### Uniaxial DIC test chart

The failure of deformation of rock materials occurs due to the development of localization of deformation in the rock system under most conditions. At present, the analysis of rock deformation and failure process through the development of deformation localization has become one of the important methods for researching rock mechanics, among which the method of digital image correlation has received wide attention from scholars at home and abroad. DIC, the digital image correlation method, is a non-destructive full-field optical measurement method, which was first proposed in the 1980s.26,27,28.

As shown in Figure 4, a uniaxial DIC test is performed to explore the influence of the loading rate of the rock, and the test system consists of a test loading system and a data image acquisition system. The loading system is realized by hydraulic servo testing machine, and displacement control is used for axial loading. Selecting the red sandstone for the sample and processing it into a cuboid as $$50\;{\text{mm}}\times 50\;{\text{mm}}\times 100\;{\text{mm}}$$. The six sides of the sample are sanded to ensure that the end of the sample is smooth. Based on related research1,29,30,31loading rates of 0.02 mm/min, 0.1 mm/min and 0.5 mm/min are specified, five sets are performed at each loading rate, and the test scheme is shown in Table 1.

The specific operating steps of the test are as follows:

1. (1)

First of all, the matte white paint is sprayed on the surface of the sandstone sample as the base color, and then the matte black paint is sprayed on the white background to form artificial spots, which are randomly distributed, and the size of one spot is greater than three pixels. The ratio of the spot area to the area of ​​the base color on the sample surface is close to 1:1.

2. (2)

Adjust the aperture and focal length of the camera to make the viewfinder roughly parallel to the surface of the sandstone sample.

3. (3)

The good quality spray spot samples are put on the testing machine, and clamped under a certain pressure.

4. (4)

Set the test machine load rate and camera acquisition frequency.

5. (5)

Collect DIC system image information, while the test device is compressed.

6. (6)

When subjected to a certain stress, obvious macroscopic damage occurs and the download and image collection stops at the same time. Select the appropriate patch on the sandstone sample for image processing and analysis, and obtain the deformation field of the sandstone sample surface.

### Analysis of Uniaxial Rock Test Results – DIC

Five sets of tests are conducted at each load rate level. Filter a set of data with better test results under each set of load rates. The stress-strain curve of the sandstone sample during loading is shown in Figure 5, and the test results are shown in Table 2.