Experimental design for teaching coal mechanical properties and crack evolution under liquid nitrogen freezing and freeze–thaw conditions

As an ultra-low-temperature medium, liquid nitrogen offers the advantages of low cost, fast freezing, and high freezing strength, which contribute to its wide use in urban underground engineering, tunnel construction, water plugging, mine construction, and production. In addition, liquid nitrogen freeze–thaw can promote the development of coal rock fractures and further increase the permeability of gas-bearing coal rock reservoirs. Therefore, it is highly important for freezing-applied construction and coalbed methane extraction engineering in cold regions to study the evolution of the mechanical properties of coal rock mass under liquid nitrogen freezing and freeze–thaw conditions.

In this study, experiments were conducted on the mechanical properties of coal under liquid nitrogen freezing and freeze–thaw conditions using a liquid nitrogen freezing coal rock experimental device. Using liquid nitrogen freezing time and liquid nitrogen freeze–thaw frequency as single-factor variables, the stress–strain curve was used to quantitatively analyze the changes in the mechanical parameters, including the compressive strength and elastic modulus of coal. The mechanisms of enhancing the mechanical strength of frozen coal and deteriorating the mechanical strength of the freeze–thaw coal were discussed based on the micropore and crack angles.

The experimental results on the mechanical properties of coal under liquid nitrogen freezing and freeze–thaw conditions showed the following: 1) liquid nitrogen freezing has a positive effect on the mechanical strength of coal. The elastic modulus of the coal sample after 80 minutes of liquid nitrogen freezing is 3.67 GPa, while the elastic modulus of the original coal sample is 1.40 GPa, with an increase of 165.69%. Liquid nitrogen freeze–thaw deteriorates the mechanical strength of coal. After 15 cycles of liquid nitrogen freeze–thaw, the elastic modulus of the coal sample is 0.52 GPa, while the original coal sample has an elastic modulus of 1.50 GPa, i.e., a decrease of 65.33%. 2) The formation of porous ice has three positive effects on enhancing the mechanical strength of coal. Porous ice has a certain mechanical strength, and the mechanical properties of ice continue to increase with the decrease in temperature. The volume expansion effect of the ice–water phase transition increases the contact area between ice and coal matrix particles, revealing the cementing role of pore ice formation. After the formation of pore ice, the length of pore cracks decreases, further decreasing the stress concentration at the crack tip, which hinders the development of the pore structure of the loaded coal. 3) The damage and deterioration of the coal body caused by liquid nitrogen freeze–thaw primarily contain three aspects. In liquid nitrogen freezing and thawing, the shrinkage expansion cycle of the middling coal matrix will eventually increase the porosity of the coal sample, damaging the interior of the coal. The volume expansion caused by the phase change of pore water and the formation of pore ice will promote the expansion and connectivity of primary fractures, and thermal stress will facilitate the formation of new pores inside the coal body, further damaging and destroying the coal structure. Freezing coal samples can reduce the stress concentration due to short cracks, while freeze–thaw coal samples can increase the stress concentration due to the increased length of internal cracks in the coal.

This experiment uses a combination of macro and microanalysis methods, with comparative and in-depth mechanism analysis as the research tools, to cultivate students’ flexible and innovative thinking to adapt to the changes in scientific research and disciplinary development. In terms of experimental organization, allowing students to participate directly in experiments requiring hands-on practice, data processing, and result analysis can greatly stimulate their learning enthusiasm and improve their subjective initiative and innovative thinking ability.