基于CT三维重构的深部煤体损伤演化规律

Characteristics of damage evolution of deep coal based on CT three-dimensional reconstruction

为研究不同应力条件下深部煤体损伤演化规律及破坏机理,以平煤十二矿己15-17220工作面的深部煤体为研究对象,进行了单轴压缩的实时CT扫描实验,结合细观统计损伤力学,提出了一种基于CT图像灰度值定义损伤变量的方法,定量分析了煤样单轴压缩过程中损伤演化规律。通过CT扫描实验、压汞实验和室内基本力学实验,建立了能够反映固体基质分布的深部非均质煤样的三维数值几何模型,进行了合理的网格划分,确定了不同材料组分的本构模型及其物理力学参数,在位移控制加载条件下开展了煤样单轴压缩的数值模拟,定性研究了煤样单轴压缩过程中的损伤演化规律及破坏机理。进一步,在单轴压缩数值模拟基础上,通过对煤样施加不同的环向应力,进行了5种不同围压条件下煤样三轴压缩的数值模拟,从应力-应变曲线形态、煤样破裂形态及破裂角大小等方面定性分析了三轴压缩条件下深部煤体损伤演化规律。结果表明:单轴压缩数值模拟的应力-应变曲线及损伤演化特征与CT实时扫描实验得到的结果具有较好的一致性。随着轴向应力的逐渐增加,煤样损伤依次经历了零损伤阶段、局部损伤产生阶段、损伤线性和非线性稳定增长阶段和损伤加速增长致使完全破坏阶段。试件最终破坏时其最大剪切应变率区域及塑性区都近似平行或垂直于煤基质和煤杂质的交界面,且损伤发生的两个主破坏面相互垂直。单轴压缩的整个过程煤样主要发生拉伸破坏,屈服应力后由于煤样的不均匀变形才发生剪切破坏。基于CT重构的煤样三轴压缩的数值模拟得到的损伤演化特征和经典的岩石损伤演化的6个阶段能够很好的吻合,煤样主要发生剪切破坏;随着围压的增大,峰值强度、扩容点应力和残余强度均逐渐增大,破裂角逐渐减小,破裂角与围压之间近似呈负线性相关。在数值模拟的网格划分、几何模型建立、材料参数和本构模型的选取以及应力应变的计算方法等方面做出了优化,取得了较好的数值模拟效果,能够消除实验样品差异性带来的影响,且能够直观准确地定性描述单轴和三轴压缩过程中的损伤演化规律。

In order to study the damage evolution characteristics and failure mechanism of deep coal under different stress conditions,the coal at the No.17220 working face of the Ji-15 coal seam in No.12 Pingdingshan Coal Mine was sampled for this study.The CT real-time testing of uniaxial compression was carried out.Combining with the meso-statistical damage mechanics,a method of defining the damage variable based on the gray value of CT images was proposed to quantitatively analyze the damage evolution characteristics during the uniaxial compression of coal samples.Based on CT scanning experiment,mercury intrusion experiment and indoor basic mechanics experiment,a three-dimensional numerical geometric model of deep heterogeneous coal sample was established to reflect the distribution of solid matrix.And,a reasonable meshing was carried out.In addition,the constitutive models and physical and mechanical parameters of different material components were determined.Initially,the numerical simulation of uniaxial compression was carried out to qualitatively study the damage evolution and failure mechanism of coal samples using displacement controlling loading.Furthermore,on the basis of numerical simulation of uniaxial compression,the triaxial numerical simulations of coal samples under five different confining pressure conditions were carried out by applying different circumferential stresses to the coal sample.The damage evolution of deep coal under triaxial compression was qualitatively analyzed from the aspects of stress-strain curve,fracture morphology and fracture angle of coal sample.The results show that the stress-strain curve and damage evolution characteristics obtained by the numerical simulation of uniaxial compression are in good agreement with the results obtained by CT real-time scanning experiment.With the increase of axial stress,the meso-damage evolution of coal sample occurs in four stages:the zero damage stage,the local damage generation stage,the damage linear and nonlinear steady growth stage and the accelerated damage growth stage.When the specimen is destroyed,its maximum shear strain rate region and plastic zone are approximately parallel or perpendicular to the interface between the coal matrix and the coal inclusion,and the two main damage surfaces are perpendicular to each other.The coal sample mainly undergoes tensile failure during uniaxial compression,and the shear failure occurs after the yield stress due to the uneven deformation.The damage evolution characteristics under triaxial compression obtained by numerical simulation based on CT reconstruction can be well matched with the six classical stages of rock damage evolution.The coal sample mainly undergoes shear failure.With the increase of confining pressure,the peak strength,the stress at the expansion point and the residual strength of coal all increase.The rupture angle decreases with the increasing confining pressure,and displays an approximately negatively linearly curve with the confining pressure.In this paper,the numerical simulation was optimized for meshing,establishment of geometric model,determination of material parameters and constitutive models and calculating method of stress and strain,and had achieved some good results of damage evolution.The numerical simulation method also can eliminate the influence of the difference of experimental samples,and can qualitatively describe the damage evolution law under uniaxial and triaxial compression intuitively and accurately.