Some Fundamental Issues in Dynamic Compression and Tension Tests of Rocks Using Split Hopkinson Pressure Bar

Accurate characterizations of rock strengths under higher loading rates are crucial in many rock engineering applications. The split Hopkinson pressure bar (SHPB) system has been used to quantify the dynamic compressive strength of rocks using the short cylindrical specimen and the dynamic tensile strength of rocks using the Brazilian disc (BD) specimen. However, SHPB is a standard tool that is suitable for metal testing; there are some fundamental issues that need to be carefully visited in applying SHPB to rock dynamic tests. This paper addresses several such critical issues, including the choice of slenderness ratio of the compressive specimen, the effect of friction between the sample and bars on the measured results of compressive strength, the necessity of dynamic force balance on the dynamic BD test, and the validity of using the standard BD equation in the data reduction. We show that with proper experimental designs that address these issues, the dynamic compressive strength and dynamic tensile strength of rocks measured using SHPB are valid and reliable.     

延脆性变化对隧道掘进机刀具破岩过程及其破坏模式影响的颗粒元模拟分析

在全断面岩石掘进机（TBM）刀具破岩的颗粒元模拟中颗粒参数的选取至关重要,其中颗粒间平行黏结切向、法向强度是关键控制性参数之一,它们之间的比值关系直接决定所模拟试样的延脆性质,影响刀具破岩过程及其破岩效果。为探讨延脆性对刀具破岩模式的影响,（1）建立9种采用不同平行黏结强度比值的数值模型,分别进行单轴压缩及巴西劈裂模拟,研究不同延脆性试样的力学行为及破坏模式的变化。（2）对9种模型进行双刀破岩,并监控其裂缝的发展情况及刀具的受力状况。（3）为减小随机性对模拟结果的影响,通过改变随机数,每种模型重复模拟5次,综合分析5次的计算结果。模拟分析发现,随着切向和法向黏结强度比值( )的增大,试样的脆性增加,破坏模式逐渐从剪切破坏转变为脆性张拉破坏,刀具破岩压碎区范围减小,张拉裂缝更容易在刀具间贯通延伸从而切割出块体更大的岩渣;随着试样脆性的增加,归一化比能减小,刀具破岩的效率增加;平行黏结强度比值相同的条件下采用不同随机数种子生成的模型中,试样的具体破坏情况有一定的差别,但总体破坏模式相似。     

Discrete element modeling of tool‐rock interaction I: rock cutting

Tool‐rock interaction processes can be classified as indentation or cutting depending on the direction of motion of the tool with respect to the rock surface. The modes of failure induced in the rock by an indenting or a cutting tool can be ductile and/or brittle. The ductile mode is associated with the development of a damage zone, whereas the brittle mode involves the growth of macrocracks. This is the first part of a series of two papers concerned with an analysis of the cutting and the indentation processes based on using the discrete element method. In this paper, numerical simulations of the cutting process are conducted to reproduce the transition from a ductile to a brittle failure mode with increasing depth of cut, which is observed in experiments. The numerical results provide evidence that the critical depth of cut d _* controlling the failure mode transition is related to the characteristic length ? = (K_Ic ∕ σ_c)² with K_Ic denoting the material toughness and σ_c its unconfined compressive strength. The nature of frictional contact between the cutter face and the rock in the ductile failure mode is also examined. It is shown that the inclination of the total cutting force is controlled by a multi‐directional flow mechanism ahead of the cutter that is related to the formation of a wedge of failed material, intermittently adhering to the cutter. As a result, the inclination of the total cutting force varies with the rake angle of the cutter and cannot be considered an intrinsic measure of the interfacial friction between the cutter and the rock. Copyright © 2012 John Wiley & Sons, Ltd.     

