冻融焦煤力学行为特征及微观结构响应规律试验研究

Experimental investigation on mechanical behaviors and microstructure responses of the coking coal subjected to freeze-thaw cycles

为深入研究煤岩在冻融与载荷作用下的力学行为特征和细观结构响应规律,开展了冻融焦煤单轴压缩试验,试验全程采用声发射和DIC系统监测,采用扫描电镜及显微镜观测了不同冻融次数焦煤细观结构及断裂面形貌。结果表明,随着冻融次数增加,煤样质量持续增长,孔隙裂隙结构显著发育,单轴抗压强度及弹性模量逐渐降低。冻融次数增加,压缩煤样声发射活动逐渐增强,且压密阶段AE事件分布密度及强度显著增加;煤样压缩全程内部拉伸微裂纹占比较大(除冻融30次外);声发射b呈“V”型变化,揭示冻融焦煤裂纹扩展速度先增后减,加载过程中b值平稳降低特征逐渐显著,微裂纹尺度不断增大。冻融次数增加,压缩煤样耗散能占比逐渐增大,弹性能占比对应减少;冻融0~20次煤样多呈劈裂拉伸破坏,冻融30次后转为剪切破坏,煤样破坏时最大主应变由17.42%降至4.65%,断面形貌由粗糙逐渐趋于光滑。建立了冻融及单轴载荷作用下冻融焦煤单轴压缩损伤本构模型,所得理论曲线与试验结果吻合度较高。研究发现,自由水冻结造成孔隙裂隙空间扩展延伸、不断强化的水岩作用以及矿物颗粒间的相互作用是造成焦煤冻融损伤的主因。

In this paper,the uniaxial compression test of coking coal with different freeze-thaw(F-T)cycles was carried out,accompanied by the monitoring of the acoustic emission(AE)system and digital image correlation(DIC)system.The results show that with the increase of F-T cycles,there is the continuous increase and obvious development of the sample mass and the pore structure respectively,causing the deterioration of mechanical properties.In addition,the AE activity of the samples gradually increased,which is more obvious in the compaction stage;furtherly,the tensile microcracks in the rock during the compression test always account for a larger proportion(except for 30 F-T cycles).For the failure characteristics,the rock sample changes from splitting tensile failure to shear failure,and the fracture surface morphology changes from rough to smooth.The damage constitutive model of coking coal subjected to the F-T effect and axial load was established,which is in good agreement with the experiment results.Finally,it is found that the initial damage of coking coal subjected to F-T cycles could be attributable to the expansion of pore and fissure space caused by the freeze of free water,the increasingly enhanced water-rock interaction and the interaction between mineral particles.