断裂韧度试样CCNBD宽范围应力强度因子标定

国际岩石力学学会（ISRM）在1995年提出了一种新型岩石断裂韧度试样--人字形切槽巴西圆盘试样CCNBD,但是,其断裂韧度计算公式中的重要参数（即无量纲应力强度因子的标定）仍存在问题。采用一种新的分片合成方法并结合有限元法,参照ISRM给出的CCNBD试样的尺寸限制,对该范围试样的应力强度因子进行了宽范围的标定,以便在试验中能因地制宜地选用不同几何参数的CCNBD试样。结果表明：分片合成方法的计算值有很高的精度,不但减少了工作量,也使标定的无量纲应力强度因子比现有文献值更加准确、可靠。     

CCNBD断裂韧度试样的SIF新公式和在尺度律分析中的应用

根据国际岩石力学学会于1995年推荐的一种测试岩石断裂韧度的新型试样－人字形切槽巴西圆盘试样,对其断裂韧度计算公式中的关键参数即无量纲应力强度因子（SIF）提出了一个改进的计算公式。采用分片合成方法结合有限元法对CCNBD试样的应力强度因子进行了宽范围标定,结果以表格的形式给出;并采用数据线性回归的方法,将标定结果以一个指数函数的形式给出。结果表明,与标定值相比,无量纲应力强度因子新公式的误差较小,并且囊括了CCNBD试样的较宽范围的尺寸,且查表使用方便,也为理论分析提供了条件。在此基础上,对岩石断裂韧度测试的尺度律进行了更进一步的探索,结果表明,利用新公式进行的尺度律分析是有效的。     

V形切槽巴西圆盘法测定岩石断裂韧度KIC的实验研究

采用国际岩石力学学会岩石断裂韧度建议测试方法(ISRM)[1]提出的V形切槽巴西圆盘试样(CCNBD),测试了一种泥质砂岩的I型断裂韧度值,给出了一套试样切割制备方案,从试验现象角度分析了该泥质砂岩的断裂力学特性,讨论了该试样类型的有效尺寸和断裂机制,并指出了该方法的特点和优劣性,得出如下结论：（1）该类岩石试样测得的I型断裂韧度值对CCNBD试样直径尺寸变化具有较大的敏感性,并且直径大于ISRM建议方法中最小有效直径（75 mm）的试样测试结果更为稳定;（2）CCNBD试样断裂机制表现为以拉张应力（间接拉伸）作用为主,兼有一定的韧带面内剪切作用的应力状态下I型裂纹扩展模式;（3）V形切槽巴西圆盘方法具有试样加工工艺简单、能承受较大临界载荷、测试的I型断裂韧度值较稳定等优点,但其没有考虑断裂过程区(FPZ)的非线性问题,建议对该方法进行非线性修正图解方案研究,以达到更准确测定岩石断裂韧度的目的。     

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

国际岩石力学学会建议了4种岩石I型断裂韧度（KIC）测试方法。将建议方法的人字形切槽巴西圆盘试样与直切槽半圆盘试样结合,可以得到具有诸多优点的人字形切槽半圆盘（CCNSCB）三点弯曲试样。近年来,CCNSCB方法受到许多关注,然而,其渐进破坏过程却尚未进行有效的评估。为此,对其进行了数值研究,其内容包括：进行细观损伤力学模拟,直观展现CCNSCB试样渐进断裂过程;考虑不同支撑跨距与直径之比（ ）的影响,发现 愈大,愈加符合测试原理,建议 取0.8;采用有限元子模型技术对CCNSCB方法（ 0.8）中计算KIC的关键参数-临界无量纲应力强度因子（ ）进行了宽范围标定,可供相关研究直接查取;细观损伤力学模拟峰值力对应的临界裂纹与有限元标定 对应的临界裂纹较为一致,证明CCNSCB方法测试原理的合理性,以及数值模拟与 标定结果的有效性。     

