Rock fracture characterization using the modified Arcan test specimen

This study presents an application of fracture mechanics to determine mixed-mode fracture properties of rock using the numerical and experimental methods. The modified version of Arcan specimen test was, in association with a special loading device, an appropriate apparatus for experimental mixed-mode fracture analysis. Using the finite-element results, correction factors were applied to the specimens and a polynomial fit was proposed to evaluate the stress-intensity factors of a modified version of Arcan specimen with a crack subjected to mixed mode loading. The finite element analyses of specimens were also studied for various materials, different thickness and crack length. The mixed-mod fracture-toughness tests were carried out by using the modified Arcan test specimens over a wide range of loading angles. Using the finite element results, non-dimensional stress-intensity factors applied to the fracture specimen. By employing experimentally measured critical loads and the aid of the finite element method, mixed-mode fracture toughness for rock under consideration determined. The fracture surfaces under different mixed-mode loading conditions were examined by optical and scanning electron microscopy (SEM) to gain insight into the failure surfaces.     

运用等效应力法建立压剪型危岩破坏判据的研究

基于断裂力学理论,构建压剪型危岩断裂力学模型。模型考虑了主控结构面的贯通段有水压和无水压两种情况,建立了两种情况下主控结构面的非贯通段等效应力场,结合D-P理论,建立了压剪型危岩的破坏判据。算例分析的结果表明:当贯通段无水压作用时,将有关参数代入破坏判据式,即可判定该危岩体是否失稳;当贯通段有水压作用时,危岩体的失稳破坏与水压应力在裂缝尖端产生的应力场距离r有关,只要求得r的具体值,就可以利用破坏判据式判断该危岩体是否失稳破坏。     

Microcrack fracturing of coal specimens under quasi-static combined compression-shear loading

Coal pillars are usually loaded under combined compression-shear stresses at underground coal mines. Their long-term stability is critical to the utilization of underground structures, such as underground reservoirs at coal mines. In this study, a modified rock property testing system was used to explore the mechanical properties of coal specimens under quasi-static combined compression-shear loading conditions. The acoustic emission technique was applied to investigating the microcrack fracturing of coal specimens at various inclination angles. The experimental results show that specimen inclination has remarkable effects on the microcrack initiation, microcrack damage and ultimate failure of the coal specimen. The failure mode of the coal specimen tends to transit from axial splitting to shear failure with increasing specimen inclination, and its peak strength is closely associated with the microcrack damage threshold. In practice, it is recommended to consider coal strength under combined compression-shear loading when using empirical pillar strength formulae so that the effect of pillar inclination can be included.     

类岩石脆性材料非闭合裂纹的Ⅰ-Ⅱ压剪复合型断裂准则研究

实际工程中,结构体裂纹常处于拉剪和压剪复合受力状态,研究适合于复合型裂纹的断裂准则和裂纹扩展机理具有重要的理论意义和实用价值。以Ⅰ-Ⅱ复合型裂纹为研究对象,基于线弹性理论,在考虑裂纹几何特征及受力形式的基础上,系统介绍了裂纹应力强度因子（SIF）的理论解。提出了适用于Ⅱ型断裂的径向剪应力准则和双剪应力准则。对于Ⅰ-Ⅱ复合型裂纹,提出用等效Ⅰ、Ⅱ型SIF比值与Ⅰ、Ⅱ型断裂韧度比值的关系判定裂纹断裂类型,并分别选择适合于Ⅰ、Ⅱ型断裂的断裂准则,计算了裂纹断裂扩展理论角度。理论断裂角与预制非闭合裂纹类岩石脆性材料压剪断裂试验结果符合得较好。     

