A HIGH-PRESSURE TECHNIQUE FOR DETERMINING THE MICROCRACK POROSITIES OF DAMAGED BRITTLE MATERIALS

The microcrack porosity of a damaged brittle material is found by accu rately measuring the compressibilities under a hydrostatic pressure to 200 MPa. Crack apertures are sensitive to the stress applied normal t o their plane and, as a result, at low confining pressures the bulk of the strain records the response of the progressive closure of these c racks. At higher pressures when much of the microcrack porosity is clo sed the observed strains are nearly the same as that expected for the solid compounds within the rock. The higher pressure response may then be used to estimate the expected solid strains that are removed from those observed to leave that portion of the strain due to microcrack p orosity. This procedure differs from that of other workers who either ignore the portion of the strain due to intrinsic compression or accou nt for it indirectly by taking the derivative of the observed strain c urves. The final results may be analyzed in tenser form to show possib le anisotropies in microcrack orientations within a given sample. An e xample of the experiment on a rock sample that was damaged during remo val from the earth by drilling yielded a microcrack porosity of 0.79%, which was a substantial fraction of the total porosity near 5%. At 20 0 MPa, the principal crack strains had magnitudes of 1.86, 2.61, and 3 .39 mstrains; the orientation of this last and greatest magnitude stra in differing from the vertical direction by approximately 20 degrees. This suggests that the planes of the predominant family of microcracks within the test piece were subhorizontal. This suggests that a portio n of the microcracks result from drilling-induced damage and not solel y from stress relief of the sample upon removal from the earth.