锥形聚晶金刚石复合片齿破岩特征与机制研究

锥形聚晶金刚石复合片(polycrystalline diamond compact,PDC)齿是一种具有较强抗冲击性和耐磨性的新型PDC齿,在坚硬、强研磨性和软硬交错地层中取得了非常好的钻井提速效果。为了揭示锥形齿破碎硬岩机制,开展锥形齿破碎花岗岩试验与数值模拟研究,分析了切削深度和前倾角对锥形齿切削力和破岩比能的影响规律,采用高速摄像机和透明K9玻璃观测了锥形齿作用下岩屑形成过程及微裂纹萌生与扩展过程,通过数值模拟分析了破岩过程中岩石应力响应与损伤演化特性,结合对切削槽和大尺寸块体岩屑表面形貌及断口微观特征分析,建立了锥形齿破碎花岗岩机制模型。结果表明,锥形齿破碎花岗岩的过程可以分为挤压成核和块体崩裂两个阶段,前倾角对岩石破碎过程影响较小,切削深度的影响显著;锥形齿周围的裂纹主要由压实核、纵向裂纹和横向裂纹组成,纵向裂纹和横向裂纹扩展的最大深度分别为切削深度的6.69倍和4.53倍;齿尖周围压应力集中,岩石发生压剪破坏,压应力区外围形成弧形条带状拉应力区,并在齿尖及压应力区边界处诱导出拉伸微裂纹;微裂纹向齿前扩展形成弧形拉伸主裂纹,发生块体岩屑崩裂,提高破岩效率,向岩石内部扩展劣化...     

加锚岩石抗弯特性试验研究

采用煤层顶板岩石作为加锚基体,用钢丝模拟锚杆,薄钢片模拟钢带,对加锚体进行了三点弯试验,并与常规试件进行对比分析。结果表明,因岩石抗压与抗拉性能的差异,试件在弯曲过程中,下表面拉应变的增长速度大于上表面压应变的增长速度,裂纹最先在下表面产生并逐渐向上发展。加锚试件,因锚杆改善了锚固区域岩石的力学性能,且钢带与锚杆共同承担了一定的拉应力,锚固试件抗弯能力有所增强。试件截面应力状态可以结合破坏过程分为3个阶段,第1阶段为弹性阶段;第2阶段为裂纹产生与扩展阶段,岩石承载能力逐渐劣化,拉应力逐渐向钢带转移,岩石裂纹发展受限,试件抗弯能力有较大提升;第3阶段为破坏阶段,锚杆失黏,挠度持续增加而试件承载能力趋于稳定。     

Three-Dimensional Numerical Investigations of the Failure Mechanism of a Rock Disc with a Central or Eccentric Hole

The diametrical compression of a circular disc (Brazilian test) or cylinder with a small eccentric hole is a simple but important test to determine the tensile strength of rocks. This paper studies the failure mechanism of circular disc with an eccentric hole by a 3D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. First, numerically simulated Brazilian tests are compared with experimental results. Special attention is given to the effect of the thickness to radius ratio on the failure modes and the peak stress of specimens. The effects of the compressive strength to tensile strength ratio (C/T), the loading arc angle (2α), and the homogeneity index (m) are also studied in the numerical simulations. Secondly, the failure process of a rock disc with a central hole is studied. The effects of the ratio of the internal hole radius (r) to the radius of the rock disc (R) on the failure mode and the peak stress are investigated. Thirdly, the influence of the vertical and horizontal eccentricity of an internal hole on the initiation and propagation of cracks inside a specimen are simulated. The effect of the radius of the eccentric hole and the homogeneity index (m) are also investigated.     

Prediction of specific energy of carbonate rock in industrial stones cutting process

Specific energy (SE) measurements of circular saws were conducted on 12 different carbonate rocks. Rock samples were collected from the factories for laboratory tests. Bulk density, apparent porosity, uniaxial compressive strength, Brazilian tensile strength, flexural strength, Schmidt rebound hardness, Shore hardness, point load strength index, Los Angeles abrasion values, and P-wave velocity values were determined in the laboratory. SE and rock properties were evaluated using simple regression analysis and empirical equations were developed. The equations were verified by statistical tests. Regression analysis showed that high correlations exist between SE and uniaxial compressive strength, Shore and Schmidt hardness, bulk density, apparent porosity, and flexural strength. It was found that the SE value of rocks in cutting process was highest for those rocks having the high density, compressive strength, flexural strength, Schmidt and Shore hardness, point load strength index, and P-wave velocity values.     