不同赋存深度岩石的动态断裂韧性与拉伸强度研究

按照国际岩石力学学会试验规范以及工程岩体试验方法标准(GB/T50266-99),对不同赋存深度的玄武岩试件分别进行动态断裂韧性测试和单轴拉伸强度测试,得到动态断裂韧性与拉伸强度之间可能存在一定的关系;并从岩石破坏的力学机制角度,分析动态断裂韧性与拉伸强度之间存在联系的根本原因:两者均是由于岩石内部微裂纹受到拉应力作用而引起微裂纹的扩展、互相贯通,从而导致岩石的破坏。根据动态断裂韧性与拉伸强度之间可能存在的关系,可以由拉伸强度的测试结果推测试件的动态断裂韧性值,将大大简化动态断裂韧性测试的繁琐性。     

Strength of massive to moderately jointed hard rock masses

The Hoek-Brown(HB) failure criterion and the geological strength index(GSI) were developed for the estimation of rock mass strength in jointed and blocky ground where rock mass failure is dominated by sliding along open joints and rotation of rock blocks. In massive, veined and moderately jointed rock in which rock blocks cannot form without failure of intact rock, the approach to obtain HB parameters must be modified. Typical situations when these modifications are required include the design of pillars,excavation and cavern stability, strainburst potential assessment, and tunnel support in deep underground conditions(around s1/s ci 0.15, where s1 is the major principal compressive stress and s ciis the unconfined compressive strength of the homogeneous rock) in hard brittle rocks with GSI ! 65. In this article, the strength of massive to moderately jointed hard rock masses is investigated, and an approach is presented to estimate the rock mass strength envelope using laboratory data from uniaxial and triaxial compressive strength tests without reliance on the HB-GSI equations. The data from tests on specimens obtained from massive to moderately jointed heterogeneous(veined) rock masses are used to obtain the rock and rock mass strengths at confining stress ranges that are relevant for deep tunnelling and mining;and a methodology is presented for this purpose from laboratory data alone. By directly obtaining the equivalent HB rock mass strength envelope for massive to moderately jointed rock from laboratory tests,the HB-GSI rock mass strength estimation approach is complemented for conditions where the GSIequations are not applicable. Guidance is also provided on how to apply the proposed approach when laboratory test data are not or not yet available.     

数字图像相关法测定岩石Ⅰ-Ⅱ复合型裂纹应力强度因子

提出了一种通过数字图像相关方法测定岩石Ⅰ-Ⅱ复合型裂纹尖端位置和应力强度因子的试验方法。试验通过数字图像相关方法测定裂纹尖端区域位移场数据,将位移场数据带入极坐标系下位移场方程,计算裂纹尖端位置和应力强度因子。试验结果表明：采用该方法可以准确的测定岩石Ⅰ-Ⅱ复合型裂纹应力强度因子、裂纹尖端位置及裂纹扩展长度,解决了以往研究方法因不能准确测定裂纹尖端位置和扩展方向,而无法准确测定岩石应力强度因子的难题。     

泥巴山隧道流纹岩加卸围压力学特性研究

 为了给泥巴山隧道开挖稳定性分析提供依据,选取钻孔深部的流纹岩,以隧道围岩实际应力环境为基础,结合单轴压缩破坏试验成果,进行常规三轴压缩破坏试验以及卸围压破坏试验。对流纹岩在不同试验条件下的应力–应变全过程曲线进行分析和比较,结果表明：在隧道部分深埋段,岩体的微裂隙也较发育;随着埋深的增加,岩石峰值强度有逐渐增大的趋势;完整的流纹岩抗压强度高且脆性特征明显;在三轴压缩试验时,流纹岩具有明显屈服特征,但经过很大的塑性变形后才发生宏观破坏,随着围压的增大,岩石从张性破坏向剪切破坏变化,破裂面倾角变缓;卸围压更容易造成岩石破坏,并且在卸围压破坏时其抗剪断参数c值比加围压破坏时下降48%,而 值增加11%。     

