Issue 52

C. Caselle et alii, Frattura ed Integrità Strutturale, 52 (2020) 247-255; DOI: 10.3221/IGF-ESIS.52.19 253 in Fig. 8b, the central part of this structure consists of a diffuse fracturing that creates a zone of crashed grains. This central band is accompanied by a larger area of fracturing that helps in the accommodation of the relative displacement of the upper and the lower portions of material. In this sample, the presence of intra-crystalline plastic structures became more evident (e.g. Fig. 8c). The crystals appear to be deformed for the effect of a kink folding, i.e. a folding mechanism with straight limbs and pointed hinges that is usually associated to materials with a fine, regular layering. In the present framework, the regular layering corresponds to the mineral cleavage along the (010) crystallographic direction. This cleavage reproduces the sandwiched crystallographic structure of gypsum, with an alternation of sheets of Ca2+ and (SO4)2- ions and double-sheets of water molecules. The weakness of the water-water chemical bonds favours the formation of the mineral cleavage. This kind of plastic mechanism seems to be activated by the lateral confinement, becoming more evident and pervasive with the increase of confining pressure. Figure 8: Micrographs of gypsum sample deformed with a confining pressure of 6 MPa. a. Failure surface, observed at optical microscope in reflected light. b. Shear band oriented as a conjugate, observed at the SEM c. Kinked grain at the SEM In the sample deformed at Pc = 8 MPa, indeed, the presence of kinked grains becomes the principal effect of strain accommodation, involving all the biggest gypsum crystals (Fig. 9). The presence of kinked grains can be easily recognized with the optical microscope at crossed nicols, since the change in orientation of the crystal implies a change in the interference colour (e.g. Fig. 9b). The kinking structures are preferentially concentred on gypsum crystals oriented in vertical direction (i.e. with the cleavage planes parallel to the principal stress σ 1). This confirms the connection between this kind of structure and the mineral cleavage. As suggested by [21], who observed similar structures in triaxially loaded samples of Volterra gypsum, the grain kinking is a hardening mechanism: after a certain amount of strain, the energy needed to further deform a specific grain becomes larger than the energy needed to start to deform a new grain. This is coherent with the transition of behaviour from strain softening to strain hardening observed in the mechanical tests. Figure 9: Micrographs of gypsum sample deformed with a confining pressure of 8 MPa. a. Kinked grain, in optical microscope with reflected light. b. Kinked grain, in optical microscope with crossed nicols. c. Gypsum crystal with kinking structure and sharp cracks along the kink borders, observed at the SEM.. As a confirmation of this idea, Fig. 9c proposes the SEM micrograph of a kinked grain with sharp cracks along the boundaries of the kink bands. This evidence suggests that each gypsum crystal can accommodate with plastic kinking only