MICROSTRUCTURE EFFECTS ON MICROCRACKING AND BRITTLE FAILURE OF DOLOMITES

In this paper the influence of microstructure on crack initiation stre ss and ultimate strength is investigated using results and analysis of 32 triaxial compression tests performed on cylindrical cores of dolom ite samples which exhibit a wide range of grain sizes and mosaic textu res. All tests were performed at a constant strain rate, under confini ng pressures between 0 to 40 MPa, The predictive capability of convent ional criteria for ultimate strength which are based on empirical fitt ing parameters such as cohesion and internal friction angle, or mechan ical properties such as unconfined compressive strength, is shown to b e quite poor, due to the influence of microstructure. Microstructure c ontrols 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 pres ent. The validity of published analytical expressions which predict fr acture initiation stress assuming the sliding crack model is tested us ing both mean and maximum grain size, and inserting the measured fract ure initiation stress as the remote stress. It is shown that these app roximate models fail to describe true behaviour because they ignore bo undary conditions which exist at the tip of the leading crack at diffe rent mosaic textures. Early attempts to discuss the influence of micro structure on rock strength have shown that ultimate strength is invers ely related to mean grain size. This study demonstrates that grain siz e alone can not be used in correlation with ultimate strength. Rather, the combination of both grain size and porosity dominate the mechanic al response of the rock. Fracture initiation stress is found to be mor e sensitive to the influence of grain size than ultimate strength, pos sibly because the length of initial cracks controls the level of stres s concentration at the tip of leading cracks. However, fracture initia tion 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 poro sity and mosaic texture, and is less sensitive to mean grain size, pos sibly because once fracture propagation is initiated, grain arrangemen t controls fracture interaction processes which lead to macroscopic fa ilure.