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1.
Cracking and coalescence behavior in a rectangular rock-like specimen containing two parallel (stepped and coplanar) pre-existing open flaws under uniaxial compression load has been numerically studied by a parallel bonded-particle model, which is a type of bonded-particle model. Crack initiation and propagation from two flaws replicate most of the phenomena observed in prior physical experiments, such as the type (tensile/shear) and the initiation stress of the first crack, as well as the coalescence pattern. Eight crack coalescence categories representing different crack types and trajectories are identified. New coalescence categories namely “New 1” and “New 2”, which are first observed in the present simulation, are incorporated into categories 3 and 4, and category 5 previously proposed by the MIT Rock Mechanics Research Group, respectively. The flaw inclination angle (β), the ligament length (L) (spacing between two flaws) and the bridging angle (α) (inclination of a line linking up the inner flaw tips, between two flaws) have different effects on the coalescence patterns, coalescence stresses (before, at or post the peak stress) as well as peak strength of specimens. Some insights on the coalescence processes, such as the initiation of cracks in the intact part of specimens at a distance away from the flaw tips, and coalescence due to the development and linkage of a number of steeply inclined to vertical macro-tensile cracks are revealed by the present numerical study. 相似文献
2.
应力比影响下的破裂角、闭锁角、摩擦系数及其耦合关系 总被引：1，自引：0，他引：1
裂纹扩展和摩擦系数分属两个学科而鲜有联系,摩擦系数理论较少,主要依赖实验获得。在Griffith椭圆形裂纹基础上讨论摩擦面状裂纹力学模型,计算了单轴、三轴压应力情况下微裂纹扩展的期望方向及应力集中,那些方向偏离宏观破裂方向较大角度的裂纹因闭锁而无法延伸,应力比 越大,可扩展的微裂纹越向宏观裂纹面方向集中,随着围压 的增加,闭锁范围增大,实现了裂纹张性向剪性的转变。摩擦系数、方向集中度、应力比呈耦合关系,一方面,摩擦系数越大越利于微裂纹方向集中;另一方面微裂纹的方向集中导致了宏观裂纹面（裂缝）上凸起角度降低,进而减小摩擦系数。围压和裂纹扩展期的应力状况是影响摩擦系数的重要因素, 往往成为裂纹扩展的应力条件,凸起斜面摩擦系数越大,临界应力比越小。裂纹粗糙度(或分维数)对形成期的应力状况具有一定记忆功能。 相似文献
3.
含双裂隙岩石裂纹演化机理的离散元数值分析 总被引：2，自引：0，他引：2
采用离散单元法探讨了预制双裂隙岩石的裂纹演化机理。用近期从试验资料提取的无胶结厚度含抗转动能力的岩石微观力学模型和相应的离散单元法商业软件,模拟了含不同预制倾角的双裂隙岩石试样在单轴压缩作用下裂纹的扩展与贯通规律,揭示了裂纹演化的宏微观机理。同时,将离散元法DEM岩石试样的裂纹的扩展和贯通规律以及强度特性与室内试验结果进行了比较分析。结果表明,预制裂隙之间以及端点处的拉应力集中是导致裂隙岩石破坏的主要原因,且DEM数值试验得到裂纹的演化规律与室内试验结果较为一致。含30°的预制裂隙的岩石试样最容易起裂,含75°的预制裂隙的岩石试样最困难起裂,造成此种现象的原因可能是裂纹在垂直于主应力方向上的长度不同导致试样受拉区域大小不同。 相似文献
4.
Optical and TEM observations were made on the active surfaces of the sliding blocks from orthoquartzite subjected to sliding friction experiments (Hayes, 1975; Dunn and Hayes, 1975) at a constant effective confining pressure of 500 bars and a shortening rate of 2.5 · 10−5 s−1, dry and in the presence of pore fluids. Surface-wear features show that the sequence in the destruction of the sliding surface is: (a) penetration, ploughing, and immediate shearing off of brittle asperities to produce gouge, (b) formation of cracks at grain boundaries and intragranular cracks, facilitating shearing of asperities, and forming pluckouts and a rough or smooth sliding surface. Microcracks occurring and terminating in dislocation-free regions indicate that crack growth through dislocation coalescence of pre-existing dislocations is invalid for orthoquartzite at room temperature. 相似文献
5.
