CRACK LOCATION IN GRANITIC SAMPLES SUBMITTED TO HEATING, LOW CONFINING PRESSURE AND AXIAL LOADING

Until now, observations of mechanically and thermally induced microcra cks in rocks could only be carried out by indirect measurements or des tructive observations on samples brought back to atmospheric pressure conditions. A special triaxial test cell was designed in order to perf orm direct observations during loading. The use of a cell in tomograph y apparatus involves new devices: (1) a movable horizontal load frame around a scanner; and (2) a test cell transparent to X-rays, able to w ithstand up to 28 MPa maximum confining pressure and temperatures of u p to 180 degrees C. Volumetric strains are compared with radiological density measurements. The first processed X-ray images locating microc racks during propagation are also presented. Mineralogical effects on the crack location can be demonstrated. Strain inferred from CT densit y measurement is clearly correlated with the strain usually measured b y a strain gauge. Different phases of mechanical behaviour are describ ed: contracted phase and failure by macrocrack formation. The principa l results obtained with this tool are the description of the porosity formation and macrocracking. Results show two principal factors locali zing the porosity. First, the diffused porosity volume is controlled b y mineralogical parameters, quartz and plagioclase grains, and boundar ies of biotite grains during the thermal and mechanical loading. Secon d, macrocracking begins at the perimeter of the central section of cor e and grows towards the sample/piston interface. It seems that the fir st macrocracking is not located in the high-porosity zone formed durin g the loading phase, but in a relatively low-porosity zone.