-hit data was recorded continuously. Comparison of AE hit versus time for each type of tests shows two sharp steps in AE activity in UC tests and three steps in the Brazilian tests. In this paper the possible mechanisms of the AE steps in UC and Brazilian.
It is generally accepted that most solids, when stressed, emit sub-audible noises or low-level seismic signals that mostly are named acoustic emission. Acoustic emissions (AEs) are the stress waves produced by the sudden internal stress redistribution of the materials caused by the changes in the internal structure. Possible causes of the internal-structure changes are crack initiation and growth, crack opening and closure, dislocation movement, twinning, and phase transformation in monolithic materials and fiber breakage in composites [1 and 2]. Most of the sources of AEs are damage-related; thus, the detection and monitoring of these emissions are commonly used to predict material failure. Over the last few decades, acoustic emission (AE) has been widely used to evaluate cracking processes in laboratory specimens [3 and 4] and engineering structures  because of its high sensitivity to crack initiation and growth. A lot investigation also carried out for in-situ stress evaluation using Keiser effect [6 and 7] and Felicity ration theories. Study on failure mechanism and hydraulic fracturing evaluation  are the other fields that used AE monitoring in last decade. Transient elastic waves generated by the rapid release of energy from localized sources within a material are named acoustic emission. The sources of these emissions in rocks are closely associated with the initiation and extension of cracks under stress . The most important application of AE to industry and civil engineering studies is evaluation of severe and sudden failures to make early warnings . AE can detect also the accumulation of micro damage inside the components, especially under service conditions. Some details of advantage and disadvantage of AE monitoring in comparison to other non destructive techniques could be observed in reference . Considering to the different fields of AE monitored investigations, the comparing of two types of tests are less investigated. This research focuses on microcrack initiation in order to figure out the failure mechanism using AE data in two most known Uniaxial compression and indirect tensile-Brazilian-tests. For this purpose, the samples core drilled from a block of North African gabbro. This rock is isotropic and very hard rock with less than 0.5% of porosity. The preliminary approach of this research program is aimed at crack initiation in uniaxial and Brazilian test and comparing the recorded AE parameters.