CHARACTERISTICS OF THE HETEROGENEOUS MECHANICAL RESPONSE OF COAL AT THE NANO AND MICRO-SCALE USING INSTRUMENTED INDENTATION EXPERIMENTS
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摘要: 煤炭是我国的主体能源, 煤矿井下冲击地压、煤与瓦斯突出等灾害的频繁严重影响煤炭的安全生产. 煤体是典型的混合物, 其内部不同组分的力学性质差异较大, 使其在外部扰动的作用下容易产生内部应力集中, 导致煤体的失稳、破坏, 形成煤矿动力灾害. 本文以非均质煤体为研究对象, 利用微焦CT、扫描电子显微镜和纳−微米压入实验, 研究了煤体微纳尺度的非均质结构和力学性质, 实验研究结果表明: 煤体是有机物和多种矿物组成的混合物, 矿物以点填充、丝状填充和条带状侵入等结构存在于煤体有机物中, 不同的矿物填充或侵入区域中矿物含量和结构具有差异, 这导致煤体微纳尺度的物理力学性质具有非均质性; 纳米尺度压入实验可以捕捉矿物在有机物中的填充或侵入结构, 测量煤体混合物中矿物和有机物单组分的力学参数, 识别两者力学性质的巨大差异; 微米尺度的压入实验可以表征煤体混合物整体的力学性质, 矿物填充量越多, 煤体混合物的力学性质越强, 同时煤体混合物微观尺度的破裂模式会受到矿物填充结构的影响. 研究结果揭示了煤体微观结构和力学性质的非均质特征, 探讨了煤体混合物的非均质结构可能引起的脆性破坏, 为煤矿井下冲击地压和煤与瓦斯突出等动力灾害的预测与防治提供了理论基础.Abstract: Coal is the main energy resource in China. The occurrence of coal and gas outburst, and rock burst in coal mines seriously impact the safety of coal production. Coal is a typical composite. The significant difference in mechanical properties among the components of coal can easily induce internal stress concentration under external stress disturbance. This may result in the instability and failure of coal and forming dynamic disasters in coal mines. This research focused on heterogeneous coal and studied the heterogeneous microstructures and mechanical properties of coal at the nano and micro-scale using micro-CT, scanning electron microscope, and instrumented nano and micro-indentation experiments. The experimental results indicated that: coal is composed of organics and multiple types of minerals with different mechanical properties. Minerals exist in coal as granular filling, filiform filling, and banded intrusion. The mineral content and structure were different among these mineral filling or intrusion areas. It led to the physical and mechanical heterogeneities of coal at the nano and micro-scale; The indentation experiments at the nano-scale could capture the mineral structures at mineral filling or intrusion areas and measure the individual mechanical parameters of minerals or organics. Their significant difference in mechanical properties could be identified by nano-indentation; The indentation experiments at the micro-scale could characterize the mechanical properties of a whole coal composite. The more mineral filling was, the stronger mechanical properties of a coal composite were. Moreover, the failure mode of a coal composite at the micro-scale depended on its mineral structure. The experimental results revealed the heterogeneous characteristics of the microstructures and mechanical properties of coal. The brittle failure induced by the special filiform structure of coal at the micro-scale was discussed. This study provides the fundamental understanding for the prediction and prevention of rock burst, coal and gas outburst during coal mining.
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图 2 不同微纳尺度仪器化压入实验的原理
Figure 2. Schematic diagram of instrumented indentation experiments at different scales
图 3 三维和二维微焦CT扫描形貌图(十字和数字表示SEM扫描的区域)
Figure 3. 3D and 2D micro-CT scanning images (The crosses and numbers represent the scanning areas of SEM)
图 4 两种煤样内部SEM观察图(黑色为有机物、白色为矿物)
Figure 4. SEM images of the two samples (the black areas represent organics, the white areas represent minerals)
图 6 3个实验区域纳米尺度压痕实验结果(载荷−深度曲线、压入模量分布)
Figure 6. Nano-indentation experimental results of three experimental areas
6 3个实验区域纳米尺度压痕实验结果(载荷−深度曲线、压入模量分布) (续)
6. Nano-indentation experimental results of three experimental areas (continued)
图 7 3个实验区域SEM扫描图和纳米尺度压入模量拼接矩阵(SEM中白色代表矿物、黑色代表有机物; 压入模量拼接矩阵中白色代表高压入模量、黑色代表低压入模量)
Figure 7. SEM images and modulus matrices of nano-indentation in three experimental areas (white areas represent minerals, black areas represent organics in the SEM images; white areas represent high indentation modulus, black areas represent organics in low indentation modulus in the modulus matrices)
图 8 随机纳米尺度压入实验结果(力学参数分布图)
Figure 8. Experimental results of random nano-indentation (distribution diagram of mechanical parameters)
图 9 B-1煤样不同区域微米尺度压入实验结果(载荷-深度曲线、压入模量分布), 从(a)至(c)矿物填充结构非均质性逐渐增加
Figure 9. Experimental results of micro-indentation in different areas of B-1 sample (loading and unloading curves, and indentation modulus distribution). The heterogeneity of the mineral structures is increasing from (a) to (c)
图 10 Z-1煤样不同区域微米尺度压入实验结果(载荷-深度曲线、压入模量分布), 从(a)至(c)矿物填充结构非均质性逐渐增加
Figure 10. Experimental results of micro-indentation in different areas of Z-1 sample (loading and unloading curves and indentation modulus distribution). The heterogeneity of the mineral structures is increasing from (a) to (c)
图 11 不同尺度条件下, 不同矿物填充结构区域的统计学力学参数
Figure 11. Mechanical parameters of different mineral filling areas at different scales based on statistical analysis
图 12 不同矿物填充结构中矿物代表尺度与微米级压痕作用面积的关系
Figure 12. The relationship between the representative size of minerals and the impact area of micro-indentation in different mineral filling areas
图 13 各矿物填充结构中典型的实验曲线(丝质组矿物填充表面B区域的加载曲线呈现出锯齿和阶梯状)
Figure 13. Typical curves of indentation experiments in different mineral structures. (The loading curves of filiform mineral embedding, surface B are zigzag and stagewise )
图 14 压入实验后不同矿物填充结构中的残余压入(丝质组丝状填充表面B的残余压入附近有明显的煤体破裂现象)
Figure 14. Residual indents of different mineral structures after indentation experiments (obvious breakages are found around the indent of filiform mineral embedding, surface B)
图 15 不同矿物填充或侵入结构压入作用下的破坏模式示意图
Figure 15. Sketches of the failure modes in different mineral embedding or intrusion structures under an indentation experiment
表 1 实验煤样工业分析
Table 1. Industrial analysis of experimental coal samples
coal sample moisture(%) ash content(%) volatiles(%) Z-1 4.53 17.55 33.41 B-1 1.88 27.35 11.90 
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表 2 压入实验方案
Table 2. Experimental scheme of indentation experiments
Experiment type Scale Force Experiment numbers grid indentation experiment nanoscale 10 mN 100 /225 random indentation experiment 81 random indentation experiment microscale 300 mN 49
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表 3 两块煤样中矿物填充的类别和结构
Table 3. Mineral types and structures in the two samples
Maceral Mineral type Mineral content Mineral structure vitrinite clay minor spot embedding quartz major spot embedding carbonate major banding intrusion inertinite
(fusinite)clay or quartz minor filiform embedding carbonate major filiform embedding
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