Experimental study on mechanical properties of saturated coal samples subjected to freezing-thawing cycles of liquid nitrogen

Liquid nitrogen injection into low-permeability coal seam to increase permeability and gas recovery of the coal bed is a new technology to improve gas dr ainage rate. As an ultra-low temperature fluid,the temperature stress acting on coal causes structural damage and changes in mechanical properties of coal,which in tur n affects coal mining safety. In order to reveal the influences of freezing-thawing cycles of liquid nitrogen on the mechanical properties of coal,raw coal taken from Nalin he Mine was made into saturated coal samples.And liquid nitrogen freezing-thawing cycles experiments and the corresponding mechanical experiments were carried ou t.The wave velocity,elastic modulus,uniaxial compressive strength,Poisson's ratio,compressive strain,failure strain and fatigue life strain of coal before and after the acti on of freezing-thawing liquid nitrogen were tested by wave velocity test,uniaxial compression test,and cyclic load test change,and the effects of liquid nitrogen freezing and freezing-thawing cycles on mechanical properties of coal were investigated.The results show that the number of liquid nitrogen cooling and freezing-thawing cycles have great influence on the mechanical properties of coal.After liquid nitrogen is frozen,the structure of coal samples is damaged and the mechanical properties are det eriorated. With one action of liquid nitrogen,the wave velocity,elastic modulus,uniaxial compressive strength,failure strain and fatigue life of coal samples were reduced by 32.9%,0.2%,29.9%,35.9% and 22.7% respectively. And Poisson's ratio and compression strain were increased by 18.2%and 2.1% respectively. Under the action of li quid nitrogen for three times,the wave velocity,elastic modulus,uniaxial compressive strength,failure strain and fatigue life of coal samples were reduced by 44.9% ,20. 8%,57.8%,44.7% and 42.3% respectively. The Poisson's ratio and compression strain were increased by 59. 1% and 16.7 %,respectively. With the increase in the numb er of freezingthawing cycles of liquid nitrogen,the structural damage and mechanical property deterioration of coal samples are aggravated.The uniaxial compressive str ength,fatigue life and the number of freezing-thawing cycles of liquid nitrogen have a negative exponential function,and the elastic modulus and compaction strain have a linear relationship with the number of freeing-thawing cycles of liquid nitrogen.