拉剪应力状态下岩体裂隙扩展的断裂力学机制及物理模型试验 总被引：3，自引：1，他引：2
开挖卸荷岩体裂隙面通常处于拉剪应力状态。在裂隙应力和变形状态分析的基础上,采用线弹性断裂力学理论和物理模型试验研究了拉剪应力状态下裂隙扩展的力学机制。岩体裂隙在拉剪应力状态下沿裂隙面间的滑动抗剪摩擦力消失,裂隙起裂沿Ⅰ型张拉裂隙断裂韧度KⅠ最小的方向起裂,并最终发展与卸荷拉应力方向垂直;拉剪应力状态下岩体的总位移方向平行于拉应力方向,通过合理的位移假设,基于能量及线弹性断裂力学理论,求解了拉剪应力状态下分支裂隙扩展过程中尖端的动态应力强度因子和扩展长度判据;通过拉剪应力状态下单裂隙扩展物理模型试验验证了理论推导的正确性。 相似文献
6.
Cracks and joints are common in rock masses and play a crucial role in rock mass stability. This study prepared specimens with multiple parallel pre-existing flaws by embedding iron sheets in rock-like materials and used the samples to investigate the crack growth characteristics of these materials. Biaxial compression experiments were performed on sixty specimens, and the influences of the number of pre-existing flaws, their angles and the lateral stress on crack growth were investigated based on video recordings of the crack growth. The results demonstrate that structural failure will occur due to crack growth when the sample contains a small number of pre-existing flaws and that as the number of cracks increases, the specimens will fail due to local failures. In addition, the types of rock bridge failures are summarized, including wing cracks, secondary shear cracks between horizontally-separated pre-existing flaws and secondary shear cracks between vertically-separated pre-existing flaws. Wing cracks play a significant role in the failure of the specimens. The results increase the understanding of crack growth in brittle materials that contain multiple parallel pre-existing flaws under biaxial compression. 相似文献
7.
MARK W. HOUNSLOW 《Sedimentology》1997,44(6):1133-1147
The burial-stress and hydrologic conditions existing during concretion formation in mudrocks are evaluated and integrated into a model for the genesis of septarian cracks. Initial concretion cement formation will lower concretion permeability through the filling of pre-existing pore space. During progressive burial, this may lead to increased excess pore pressure, localized within the concretion body causing a reduction of the effective stress. Analysis of the stress conditions and crack morphology suggests that cracks in septarian concretions result from tensional failure (sub-critical crack growth), as a consequence of this localized excess pore pressure. Conditions suitable for crack formation will depend upon the magnitude of the excess pore pressure and the stress corrosion limit of the concretion body. A review of the likely strength of such concretions indicates that cracking could be initiated at depths less than 10 m. A variety of observed crack morphologies can be explained with this model, depending upon the spatial distribution of strength and effective stress in the concretion. Crack orientations mostly reflect stress anisotropy, but are also influenced by directional anisotropy in the crack growth rates. Locally increased pore pressure also likely occurs in non-septarian concretions, but is not sufficient to cause cracking. This enhanced local pressure may assist the crystal surface growth reactions of the carbonate cement. Through this enhancement process, the shape of concretions may be a response to the local anisotropic pore-pressure contours, which reflect the permeability anisotropy of the concretion and surrounding mudrock. 相似文献
8.
Rock strengths are directly influenced by the open or closed flaws widely distributed in rock masses. Extensive studies have been conducted on the propagations of open flaws in rocks. However, few concerns are paid on the propagation of closed flaws, the influence of the surface friction on the initiation and propagation of closed flaws should be investigated systematically. In present article, the crack initiation and propagation in rock like material subjected to compressive loads have been investigated. The effects of crack surface friction on crack initiation and propagation have been quantified with the help from extended finite element method which is efficient and accurate. Based on the analysis on stress distribution and propagation patterns, following results are obtained: Firstly, minor effects are exerted by crack surface friction on the stress distribution around the flaws when the flaws inclination angle is 30° and 45°. However, as the inclination angle increases to 60°, the effects are much more significant. Secondly, as the inclination angle ranges from 30° to 60°, the most favorable angle for crack propagation is 45°. Thirdly, the initiation location and angle of the wing cracks will not be influenced by the frictions. However, the propagation length will be greatly influenced by the friction and the inclination angle. 相似文献
9.
