Issue 52

C. Caselle et alii, Frattura ed Integrità Strutturale, 52 (2020) 247-255; DOI: 10.3221/IGF-ESIS.52.19 249 respect to the initial volume of the sample. At the end of the test, samples were cut to prepare thin sections for the analysis with optical microscope and SEM (Scanner Electron Microscope). The shape of the samples (prismatic or cylindrical – Fig. 1) depends, therefore, on the performed test and on the method used for the investigation of failure mechanisms. All the samples have height of 100 mm and diameter (or side) of 50 mm. The upper and lower faces were polished and rectified in order to obtain flat and parallel surfaces. Figure 1: Samples for the uniaxial tests (a) and triaxial tests with confining pressure of 4 MPa (b), 6 MPa a(c) and 8 MPa (d). R ESULTS ig. 2 shows the stress-strain curves for the four tested samples. The curves are ordered following the increasing confining pressure, from 0 MPa (i.e. Uniaxial Compression) to 8 MPa. Resulting peak stresses are reported in Tab. 1. The curves describe a clear change in behaviour: in uniaxial conditions, the material reaches a strength peak and is then characterized by a strain softening deformation. The post-peak strain softening curve consists of a series of stress- drops, followed by a short recovering phase, then interrupted by a new drop and so on. With the introduction of a confinement, the strain to accommodate significantly increases: the strain softening is replaced by a long phase of stress stability that gives place to a slight strength decrease only around a strain of 5%. When the confining pressure reaches 8 MPa, no strain-softening is observed for the entire duration of the test (i.e. axial strain higher than 6%). The stress-drop behaviour is also observed in triaxial loading condition, particularly in the sample deformed at Pc = 6 MPa. The decrease of strain softening suggests a gradual disappearance of the macroscopic fragile cracks in the post-test samples. On this regard, the macroscopic features of the samples show a good agreement with the stress-strain curves. Fig. 3 shows the final images of the samples associated with a schematic representation of their shape and fracturing. With the increasing of confining pressure and the hardening of the material, we registered a transition from a structure characterized by a single, localized failure surface (Fig. 3a-b) to the presence of a more diffuse fracturing, with conjugate structures (Fig. 3c), to the total absence of visible fracturing (Fig. 3d). Both the localized failure surfaces of Fig. 3a-b and the diffuse fracturing in Fig. 3c are well evident on both the sides of the samples, while sample in Fig. 3d does not show any macroscopic crack on any side of the sample surface. A constant angle with the main loading direction can be observed. In Fig. 3a, 3b and 3c an angle α with the horizontal surface of 60° was measured. Considering a failure envelope estimated on the base of the results of triaxial tests (Fig. 4), the angle of cracking on the samples is coherent with the friction angle φ of the material, following α = φ /2 + π /4 The final shape of the samples is also influenced by the confining pressure: in the uniaxial test the shape is almost unchanged, while samples under triaxial loading show an increasing propensity to localize a volumetric strain in the central part (barrel shape). F