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。     

岩石巴西圆盘试验中的空间拉应力分布

指出目前人们使用了40多年的岩石巴西圆盘试验拉伸强度公式来自二维弹性力学解答,该公式不适用于实际情况所对应的三维弹性力学问题。为了得到三维条件下试样内部的应力分布规律,采用三维有限元分析软件Marc对试样进行了弹性受力分析,结果表明,试样横截面上的拉应力分布规律与二维条件下的情况相类似,但横截面上的应力值沿试样厚度方向是有变化的,越靠近两端面,水平拉应力越大。分析发现,对于高径比为1、泊松比为0.25的巴西圆盘试样,按二维公式计算的抗拉强度比真实值小23.3 %     

Ⅰ型断裂韧度模拟方法及细观影响因素研究

既有离散元参数敏感性分析大多集中在压缩试验及巴西劈裂试验,对I型断裂韧度 试验细观影响因素及3D破裂过程系统分析的报道较少。采用三维平节理模型（FJM3D）研究微观结构参数及黏结细观参数对不同切槽形状的I型断裂韧度试验的影响。微观结构参数包括晶粒平均半径的平方根 、模型分辨率Ψ和最大/最小晶粒直径 。黏结细观参数包括平均配位数CN、S类型单元比例 、黏结抗拉强度 、黏结内聚力 、摩擦系数 和摩擦角 。参数敏感性分析结果表明, 与 、CN及 正相关,与 、 负相关,而与 、 、 和 无明显的线性关系,此外为获得较低的 波动水平,给出了参数 和 的建议范围。根据参数敏感性分析结果,校核匹配了Kowloon花岗岩直切槽半圆盘（SCB）和人字形切槽半圆盘（CCNSCB）试样的宏观力学性质。从细观角度直观、深入分析不同切槽形状I型断裂韧度试验破裂机制,得出SCB试验曲线的峰前和峰后行为与室内试验更为吻合。     

脆性岩石试样断裂过程区及宏观疲劳裂纹行为研究

通过脆性岩石试样巴西圆盘试验研究了疲劳效应对脆性岩石断裂韧度KIC的影响,阐释了脆性岩石的疲劳损伤机制,首次展示了脆性岩石破坏前宏观裂纹的张开和闭合行为。巴西圆盘试验结果表明,循环荷载作用下,脆性岩样的KIC降低了35%,巴西劈裂拉伸强度降低了30%。通过高速相机数小时的观测并记录到了岩石力学领域从未观测到的过程,即脆性岩样破坏前,疲劳裂纹在正弦荷载作用下弹性张合。扫描电镜和计算机断层扫描结果显示,巴西圆盘和V形切槽巴西圆盘试样的破坏是由断裂过程区(fracture process zone,简称FPZ)引起的,FPZ中包含许多异于循环荷载作用下单条宏观裂纹的微裂纹,切槽裂纹尖端FPZ的形成导致未破坏脆性岩样中可视疲劳裂纹弹性张合。室内试验和数值计算结果表明,切槽裂纹倾角为60°时获得最大FPZ(即FPZmax),这表明最大FPZ的形成可能与Ⅰ-Ⅱ型(拉伸和剪切)组合加载模式有关。     

点荷载强度指数估算与岩浆岩的单轴压缩强度的关系

单轴压缩强度是岩石工程建设中广泛使用的力学参数,直接确定单轴压缩强度相对耗时且较为麻烦。点荷载试验可以间接估算岩石的单轴压缩强度（UCS）,试验方式简洁。通过收集到的岩浆岩点荷载试验成果,分析结果表明,（1）大部分转换公式在点荷载强度指数较大时得到偏高的单轴压缩强度值,特别是对部分幂函数;（2）ISRM（American Society for Testing and Materials）建议的取值范围仍然高估了岩石的单轴压缩强度值。为更准确地估算岩石的单轴压缩强度,建议采用点荷载强度指数 的二次函数（见式（1））估算岩石的单轴压缩强度,适用范围为0.0 MPa < <15.0 MPa。     

