煤样三点弯曲裂纹扩展及断裂力学参数研究

doi:10.13722/j.cnki.jrme.2020.0736

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摘要煤样裂纹尖端存在着微裂纹聚集的非线性过程区，其影响着试样强度及结构稳定性，并使得煤样断裂过程无法用经典线弹性断裂力学模型来描述。为了更好地认识煤样裂纹扩展非线性力学行为，本文开展以含中心切口煤样为测试对象的三点弯曲实验，实现煤样I型张拉裂纹缓慢扩展控制，综合采用声发射和数字图像匹配技术追踪可视化了煤样裂纹扩展全过程，基于等效线弹性断裂力学模型计算煤样断裂力学参数，分析煤样裂纹扩展准则及扩展速度，并与实验测试结果进行对比验证。结果表明：峰前阶段煤样裂纹尖端过程区不断发育，完全发育的过程区长度为7.5 mm，临界张开量为38.4 μm；而峰后裂纹扩展阶段过程区长度和临界张开量值保持不变，无应力约束裂纹长度逐渐增大，导致有效裂纹长度也随之变大。整个加载过程中绝大多数声发射事件定位于过程区内，表明声发射主要来源为过程区内的微裂纹活动。根据等效线弹性断裂力学模型计算得出煤样断裂韧度KIC均值为0.316 MPa·，临界能量释放率Gc值为39.6～40.4 N/m，这与由RILEM标准计算的煤样断裂能Gf = 36.2～39.6 N/m较接近。煤样峰后裂纹扩展满足考虑过程区影响的KI = KIC断裂准则，根据该准则得到理论CMOD–载荷曲线与实验结果吻合较好；且理论和实验结果均表明裂纹扩展速度是非线性的：随着裂纹长度的增大，裂纹扩展速度逐渐降低。

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	王笑然1
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	王恩元1
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	刘晓斐1
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	李楠3
	周鑫1
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关键词 ：采矿工程, 三点弯曲, 声发射, 数字图像匹配技术, 等效线弹性断裂力学, 过程区, 断裂力学参数

Abstract：Fracture of quasi-brittle materials such as coal involves a distributed microcracking region around the crack tip，called fracture process zone(FPZ). This non-linear FPZ influences both the strength and the stability of a coal structure，and often makes the linear elastic fracture mechanics(LEFM) fail to describe the fracture process of coal samples. To better understand the non-linear phenomena of coal fractures，three-point bending tests of coal beams with a center notch were performed. The integrated acoustic emission(AE) and digital image correlation(DIC) techniques were used to track the crack propagation during pre-peak and post-peak stages visually. Based on the equivalent linear elastic fracture mechanics(ELEFM)，the fracture parameters were calculated，and the fracture criterion and the crack propagation speed were also analyzed and discussed. Additionally，comparison between theory and test results was performed. The results indicate that，at the pre-peak regime，the FPZ of coal is gradually developing with a fully developed FPZ length of 7.5 mm and a critical opening displacement of 38.4 μm from three separate tests of the same material. During the crack propagation in the post-peak regime，the FPZ length and the critical opening displacement keep constant，while the traction-free crack length increases with CMOD，leading the increment of the effective crack length. Most of the AE events are located within or surrounding the FPZ，which indicates that the AE are mainly from the microcracking activities in the FPZ. Using ELEFM，the average calculated fracture toughness KIC is 0.316 MPa·，and the critical energy release rate Gc by Irwin¢s relation is in the range of 39.6–40.4 N/m which is very close to the fracture energy Gf = 36.2–39.6 N/m determined following a RILEM recommendation. The post-peak response during crack propagation follows the fracture criterion of KI = KIC if FPZ is considered，from which the theory CMOD-load curve agrees well with the experiment result. Both theory and experimental results show that the crack speed decreases as the crack length increases.

Key words： mining engineering three-pending bending acoustic emission(AE) digital image correlation(DIC) equivalent linear elastic fracture mechanics(ELEFM) fracture process zone(FPZ) fracture parameters

引用本文:

王笑然1，2，王恩元1，2，刘晓斐1，2，李楠3，周鑫1，2. 煤样三点弯曲裂纹扩展及断裂力学参数研究[J]. 岩石力学与工程学报, 2021, 40(4): 690-702.
WANG Xiaoran1，2，WANG Enyuan1，2，LIU Xiaofei1，2，LI Nan3，ZHOU Xin1，2. Three-point-bending test of crack propagation and fracture parameters of coal specimens. , 2021, 40(4): 690-702.

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http://rockmech.whrsm.ac.cn/CN/10.13722/j.cnki.jrme.2020.0736 或 http://rockmech.whrsm.ac.cn/CN/Y2021/V40/I4/690

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