半圆盘三点弯曲法测定页岩断裂韧度(K_(IC))的实验研究
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摘要
为了准确测试页岩的I型断裂韧度K_(IC),分别采用直切槽半圆盘试样(NSCB)和人字形切槽半圆盘试样(CCNSCB),在3种预制切槽布置模式(splitter、arrester、divider)下,开展三点弯曲加载实验。通过标准差和变异系数分析两类方法 K_(IC)测算值的离散性,并评价各自适用性。使用光学显微镜观察试件破裂后的裂纹扩展路径和破裂面断口形态。基于断裂力学理论,给出页岩试件粗糙起伏断口表面能计算公式,分析页岩试件实测值K_(IC)离散的原因,并用最小耗能原理解释裂缝在页岩试件扩展过程中形成粗糙断裂面的机理。对比分析两种试样类型测试的K_(IC)值,认为两种方法在切槽平行层理布置的情况下测算值离散程度一致,二者都适用;对于切槽垂直层理两种布置方式下NSCB试件更容易发生裂缝偏转,导致产生Ⅰ-Ⅱ复合型断裂,使试验失效;CCNSCB试件的韧带具有良好的裂缝扩展导向作用,某种程度上抵消了加载配置非对称性带来的不利影响,CCNSCB比NSCB测得的K_(IC)值离散性更小,更适用于各向异性页岩的K_(IC)测试。
In order to accurately test the mode I fracture toughness(K_(IC)) of shale, three point bending loading experiments are carried out using notched semi-circular bend(NSCB) and cracked chevron notched semi-circular bend(CCNSCB) samples under three types of prefabricated notches(splitter, arrester and divider), respectively. According to statistical results of standard deviation and coefficient of variation, discreteness of each group of K_(IC) values is analyzed; and applicability of the two methods is valued respectively. The crack propagation path and fracture surface morphology are observed by optical microscope. Based on fracture mechanics, we give the calculation formulas of the rough undulating fracture surface of shale specimens. The reason for K_(IC) value scattering of the tested shale specimens is analyzed; and the formation mechanism of rough fracture surface in the expansion process is explained using the principle of minimum energy dissipation. By comparing the K_(IC) values of the two sample types, CCNSCB and NSCB methods yield K_(IC) values of similar discreteness when notches are parallel to bedding planes. When notches are vertical to layers, K_(IC) values from CCNSCB specimens is more consistent than that from NSCB. The ligaments of CCNSCB specimens are in favor of crack propagation, which counteracts the adverse influence caused by asymmetry loading configuration to some extent. Therefore, CCNSCB specimens are more suitable to test fracture toughness of anisotropic shale than NSCB.
In order to accurately test the mode I fracture toughness(K_(IC)) of shale, three point bending loading experiments are carried out using notched semi-circular bend(NSCB) and cracked chevron notched semi-circular bend(CCNSCB) samples under three types of prefabricated notches(splitter, arrester and divider), respectively. According to statistical results of standard deviation and coefficient of variation, discreteness of each group of K_(IC) values is analyzed; and applicability of the two methods is valued respectively. The crack propagation path and fracture surface morphology are observed by optical microscope. Based on fracture mechanics, we give the calculation formulas of the rough undulating fracture surface of shale specimens. The reason for K_(IC) value scattering of the tested shale specimens is analyzed; and the formation mechanism of rough fracture surface in the expansion process is explained using the principle of minimum energy dissipation. By comparing the K_(IC) values of the two sample types, CCNSCB and NSCB methods yield K_(IC) values of similar discreteness when notches are parallel to bedding planes. When notches are vertical to layers, K_(IC) values from CCNSCB specimens is more consistent than that from NSCB. The ligaments of CCNSCB specimens are in favor of crack propagation, which counteracts the adverse influence caused by asymmetry loading configuration to some extent. Therefore, CCNSCB specimens are more suitable to test fracture toughness of anisotropic shale than NSCB.
引文
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[13]戴峰,魏明东,徐奴文,等.岩石断裂韧度CCNSCB方法渐进破坏机制与无量纲应力强度因子宽范围标定[J].岩土力学,2016,37(11):3215-3223.DAI Feng,WEI Ming-dong,XU Nu-wen,et al.Progressive fracture mechanism of CCNSCB rock fracture toughness specimens and calibration of wide-range dimensionless stress intensity factors[J].Rock and Soil Mechanics,2016,37(11):3215-3223.
[14]CUI Z D,LIU D A,AN G M,et al.A comparison of two ISRM suggested chevron notched specimens for testing mode-I rock fracture toughness[J].International Journal of Rock Mechanics and Mining Sciences,2010,47(5):871-876.