Application of the Christensen Failure Criterion to Intact Rock

The Christensen criterion, originally introduced in materials science, has a simple mathematical form and uniaxial tensile and compressive strength as the only parameters, making it an attractive candidate for rock engineering purposes. In this study, the applicability of the criterion to rock materials is examined. Explicit equations for application of the criterion under biaxial, triaxial compression, triaxial extension, and polyaxial states of stresses are derived. A comprehensive strength data set including the results of tests on synthetic rock, chert dyke, Carrara marble and Westerly granite is utilized to examine the accuracy of the Christensen criterion to the failure of rock material. The two surprising findings about the Christensen criterion are the zero values of tensile strength and the very low slopes of the failure envelope obtained from fitting analyses for chert dyke and Westerly granite. It is shown that the two problems are interrelated and the values of tensile strength tend to zero to produce higher slopes. It is then mathematically proven that the maximum initial slope of the Christensen failure envelope is limited to 4 in triaxial compression and 2.5 in triaxial extension which is considerably lower than the slope of experimental data. The accuracy of the Christensen criterion was found to be significantly lower than the well-established Hoek–Brown criterion. The circular π-plane representations and brittle-to-ductile transition limits from the Christensen criterion are also inconsistent with the observed behavior of rocks.     

不同围压下岩石抗压强度与变形参数尺寸效应的数值模拟

岩石抗压强度和变形参数是岩石工程设计的重要指标。由于岩石是典型的非均质材料,其强度和变形特性随样品尺 寸的变化而不同。本文采用PFC2D程序模拟了不同围压下不同尺寸岩样的压缩试验。结果表明（1） 岩样具有明显的尺寸效 应。同一围压下,尺寸越大,岩石强度、峰值应变和压缩模量越小,尺寸的变化对岩样的破坏模式影响较小;（2） 岩样具 有明显的围压效应。同一尺寸的岩样,随着围压的增大,岩石强度、峰值应变和压缩模量均增加,其中强度和峰值应变随 围压的增加呈线性增加。同时,随着围压的增大,岩石破裂模式由轴向劈裂破坏向剪切破坏变化;（3） 围压的存在会影响 岩样的尺寸效应。不同尺寸岩样的强度和峰值应变在相同围压区间内的增量基本相同,同时随着围压的增大,其强度和峰 值应变增加,进而使岩石强度和峰值应变的尺寸效应弱化;而不同尺寸岩样的压缩模量在相同围压区间内的增长率大致相 同,因而造成围压对压缩模量尺寸效应的影响较小     

受弯条件下薄层灰岩的力学响应行为试验研究

鉴于薄层岩体在实际地下工程中经常发生弯折破坏,利用三点弯曲试验对单层灰岩及含弱面灰岩试样进行抗弯性能测试,探究岩层受弯作用时的变形及破坏特征。试验结果表明,薄层灰岩的抗弯性能与层厚密切相关,单宽峰值弯矩及峰值力对应的岩梁挠度随层厚变化规律基本符合材料力学理论公式计算结果。薄层灰岩弯折破裂模式呈现多样化,既有垂直切穿层理的I型裂纹,也有在弱面处发生转折并沿弱面扩展的断续裂纹,以及与层理斜交的复合型裂纹等。理论分析表明,薄层岩体中的裂纹扩展路径主要受层面拉、剪强度（或断裂韧度）与岩石抗拉强度（或断裂韧度）的相对大小控制。利用双模量理论结合弯曲试验求解岩石拉伸模量需要进行迭代计算,拉压模量相差越大,迭代次数越多,收敛值越偏离初值。试验所得规律有助于深入理解薄层岩体弯曲变形特征及弯折破坏机制,并可作为对薄层岩体受弯作用理论及数值计算的有益补充。     

