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。     

有限宽切槽对CCNBD断裂试样应力强度因子的影响

国际岩石力学学会（ISRM）在1995年提出一种新型的岩石断裂韧度试样--人字形切槽巴西圆盘试样（cracked chevron notched Brazilian disc--CCNBD）,对该试样的一个重要力学参数即最小无量纲应力强度因子的标定,以前的分析和计算都没有考虑切槽宽度的影响。然而试样切槽的宽度受切割刀具厚度所限,不能为零。当试样较小时,切槽宽度则相对较大。通过三维边界元计算分析表明,切槽宽度越大,无量纲应力强度因子的标定值就越大;对于ISRM推荐的CCNBD标准试样,得出其最小无量纲因子值为0.954,这比ISRM给出对应值0.84要大13.6 %。同时,小裂纹应力强度因子曲线的变化趋势也发生了质的变化,这可能会导致实验的失败。推荐最小无量纲应力强度因子的标定采用考虑切槽的三维分析。     

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。     

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。     

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。     

用5种圆盘试件的劈裂试验确定岩石断裂韧度

用5种不同形状的圆盘试件测定了大理岩张开型断裂韧度。5种圆盘试件分别为平台巴西圆盘、带有中心圆孔的平台巴西圆盘、人字型切槽巴西圆盘、直切槽巴西圆盘和圆孔切槽平台巴西圆盘。加载模式是对径压缩劈裂。介绍了试件的制作方法,提出了用每种圆盘确定断裂韧度的公式。结果表明,含有切槽圆盘的断裂韧度值在0.78～0.91 MPa•m1/2之间,不含切槽圆盘测得的值在1.01～1.04 MPa•m1/2之间。有3种含有切槽圆盘测得的断裂韧度值比较稳定,其中孔槽式平台巴西圆盘能够制作理想的宽度较小的切槽。     

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。     

中心直裂纹半圆盘试样的石灰岩断裂韧度尺寸效应试验研究

为了考察试样尺寸和预制裂缝长度对岩石断裂韧度测试值的影响,采用4种圆盘半径分别为25.0、37.5、50.0、75.0 mm,预制裂缝长度与圆盘试样半径比值(无量纲裂缝长度)在0.3～0.7范围内的中心直裂纹半圆盘石灰岩试样(NSCB)进行了三点弯曲断裂试验。结果表明:NSCB试样的破坏过程表现出明显的"脆性"破坏特征。与预制裂缝长度相比,试样尺寸对峰值载荷和载荷-位移曲线的形态影响更显著;随着试样尺寸的增加,试样的断裂韧度测试值不断增加,增加幅度随着预制裂缝长度的增加而加剧;当试样尺寸一定时,随着无量纲裂缝长度的增加,断裂韧度测试值有一定程度降低;为减小尺寸效应的影响,建议采用圆盘半径大于37.5 mm,无量纲预制裂缝长度范围在0.4～0.6内的NSCB试样测试岩石的断裂韧度。所得结果为推广NSCB试样国际建议方法及准确测试岩石的断裂韧度提供了一定的参考。     

砂岩Ⅰ型断裂韧度及其与强度参数的相关性研究

岩石的破坏和断裂是密切相关的,岩石强度准则的材料参数和断裂理论的断裂韧度是存在特定关系的,基于此,设计了紧凑的试验方案,对干燥和饱水状态下的砂岩试样进行了3点弯曲断裂韧度试验和抗压、抗拉强度试验,试验结果表明,饱水后,砂岩的Ⅰ型断裂韧度软化效应明显,饱水试样和干燥试样的 比值与砂岩抗压强度的软化系数相近,具有相似的软化效应。同时,从理论上分析了岩石I型断裂韧度与抗拉强度之间的关系,并结合大量的试验数据进行了验证,相关结论为以往的岩石I型断裂韧度与抗拉强度之间的数据统计拟合公式提供了理论基础。最后,统计分析了砂岩Ⅰ型断裂韧度与抗压强度、黏聚力、内摩擦角等强度参数之间的关系。研究成果对于把握砂岩Ⅰ型断裂韧度及其与强度参数的相关性具有较大的参考价值,相关的分析结论、试验方法和试验结果也可以为其他类型岩石的研究提供有益的参考。     

采用NSCB试样进行石灰岩断裂韧度尺寸效应的试验研究

为了考察试样尺寸和预制裂缝长度对岩石断裂韧度测试值的影响,采用四种圆盘半径分别为25mm、37.5mm、50mm、75mm,预制裂缝长度与圆盘试样半径比值(无量纲裂缝长度)在0.3～0.7范围内的中心直裂纹半圆盘石灰岩试样(NSCB)进行了三点弯曲断裂试验。结果表明:NSCB试样的破坏过程表现出明显的“脆性”破坏特征,与预制裂缝长度相比,试样尺寸对峰值载荷和载荷-变形曲线的形态影响更显著;随着试样尺寸的增加,试样的断裂韧度测试值不断增加,增加幅度随着预制裂缝长度的增加而“加剧”;当试样尺寸一定时,随着无量纲裂缝长度的增加,断裂韧度测试值有一定程度降低;为减小尺寸效应的影响,建议采用圆盘半径大于37.5mm,无量纲预制裂缝长度范围在0.4～0.6内的NSCB试样测试岩石的断裂韧度。所得结果为推广NSCB试样国际建议方法及准确测试岩石的断裂韧度提供了一定的参考。     

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.     

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.     

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.     

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.     

动载确定方法对岩石动态断裂韧度测试的影响

为了考察不同方法确定的动态载荷对测试岩石动态断裂韧度的影响,在SHPB压杆系统上动态冲击直径80 mm的大理岩圆孔裂缝平台巴西圆盘,获得了弹性压杆上的应力波形,间接计算得到3种不同的作用在圆盘端部的动态载荷。将载荷输入ANSYS动态有限元模型中,求得了相应的动态应力强度因子,并根据试验-数值分析方法确定了岩石的动态断裂韧度测试值。结果表明,在加载速率约为4.0×104 MPa•m1/2/s的条件下,采用三波法确定的大理岩的平均动态断裂韧度为 3.92 MPa•m1/2,采用一波法比三波法计算的结果偏低11.22%,采用二波法比三波法计算的结果偏高20.15%,3种方法得到的结果差异较大。应力波在传播过程中,通过圆盘表面和预制裂缝面发生散射,部分能量不断发生释放是造成圆盘试件两端加载载荷不相等的主要原因。三波法是3种方法中比较理想的动态载荷确定方法,但需要考察试件的动态应力平衡性。     

边缘裂缝圆盘砂岩试体III型断裂韧度量测之研究

针对含边缘裂缝之砂岩试体受反平面剪力情形进行了III型断裂韧度量测试验,搭配相关所推导之理论公式求取砂岩III型断裂韧度。研究试验条件为无围压及单轴荷重加载下进行III型断裂试验,且针对试体受高温后之性质改变为变量条件加以探讨。研究结果表明,以该方式进行III型断裂韧度之量测,为可靠且稳定;砂岩经历温度在200 ℃前,试体III型断裂韧度会提升,之后呈现下降,到600 ℃,下降趋于平缓;随着烧结温度的提升,单压强度、动态杨氏模量、动态泊松比与III型断裂韧度皆有下降之趋势,且试体出现熔融样态,表面出现明显微裂纹。