液氮冻融循环作用下饱水煤样力学特性试验研究

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

液氮注入低渗煤层致裂、增透是提高瓦斯抽采率的新技术。液氮是超低温流体,与煤作用的温度应力引起煤结构损伤和力学特性改变,进而影响煤矿安全开采。为揭示液氮冻融循环作用对煤力学特性的影响,将取自于纳林河矿的原煤,制成饱水煤样,开展液氮冻融循环和力学特性试验。通过波速测试、单轴压缩试验和循环荷载试验测试液氮作用前后煤的波速、弹性模量、单轴抗压强度、泊松比、压密应变、破坏应变和疲劳寿命的变化,分析液氮冷冻和冻融循环次数对煤力学特性的影响规律。结果表明,液氮冷冻和液氮冻融循环次数对煤力学特性有很大影响。液氮冷冻后,煤样结构损伤,力学性能劣化。液氮作用1次,煤样波速、弹性模量、单轴抗压强度、破坏应变、疲劳寿命分别降低了32.9%、0.2%、29.9%、35.9%和22.7%,泊松比和压密应变分别增加了18.2%和2.1%。液氮作用3次煤样波速、弹性模量、单轴抗压强度、破坏应变、疲劳寿命分别降低了44.9%、20.8%、57.8%、44.7%和42.3%,泊松比和压密应变分别增加了59.1%和16.7%。随着液氮冻融循环次数的增加,煤样结构损伤和力学性能劣化加剧。单轴抗压强度、疲劳寿命与液氮冻融循环次数呈负指数函数关系,弹性模量和压密应变与液氮冻融循环次数呈线性关系。

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 drainage 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 turn 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 Nalinhe Mine was made into saturated coal samples.And liquid nitrogen freezing-thawing cycles experiments and the corresponding mechanical experiments were carried out.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 action 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 deteriorated.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 liquid 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 number of freezingthawing cycles of liquid nitrogen,the structural damage and mechanical property deterioration of coal samples are aggravated.The uniaxial compressive strength,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 freezing-thawing cycles of liquid nitrogen.