液氮作用下冻结态与融化态煤体力学特征及劣化机理研究

Study on mechanical characteristics and deterioration mechanism of frozen and melted coal under the action of liquid nitrogen

液氮可通过改变煤层内部结构从而影响煤体力学性能。为研究液氮作用下煤体劣化机理,以冻结态和融化态煤样为研究对象,采用扫描电镜观测煤体内部裂隙形态,通过开展巴西劈裂试验、单轴抗压试验测得其力学特征变化。研究结果表明:单次冻结50 min以上,煤体抗拉、单轴抗压强度增加,随着冻结时间增长,煤体强度、弹性模量呈先增后减趋势;冻融循环次数增加,煤体力学性能降低,初期(0~10循环)降幅较大,后趋于平缓;冻融循环造成的煤体损伤更严重,融化态煤样更易形成致密裂隙网格;为评价不同状态煤体冻融损伤,计算相对弹性模量E,拟合冻融次数和相对抗拉强度σs、相对抗压强度σu的关系,随着冻融次数增加,E、σs和σu均在冻融初期大幅下降,后趋于平缓,且融化态煤体后期降低幅度大于冻结态煤体。

Liquid nitrogen can affect the mechanical properties of coal by changing the internal structure of coal seam.In order to study the mechanism of coal degradation under the action of liquid nitrogen,the frozen and melted coal samples are taken as the objects for research.The results show that the tensile and uniaxial compressive strength of coal body increases when frozen for more than 50 minutes at a time.With the increase of freezing time,the strength and elastic modulus show a trend of increasing and then decreasing;with the increase of freezethaw cycles,the coal body mechanics is reduced.The initial(0-10)cycles decline dramatically and the decline tends to be moderate.When the coal body damage caused by the freezethaw cycle is more serious,it is more likely for the thawed coal sample to form a dense fracture grid.In order to evaluate the freezethaw damage of coal in different states,the relative elastic modulus E is calculated,and the relationship between the number of freezethaw cycles and the relative tensile strengthσs and relative compressive strengthσu is fitted.It decreases sharply in the early stage of freezing and thawing,and then tends to be gentle.