Fracture characterization of chalk in uniaxial and triaxial tests by rock mechanics, fractographic and electromagnetic radiation methods

This study relates to fracture characterization of chalk in uniaxial and triaxial tests by combined application of rock mechanic, fractographic and electromagnetic radiation (EMR) methods. We found high strength results (sigma (1) - sigma (3)) that ranged from 37.7 to 52.6 MPa. We identified several 'key surface morphologies' that were distinct from each other, and expanded on their characterization according to tensile and shear categories. Tensile features include plumes (or striae) and stairs. Shear features include a sugary texture, ridges and grooves and steps. Fracture surfaces induced by tension and shear show distinct morphologies. Longitudinal splitting domains can occur continuously on an axial, tensile plane or adjacent parallel planes. On the other hand, shear failure into meso-fractures (up to about 10 cm in length) is a process that combines a series of distinct domains on separate surfaces that slightly differ in curvature and orientation, and are inclined to the sample axis. Four fracture sequences that initiated either in tensile or in shear surfaces were distinguished in the failed samples. Results obtained by rock mechanics and by EMR methods are well correlated for microcracks and for meso-fractures and may vary over five orders of magnitude in size, and this correlation is partially supported by fractography. In particular, a very good linear relationship was obtained between the ratio of the time interval to reach EMR pulse envelope maximum and the pulse frequency (T'/omega) and the largest fractures formed. Extrapolating this result enabled us to calculate fracture sizes when the value of T'/omega was known. (C) 2001 Elsevier Science Ltd. All rights reserved.