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。     

用SCDC试样测试岩石动态断裂韧度的新方法

 利用大直径(?100 mm)分离式霍普金森压杆对大尺寸(150 mm×80 mm)压缩单裂纹圆孔板(SCDC)试样冲击加载,采用实验–数值–解析法测定了青砂岩的I型动态起裂韧度和动态扩展韧度。试样的起裂时刻和裂纹扩展速度由黏贴在裂尖附近的裂纹扩展计确定,通过对比发现,裂纹扩展计的准确性和灵敏性都比黏贴在同一试样对应位置的普通应变片更好。实验–数值–解析法根据实验数据获取试样两端的加载历程,利用有限元数值计算和普适函数的半解析修正,综合考虑材料惯性效应和裂纹扩展速度对动态应力强度因子的影响,较准静态方法更适于采用大尺寸试样确定岩石动态断裂韧度。实验–数值–解析法所确定的高加载率和高裂纹扩展速度下砂岩的动态断裂韧度值分别随动态加载率和裂纹扩展速度的提高而增加。最后,通过对SCDC试样裂纹扩展路径上应变片的断裂时间分析,确定了利用SCDC试样实现动态止裂的可能性。     

Dynamic cracked chevron notched Brazilian disc method for measuring rock fracture parameters

The cracked chevron notch Brazilian disc (CCNBD) method is widely used in characterizing rock fracture toughness. We explore here the possibility of extending the CCNBD method to dynamic rock fracture testing. In dynamic rock fractures, relevant fracture parameters are the initiation fracture toughness, the fracture energy, the fracture propagation toughness, and the fracture velocity. The dynamic load is applied with a split Hopkinson pressure bar (SHPB) apparatus. A strain gauge is mounted on the sample surface near the notch tip to detect the fracture-induced strain release, and a laser gap gauge (LGG) is used to monitor the crack surface opening distance (CSOD) during the test. With dynamic force balance achieved in the tests, the stable–unstable transition of the crack propagation crack is observed and the initiation fracture toughness is obtained from the peak load. The dynamic fracture initiation toughness values obtained for the chosen rock (Laurentian granite) using this method are consistent with those measured using other methods. The dynamic fracture initiation toughness is in the range 2.5–4.6 MPa m^1/2 and the propagation fracture toughness is in the range 7.1–10.6 MPa m^1/2, which is consistently larger than the initiation toughness.     

Stanstead花岗岩动态断裂性能

 采用一种新的方法研究Stanstead花岗岩的动态断裂性能,包括起裂韧度、断裂能、传播韧度和裂纹传播速度。该方法采用分离式霍普金森压杆加载的带预制裂纹的半圆盘三点弯试样,同时采用激光位移计监测试样的裂纹面张开位移。在动态力平衡的条件下,起裂韧度由准静态公式计算得到。通过裂纹面张开位移数据推算出2个碎片的残余动能,从而计算出平均传播断裂能和传播韧度。裂纹传播平均速度由黏接在试样上的一系列裂缝计测量得到。试验结果表明,该花岗岩的起裂韧度和传播韧度都与加载速率有关,传播韧度大于起裂韧度,传播韧度随着裂纹传播速度的提高而提高。通过裂纹传播速度和传播韧度的关系拟合得到材料的止裂韧度及裂纹传播极限速度。得到的Stanstead花岗岩与Laurentian花岗岩结果对比表明,Stanstead花岗岩颗粒较大,起裂、止裂韧度较小;Laurentian花岗岩颗粒较小,传播韧度较小,裂纹传播极限速度较大,裂纹容易传播。     

Fracture mechanics analysis of cracked discs of anisotropic rock using the boundary element method

This paper is the second of a series of two papers dealing with the determination of the deformability, tensile strength and fracturing of anisotropic rocks by diametral compression (Brazilian test) of discs of rock. It is shown how a new formulation of the Boundary Element Method (BEM), proposed recently by the authors, can be used to determine the stress intensity factors (SIFs) and the fracture toughness of anisotropic rocks from the results of diametral compression tests on initially cracked discs. Crack initiation angles and propagation paths can also be predicted using a numerical procedure based on the BEM and maximum tensile stress criterion. Numerical examples of calculation of mixed mode SIFs are presented for both isotropic and anisotropic media. The calculated SIFs for the special isotropic case are found to be in good agreement with those reported by previous authors. Diametral loading tests were conducted on Cracked Straight Through Brazilian Disc (CSTBD) specimens of a shale in order to evaluate its fracture toughness, the angle of crack initiation and the path of crack propagation. It was found that the numerical simulations of crack initiation and propagation in the CSTBD specimens of the shale were in good agreement with the experimental observations.     