中心孔径对岩石动态断裂韧度测试值的影响

 为了考察圆盘试件不同中心孔径对岩石动态断裂韧度测试值的影响,采用直径80 mm含有不同中心圆孔孔径的圆孔裂缝平台巴西圆盘试件,在Hopkinson压杆系统上进行径向冲击试验,获得岩石的动态断裂韧度。结果表明,当中心圆孔孔径与圆盘直径之比r0/R∈(0.10,0.30)时,岩石动态断裂韧度的平均值为4.57 MPa•m1/2,测试值受中心圆孔孔径变化的影响并不明显。试件的断裂模式有一定差异,当圆孔孔径较小时,在主裂纹扩展的过程中萌生较多的次生裂纹;随着孔径的增大,次生裂纹减少,试件呈现更加明显的宏观拉贯通破坏。对于推广中心圆孔裂缝平台巴西圆盘测试岩石动态断裂韧度的方法,以及掌握岩石受到动态冲击时的破坏特性具有重要的意义。     

层理方向对砂岩断裂模式及韧度的影响规律试验研究

为了探索具有层理面的砂岩断裂力学性质的各向异性,开展了具有不同层理方向的半圆形砂岩试样在不同切缝角度下的三点弯试验研究,揭示了层理方向对砂岩应力强度因子、断裂韧度及破裂模式的影响规律。试验结果表明试样破裂模式受层理面与荷载方向夹角θ控制:θ=0°时,沿层理面张裂破坏;θ=30°时,沿层理面剪切破坏;θ=45°,60°时,切层和沿层理面混合破裂;θ=90°时,切层破坏。不同层理角度的试样测得的断裂韧度差异较大,切缝角α=0°时,θ=90°试样断裂韧度最大,θ=0°试样断裂韧度最小,且KImax/KImin=2.36。运用有限元计算了各试样的无量纲化应力强度因子,结果表明切缝角α=0°时,无量纲化II型应力强度因子YII受层理面与荷载方向夹角θ影响显著:θ=0°,90°试样YII=0,呈现I型断裂;θ=45°,60°试样YII≠0,呈现出I-II复合型断裂;θ=30°试样YII最大,以II型断裂占主导,其余切缝角度下试样无量纲化I型应力强度因子与II型应力强度因子随层理角度θ的变化呈现不同的变化规律。通过扩展有限元XFEM计算出的试样起裂角、断裂韧度及断裂路径与试验结果吻合较好,结果表明各试样的起裂角随层理面与荷载方向夹角θ及切缝角α的变化呈现一定的各向异性。试验所得规律有助于更全面理解具有层理面岩石的断裂特性,并可作为对各向异性岩石断裂力学理论研究和数值计算的有益补充。     

利用测井资料预测岩石II型断裂韧性 的方法研究

 根据直缝巴西圆盘(SNBD)试验获取应力强度因子的计算原理,设计组建一套岩石II型断裂韧性测试系统,测定20块取自WG油田岩石试样的II型断裂韧性。基于试验数据,分析II型断裂韧性和围压、抗拉强度的关系,建立利用测井资料预测断裂韧性的模型。利用H341井声波测井、密度测井和伽马测井资料预测水平地应力和抗拉强度,结合建立的断裂韧性模型成功预测II型断裂韧性连续值,并在压裂实践中得到验证。该方法解决了现场压裂作业缺少断裂韧性全井筒连续数据的难题。     

水压诱发裂缝拉伸与剪切破裂的理论模型研究

 基于断裂力学理论,分别研究水压诱发的拉伸与剪切破坏特征。在水压诱发拉伸破坏方面,建立考虑井筒影响的地应力与水压耦合下的射孔尖端裂缝应力强度因子计算方程,推演该条件下拉剪复合应力状态下的断裂准则以及临界水压和启裂角的计算式。运用该方法从理论上分析了井筒周围存在对称的2条裂纹时,启裂水压和角度受射孔倾角、射孔长度、井筒半径及主应力差的影响。研究结果表明,依据推演的拉剪复合型断裂准则计算得到的射孔启裂水压和角度与试验结果吻合良好。参数分析表明：随着射孔长度或井筒半径的增加,临界水压减小。在水压诱发剪切破坏方面,则根据裂纹面的应力状态,分别推导裂纹处于压剪闭合状态和压剪不闭合状态下,水压诱发剪切破坏的断裂准则,并考虑非奇异应力对剪切断裂的影响,进一步推导压剪闭合应力状态下发生剪切破坏的临界水压计算公式。     

