The paper describes the mechanisms and conditions for the Kaiser damage-memory effect in rocks subjected to a three-dimensional disproportional cyclic loading with changes in the rocks’ shape and the orientation of the Lamé ellipsoid. The experiments with the cubic samples taken from polymictic sandstone were conducted on Triaxial Independent Loading Testing System with continuous recording of an acoustic emission (AE) signals [1-5]. The results of a disproportionate triaxial compression under the developed two protocols, they are 4- and 9-cycle loading programs, have shown that a dominate mechanism of the damage memory effect in each ensemble of cracks (vectored differently) is the development of micro-cracks of opening fracture mode oriented subnormally to the minimum main stress. It was found that the Kaiser damage-memory effect is detected not so much to the fact of opening cracks, friendly oriented, as to a discrete growing (increase of length) of already existing and newly emerging micro-cracks. The experiments on the cyclic strain of the sandstone at constant average stress and a changing level of a stress deviator allow the authors to conclude that the Kaiser damage-memory effect is not influenced by the intensity level of a shearing stress forcing on the sample. The obtained results can be considered as a trigger for models development oriented to strain and destruction of rocks, considering the anisotropic nature of damage accumulation.
The rocks destruction, the inhomogeneous structure of which determines the anisotropic features of their properties, has a multi-scale and multi-stage nature. At present, numerous experimental data on the strain and strength properties have been accumulated and systematized, however, insufficient attention has been paid to the influence of orientational nature of rocks under cyclic (active loading - unloading) multi-axial loading with different intensities. Mostly, it is explained by the fact that the equipment available to researchers allows conducting experiments for the simplest loading protocols (compression and extension in one direction, three- and four-point bending), whereas the facilities for conducting an independent triaxial loading are unique, available to a small number of research centers in the world. Experiments on the triaxial independent loading that the authors of this research carried out allow reproducing in laboratory setting the natural conditions corresponding to natural rocks deposit when the complicating factors related to tectonic and anthropogenic processes have been considered. In addition, installations of independent triaxial loading of rocks allow to simulate the evolution of complex three-dimensional in-space-rotating mechanical impacts that take place under hydrocarbon production in deep depth conditions and with complex geological settings (continental shelf, major fault zones, etc.). The experiments, carried out by the authors, allowed to advance the insight about the orientational nature of the crack damage memory effect under complex three-dimensional impacts and will serve as fundamentals for creating an effective tool for geo-mechanical modeling in mineral deposits development. The experimental results can be also of interest for specialists who do research in the field of physics and geo-mechanics (fault zones), and for experts engaged in forecasting the rock burst behavior.