A New Rock Brittleness Index Based on Crack Initiation and Crack Damage Stress Thresholds

Abstract

The brittleness of rocks is an important factor in the design of rock engineering projects, such as hydraulic fracturing, rockburst prediction, wellbore stability, and drillability. Therefore, an accurate assessment of rock brittleness is of high practical importance. However, the definition and the measurement of rock brittleness are not yet standardized. The present study proposes a new index for describing rock brittleness using crack initiation and crack damage stress thresholds for rocks with Class I stress‒strain curves. Uniaxial compressive strength (UCS) tests were conducted on fine-grained, medium-grained, and coarse-grained dolomite rock specimens in order to evaluate the performance of this index on describing brittleness. Based on the results, first, the effect of grain size on the crack initiation and crack damage stress was evaluated. Subsequently, the effect of grain size on common brittleness indexes was examined and compared to the proposed index. The results indicate that an increase in the grain size in the studied rocks increases the crack initiation, crack damage, and peak stress. It was determined that the ratio between the axial strains corresponding to the peak and crack damage stresses during the uniaxial compressive test is almost constant for all three rock types. Additionally, it was concluded, using the conventional and proposed brittleness indexes, that the brittleness index increases with an increase in grain size.