An Experimental Study of Crack Coalescence Behaviour in Rock-Like Materials Containing Multiple Flaws Under Uniaxial Compression 总被引：4，自引：1，他引：3
Experiments on man-made flawed rock-like materials are applied extensively to study the mechanical behaviour of rock masses as well as crack initiation modes and crack coalescence types. A large number of experiments on specimens containing two or three pre-existing flaws were previously conducted. In the present work, experiments on rock-like materials (formed from a mixture of sand, plaster, limestone and water at mass ratio of 126:9:9:16) containing multiple flaws subjected to uniaxial compression were conducted to further research the effects of the layout of pre-existing flaws on mechanical properties, crack initiation modes and crack coalescence types. Compared with previous experiments in which only three types of cracks were found, the present experiments on specimens containing multiple flaws under uniaxial compression revealed five types of cracks, including wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane tensile cracks and out-of-plane shear cracks. Ten types of crack coalescence occurred through linkage among wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane shear cracks and out-of-plane tensile cracks. Moreover, the effects of the non-overlapping length and flaw angle on the complete stress–strain curves, the stress of crack initiation, the peak strength, the peak strain and the elastic modulus were also investigated in detail. 相似文献
10.
Analyses of Inclined Cracks Neighboring Two Iso-Path Cracks in Rock-Like Specimens Under Compression
Abolfazl Abdollahipour Mohammad Fatehi Marji 《Geotechnical and Geological Engineering》2017,35(1):169-181
Crack propagation process in pre-cracked rock like specimens has been studied experimentally and numerically considering three cracks in the middle part of each specimen. The rock-like specimens are specially prepared from Portland pozzolana cement, fine sands and water. These pre-cracked cylindrical specimens (each containing a single inclined crack in the neighborhood of two iso-path cracks) are experimentally tested under compressive loading. The same problems are numerically simulated by a modified displacement discontinuity method using higher order displacement discontinuity elements and higher order special crack tip elements for crack tip treatment to increase the accuracy of the Mode I and Mode II stress intensity factors obtained based on linear elastic fracture mechanics theory. The crack propagation and coalescence paths of the inclined crack are estimated by implementing a suitable iteration algorithm of incremental crack length extension in a direction predicted by using the maximum tangential stress criterion. The numerical and analytical crack extension analyses are compared which are in good agreement and show the validity, applicability and accuracy of the present work. 相似文献
11.
Effects of Grain Size on the Initiation and Propagation Thresholds of Stress-induced Brittle Fractures 总被引：13，自引：5，他引：8
Summary The microstructure of rock is known to influence its strength and deformation characteristics. This paper presents the results
of a laboratory investigation into the effects of grain size on the initiation and propagation thresholds of stress-induced
brittle fracturing in crystalline rocks with similar mineralogical compositions, but with three different grain sizes. Strain
gauge and acoustic emission measurements were used to aid in the identification and characterization of the different stages
of crack development in uniaxial compression. Results indicate that grain size had only a minor effect on the stress at which
new cracks initiated. Crack initiation thresholds were found to be more dependent on the strength of the constituent minerals.
Grain size did have a significant effect, however, in controlling the behaviour of the cracks once they began to propagate.
The evidence suggests that longer grain boundaries and larger intergranular cracks, resulting from increased grain size, provide
longer paths of weakness for growing cracks to propagate along. This promoted degradation of material strength once the longer
cracks began to coalesce and interact. Thus, rock strength was found to decrease with increasing grain size, not by inducing
crack initiation at lower stresses, but through a process where longer cracks propagating along longer planes of weakness
coalesced at lower stresses. 相似文献
12.
Michele Dragoni 《地学学报》1992,4(4):501-508
A model is proposed for studying the mechanical behaviour of faults during their interseismic periods. The model considers a plane fault surface in an elastic medium, subject to a uniform shear stress which increases slowly with time. A1-D friction distribution is assumed on the fault, characterized by asperities and a weaker zone. The traction vector on the fault plane has an arbitrary orientation: in particular, it can be nonperpendicular to the asperity borders. Aseismic fault slip takes place when the applied stress exceeds the frictional resistance: slip starts in weak zones and is confined by asperities, where it propagates at increasing velocity. Propagation into asperities is characterized by a dislocation front, advancing perpendicularly to the asperity border. Fault slip does not take prate in the direction of traction, except when traction is perpendicular or parallel to the asperity border. The propagation of such aseismic dislocations produces a stress redistribution along the fault and can play a key role in determining the conditions which give rise to earthquakes. 相似文献
13.
