# Study of Rock Crack Extension under Liquid Nitrogen Low-Temperature Fracturing
## 2 Visual Physical Experimental Analysis of Liquid Nitrogen Effect
### Crack Initiation and Propagation Process  
The visualization experiment of the liquid nitrogen effect was carried out on samples 1 and 2. The settings of the two samples were consistent, except for the difference in the liquid nitrogen impact amount and impact time during the test. During the test process of specimen 1, the liquid nitrogen shocked for 9 min and consumed 0.55 kg of liquid nitrogen  
Figure 5: Locations of the thermocouple tips and temperature evolutions during the cryogenic thermal shock experiments (sample 1). Figure 6: Temperature versus time during the unconfined low-pressure LN2 test. The sample picture (top right of the figure) is where the thermocouples were located on the sample (from [PERSON] et al. [16]). (about 0.68 L), and the failure rate (the ratio of crack propagation volume to total volume) was 0.084%. Meanwhile, in specimen 2, liquid nitrogen shocked for 6 min and consumed 1.47 kg of liquid nitrogen, and the failure rate was 0.375%. The reason for this difference is that liquid nitrogen is easily boiled at room temperature. Accordingly, liquid nitrogen mainly exists in the form of gas in the first few minutes of the experiment and then mainly occurs in the form of a gas-liquid mixture. Although the impact time of sample 2 was short, the liquid nitrogen in the borehole appeared more rapidly. Then, the temperature of sample 2 immediately decreased, which resulted in a higher thermal gradient and caused higher thermal stress and failure rates. Figure 7 shows the crack morphology and tensile stress diagram, where (a) and (b) are samples 1 and 2, respectively. Two different crack morphologies were observed: the horizontal plane crack and the vertical tensile crack. The two different crack growth modes were the horizontal plane radial propagation caused by longitudinal thermal shrinkage and the vertical propagation caused by circumferential shrinkage. The crack in the vertical direction originated or formed between the two horizontal cracks and bridged due to circumferential thermal shrinkage. The crack's strength was smaller compared with the crack in the horizontal direction. A certain interval length existed between the horizontal cracks because a group of cracks could not be formed within a certain length due to the limited thermal shrinkage. This kind of spacing crack also appears in other phenomena. The typical examples are the equal spacing crack of the layered rock after a long geological action, the equal spacing crack of the pavement structure under the action of temperature, ceramic cracks, rock frost heave, and ice splitting phenomena. In some rocks under certain temperature conditions, thermal shrinkage causes equidistant cracks, and the crack length and spacing also follow the law of power scaling. This characteristic can be preliminarily observed in sample 2. Figure 8 shows the crack propagation process diagram of sample 1. During the test, the crack germination and development were accompanied by clear fracture sounds. When the tensile stress generated in the borehole reached a certain threshold, the crack rapidly expanded in a short time (the first horizontal crack at the bottom expanded for only 0.1 s, and the second horizontal crack expanded for only 0.01 s). The propagation of low-temperature cracks was sudden due to the brittleness of the specimen at low temperature and the continuous accumulation and release of the tensile stress in the borehole at the crack tip. During the two tests, a major horizontal crack first appeared at the bottom of the borehole.