Identification of critical state of deformed rocks

The article deals with the laboratory investigations of the deformation of heterogeneous brittle materials (rocks) at different stages of loading performed by acoustic emission (AE) and X-ray microtomography. Quasistatic tests of cylindrical (d = 10 mm, h = 20 mm) samples of Westerly granite were carried out under the conditions of uniaxial compression. To monitor continuously the acoustic emission during deformation, an Amsy-5 Vallen-Systeme was used. The granite sample underwent deformation in several stages. At each stage, the sample was pressed and the force of the pressure gradually increased to a certain level, after which it was kept under the given pressure. After each stage, they released the sample from pressure and conducted tomographic surveys. Each of the eleven samples underwent ten stages of pressing and tomographic surveys. The distribution of the AE signals by coordinates enabled one to identify the area of the sample with the increased concentration of the AE signals. The distribution of the AE signals by energy in the area had a power appearance. It testifies to the fact that, in this area of the sample, the system of defects emerged in the state of self-organised criticality. It was ascertained by the computer tomography that there appeared cracks in the same area. In other parts of the sample, the energy distribution of the AE signals was approximated by the exponential function. Thus, it has been demonstrated and confirmed by the data of tomography that the analysis of the functional appearance (power or exponential one) of the energy distribution of the AE signals enables one to determine the area of the sample, in which large cracks appeared.