Issue 52

C. Caselle et alii, Frattura ed Integrità Strutturale, 52 (2020) 247-255; DOI: 10.3221/IGF-ESIS.52.19 251 only in late phases of the test, is an important element in the strain accommodation (intended as the set of processes occurring in the rock during the application of an external load, allowing for the shortening of the sample). Figure 4: Mohr circles and failure envelope estimated on the base of the results of triaxial tests. Figure 5: a. Stress-strain curve of the uniaxial test, with location of the initial point for the DIC analysis (black arrow) and of the strain maps in Figs. b and c (black point). b. Map of axial strains. c. Map of lateral strains. As already noticed, several stress drops characterize the post-peak stress-strain behaviour in uniaxial conditions. Fig. 6 shows the maps of lateral strains obtained with DIC analysis in this part of the stress-strain curve. Each line of the Figure reports three images before and three images after the drop. The stress-strain graphs show the correspondent position of the drops. The image attests a step-propagation of the crack coalescence: in each line, a sudden advancement of the failure is registered in correspondence of the stress drop. The three images before the drop and after the drop are almost identical and only at the moment of the drop itself the failure coalescence has a new step. This kind of behaviour is consistent for all the duration of the strain softening, suggesting a mechanism of stepping- coalescence of the failure surface. Fig. 6d refers to the biggest stress drop in the curve. It marks a change in the curve slope, starting a phase of faster strength degradation. This crucial point of the stress-strain path corresponds to the complete development of the failure surface that, in the last step in Fig. 6d, connects the two sides of the sample. The introduction of a confining pressure does not inhibit the possibility of a failure coalescence. Up to a confining pressure of 6 MPa, a clear failure surface is recognizable in the samples, both at the macroscale (Fig. 3b-c) and at the microscale (Fig. 7 and 8). At a microscopic insight, these failure surfaces appear to be accompanied by a larger deformation area, consisting of crashed grains, as highlighted in the micrograph in Fig. 7c. The phenomenon of grain crashing, however, is not only localized along the failure surface, but also interests other portions of the rock. As instance, Fig. 7a shows an intense crashing phenomenon on a gypsum crystal, which left edge gradually turns into fine-grained crashed gypsum. Similar evidences characterize several portions of the sample and seem to be particularly concentrated in the finer layers of the rock, in accordance with the sub-horizontal crack observed in the uniaxial sample.