粉砂质板岩力学特性及各向异性特性

为研究鄂西北地区板岩的力学特性及各向异性特征,开展了志留系粉砂质板岩的单轴压缩和巴西劈裂试验,分析了试样的力学特性和各向异性特征以及在不同荷载作用下的变形破坏模式,揭示了不同破坏模式的力学机制,并通过数值分析研究了不同板理面角度板岩边坡破坏模式及力学机制。研究结果表明,粉砂质板岩中的板理面是影响岩体力学行为的弱面,使得粉砂质板岩表现出明显的各向异性的特征。在单轴压缩条件下,粉砂质板岩在垂直板理面方向比平行板理面方向更易变形,变形量更大;平行板理面方向加载时试样破坏类型为竖向劈裂型张拉破坏,其实质是压杆失稳;垂直板理面加载时试样的破坏类型为穿切板理面的劈裂型剪切破坏;所测得的力学参数各向异性也较明显。在劈裂荷载作用下,粉砂质板岩的破坏模式主要有张拉劈裂破坏和沿板理面剪切破坏两种,所得抗拉强度在平行板理面方向上最大,在垂直板理面方向上最小,两个方向上的抗拉强度均小于抗压强度。由于板理面间的抗拉强度极低,在受到与板理面呈小角度相交的劈裂荷载作用时,容易产生沿板理面的张拉劈裂或拉剪破坏,在实际工程中应尽量避免板理面间的受拉破坏和沿板理面的拉剪破坏。在边坡工程中,板理面倾向、倾角对边坡破坏模式及力学机制有较大影响,边坡防护治理需充分考虑这一影响。上述研究为粉砂质板岩区岩质边坡防护治理以及其他岩石工程设计与施工提供理论依据与技术基础。     

岩石三维破坏数值模型及形状效应的模拟研究

应用RFPA3D分别模拟了理想光滑端部加载和限制性加载端部情况下不同形状的岩石在单轴压缩下的破坏过程。模拟结果表明,试样形状对岩石的抗压强度、变形特征以及破坏模式有很大影响。岩石试样的强度随着长宽比的增加而减小,当长宽比超过2.5以后单轴压缩强度趋于稳定。理想端部下,长宽比较大的试样主要是剪切破坏模式;而长宽比较小时,主要是因为拉伸引起破坏。端部效应是引起试样拉伸破坏的一个重要因素,但是即使采用光滑端部,试样的形状效应依然存在。长宽比的逐渐增加使岩石逐渐由延性破坏向脆性破坏转变。     

单轴压缩条件下含三叉裂隙类岩石试样力学特性的细观研究

三叉裂隙是自然界普遍存在的一种岩体缺陷形式,其对岩体的力学特性有重要影响。对含预制三叉裂隙的水泥砂浆试样进行室内单轴压缩试验,配合使用摄像机拍摄裂纹的起裂、扩展、贯通过程,通过数字图像技术处理获取试样的应变场云图,并结合PFC2D程序研究不同?、? 条件下试样的强度特征、裂纹模式和裂纹演化扩展规律。研究表明：三叉裂隙对试样单轴抗压强度有明显的削弱作用。当? 恒定为120°时,试样在? = 30°时达到最大抗压强度;当? 恒定为90°时,随?增大,试样抗压强度呈先减小后增大的趋势,且当? = 45°时达到最大抗压强度。试样产生的裂纹可分为3类,分别是张拉型裂纹(Ⅰ型裂纹)、剪切型裂纹(Ⅱ型裂纹)、混合型裂纹(Ⅲ型裂纹)。这3类裂纹通常从裂隙尖端开始产生,并且Ⅰ型裂纹沿加载方向扩展,通常未扩展至试样边界;Ⅱ型和Ⅲ型裂纹通常与加载方向呈一定角度扩展至试样边界。通过对裂纹的几何形态和组成宏观裂纹的微裂纹成分的分析,得知导致含三叉裂隙试样在单轴压缩条件下失效的是张拉破坏。数字图像技术得到的应变云图表明,当载荷达到一定阶段,裂隙尖端出现应力集中,微破裂开始发育并聚集成微破裂区,微破裂区扩大产生宏观裂纹。通过对主应变和剪应变云图分析,发现导致试样失效的是张拉破坏,剪应变在裂纹扩展过程中的影响较小。     