A pore crack model for the mechanical behaviour of porous granular rocks in the brittle deformation regime

A model is developed for predicting and simulating the mechanical behaviour and failure mode of brittle porous granular rocks loaded in compression. It is based on a fracture mechanics analysis applied to cracks emanating from the surface of cylindrical pores in two-dimensional, which is well suited to the microstructure of such rocks. It is also consistent with the usual experimental procedure used for biaxial compression tests since the numerical scheme is implemented under the assumption of imposed axial strain. The model takes into account interactions between neighbouring cracks, which grow when their stress intensity factor reaches the fracture toughness of the rock. The simulation of crack growth from cylindrical holes, associated with a failure criterion based on the coalescence of interacting cracks, allows one to calculate the critical stress at rupture and to derive theoretical stress–strain curves.The present model is then used to compare theoretical results with laboratory data obtained on four sandstones with porosity ranging between 13% and 25.5% which were deformed under conventional triaxial compression conditions at confining pressures between 0 and 35 MPa. The comparison shows that by using a small number of parameters (pore size, pore density, fracture toughness, and elastic moduli), the model is able to predict the rock behaviour during the compression tests and the stress level at rupture in a quite accurate way, when the microstructural parameters introduced are consistent with the observations.     

深部高应力圆形隧洞内部卸荷条件下岩爆模拟试验和强度弱化效应研究

为了模拟深部高应力圆形隧洞在内部卸荷条件下洞壁发生岩爆的过程,以具有中等岩爆倾向性的红砂岩作为试验材料,利用TRW–3000岩石真三轴电液伺服诱变试验系统,对100 mm×100 mm×100 mm立方体红砂岩试样开展了先加载后钻孔卸荷条件下的岩爆模拟试验。试验模拟500 m深度的二维应力状态,首先对试样加载至设定的初始应力状态,然后利用自主研发的岩石钻孔卸荷试验装置进行岩石内部钻孔卸荷(孔洞直径为25mm),之后在竖直方向加载至洞壁发生破坏,达到模拟效果后主动卸载。试验过程中,利用微型摄像机监控并记录洞壁的整个破坏过程。为了对比卸荷作用的影响,对预先贯穿孔洞(孔洞直径为25mm)的同尺寸红砂岩试样开展了先开孔后加载条件下的岩爆模拟试验。试验结果表明,2种试验条件下均可实现岩爆过程的模拟,整个试验过程均可以划分为平静阶段、颗粒弹射阶段、岩片剥落阶段,最终在洞壁两侧形成V型槽。与先开孔后加载试验相比,先加载后开孔试验中洞壁的初始破坏应力较低,洞壁更容易发生破坏,并且洞壁发生岩爆破坏的严重程度较强,产生的岩片尺寸较大,洞壁剥落岩片的总质量较高,形成的V型槽深度较深,破坏范围较广。试验对比结果表明,高应力岩石内部卸荷会对围岩造成一定程度和范围的损伤,诱发围岩产生明显的强度弱化效应。     

