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

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 degrees 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.