[15]崔振东,刘大安,安光明,等.V形切槽巴西圆盘法测定岩石断裂韧度K_(IC)的实验研究[J].岩土力学,2010,31(9):2743-2748.CUI Zhen-dong,LIU Da-an,AN Guang-ming,et al.Research for determining mode I rock fracture toughness KIC using cracked chevron notched Brazilian disc specimen[J].Rock and Soil Mechanics,2010,31(9):2743-2748.
[16]WANG H J,ZHAO F,HUANG Z Q,et al.Experimental study of mode-I fracture toughness for layered shale based on two ISRM—suggested methods[J].Rock Mechanics&Rock Engineering,2017,50(7):1933-1939.
[2]LEE Y K,PIETRUSZCZAK S.Application of critical plane approach to the prediction of strength anisotropy in transversely isotropic rock masses[J].International Journal of Rock Mechanics and Mining Sciences,2008,45(4):513-523.
[3]袁俊亮,邓金根,张定宇,等.页岩气储层可压裂性评价技术[J].石油学报,2013,34(3):523-527.YUAN Jun-liang,DENG Jin-gen,ZHANG Ding-yu,et al.Fracability evaluation of shale-gas reservoirs[J].Acta Petrolei Sinica,2013,34(3):523-527.
[4]赵金洲,许文俊,李勇明,等.页岩气储层可压性评价新方法[J].天然气地球科学,2015,26(6):1165-1172.ZHOU Jin-zhou,XU Wen-jun,LI Yong-ming,et al.A new method for fracability evaluation of shale-gas reservoirs[J].Natural Gas Geoscience,2015,26(6):1165-1172.
[5]LI Q H,CHEN M,JIN Y,WANG F,WANG M Y.A calculation tool for hydraulic fracture networkpropagation in layered shale formation[J].International Review on Computers&Software,2012,7(5):2528-2533.
[6]OUCHTERLONY F.Suggested methods for determining the fracture toughness of rock[J].International Journal of Rock Mechanics and Mining Sciences&Geomechanics Abstracts,1988,25(2):71-96.
[7]FOWELL R J.Suggested method for determining mode I fracture toughness using cracked chevron notched Brazilian disc(CCNBD)specimens[J].International Journal of Rock Mechanics and Mining Sciences,1995,32:57-64.
[8]KURUPPU M D,OBARA Y,AYATOLLAHI M R,et al.ISRM—suggested method for determining the mode I static fracture toughness using semi-circular bend specimen[J].Rock Mechanics&Rock Engineering,2014,47(1):267-274.
[9]KURUPPU M D.Fracture toughness measurement using chevron notched semi-circular bend specimen[J].International Journal of Fracture,1997,86(4):33-38.
[10]AYATOLLAHI M R,MAHDAVI E,ALBORZI M J,et al.Stress intensity factors of semi-circular bend specimens with straight-through and chevron notches[J].Rock Mechanics&Rock Engineering,2015,49(4):1-12.
[11]WEI M D,DAI F,XU N W,et al.Experimental and numerical study on the cracked chevron notched semi-circular bend method for characterizing the mode I fracture toughness of rocks[J].Rock Mechanics&Rock Engineering,2016,49(5):1595-1609.
[12]CHONG K P,KURUPPU M D,KUSZMAUL J S.Fracture toughness determination of layered materials[J].Engineering Fracture Mechanics,1987,28(1):43-54.
[13]戴峰,魏明东,徐奴文,等.岩石断裂韧度CCNSCB方法渐进破坏机制与无量纲应力强度因子宽范围标定[J].岩土力学,2016,37(11):3215-3223.DAI Feng,WEI Ming-dong,XU Nu-wen,et al.Progressive fracture mechanism of CCNSCB rock fracture toughness specimens and calibration of wide-range dimensionless stress intensity factors[J].Rock and Soil Mechanics,2016,37(11):3215-3223.
[14]CUI Z D,LIU D A,AN G M,et al.A comparison of two ISRM suggested chevron notched specimens for testing mode-I rock fracture toughness[J].International Journal of Rock Mechanics and Mining Sciences,2010,47(5):871-876.
[15]崔振东,刘大安,安光明,等.V形切槽巴西圆盘法测定岩石断裂韧度K_(IC)的实验研究[J].岩土力学,2010,31(9):2743-2748.CUI Zhen-dong,LIU Da-an,AN Guang-ming,et al.Research for determining mode I rock fracture toughness KIC using cracked chevron notched Brazilian disc specimen[J].Rock and Soil Mechanics,2010,31(9):2743-2748.
[16]WANG H J,ZHAO F,HUANG Z Q,et al.Experimental study of mode-I fracture toughness for layered shale based on two ISRM—suggested methods[J].Rock Mechanics&Rock Engineering,2017,50(7):1933-1939.