用P-CCNBD试样测定岩石动态扩展韧度和观察动态止裂现象

关于动态断裂的全过程,包括动态起裂、扩展和止裂的报道在科学文献中受到日益增长的关注。研究岩石的动态扩展和动态止裂,用分离式霍普金森压杆径向冲击大直径(φ160 mm)的预裂人字形切槽巴西圆盘(pre-cracked chevron notched Brazilian disc——P-CCNBD)试样的试验,测定了大理岩的I型动态扩展韧度,而不仅是动态起裂韧度。确定的方法是试验―数值―解析法,而不是通常的准静态法,因为,大尺寸试样在动态试验中不满足应力平衡条件。裂尖起裂时刻和裂纹扩展速度采用应变片和裂纹扩展计监测;将有关试验数据,包括霍普金森压杆施加在试样上的的动态荷载输入有限元程序,对P-CCNBD进行三维动态数值模拟,再结合普适函数的半解析修正,得到动态应力强度因子的时间历程。结果表明,P-CCNBD大理岩试样的动态起裂韧度随加载率的增大而增大,试样的动态扩展韧度略低于动态起裂韧度,且扩展韧度随裂纹扩展速度的加快而增大。此外,对裂纹扩展速度振荡和扩展路径曲折现象进行了分析,讨论了P-CCNBD试样实现动态止裂的可能性。     

Recalibration and Clarification of the Formula Applied to the ISRM-Suggested CCNBD Specimens for Testing Rock Fracture Toughness

The cracked chevron-notched Brazilian disc (CCNBD) was proposed by the International Society for Rock Mechanics (ISRM) to test the mode I (opening mode) fracture toughness of rock. The test method has been vigorously discussed and debated, despite being the subject of intensive research for decades. The minimum (critical) dimensionless stress intensity factors affiliated with the formula for calculating the fracture toughness using CCNBD specimens with different geometric parameters remain elusive and complex. The matter cannot be resolved by simply replacing the diameter in the original formula with the radius, as claimed by several authors. In this paper, the formula is fundamentally improved, as wide-ranging minimum dimensionless stress intensity factors pertaining to diversified CCNBD geometries are recalibrated by three-dimensional finite element analysis, and an expression with tabulated coefficients is obtained through curve-fitting the data obtained from the numerical calibration. The present results are shown to be more accurate than those in the literature. Furthermore, the importance of the reasonability of the results is highlighted; a comprehensive comparison of different values shows that the upper bounds of minimum stress intensity factors are violated by the above claim. The confusion resulting from the claim is, thus, clarified conclusively.     

Experimental Calibration of ISRM Suggested Fracture Toughness Measurement Techniques in Selected Brittle Rocks

Summary A wide variety of specimen types and methods are employed in fracture toughness measurement of rocks, which result in scattered values for the same rock type. In order to provide some consistency to the values, the International Society for Rock Mechanics (ISRM) recommended three suggested methods using core based specimens, the Chevron Bend (CB) test, the Short Rod (SR) test and the Cracked Chevron Notch Brazilian Disc (CCNBD) test. This standardization helped obtain more consistent values but still a variation of 20–30% was observed in the values of fracture toughness obtained with the CB and SR methods. The values obtained with the CCNBD method were found to be consistently lower (30–50%) than those of the other two methods (CB and SR). Many reasons have been offered to explain this deviation. These include size of the specimen, anisotropy of rock, a dimensionless parameter in the fracture toughness calculation equation for the CCNBD test, etc. A comprehensive test program was initiated to identify the cause of these discrepancies between the CB and CCNBD methods. Three brittle rock types were selected for the study and more than 200 tests were conducted to measure the values of fracture toughness. A rigorous statistical analysis was carried out to determine the confidence level and find the significance of the test results. It was found that the CB and CCNBD methods were very comparable provided the correct equation for fracture toughness calculation was used for the CCNBD method and the size of the specimens was selected carefully. The error in the ISRM 1995 formula of fracture toughness for the CCNBD method could be the major factor responsible for the consistently lower values obtained with the method.     

