## 1 Introduction
[28] performed small-scale blasting tests on granite bench specimens, and the experimental results showed that the worst result was obtained by simultaneous detonation. They found that fragmentation improved with increased delay time, with fragment size stabilizing when delay time was longer than 11 ms/m of burden. [PERSON] and [PERSON] [29] investigated the effect of timing on fragmentation. Their results indicated that the best fragmentation could be obtained by detonating the second blast hole when the damage process of the first blast hole reached its final state. [PERSON] et al. [30] used double-hole bench specimens to study the influence of delay time on rock fragmentation. Their study indicated that a long delay without stress wave superposition obtained the best blasting excavation quality. [PERSON] and [PERSON] [31] investigated the effect of accurate detonation on rock fragmentation through the use of a 3D model in LS-DYNA. In their numerical study, the influence of stress wave interaction on fragmentation was limited and occurred in localized zones. In the theoretical and numerical studies of [PERSON] et al. [32; 33], they indicated that when the stress waves between two blast holes are superposed, tensile stress increases in a small region around the interaction point, which cannot improve fragmentation. [PERSON] [34] questioned the positive influence of stress wave superposition on fragmentation, stating that stress wave interactions were uncertain in the 3D model and that stress wave superposition had a limited effect on fragmentation. In mining and tunneling, the purpose of optimizing rock blasting projects is to obtain better rock fragmentation. Compared to poor fragmentation, uniform rock fragmentation can improve ore recovery and reduce the energy consumption of crushing and grinding processes in the practice of mining operations. As rock fragmentation by blasting influences a realistic mine-to-mill process, fragment size distribution is an important issue to study. [PERSON] et al. [35] investigated rock fragmentation under different constraint conditions by analyzing fragment size distribution. [PERSON] et al. [36] performed blast experiments using cylinder specimens with different decoupling ratios. The analysis of fragment size distribution showed that the decoupling ratios and the filling materials in the blast hole have a significant influence on rock fragmentation. [PERSON] et al. [37] conducted two bench blasting experiments in the field and estimated fragment sizes using sieving data and image analysis. Furthermore, they proposed a numerical simulation method to predict the fragment size distribution. In the 12 full-scale bench blast tests performed by [PERSON] et al. [38], the fragmentation results were compared using the fitted curve of fragment size distribution. In addition, they used the fragmentation-energy fan concept to establish a relationship between the fragment size distribution and the explosive energy concentration. [PERSON] and [PERSON] [20] investigated rock fragmentation in multi-hole blasting through the use of LS-DYNA numerical code. In their study, the influence of stress wave interaction on fragmentation was investigated by fragment size distribution analyzed from WipFrag. The objective of this paper is to study the effect of delay time on rock fragmentation in multi-hole bench blasting. Six small-scale blast tests were conducted with the purpose of providing experimental data. In Section 2, the material properties of the granite, the dimensions of the bench specimen, the devices of the experiment, and the combination measurement system of the high-speed cameras and the 3D-DIC are illustrated. In Section 3, the surface strain fields of three bench specimens with different delay times are compared and analyzed. The post-blast pattern and blast excavation weight of specimens with different delay times are compared. Furthermore, the extended Swebrec function is employed to fit the curves of the fragment size distribution for all specimens.