中低速冲击载荷下巷道内裂纹的动态响应

 由于爆破开挖,巷道内常含有径向裂隙,并影响巷道的稳定性,为了详细地研究含径向裂纹巷道在冲击载荷作用下的动态断裂行为,采用砂岩材料制作巷道模型试样进行中低速冲击动态断裂试验,并采用AUTODYN有限差分软件进行数值模拟分析。分析巷道对称轴线上的径向裂纹在冲击荷载作用下的扩展特性及止裂现象,并采用试验–数值–解析法计算出裂纹的起裂韧度及扩展速度等参数。研究结果表明：(1) 巷道围岩在静力载荷作用和动力载荷作用下的破坏行为有较大差异,动力载荷下破坏仅是裂纹尖端处的起裂、扩展;而静力载荷下破坏除了发生在裂纹尖端处,也会在巷道拱肩、拱脚及两侧帮处发生破坏。(2) 巷道对称轴线上的裂纹在冲击载荷下的扩展路径大致沿着裂纹的原方向扩展,扩展路径中存在明显的止裂现象。(3) 采用试验–数值–解析法能够较好地计算出裂纹的起裂速度及扩展速度,进一步采用位移外推法能够求解出巷道内裂纹的动态应力强度因子时程曲线,利用测试的裂纹起裂时间确定起裂韧度。     

Applications of rock failure process analysis （RFPA） method

Brittle failure of rocks is a classical rock mechanical problem.Rock failure not only involves initiation and propagation of single crack,but also is associated with initiation,propagation and coalescence of many cracks.The rock failure process analysis(RFPA)tool has been proposed since 1995.The heterogeneity of rocks at a mesoscopic level is considered by assuming that the material properties follow the Weibull distribution.Elastic damage mechanics is used for describing the constitutive law of the meso-le...     

采用中心圆孔裂缝平台圆盘确定岩石的动态断裂韧度

由于带有预制裂缝岩石试件的难于制作以及动态研究的复杂性,岩石动态断裂韧度在研究方法上一直也没有统一的标准,有必要对其测试方法进行研究。采用大理岩制作了一种含有中心圆孔预制裂缝宽度小于1mm的平台圆盘试件,在霍普金森压杆系统上进行了动态冲击试验,并采用实验-数值方法,确定其动态断裂韧度。该方法基于一维应力波理论,采用Hopkinson弹性压杆上应变片获得作用在试件两端面的动态载荷P(t),输入此载荷,利用动态有限元法求得试样内动态应力强度因子KI(t)随时间的变化历程,对应于试件上应变片测得的起裂时间tf的动态应力强度因子KI(tf)即为材料的动态起裂断裂韧度KId。     

加载速率对岩石动态断裂韧度影响的实验研究

为了获得岩石在高加载速率作用下的动态断裂韧度值并分析加载速率的影响,由分离式霍普金森压杆入射杆杆端附加劈尖及其基座对边切槽圆盘试样施加动态劈裂载荷。把应变片粘贴在裂纹尖端附近获得裂纹扩展时间;将劈裂载荷时间历程及裂纹扩展时间输入有限元计算模型,获得试样的起裂动态断裂韧度值。结果表明,在加载速率18.85×104MPa.m1/2s-1以下,大理岩的动态断裂韧度值随着加载速率的增大而上升,但上升趋势逐渐减弱。断裂韧度数值在高加载速率下呈现出明显的离散性。     

