In order to study the effect of prefabricated elliptical holes on rock failure characteristics, static and dynamic compressive tests were conducted on rocks with elliptical holes of different geometries combined with digital image correlation (DIC), considering axial ratios, inclination angles and hole relative angles. The results show that the mechanical properties of rocks are significantly affected by hole geometries and have different sensitivity to their changes. Observations show that the process of crack initiation, propagation and coalescence in rocks at different stress stages can be well characterized by strain localization based on DIC. Under static load, the fracture proceeds from primary cracks through secondary cracks to shear cracks, and the primary cracks initiate from the top and bottom locations surrounding the hole. Under dynamic load, cracks mainly initiate at or near the hole tips and then propagate in different paths with varying hole geometries, resulting in three typical failure modes based on crack coalescence between the two pre-existing holes.
Rock mass is a complex natural medium containing various initial defects, such as joints, fissures and holes. Under external loads, new cracks always initiate and propagate from these initial defects due to stress concentration, and finally coalesce with each other into macro cracks, leading to the failure and instability of rock structures such as underground mining and tunneling. Therefore, the investigation of mechanical behavior and fracture process of brittle rock materials with defects is of great interest for geomechanics and rock engineering.
Many theoretical (Griffith 1921; Horii & Nemat-Nasser 1985; Sammis & Ashby 1986; Zhao et al. 2011), experimental (Lajtai & Lajtai 1975; Carter et al. 2010; Yang et al. 2012; Li et al. 2017; Zhu et al. 2019) and numerical (Wong et al. 2006; Bobet 2000; Wong & Lin 2015) researches have been carried out on the mechanical properties, crack propagation and failure patterns of rock or rocklike materials with defects under static or quasi-static loading. They investigated the initiation, propagation and coalescence of cracks and fracture mechanism leading to the failure of defected rock specimens under static loading. It has been widely accepted that the mechanical behavior and failure characteristics of rocks are closely influenced by geometric parameters such as shape, size, number, angle and distribution of defects.