Numerical Observation of Three-Dimensional Wing Cracking of Cracked Chevron Notched Brazilian Disc Rock Specimen Subjected to Mixed Mode Loading

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by International Society for Rock Mechanics for measuring mode I fracture toughness of rocks. Subsequently, this specimen geometry has been widely extended to conduct mixed mode fracture tests on rocks as well. A straight through crack front during the fracturing process upon the root of the chevron notch is assumed in the testing principle, but has never been thoroughly evaluated before. In this study, for the first time, the progressive rock fracture mechanism of the CCNBD rock specimen under mixed mode loading is numerically simulated. Specimens under representative mixed mode loading angles are modelled; and the assumption of the straight through crack front growth is critically assessed. The results show that not only the notch tip but also the saw-cut chevron notch cracks during the experiments, yielding a prominent twisted front, far from being straight. The crack front never grows up to the root of the notch ligament and the straight through crack front assumption is never satisfied in the realistic rock fracture progress of this chevron notched specimen subjected to mixed mode loads. In contrast, the fracture progress features typical three-dimensional wing cracking towards the loading ends. The numerically observed progressive fracture mechanism reveals that the measuring principle of mixed mode fracture tests employing CCNBD specimens is significantly violated and the measures of both modes I and II fracture toughness are uncertain.     

基于P-CCNBD试样的岩石动态断裂韧度测试方法

预裂的人字形切槽巴西圆盘(Pre-cracked chevron notched Brazilian disc,简称P-CCNBD)是将人字形切槽巴西圆盘(cracked chevron notched Brazilian disc,简称CCNBD)的切槽尖端再稍加切削制成直裂纹前沿的试样。利用霍普金森压杆对P-CCNBD砂岩试样进行径向冲击,完成I型动态断裂试验后再做数值分析得到岩石的动态断裂韧度。为了验证数值模拟的可靠性,先进行了无限平面中一条有限尺寸裂纹表面受冲击拉伸作用的动态有限元分析,结果表明,数值模拟的结果与Shi得到的结果非常吻合。将试验-数值法和他人的准静态法分别确定的砂岩的动态起裂韧度进行对比,两种方法得到的结果有一定的差异。采用试验-数值法,将比较成熟的直裂纹巴西圆盘(cracked straight-through Brazilian disc,简称CSTBD)和P-CCNBD两种试样测得的结果进行对比,两者吻合较好。得到的动态起裂韧度都有随着加载率的增加而增大的加载率效应。分析了准静态法的缺陷,认为试验-数值法得到的结果更为合理。     

Use of the Boussinesq Equation for Determining the Distribution of Stress Within a Diametrical Point Load Test

Summary The maximum tensile stress at failure for a dry specimen, as determined by the Boussinesq equation for the diametrical point load test, was found to be in very good agreement with the diametrical point load tensile strength (Is) as defined by ISRM (1985). The force at failure for specimens of different geometry was used to determine the stress distribution along the line of loading. Distinctive tensile stress gradients dominate almost 84 percent of the specimen radius regardless of the size of the specimen. The maximum tensile stress is located away from the centre of the specimen at a distance approximately 76 percent along the specimen radius, measured from the centre. The stress magnitude at the centre of the specimen is small and represents about 13 percent of the maximum tensile stress calculated, which suggests that the initiation of the fracture is not from the specimen centre. At the zone of contact between the specimen and the loading cones there exists great compressive stress in areas where much material destruction occurs under the loading platen cones. The value of this compressive stress varies from specimen to specimen and, for the material used in these experiments (Oolitic limestone), ranges from 5.3 to 7.2 times the dry unconfined compressive strength of the material. According to the ISRM Suggested Method for Point Load Test, Is ₍₅₀₎ is approximately 0.8 times the uniaxial tensile strength. The maximum tensile stress revealed by the Boussinesq equation (Bs) was correlated with Is ₍₅₀₎ and found to be in the order of 0.9 times the uniaxial tensile strength.     

The effect of specimen size and stress rate for the Brazilian test—A statistical analysis

Summary The Brazilian test is a widely accepted method for the determination of the tensile strength of intact rock. Specifications for the Brazilian tensile strength test have been established by the American Society for Testing and Materials (ASTM), ASTM D 3967-86 and a suggested approach is provided by the International Society for Rock Mechanics (ISRM). The ASTM and ISRM allow a relatively wide range of values for specimen geometry defined in terms of length to diameter ratio and loading rates defined as either time to failure or stress rate.A statistical study was carried out on a coal measure sandstone to determine whether the tensile strength determined by the Brazilian test is independent of the specimen geometry and the stress rate.