温度对岩石动态断裂韧度的影响研究

根据Central Cracked Circular Disk-Split Hopkinson Pressure Bar(CCCD-SHPB)测试原理,采用平台巴西圆盘开展温度对岩石类材料动态断裂性能影响的实验研究。实验中控制加载速率基本一致,仅改变试件测试的温度,实现了岩石材料在同一加载速率、不同温度下的动态断裂实验,进而开展岩石材料动态断裂韧度的温度相关性研究。实验结果表明,当温度处于8～100℃时,动态断裂韧度随着温度的升高而逐步下降,近似呈线性关系。     

Numerical study on static and dynamic fracture evolution around rock cavities

In this paper, a numerical code, RFPA^2D (rock failure process analysis), was used to simulate the initiation and propagation of fractures around a pre-existing single cavity and multiple cavities in brittle rocks. Both static and dynamic loads were applied to the rock specimens to investigate the mechanism of fracture evolution around the cavities for different lateral pressure coefficients. In addition, characteristics of acoustic emission (AE) associated with fracture evolution were simulated. Finally, the evolution and interaction of fractures between multiple cavities were investigated with consideration of stress redistribution and transference in compressive and tensile stress fields. The numerically simulated results reproduced primary tensile, remote, and shear crack fractures, which are in agreement with the experimental results. Moreover, numerical results suggested that both compressive and tensile waves could influence the propagation of tensile cracks; in particular, the reflected tensile wave accelerated the propagation of tensile cracks.     

脆性岩石破坏试验研究

 对不同加载速率控制条件下标准试样以及带中心圆孔的花岗岩岩板进行单轴压缩试验,研究岩石破坏的全过程并进行声发射特征分析。试验结果表明：岩石材料破坏过程是内部微裂纹产生和扩展过程的宏观反映;声发射信号与应力–应变曲线有良好的对应关系,根据声发射信号可以判断岩石内部裂纹扩展演化的情况;在不同的加载速率条件下对应不同的承载能力和不同的破坏形态。根据试验结果,建立弹脆性损伤本构模型,基于ABAQUS平台,采用与试验一致的控制条件对带孔岩板进行数值模拟,并与试验结果进行比较。结果表明,数值模拟真实地反映了岩石变形破坏的全过程,研究成果对研究脆性岩石的破坏以及脆性岩石的岩爆机制具有重要的指导意义。     

用非连续变形分析方法模拟冲击荷载作用下巴西圆盘的破坏过程

 为研究岩石在冲击荷载作用下岩石的破裂过程,运用岩体裂纹扩展破坏二维分析程序DDARF(Discontinuous Deformation Analysis for Rock Failure),对大理石巴西圆盘试样在分离式霍普金森压杆(SHPB-Split Hopkinson Pressure Bar)试验中动态破裂全过程进行了数值模拟分析研究。模拟结果形象展示了试样在不同入射波作用下裂纹的萌生、演化、扩展及贯通破坏情形,与试验结果有较好的吻合。对裂纹产生的力学机理、扩展过程及伴生现象做出了解释。研究结果表明：(1) 改进的微观破裂准则不仅适用于模拟岩石静载破裂,而且可以用于模拟动载破坏;(2) 巴西圆盘试样在受到冲击荷载作用时,主裂纹首先从一端产生,然后逐渐沿径向加载方向向中心延伸、扩展至另一端贯通破坏,裂纹尖端的拉应力是导致岩石开裂的原因;(3) 主裂纹拓展过程中伴随着次生损伤微裂纹的产生,次生微裂纹主要集中在主裂纹两侧附近区域;(4) 试样两端与入射杆、透射杆接触部分会产生三角形破裂区,且随着入射波幅值的增大,三角形破裂区域面积有增大的趋势。     

Coupled hydro-thermo-mechanical modeling of hydraulic fracturing in quasi-brittle rocks using BPM-DEM

