# Evaluation of Rock Brittleness Index under Dynamic Load
## 1 Introduction  
Rock is a natural material, composed of various minerals and formed through long-term geological processes. The differences in properties among its components and internal microfissures determine the mesoscopic heterogeneity of rock, which is the internal basis for the differences in the failure of different types of rock. Plasticity usually means that the material can maintain a complete and continuous deformation property, while brittle failure means that the material loses continuity and the initiation and expansion of cracks occurs. The brittleness characteristics of rock have important guiding significance for hard rock resource exploitation, rock burst prediction, mechanical excavation efficiency, and unconventional oil and gas resource exploitation [1; 2; 3; 4; 5; 6; 7]. Therefore, the correct evaluation of rock brittleness is crucial for the safe design and selection of mining machinery, effective reconstruction of unconventional shale gas reservoirs, and stability evaluation of the surrounding rock [8]. For the definition of brittleness, researchers mainly begin with the characteristics of brittle materials. [PERSON] (1944) [9] and [PERSON] (1950) [10] defined brittleness as a lack of ductility. [PERSON] (1960) [11] defined brittleness as a behavior that produces little or no plastic strain during rock failure or fracture. [PERSON] and [PERSON] (1967) [12] defined brittleness as the occurrence of fracture failure at or just above the yield stress. [PERSON] and [PERSON] [13] (1984) believed that brittleness was an inherent property of materialswhich was characterized by the fracturing of materials with little or no plastic deformation. They interpreted fractured rocks with deformation or strain less than 3-5% as brittle rocks. [PERSON] [14] (1995), [PERSON] [15] (2009), [PERSON] [16] (1966), and [PERSON] et al. [17] (1980) agreed that brittleness should be defined as the fracture ability of a material without obvious permanent deformation in tensile or compression tests. In addition, [PERSON] and [PERSON] [18] (1956) used fissure formation capacity to reflect rock brittleness. [PERSON] and [PERSON] [19] (1979) defined brittleness as the ease of crack propagation. [PERSON] [20] (1991) connected the brittleness of rock with the loss of cohesion in the elastic range of rock under load. Brittleness was considered to be the ability of rocks of a certain strength to lose cohesion along certain surfaces when the stress exceeds the elastic limit. [PERSON] [21] (1998) defined brittleness as the ease with which rocks form debris during the indentation process. [PERSON] and [PERSON] [22] (1979) referred to brittleness as one of the most elusive mechanical properties, measuring the competition between deformation (residual or plastic indentation) and fracture under indentation. In order to quantify the changes in rock brittleness, domestic and foreign scholars put forward the concept of a brittleness index. However, due to the inconsistent definition of brittleness and the different application fields, there is no internationally recognized standard for the determination of a rock brittleness index [7]. Some brittleness indices are shown in Table 1. Based on the relationship between the tensile and compressive strength of rocks and brittleness, [PERSON] and [PERSON] [22] proposed the classical brittleness index, \(B_{1}\). [PERSON] [23; 24] studied the uniaxial tension\(-\)uniaxial compressive strength curve and found that the area surrounded by the curve could be used to quantify the brittleness of rock, so he proposed a brittleness index, \(B_{2}\), based on the uniaxial tension\(-\)uniaxial compressive strength curve. Based on the strain characteristics of brittle rocks, [PERSON] and [PERSON] [22] proposed that the ratio of elastic strain to total strain be used as a brittleness index, \(B_{3}\), to measure the brittleness of rocks.