拉-压应力状态下脆性岩石强度及破坏机制颗粒流模拟

高地应力区地下岩体工程开挖常形成围岩拉-压应力状态,发生岩体张性破坏灾害。本文针对传统PFC平行黏结模型不能模拟脆性岩石高单轴压缩与拉伸强度比的问题,建立双抗拉强度参数的平行黏结强度准则,开展岩石拉-压数值模拟试验,得到了与物理试验接近的拉-压强度,实现了岩石高压拉强度比的模拟,并深入分析了破坏机制。研究结果表明随着围压的增加,破裂面倾角逐渐增大,由拉伸破裂转化为拉-剪破裂,发现了拉-压应力状态下破裂面处的雁行裂纹。根据细观颗粒位移场揭示了破裂面力学性质,随着围压的增加(破裂面倾角逐渐增大),破裂面张性逐渐减弱而剪性增强。可将拉-压应力状态下岩石损伤演化过程大致分为弹性变形阶段、稳定破裂发展阶段、不稳定破裂发展阶段和整体破裂阶段(峰后应力跌落及残余阶段)。围压较大时弹性变形和稳定破裂发展阶段相对较短,不稳定破裂发展阶段相对较长较剧烈,峰后残余阶段破裂面摩擦更强、应力波动较大。     

冻土的单轴抗压、抗拉强度特性试验研究

在寒区工程建筑物设计中,冻土的抗压、抗拉强度是两个重要的力学指标.在负温条件下,对粉质黏土、黄土和砂土进行单轴抗压和劈裂抗拉试验,研究冻土破坏时的破坏形态、破坏机理、应力-应变曲线和拉应力与径向位移关系曲线的形式,分析单轴抗压强度和劈裂抗拉强度的差异以及这两种强度随土质特性和温度的变化规律.试验结果表明：单轴载荷作用下试样破坏后呈鼓状,且表现为应变软化型塑性破坏特征;劈裂作用下产生沿直径向试样两侧延伸的裂缝,不同土质破坏后裂缝扩展的宽度和深度不同;冻土的抗压强度与抗拉强度均与负温存在很好的线性相关性,随温度的降低而增大;在相同温度条件下,冻土的抗压强度大于其抗拉强度;对于同一种冻土,其抗压强度的温度效应比抗拉强度的温度效应显著.本试验分析结果可为寒区工程的实际应用提供参考.     

切槽对TBM刀具破岩机制的影响研究

高压水射流与机械滚刀相联合破岩技术的出现,改变了传统隧道掘进机（tunnel boring machine,简称TBM）的作业方式。以高压水射流在滚刀两侧岩体切槽的破岩模式为研究对象,开展常截面滚刀压头贯入不同预切槽深度白砂岩板状试样的试验和数值模拟计算分析,对破裂后图像进行DIC分析,研究发现：切槽的存在,阻断了刀具贯入裂纹的拓展,使能量能够更加集中于压头下方的局部岩石块体,有利于形成“八”字形贯通裂纹,促进岩石的破碎;随着槽深增加,压头下方岩石内部的应力状态和力学响应分区逐渐过渡改变。槽深较大时,压头下方的力学响应区域在原有裂纹扩展区、弹性区之间增加了破坏过渡区,该区域内微裂纹被压密,区域内岩石存在较大变形,但未出现明显破坏;切槽后,滚刀压头下方的岩体破坏机制由无切槽试样挤压剪切为主导的径向裂纹拓展,演变为由刀具和切槽共同控制作用—拉伸剪切为主导的主裂纹扩展。     

A three-dimensional numerical investigation of the fracture of rock specimens containing a pre-existing surface flaw

