QIN L, LIU P F, LI S G,et al.Mechanical behavior of coal subjected to liquid nitrogen freeze-thaw cycles under triaxial stress conditions[J].Journal of Henan Polytechnic University(Natural Science) ,2026,45(5):136-144.

doi:10.16186/j.cnki.1673-9787.2024010035

Received:2024/01/12

Revised:2024/05/09

Published:2026-07-24

Mechanical behavior of coal subjected to liquid nitrogen freeze-thaw cycles under triaxial stress conditions

Qin Lei1,2, Liu Pengfei1,2, Li Shugang1,2, Wang Hui1,2, Mu Miao1,2, Lin Haifei1,2

1.College of Safety Science and Engineering， Xi’an University of Science and Technology， Xi’an  710054， Shaanxi， China；2.Key Laboratory of Western Mine Exploitation and Hazard Prevention， Ministry of Education， Xi’an University of Science and Technology， Xi’an  710054， Shaanxi， China

Abstract: Objectives This study aims to investigate the effects of different liquid nitrogen （LN₂） freeze–thaw treatments on the mechanical behavior of coal under triaxial stress conditions.  Methods Coal samples subjected to different LN₂ freeze-thaw treatments were tested using a triaxial servo-controlled loading system. The axial stress-strain response was analyzed to investigate the effects of LN₂ freeze-thaw treatments on mechanical properties， axial energy evolution， and pre-peak brittleness index.  Results The results show that： （1） Under the condition of identical absolute liquid nitrogen freezing time， the axial compressive strength and elastic modulus of frozen coal samples are greater than those of thawed coal samples. This is attributed to the combined effects of liquid nitrogen cryogenic action， pore water phase transition， and liquid nitrogen vaporization expansion. Furthermore， the damage degree of coal samples subjected to liquid nitrogen cyclic freeze-thaw is greater than that of single liquid nitrogen treatment. （2） As coal samples transition from frozen to thawed states， the closure strain， its rate of change， the reduction rate of compressive strength， and elastic modulus reduction rate all gradually decrease. （3） During axial loading， energy absorption and dissipation in frozen samples are higher than those in thawed samples. With phase transition， the proportion of elastic strain energy decreases while that of dissipated energy increases. （4） Different LN₂ freeze-thaw treatments lead to variations in water content， resulting in a higher pre-peak brittleness index in frozen samples than in thawed samples.  Conclusions The research findings can provide a theoretical basis and experimental data support for the engineering application of liquid nitrogen fracturing technology in coal seam permeability enhancement.

Key words:absolute freezing duration;LN freeze-thaw cycles;mechanical behavior;energy evolution;brittleness index