## 1 Introduction
[37] analyzed the surface deformation of granite specimens under single-hole explosion by using the 2D-DIC technique. In the study, the crack initiation time on the free surface was determined by DIC analysis and strain gauge measurements. [PERSON] et al. [38] studied full-field strain characterization of the bench slope surface under simultaneous detonation. The DIC analysis showed that the distribution of strain fields in the vertical direction correlates with the propagation direction of the dominant horizontal crack. [PERSON] and [PERSON] [39] obtained the strain fields of single-hole and double-hole rock specimens under blast loading from the 3D-DIC technique. The results showed that the stress waves superposition caused by symmetrical loading made the radial crack between the two holes easily and fully fractured. The application of the DIC technique for measuring deformation is ideal for obtaining information on the dynamic deformation of rock materials because of its accuracy, applicability, and operability [40]. Fragment size distribution is an important indicator for evaluating the fragmentation of rock. [PERSON] et al. [41; 42] studied the effects of different boundary and stemming conditions on the fragmentation with cylindrical granite specimens by analyzing the particle size of fragments. Many empirical models were proposed to describe and predict the fragment size distribution [43]. The Swebrec function proposed by [PERSON] [44] can effectively characterize the size distribution of post-blast fragments, including fine and coarse particles. [PERSON] et al. [45] analyzed 448 groups of sieving data for blasting fragments and compared the fitted results of the Weibull, Swebrec, Gilvary, Grady, and Lognormal models and their two-component versions. The results indicated that the Swebrec function was one of the most suitable functions to describe the fragment size distribution of rock blasting. [PERSON] et al. [46; 47] used Wipfrag software to process the numerical damage contours and used the Swebrec function to obtain the overall fragment size distribution. In this paper, the Swebrec function was used to fit and analyze the experimental data about fragment size, and the blasting quality was evaluated by the fragment size distribution. The aim of this study was to investigate the influence of delay time on fragmentation. Eight experiments of bench blasting models with double holes were carried out to provide the experimental data. In Section 2, the size of the bench specimen, the charge structure, the experimental device, and the monitoring system are introduced in detail. In Section 3, the full-field strains of three typical specimens are calculated by the 3D-DIC technique. The progressive breakage process captured by two high-speed cameras and the post-blast pattern with fragments backfilled under different delay times are compared and analyzed. Furthermore, the fragment size distribution of excavated rock is fitted by the extended Swebrec function. In Section 4, the influence of the delay time on fragmentation is discussed based on the full-field strains, high-speed images, post-blast pictures, and fragment size distributions. Finally, the conclusions of this study are provided in Section 5.