# Influence of Crack Geometry on Dynamic Damage of Cracked Rock: Crack Number and Filling Material
## 1 Introduction  
With limited land and mineral resources, mining and geotechnical engineering are continuously being developed; however, this development has brought increasing challenges. A large number of geological disasters have been induced by dynamic disturbance [1, 2]. Many studies have indicated that the dynamic damage of cracked rock bears significant adverse impacts and can lead to geological disasters. The strength deterioration and deformation of cracked rock are the main triggers of geological disasters [3, 4, 5]. Understanding the dynamic properties of cracked rock is thus necessary because it helps to prevent the potential for geotechnical failures.  
Cracks generally exist in natural rock masses, and their physical and geometrical characteristics have considerable influence on the properties of cracked rock, such as surface roughness, filling material, rock type and crack length [6, 7, 8, 9, 10, 11, 12, 13]. It is well accepted that the dynamic properties of cracks control the dynamic damage of cracked rock [14]. Moreover, the propagation of stress waves in cracks accelerates the failure of cracked rock. Many publications focus on the influence of filling materials on wave attenuation [15, 16, 17, 18, 19, 20]. The research results show that the attenuation of a stress wave is strongly affected by its reflection and transmission at the interfaces as well as by the dynamic properties of filling materials. The number of cracks is also an important geometrical characteristic influencing wave propagation [21, 22, 23, 24]. From previous studies, we can see that the transmission coefficient across fractures is a function of the fracture spacing for different numbers of cracks. The presence of cracks in a rock mass is clearly of paramount importance as it dramatically affects the dynamic behavior of cracked rock.  
[PERSON] and his son, [PERSON], invented a pressure bar to obtain the pressure-time curve with the dynamic load exerted by detonation ([PERSON], 1914). The Hopkinson bar was further developed by [PERSON] (1949) into the split Hopkinson pressure bar (SHPB). SHPB has become a reliable high strain rate loading apparatus to measure the dynamic response of brittle materials under impact loadings. It has been suggested recently as a standard method to measure the dynamic mechanical properties of rocks by the International Society for Rock Mechanics and Rock Engineering (ISRM). As a widely used device to quantify the dynamic properties of various brittle materials at high loading or strain rates, the SHPB system is used to study the compressive response [25; 26], tensile failure [27; 28; 29], shear strength [30; 31; 32] and fracture characteristics of rocks [33; 34]. There is no doubt that the SHPB system is a technique available to measure the dynamic compressive response of cracked rock under impact loadings.  
The previous studies on cracked rock focused primarily on the significant effects of cracks' physical characteristics on the dynamic properties. Research on the quantitative relation between the dynamic damage of cracked rock and the spatial geometry of cracks is limited. Therefore, this study focused on the dynamic damage of cracked rock with different spatial geometry. The influences of the number of joints and filling material on the dynamic properties of cracked rock were tested by the split Hopkinson pressure bar (SHPB) apparatus. The energy absorption in the dynamic impact process was analyzed, and the damage variable was obtained.