This paper presents an improved understanding of coupled hydro-thermo-mechanical(HTM) hydraulic fracturing of quasi-brittle rock using the bonded particle model(BPM) within the discrete element method(DEM). BPM has been recently extended by the authors to account for coupled convective econductive heat flow and transport, and to enable full hydro-thermal fluidesolid coupled modeling.The application of the work is on enhanced geothermal systems(EGSs), and hydraulic fracturing of hot dry rock(HDR) is studied in terms of the impact of temperature difference between rock and a flowing fracturing fluid. Micro-mechanical investigation of temperature and fracturing fluid effects on hydraulic fracturing damage in rocks is presented. It was found that fracture is shorter with pronounced secondary microcracking along the main fracture for the case when the convectiveeconductive thermal heat exchange is considered. First, the convection heat exchange during low-viscosity fluid infiltration in permeable rock around the wellbore causes significant rock cooling, where a finger-like fluid infiltration was observed. Second, fluid infiltration inhibits pressure rise during pumping and delays fracture initiation and propagation. Additionally, thermal damage occurs in the whole area around the wellbore due to rock cooling and cold fluid infiltration. The size of a damaged area around the wellbore increases with decreasing fluid dynamic viscosity. Fluid and rock compressibility ratio was found to have significant effect on the fracture propagation velocity.     

Fracture propagation in sandstone and slateeLaboratory experiments,acoustic emissions and fracture mechanics

Fracturing of highly anisotropic rocks is a problem often encountered in the stimulation of unconventional hydrocarbon or geothermal reservoirs by hydraulic fracturing. Fracture propagation in isotropic material is well understood but strictly isotropic rocks are rarely found in nature. This study aims at the examination of fracture initiation and propagation processes in a highly anisotropic rock, specifically slate. We performed a series of tensile fracturing laboratory experiments under uniaxial as well as triaxial loading. Cubic specimens with edge lengths of 150 mm and a central borehole with a diameter of13 mm were prepared from Fredeburg slate. An experiment using the rather isotropic Bebertal sandstone as a rather isotropic rock was also performed for comparison. Tensile fractures were generated using the sleeve fracturing technique, in which a polymer tube placed inside the borehole is pressurized to generate tensile fractures emanating from the borehole. In the uniaxial test series, the loading was varied in order to observe the transition from strength-dominated fracture propagation at low loading magnitudes to stress-dominated fracture propagation at high loading magnitudes.     

基于能量耗散原理的盐岩动力特性及破坏特征分析

 为研究盐岩的动力特性和破坏特征,利用带围压的分离式Hopkinson压杆(SHPB)试验装置,对盐岩进行不同围压(5,15和25 MPa)下的冲击试验,并基于能量耗散原理来研究盐岩动态力学性能以及破坏特征,分析整个试验过程中的能量传递与转化,探究围压和输入能量对试件吸能及破坏的影响。研究结果表明：在同一围压下,随着入射能的增加,盐岩硬化效应越明显,表现为能量反射率增高而透射能和吸收能降低;在相同或相近的入射能下,随着围压的升高盐岩的流塑性变得越明显,但在动力荷载下盐岩由流塑性向脆性转变,最后发生脆性破坏;随着吸收能的增加,盐岩的峰值应力因围压不同而表现出不同的变化趋势,低围压时,吸收能越大,峰值应力越高,而高围压时,吸收能越大,峰值应力却越小;在有围压状态下,盐岩的冲击破坏形态与其他的脆性岩石相似,但在破坏机制上存在很大差异。     

Correlating mode-I fracture toughness and mechanical properties of heat-treated crystalline rocks

For the effect of thermal treatment on the mode-I fracture toughness(FT), three crystalline rocks(two basalts and one tonalite) were experimentally investigated. Semi-circular bend specimens of the rocks were prepared following the method suggested by the International Society for Rock Mechanics(ISRM)and were treated at various temperatures ranging from room temperature(25 ℃) to 600 ℃. Mode-I FT was correlated with tensile strength(TS), ultrasonic velocities, and Young's modulus(YM). Additionally,petrographic and X-ray diffraction analyses were carried out to find the chemical changes resulting from the heat treatment. Further, scanning electron microscopy(SEM) was conducted to observe the micro structural changes when subjected to high temperatures. These experiments demonstrate that heat treatment has a strong negative impact on the FT and mechanical properties of the rocks. From room temperature to 600 ℃, mode-I FT values of massive basalt, giant plagioclase basalt, and tonalite were reduced by nearly 52%, 68%, and 64%, respectively. Also, at all temperature levels, FT and mechanical properties are found to be exponentially correlated. However, the exact nature of the relationship mainly depends on rock type. Besides, TS was found to be a better indicator of degradation degree than the mode-I FT. SEM images show that micro crack density and structural disintegration of the mineral grains increase with temperature. These physical changes confirm the observed reduction in the stiffness of heat-treated crystalline rocks.     

