Crack location in granitic samples submitted to heating, low confining pressure and axial loading

Abstract

Until now, observations of mechanically and thermally induced microcracks in rocks could only be carried out by indirect measurements or destructive observations on samples brought back to atmospheric pressure conditions. A special triaxial test cell was designed in order to perform direct observations during loading. The use of a cell in tomography apparatus involves new devices: (1) a movable horizontal load frame around a scanner; and (2) a test cell transparent to X‐rays, able to withstand up to 28 MPa maximum confining pressure and temperatures of up to 180 °C. Volumetric strains are compared with radiological density measurements. The first processed X‐ray images locating microcracks during propagation are also presented. Mineralogical effects on the crack location can be demonstrated. Strain inferred from CT density measurement is clearly correlated with the strain usually measured by a strain gauge. Different phases of mechanical behaviour are described: contracted phase and failure by macrocrack formation. The principal results obtained with this tool are the description of the porosity formation and macrocracking. Results show two principal factors localizing the porosity. First, the diffused porosity volume is controlled by mineralogical parameters, quartz and plagioclase grains, and boundaries of biotite grains during the thermal and mechanical loading. Second, macrocracking begins at the perimeter of the central section of core and grows towards the sample/piston interface. It seems that the first macrocracking is not located in the high‐porosity zone formed during the loading phase, but in a relatively low‐porosity zone.

Influence of mineralogy on granite decay induced by temperature increase: Experimental observations and stress simulation

“…While heating under this threshold may lead to crack closure and a decrease in permeability (Dwivedi et al, 2008), microcracks develop above this temperature (Homand-Etienne and Troalen, 1984), which results in an increase in porosity or a decrease in Vp (Geraud et al, 1998;Gómez-Heras et al, 2006a,b;Menéndez et al, 1999). Although microcracking may develop below 400°C, this temperature would be the lowest to produce visible damage under the microscope.…”

4D imaging and quantification of pore structure modifications inside natural building stones by means of high resolution X-ray CT

“…use during pore modification processes in a natural building stone in full 3D. Many previous studies have involved already real-time CT measurements of compression and unloading experiments (Geraud et al, 1998;Viggiani et al, 2004;Zhou et al, 2008), investigations of porosity changes before and after protective treatments (Bugani et al, 2008;Cnudde et al, 2004), bacterial weathering (De Graef et al, 2005) and time-lapsed micro structural imaging at synchrotron facilities in medical (Nazarian and Muller, 2004;Voide et al, 2009), as well as geomaterial applications. To the best of the author's knowledge, this study provides, for the first time, timelapse 3D quantification on micrometer resolution of specific weathering features.…”

High-resolution X-ray computed tomography in geosciences: A review of the current technology and applications

“…a full strain map). The implementation of triaxial test cells in a micro-CT scanner (Charalampidou et al, 2011;Geraud et al, 1998;Hall et al, 2010;Lenoir et al, 2004;Otani et al, 2010;Raynaud et al, 2010;Siddiqui et al, 2010;Viggiani et al, 2004;Watanabe et al, 2009) can help to understand the stress-and timedependent deformation of porous structures (Figure 7). The magnitude of the rotation of individual grains during triaxial compression tests, plotted for the vertical slices through the middle of the scanned specimen is shown in Figure 7a.…”