Postpeak Deformability Parameters of Localized and Nonlocalized Damage Zones of Rocks under Cyclic Loading

Previous studies on the effects of cyclic loading on the strength and deformability behavior of rocks under uniaxial conditions, i.e., strength damage together with the degradation of rocks' mechanical properties, have solely taken into account the postpeak stress strain regime. Nevertheless, rock may experience cyclic loading history in the postpeak regime as well. In this study, the effects of cyclic loading on the deformational characteristics of both the nonlocalized damage zone (NLDZ) and the localized damage zone (LDZ) of rock in the postpeak regime are investigated. A series of postpeak damage-controlled tests were carried out on three different rock types, including sandstone, limestone, and granite specimens. Three-dimensional digital image correlation was implemented to measure the strain development in the surface of the specimens. Strain localization was found to be more significant in the lateral direction and less occurrent in the axial direction. In addition, it was observed that the material located in the NLDZ experiences much less damage in contrast to material located in the LDZ. Both large irreversible strain accumulation and Young's modulus degradation were found to take place progressively in the LDZ, initially occurring at a lower rate and then faster as the number of loading-unloading cycles increased. In this regard, the extent of irreversible deformation and stiffness degradation in the LDZ is considered to be a major contributor to the global loss of strength of the entire specimen.