## 1 Introduction  
As a common method of rock excavation, the borehole blasting technique is widely used in mining engineering. However, inappropriate blasting designs depending on experience may result in high ground vibration, poor fragmentation, and even safety risks [1; 2]. In bench blasting, poor fragmentation will directly increase downstream costs and reduce ore utilization [3; 4; 5]. Many studies have been carried out by relevant practitioners and researchers on bench blasting to obtain better fragmentation [6; 7; 8; 9; 10]. It was found that delay time is one of the important factors affecting the fragmentation of rock [8]. With the development of electronic detonators with a highly precise delay time, controlling stress wave interactions to improve blasting fragmentation has become a hot topic. [PERSON] [11; 12] studied the stress wave interaction between two adjacent blast holes by the Lagrange diagram based on stress wave theory, and he suggested that the superposition of stress waves induced by short delay times has a positive effect on rock fragmentation. [PERSON] and [PERSON] [13] carried out a series of short-delay blasting experiments by using electronic detonators to enhance the tensile effect of the stress wave tail. Field experiments showed that the mean size of fragments was improved by 45.6% than that with a long delay time. [PERSON] [14] believed that the optimum delay time should be short enough to make the stress waves generated by adjacent holes interact between blast holes and validated this conclusion based on image analysis and fragment size statistics. [PERSON] [15] suggested a delay time of 3 ms between the blast holes of the same row, which can ensure the superposition of stress waves and improve rock fragmentation. Unfortunately, the view of stress wave superposition improving fragmentation cannot be widely accepted. [PERSON] [16; 17] stated that the potential of stress wave interactions was overestimated by adopting the simplified two-dimensional analysis. Specifically, the stress wave interaction was uncertain in realistic three-dimensional models, and the superimposed region had a limited effect on the fragment size distribution. [PERSON] et al. [18] carried out small-scale blasting experiments with granite. The results showed that the blasting fragmentation of simultaneous initiation was the worst. When the delay time between adjacent blast holes was longer than 1 ms, corresponding to 11 ms/m of burden, the fragmentation was improved. [PERSON] and [PERSON] [7] used bench models with a single free surface to conduct a series of delay blasting experiments. The statistical results of fragments indicated that the stress wave superposition had no significant improvement for fragmentation. By comparing experiments with simultaneous initiation, short delay time, and long delay time, [PERSON] [19] found that the fragment size distribution with long delay time is better than that with short delay time. [PERSON] [20] screened fragments under different delay times and found that the relatively long delay time would improve fragmentation. In the numerical study of [PERSON] and [PERSON] [21], the stress wave interactions were not significant enough to improve fragmentation, which was contradictory to the views of [PERSON] [11; 12]. Therefore, it is necessary to study the influence of delay time on rock fragmentation. The evolution of the strain fields and the dynamic breakage process can assist the study of rock fragmentation [22; 23; 24]. Many techniques have been developed to detect surface deformation and crack growth in brittle materials under different loading [25; 26]. With the development of high-speed cameras and the DIC technique, a non-contact measurement method successfully captured the full-field on the specimen before fracture occurred [27; 28; 29; 30; 31; 32; 33; 34; 35]. In the experimental study of blast loading, [PERSON] et al. [36] obtained the strain evolution process of rock blasting under a high-ground stress state using the 2D-DIC technique in model tests. [PERSON] et al.