# Mechanical Behavior and Fracture Evolution of Artificial Rock Specimens Within 3D-Printed Fractures
## 3 Experimental Tests
### Fracture Patterns  
The uniaxial compression failure mode of normal temperature samples was shown in Figure 6. The speckle is prepared manually, and the density is nearly 40-50 /cm\({}^{2}\). The ROI area was chosen based on the whole area of the rock specimens. Figure 6a shows the failure mode diagram of the standard cylindrical specimen. Two typical failure modes, longitudinal splitting and shear failure, occurred in the compression process of the specimen. The B2 sample, with a content ratio of AB glue and pulverized coal of 1:1.2, contained two tensile fracture planes and a shear failure plane that traversed the entire rock sample. The B1 sample, with a ratio of 1:1, was not pressurized after being compacted to produce transverse cracks. Sample B3 with a 1:1 ratio experienced shear failure, and the form of failure was section shear failure, accompanied by a local shear failure surface. Comparison of the damage patterns of the B2 and B3 specimens at low temperatures revealed a tendency for specimen brittleness to increase proportionally with the coal dust content and to produce indicative rock compression damage. Figure 6b shows the failure mode diagram of the cubic specimen. Different material ratios of samples demonstrated distinct failure modes. Both specimen A1 and solid specimen C exhibited varying surface fracture shapes under compression. The A1 specimen exhibited shear failure along the joint, while the C specimen of the solid model revealed two cracks at the apex, but no penetration. The above findings demonstrated that the incorporation of a complex fracture network modified the sites of crack formation in the sample. The A2 sample, with a 1:1 AB glue to pulverized coal ratio, presented longitudinal fractures, while the middle of the sample exhibited incomplete microcracks. Comparison of the failure modes of A1 and A2 samples revealed that the adjustment of the material ratio altered the samples' failure modes. Furthermore, the uniaxial compressive strength of the samples increased with the increase in pulverized coal content. The WAW-300 universal testing machine and strain collector (Figure 3) were used for loading, and the maximum loading was 300 kN. A 1920 \(\times\) 1080 resolution camera was used for video recording during the experiment. Uniaxial compression tests were performed on three different model specimens. The initial loading rate was 1 kN/s by load control and displacement control, and when it reached 250 kN, it was changed to displacement control to ensure that the loading rate of the target load was 0.2 mm/min. The open MATLAB v2018b source, termed as the Ncorr code [27], is used to calculate displacement and strain by using the digital image correlation function. The strain error of this program is less than 0.05%. Figure 7 displays the y-direction strain nephgram of samples with similar material ratios. The y-direction strain cloud of the specimen changed uniformly during the compaction stage and the linear elastic stage. The compressive strain of the B2 specimen increased uniformly. There was a strain localization area in the upper right corner (Area I), which corresponded to the crack position of the specimen. No strain localization band appeared in the other regions. The strain cloud shows a downward strain (Area I) at the top, which corresponds to the strain localization area inside the B3 sample after compression. The uneven strain distribution of the A1 and A3 samples caused failure along the prefabricated discrete fracture network (DFN) cracks. Localized areas appeared in the center of the samples, corresponding to the surface cracks of the samples. A downward-inclined strain localization band that was 45 degrees inclined appeared in the strain cloud of specimen C, corresponding to the surface crack area. The strain nephogram analysis was combined with an investigation of various stages of specimen failure and the evolution of cracks.