钻爆施工条件下岩溶隧道掌子面突水机制及最小安全厚度研究

岩体裂纹的水力劈裂破坏是岩溶隧道突水灾害发生的主要影响因素之一。从断裂力学角度分析了在钻爆施工条件下爆炸应力波对含水裂纹岩体扩展的影响,结果表明爆炸应力波通过增大孔隙水压力影响裂纹扩展模式;通过建立的岩体含水裂纹扩展计算模型,推导了在爆炸应力波作用下裂纹发生压剪扩展破坏时的临界水压力Pc,并被实例所验证。基于理论分析、数值试验及工程实例,得出了岩体含水裂纹压剪扩展破坏突水存在滞后效应,施工人员可利用这一特点,在发生突水灾害前,及时停工并迅速撤离,保证人员与机械设备的安全。最后,通过对岩溶隧道掌子面与高压含水体之间裂隙岩体的防突最小安全厚度的研究,提出"两带"理论,推导了合理反映爆破开挖扰动与水压作用下裂纹岩体最小安全厚度计算公式,并为工程实例所验证,其结果可为高风险岩溶地区隧道突水预测及防治提供理论依据。     

Shear testing on rock tunnel models under constant normal stress conditions

Large shear deformation problems are frequently encountered in geotechnical engineering. To expose the shear failure mechanism of rock tunnels, compression-shear tests for rock models with circular tunnel were carried out, including single tunnel and adjacent double tunnels. The failure process is recorded by the external video and miniature cameras around the tunnel, accompanied by real-time acoustic emission monitoring. The experiments indicate that the shearing processes of rock tunnel can be divided into four steps: (i) cracks appeared around tunnels, (ii) shear cracks and spalling ejection developed, (iii) floor warping occurred, and (iv) shear cracks ran through the tunnel model. Besides, the roughness of the sheared fracture surface decreased with the increase in normal stress. Corresponding numerical simulation indicates that there are tensile stress concentrations and compressive stress concentrations around the tunnel during the shearing process, while the compressive stress concentration areas are under high risk of failure and the existence of adjacent tunnels will increase the degree of stress concentration.     

The influence of water on the shear behaviors of intact sandstone

The shear behavior of rock is markedly affected by the presence of water. To study the influence of water on the shear behaviors of rock, a series of direct shear tests were conducted on intact sandstone specimens with different water soaking durations (from dried to saturated) under three levels of normal stresses. Before direct shear tests were performed, water absorption experiments were conducted to guide the different levels of water saturation required for the direct shear tests. The influences of water on shear stress-shear strain responses and peak and residual shear strength envelopes were investigated. The results show that soaking duration significantly impairs the peak and residual shear strengths and other shear properties (e.g., cohesion, friction angle and shear stiffness). The reductions in fracture energy and friction coefficient and the chemical and corrosive deterioration are the main mechanisms for the decrease of intact sandstone’s shear strength. A novel observation is that the failure patterns of specimens are sensitive to different water distributions. The results provided in this study reveal that the stability of rock engineering is affected by changes in water levels.

     

中国深层岩石力学研究及在石油工程中的应用

论述了深层岩石力学的研究范围、特点，介绍了深层岩石力学的全尺寸钻井模拟试验，地应力、岩石断裂韧性、动静态岩石力学参数获取方法和技术，以及在钻井过程中的井壁稳定预测检测技术、水力压裂的室内物理模拟技术、数值模拟技术等问题的研究进展，并提出了深层岩石力学面临的挑战和需要解决的技术问题。     

基于半圆弯拉试验的煤样抗拉及断裂性能研究

 为研究准静态加载条件下煤的抗拉及断裂性能,采用巴西圆盘劈裂法和半圆弯拉法对煤样进行抗拉性能对比测试;并开展不同切缝深度的半圆弯拉煤样断裂性能测试分析,探讨平面应变断裂韧度KIC和J积分断裂韧度对评价煤的断裂性能的适用性,研究切缝深度对半圆弯拉煤样的2种断裂韧度测定结果的影响;并结合受载过程中煤样表面应变监测和破坏后试样的工业CT扫描图像分别对煤样受载变形特征和破坏后裂纹展布规律进行分析。研究表明：半圆弯拉试验更适于测定煤的抗拉强度;当量纲一化的切缝深度 = 0.28时半圆弯拉煤样平面应变断裂韧度KIC离散度最小;煤样的J积分断裂韧度离散度更小,且更适用于评价煤的断裂性能。