Microstructure effects on microcracking and brittle failure of dolomites

In this paper the influence of microstructure on crack initiation stress and ultimate strength is investigated using results and analysis of 32 triaxial compression tests performed on cylindrical cores of dolomite samples which exhibit a wide range of grain sizes and mosaic textures. All tests were performed at a constant strain rate, under confining pressures between 0 to 40 MPa. The predictive capability of conventional criteria for ultimate strength which are based on empirical fitting parameters such as cohesion and internal friction angle, or mechanical properties such as unconfined compressive strength, is shown to be quite poor, due to the influence of microstructure. Microstructure controls ultimate strength to such a degree that an assumed mechanical property such as unconfined compressive strength may vary by more than a factor of two, where two different microstructure patterns are present. The validity of published analytical expressions which predict fracture initiation stress assuming the sliding crack model is tested using both mean and maximum grain size, and inserting the measured fracture initiation stress as the remote stress. It is shown that these approximate models fail to describe true behaviour because they ignore boundary conditions which exist at the tip of the leading crack at different mosaic textures. Early attempts to discuss the influence of microstructure on rock strength have shown that ultimate strength is inversely related to mean grain size. This study demonstrates that grain size alone can not be used in correlation with ultimate strength. Rather, the combination of both grain size and porosity dominate the mechanical response of the rock. Fracture initiation stress is found to be more sensitive to the influence of grain size than ultimate strength, possibly because the length of initial cracks controls the level of stress concentration at the tip of leading cracks. However, fracture initiation stress is shown to be inversely related to both porosity and mean grain size, thus the importance of porosity in the initiation process must be recognized. Ultimate strength is influenced primarily by porosity and mosaic texture, and is less sensitive to mean grain size, possibly because once fracture propagation is initiated, grain arrangement controls fracture interaction processes which lead to macroscopic failure. Kevwords: micromechanics; brittle failure; dolomites; porosity; grain size * Corresponding author. Tel.: +972 (7) 647-2621; fax: +972 (2) 647-2997; e-mail: [email protected] 0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0040-195 ! (97 )00073-5 142 Y. H. Hatzor et al./Tectonophysics 281 (1997) 141-161