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.

1. Introduction

The brittleness is one of the most important mechanical properties of materials and was firstly defined as the lack of ductility or the lack of ability to endure a large plastic deformation without fracture (Hetenyi 1950). Brittleness has an important application in rock bursting identification (Hucka and Das 1974; Zhao et al. 2014). The brittleness or its index has been used in different rock engineering projects including hydraulic fracturing operation for simulating the oil and gas reservoirs (Feng et al. 2020; Zhang et al. 2016), predicting hard rocks drill-ability and cutting efficiency to determine the progress of excavation in civil and mining structures (Altindag 2002; Altindag 2003) and estimating the tunnel boring machine (TBM) performance. Although the brittleness is a tangible concept, its quantification has long been ambiguous as it depends on a number of intrinsic and mechanical characteristics. Multiple factors need to be considered to conceive the brittleness of rocks including mineral composition (Jarvie et al. 2007; Wang and Gale 2009), mechanical and strength properties (Bishop 1967; Sun et al. 2013), size and time (Hajiabdolmajid and Kaiser 2003), and fracture energy (Tarasov and Potvin 2013). Providing a method to measure the brittleness which covers all the above-mentioned factors is not applicable. Thus, it has been attempted to consider only the most influential factors in the rock brittleness approaches. Significant attempts have been undertaken to quantify the brittleness by scholars since 1967 leading to establishment of over 80 different formulas in which each has its own limitations based on the selected influential factors. It is evident that one model alone cannot include all the affecting factors.

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