Three-dimensional surface crack initiation and propagation in two kinds of heterogeneous rocks were numerically investigated via parallel finite element analysis using a supercomputer. Numerically simulated rock specimens containing a pre-existing flaw were subjected to uniaxial compression until failure. The initiation and propagation of wing cracks, anti-wing cracks, and shell-like cracks were reproduced by numerical simulations. The numerically simulated results demonstrate that the further propagation of wing cracks and shell-like cracks stop due to their wrapping (curving) behavior in three-dimensional spaces, even if the applied loads continue to increase. Furthermore, rock heterogeneity could significantly influence crack propagation patterns and the peak uniaxial compressive strengths of rock specimens. Moreover, anti-wing cracks only appeared in relatively heterogeneous rocks, and the peak uniaxial compressive strengths of the specimens were observed to depend on the inclination of the pre-existing flaw. Finally, the mechanism of surface crack propagation is discussed in the context of numerically simulated anti-plane loading tests, wherein it was identified that Mode III loading (anti-plane loading) does not lead to Mode III fracture in rocks due to their high ratio of uniaxial compressive strength to tensile strength. This finding could explain the lateral growth of an existing flaw in its own plane, which is a phenomenon that has not been observed in laboratory experiments.     

基于软硬石模板库的土石混合体细观损伤与力学特性分析

为了探究含软、硬两种岩性碎块石土石混合体的变形破坏过程与力学性质,发展了一种基于软硬石模板库的土石混合体建模方法。通过在PFC2D中建立符合原位结构特征的软硬石模板库,利用此模板库生成了不同含石量土石混合体颗粒流模型,对土石混合体进行了单轴压缩颗粒流模拟。结果表明：土石混合体破坏始于土-石接触面,土石介质分离大多因剪切破坏导致,加载前期微裂纹迅速增长,中、后期微裂纹增长速率减缓,但宏观变形渐趋显著;随着含石量的增加,土石混合体单轴抗压强度逐渐降低,块石的转动与侧向移动会加剧土石混合体的整体破坏;相同含石量下,随着软石比例的增加,单轴抗压强度呈下降趋势,试验后试样破裂率与单个软石颗粒的破裂程度均保持在较高水平,软石颗粒破裂是引起土石混合体单轴抗压强度降低的重要原因。     

Numerical modelling of heat shock-assisted rock fracture

This paper presents a numerical investigation on the effects of thermal shock as a pretreatment of rock prior to comminution. More specifically, the effect of heat shock-induced cracks on the uniaxial compressive strength of rock is numerically studied. The chosen constitutive model of rock employs a (strong) embedded discontinuity finite element formulation to describe cracks. The thermomechanical problem that governs the heat shock pretreatment of rocks is considered as an uncoupled problem because of a highly dominating role of the external heat influx. Two solution methods of the global problem are presented: an explicit-explicit dynamic scheme and an implicit-implicit quasi-static scheme. The model performance is tested in simulations on heterogeneous numerical rock samples subjected first to a heat shock pretreatment and then to a mechanical compression test. According to the results, the compressive strength of intact granite rock having the axial splitting failure mode can be substantially reduced by heat shock pretreatment.     

全断面岩石掘进机滚刀最优刀间距计算公式研究

基于剪切与张拉破岩机制,提出拉剪综合失效机制假设;建立了非协同切削与协同切削体积模型,推导了协同切削模式的滚刀比能耗理论模型;通过离散元仿真得到滚刀侵入岩石裂纹长度与贯入度的映射关系;推导出滚刀最优刀间距理论公式;以全断面岩石掘进机（TBM）回转切削试验台为基础,水泥模拟料为切削对象,通过多次压痕试验得到滚刀贯入度和岩石裂纹长度的趋势曲线,验证了仿真结果;通过滚刀滚动破岩进行了12组不同贯入度与刀间距的物理切削试验,统计滚刀做功与岩石破碎体积,拟合得到比能耗关系曲线,验证了最优刀间距理论公式的结论。研究表明：TBM滚刀最优刀间距计算公式综合考虑了岩石特性与滚刀结构特性,适应性较广泛;刀间距超过滚刀协同工作距离时,滚刀破岩以剪切破碎为主;刀间距小于2倍岩石裂纹长度时,张拉破碎的影响更加明显;随着贯入度的增加,最优刀间距逐渐增大,而最优滚刀间距S与贯入度P之比（S/P）值则逐渐减小。