دانلود رایگان مقاله مطالعه چند مقیاسی تکامل و تغییر شکل شکستگی نمونه سنگ سفال

Abstract

The aim of this paper is to compare and discuss the values of strains and crack apertures associated with desiccation cracks measured in Tournemire clay rock at different scales (micrometer to decimeter). Experimental investigations in the laboratory were conducted on one clay rock sample subjected to a desiccation process. Two faces with dimensions of 20 × 20 mm2 (i.e., macroscopic scale) and 5.1 × 4.1 mm2 (i.e., mesoscopic scale) were analyzed. The induced hydric strains and desiccation cracking were monitored by digital image correlation combined with a new algorithm (H-DIC). The results were compared with the data of Hedan et al. (2014) at the gallery scale (decimeter) and those of Wang et al. (2013) at the microscopic scale (micrometer). Our laboratory study yielded the following phenomenological results. First, the displacement fields revealed the presence of sub-horizontal cracks associated with the direction of bedding planes and sub-vertical cracks, as previously observed in a gallery front in Tournemire Station. Second, when the relative humidity (RH) decreased between 98% and 33%, the crack aperture kinematics at the macroscopic scale (centimeter) was divided into three steps: (i) a phase of opening and closure, (ii) a phase of only gradual closure, and (iii) a final phase in which the desiccation cracks closed. Only phases (ii) and (iii) were observed at the mesoscopic scale (millimeter), revealing that the kinematics of cracks depends on the scale observed. The comparison of the strains at the mesoscopic and the macroscopic scales also highlights that their values depend on the study scale: the presence of cracks at the mesoscopic scale leads to a large overestimation of the values of the strains calculated at the macroscopic scale. In contrast to the observations in the laboratory, the desiccation cracks detected in the gallery systematically open when RH decreases. This difference and the differences observed in the geometrical organization of crack networks are explained by the different boundary conditions prevailing in both cases (i.e., free swelling/shrinkage in laboratory versus constrained swelling/shrinkage in the gallery). The interpretation of the entire dataset emphasizes the need for a multi-scale approach to understand and model desiccation cracking mechanisms and the associated hydric strains in clay rocks.

6. Conclusions

A non-invasive DIC method was used on an argillaceous rock sample subjected to a desiccation process under measured and controlled RH and temperature. A new experimental set-up was designed to investigate the kinematic fields and patterns (displacement, strains, crack opening and closure) at two scales: mesoscopic (millimeter) and macroscopic (centimeter). This experimental laboratory investigation permitted the monitoring of deformations at both scales for (a) the spatial clay rock deformations and (b) the crack evolution (opening and closure) to calculate their aperture during desiccation. From a phenomenological perspective, several conclusions have been reached. As observed in the Tournemire site (crack aperture ≤ 500 μm), our laboratory work demonstrated that hydric changes induce sub-horizontal cracks and sub-vertical cracks whose apertures were between 3.5 and 63.5 μm. At the beginning of the desiccation (b0.025 days, i.e., b40 min), some cracks opened rapidly, whereas other cracks closed. One half day after the data acquisition began, all of the desiccation cracks were closed; the desiccation process induced the closure of all cracks under free deformation and drained conditions. Under free deformation conditions, our observations suggest shrinkage from the surface to the center of the sample. Moreover, the comparison between the crack apertures and the distance between cracks at four scales (from the microscopic to the gallery scale) reveals they are multiscale parameters of clay rocks. The crack kinematics also exhibited multiscale behavior from the microscopic scale to the gallery scale, but this behavior appeared to depend on boundary conditions.