Occurrence of Permanent Strain of Granitic Rocks and Gabbro under Uniaxial Tension: Implicating Damages Undergone in Failure Process

In order to understand mechanisms of rock failure under uniaxial tension, we analyzed permanent strains occurred post ultimate failure in uniaxial tension tests for several plutonic rocks included tonalite, granodiorite, granite and gabbro following our previous studies which just analyzed the test data until the ultimate failure. Usually, a specimen under uniaxial tension test broke locally, but the other part of the specimen looked no obvious visual changes with that before uniaxial tension test. From the post failure stress - strain curves, we found that extensional permanent strains occurred in apparently intact parts of the broken specimens, i.e., the tensile stress was unloaded completely. In addition, the permanent strain occurred in both axial and latera directions, i.e., in directions parallel and perpendicular to uniaxial tensile loading. We interpret that the extensional permanent strain occurred by mixed cumulative effects of new microcrack occurrence and developing and/or opening of pre-existing microcracks accompanying with increase of the tensile stress. Therefore, these extensional permanent strains implicate ‘damages’ that the specimens undergone in their failure process under uniaxial tension. Consequently, the failure process of uniaxial tension may be considered as that the minor damages occurred over the entire part of the specimen at first, then a concentrated damage zone was formed and finally proceeded to the ultimate failure.