## 1 Introduction  
The number of deep rock engineering structures, such as mines and tunnels, is increasing [1; 2]. Additionally, it has been found that geostress gradually increases with depth [3]. During excavation using the drilling-and-blasting method, rock blasting is conducted under the combined actions of high static stresses and blasting-induced dynamic loading. Compared with shallow operations, the blasting for deep rock engineering projects is more complex owing to the high static stresses. The uneven stress distribution near the working face leads to a significant difference in blasting efficiency, severe over- or under-excavation, and difficulty in the control of the final shapes of the deep rock structures [4; 5]. Therefore, it is necessary to study the effect of high static stress on rock blasting [6; 7; 8; 9; 10]. The crater blasting theory and tests are effective means to understand the rock fragmentation mechanism under blasting, which has been extensively investigated [11; 12]. [PERSON] and [PERSON] [13] have simulated the two-dimensional (2D) crack propagation under blasting with a single free surface based on the Johnson-Holmquist material model using the LS-DYNA software and studied the effects of the loading rate, uniaxial stress field, and joints on the blasting performance. [PERSON] et al. [14] have simulated the damage evolution and crack development in rock upon blasting under confining in-situ stresses and reported damage development trends toward the direction of the high applied initial pressure. [PERSON] and [PERSON] [15] have numerically modeled the explosion of a cylindrical charge based on the Mohr-Coulomb failure criterion using the FLAC3D program and the results have indicated that the anisotropy in the shape of the cracked zone has significantly increased with the anisotropy of the static stress field. The miniaturized crater blasting model experiment can be used to study the whole blasting process, such as the initiation and propagation of cracks. In order to study the influence of static stress on blasting, many scholars have carried out miniaturized crater blasting model experiments [16; 17; 18; 19]. [PERSON] et al. [16], [PERSON] and [PERSON] [17] investigated the effect of static stress on the propagation of blasting cracks using PMMA (polymethyl methacrylate) specimens with dimensions of \(315\times 285\times 6\) mm\({}^{3}\), and the results indicated that static stress plays an important role in crack formation and propagation. [PERSON] and [PERSON] [18] used \(250\times 250\times 100\) mm\({}^{3}\) rock model specimens to study the characteristics of free surface cracks in single-hole and double-hole blasting. [PERSON] et al. [19] have carried out experimental studies on crater blasting under various static stresses and reported that the static stress has promoted crack propagation during the blasting. Hence, the high static stress is one of the main factors influencing the blasting efficiency in deep rock engineering. From previous studies, it can be seen that static stresses play an important role in the crack initiation. Although numerous studies have been focused on the influences of static stress on crack initiation and propagation, no extensive studies have been conducted on the strain field evolution characteristics before crack initiation. However, at the initial stage of blasting, cracks do not emerge and the strain field has a significant role in the crack initiation and blasting crater shape. [PERSON] et al. [20] used high-speed camera and 2D-DIC technology to observe the blasting process of crater blasting under the coupling of dynamic and static loads processing, including strain field evolution, crack propagation process, blasting crater shape and blasting fragmentation. However, this paper only conducted qualitative analysis on the evolution of the strain field, and the mechanism of static stress affects blasting crack formation is still unclear.