裂缝长度对岩石动态断裂韧度测试值影响的研究

 为了考察裂缝长度对试件动态断裂韧度测试值的影响,采用圆盘直径为80 mm变化裂缝长度的大理岩中心圆孔裂缝平台巴西圆盘试件,在霍普金森压杆系统上进行动态冲击劈裂试验。对不同裂缝长度试件动态试验时弹性压杆上测得的应变波形以及试件的断裂模式进行分析,用试验–数值的方法确定大理岩的动态断裂韧度。结果表明,在平均加载率为2.96×104 MPa·m1/2·s－1的条件下,大理岩动态断裂韧度均值是其静态断裂韧度均值的2.6倍,随着裂缝长度的增加,动态测试值没有静态测试值的变化显著,最后对与试件尺寸和构形无关的动态断裂韧度的确定方法进行讨论。     

Numerical investigation of blasting-induced damage in cylindrical rocks

In order to investigate rock fracture and fragmentation mechanisms under dynamic loading, a cylindrical rock model with a centralized borehole is developed through the use of AUTODYN code. According to the material properties and loading conditions, four kinds of equation of state (EOS), linear, shock, compaction and ideal gas, are applied to the four kinds of materials employed in this numerical model. A modified principal stress failure criterion is applied to determining material status, and a well-behaved explosive, PETN, and a relatively homogeneous igneous rock, diorite, are used in this rock model. A single centrally located line source of explosive is fired numerically to produce the dynamic loadings operating on the surrounding rocks. This numerical model is applied to actual blasting conditions. The rock failure mechanism under dynamic loading is first analyzed, and then the influences of the following factors on rock fracturing are discussed: (a) coupling medium, (b) confinement, (c) boundary condition, (d) initiation location in an explosive column, and (e) air ducking. The results show that all these factors have significant effects on rock fracturing under dynamic loading.     

Effects of high temperatures on dynamic rock fracture

The dynamic fracture toughness of Fangshan gabbro and Fangshan marble subjected to high temperature was measured by means of the split Hopkinson pressure bar (SHPB) system. The specimens for measuring the fracture toughness were manufactured according to the requirements for the Short Rod (SR) specimen suggested by ISRM. Two cases were investigated: (1) the SR specimens of the gabbro and marble were fractured at high temperature (100–330°C), and (2) the specimens of the rocks were first pre-heat-treated at 200°C for the marble and 600°C for the gabbro, and then fractured at room temperature. The experimental results showed that under dynamic loading the fracture toughness of both the gabbro and the marble tested in the above-mentioned cases increased with increasing loading rates. The relationship between the fracture toughness and the loading rates in the two cases is similar to that obtained in the room temperature environment, i.e., without high temperature. (This is defined as the third case.) It can be concluded that temperature variation affects the dynamic fracture toughness of the two rocks to a limited extent within the temperature ranges tested. This is different from the results obtained under the static loading condition. Furthermore, by means of the scanning electronic microscope (SEM), the vertical sections of the fracture surfaces for some gabbro specimens were examined. In addition, the fractal dimensions of the fracture surfaces of some specimens were measured by means of fractal geometry. The results showed that under dynamic loading: (1) macro-crack branching near the fracture surfaces was universal; (2) the fractal dimensions increased with increasing loading rates; (3) in the sections of the specimens tested at high temperature there were many micro-cracks that were probably induced by thermal cracking. On the basis of the above macro- and micro-experimental investigation, an energy analysis of the process of dynamic rock fracture was performed. The results showed that the energy utilisation in dynamic fracture was much lower than that in static fracture.