Comparative study on the failure mechanisms and brittleness of rocks with various geological origins

Understanding the true failure mechanism of rock and rock-like materials has been a significant challenge in rock mechanics particularly in the rocks with high brittleness where the sample fails rapidly under complex mixed mode of shearing and multiple tensile cracking. This study presents the results of an experimental investigation on the failure mechanisms of two different rock types from sedimentary and igneous origins. A set of commonly used brittleness indices (BI) were deployed to measure the rocks’ brittleness, then with the aid of high-speed imaging technique, the failure mechanisms of sandstone and granite were compared. The behavior of samples was monitored from the commencement of loading until the complete failure using the Phantom high-speed camera under uniaxial compression. The various stages of cracking process were captured with high resolution including crack initiation, coalescence and failure. Finally, it was found that the granite samples have the most violent failure mechanism under both uniaxial compressive and indirect tensile loadings being 17 times faster failure rate than the sandstone samples under uniaxial compression. Majority of the selected brittleness indices recognized granite as the most brittle rock which was in a good agreement with the data obtained from the high-speed imaging.