## 1 Introduction
[PERSON] [13], [PERSON] [14], and [PERSON] [15] proposed and explored the physical parameters related to the time effect in the dynamic fracture process of brittle materials, called incubation time. Based on the concept of incubation time, [PERSON] et al. [11; 16] proposed the structure-time fracture criterion. The dynamic strength of rock and concrete-like brittle materials was studied against this criterion and its simplified form. In [PERSON]'s research [17], a unified interpretation of sensitivity to strain or stress rate of the mechanical properties of fiber-reinforced concrete was presented. The dynamic compressive strength, split tensile strength, and dynamic toughness of the concrete can be predicted by the incubation time criterion. Similarly, the incubation time criterion has been applied to analysis of the dynamic strength characteristics of rock materials. The incubation time was considered to be an important temporal property that affected the dynamic strength of rock when comparing the experimental results of different types of rocks [18]. Then, in the experiments on rock specimens, different saturations and temperatures demonstrated the influence of physical condition changes on incubation time [19; 20; 21]. The measured temporal waveforms of strain on incident and transmitted bars in the SHPB system are the basic experimental data. When strain gauges are mounted on the specimen, deformation history at a given location can be captured. As a standard measurement technique, strain gauges can only measure the average strain in a limited region. A non-contact full-field measurement method such as digital image correlation (DIC) based on image processing has been used for determining rock deformation [22]. With the development of the high-speed camera, the strain field in the dynamic response processes of rocks can be calculated by DIC [6; 23; 24]. The DIC technique in conjunction with the high-speed camera and strain gauges were simultaneously used to determine the deformation of specimens in dynamic notched semi-circular bending (NCSB) and Brazilian tests. Comparison of the determination of strain gauges and the DIC indicated that the DIC technique provided reliable results. [PERSON] et al. [25] captured the deformation preceding the dynamic fracture of Brazilian test on Kuru granite rock by combining the high-speed camera with DIC. [PERSON] et al. [26] obtained the crack-tip position and dynamic stress intensity factors from DIC analysis of dynamic NSCB tests on Laurentian granite. In contrast to NSCB and Brazilian tests, dynamic compression tests observe a cylindrical surface of rock specimen with a high-speed camera. The comparison of the resulting strain fields from the 2D- and 3D-DIC techniques in the same experiment indicated that 3D-DIC technology obtained more accurate data of strain fields on the arc surface of specimen [27]. Wave propagation, dispersion, and radial inertial effects on specimens during the dynamic compression process were found by the DIC technique. A detailed description of high-speed cameras and the DIC technique for geomaterials provides a specific reference of the relevance [28]. In the description of the dynamic fracture behaviors of rock materials, a single indicator of strength is not satisfactory. The morphological characteristics of crack initiation, propagation, penetration, and formation are equally important. In the dynamic Brazilian testing, the DIC technique can be used to analyze the full-field strain distribution on the surface of the rock specimen based on the high-speed images at the crack initiation stage. When the central main crack is formed, the secondary crack will accompany it at the contact end. The cracks' interaction forms the fragmentation of the rock specimen, which is clearly recorded by high-speed camera. As a widely acknowledged dynamic fracture pattern, the fracture morphology of Brazilian specimens is often characterized simply and qualitatively as central cracking.