Crack evolution is initiated by the occurrence of tensile wing cracks and is then further promoted due to the crack coalescence caused by the extension of a central tensile crack segment between two relatively adjacent flaws. To understand such progressive failures in rock, a parallelized peridynamics coupled with a finite element method is utilized. Through this method, the initiation position of tensile wing cracks is observed with respect to varying inclination angles of a flaw, and then its corresponding shifting mechanism is investigated. In addition, the phenomenon of the position shifting being sensitive to various flaw shapes is discussed. Moreover, it is observed that the inclination angle of a central flaw affects the initiation position of other flaws; therefore, the initiation positions of tensile wing crack emanating from other neighboring flaws are analyzed with their angles. Following tensile wing cracks, a central tensile crack segment occurs in the bridging region between a central flaw and other neighboring flaws; the developmental patterns caused by the crack segment are discussed as well. Finally, the role a central tensile crack segment plays in the formation of crack coalescence and specimen failure is investigated in detail. The numerical results in this paper demonstrate good fidelity with established physical test results and complement them, thereby expanding the understanding of fracturing morphology in rock specimens with various flaws. 相似文献
14.
Crack initiation in uniaxial compressive loading of rocks occurs well before the peak strength is reached. The factors that may influence the onset of cracking and possible initiating mechanisms were explored using a discrete element numerical approach. The numerical approach was based on grain-based model that utilized the Voronoi tessellation scheme to represent low porosity crystalline rocks such as granite. The effect of grain size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to 2.25 mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined. The modelling revealed that crack initiation appears to be a tensile mechanism in low porosity rocks, and that shear cracking along grain boundaries is only a prominent mechanism near the peak strength. It was also shown that the heterogeneity introduced by the grain size distribution had the most significant effect on peak strength and crack initiation stress. The peak strength ranges from 140 to 208 MPa as the grain size distribution varies from heterogeneous to uniform, respectively. However, the ratio of crack initiation to peak stress showed only minor variation, as the heterogeneity decreases. The other factors investigated had only minor effects on crack initiation and peak strength, and crack initiation ratio. 相似文献
15.
The variation of the shear strength of infilled rock joints under cyclic loading and constant normal stiffness conditions is studied. To simulate the joints, triangular asperities inclined at angles of 9.5° and 18.5° to the shear movement were cast using high-strength gypsum plaster and infilled with clayey sand. These joints were sheared cyclically under constant normal stiffness conditions. It was found that, for a particular normal stiffness, the shear strength is a function of the initial normal stress, initial asperity angle, joint surface friction angle, infill thickness, infill friction angle, loading direction and number of loading cycles. Based on the experimental results, a mathematical model is proposed to evaluate the shear strength of infilled rock joints in cyclic loading conditions. The proposed model takes into consideration different initial asperity angles, initial normal stresses and ratios of infill thickness to asperity height. 相似文献
16.
H. Bock 《Engineering Geology》1979,14(4):241-254
The subject under investigation is the strength of a single shear plane which exhibits a regular, asymmetric roughness pattern. In the shear direction the asperities are so steeply inclined that the joint becomes mechanically non-effective with the result that the asperities are sheared off. Against the shear direction the asperities are only gently inclined. It is shown that this particular roughness pattern is of some general importance in geomechanics (examples: unconfined compression test; shear plane with secondary fractures).
Simple analytical considerations allow the formulation of a shear criterion, which is dependent on friction angle øm and cohesioncm of the intact rock and on the inclination γ of the gently inclined parts of the asperities which are dipping against the shear direction. In the Mohr-diagram the criterion results in envelopes which converge at high normal stresses against the envelope of intact rock. Furthermore, the criterion expresses that both the slope of the envelopes and the dilation angle continuously decrease with increasing normal stress. Therefore the criterion adequately describes features which are regarded as most important when shearing rough joints or compound shear surfaces. 相似文献
17.
Evolution of Rock Cracks Under Unloading Condition 总被引：2，自引：0，他引：2
Underground excavation normally causes instability of the mother rock due to the release and redistribution of stress within the affected zone. For gaining deep insight into the characteristics and mechanism of rock crack evolution during underground excavation, laboratory tests are carried out on 36 man-made rock specimens with single or double cracks under two different unloading conditions. The results show that the strength of rock and the evolution of cracks are clearly influenced by both the inclination angle of individual cracks with reference to the unloading direction and the combination geometry of cracks. The peak strength of rock with a single crack becomes smaller with the inclination angle. Crack propagation progresses intermittently, as evidenced by a sudden increase in deformation and repeated fluctuation of measured stress. The rock with a single crack is found to fail in three modes, i.e., shear, tension–shear, and splitting, while the rock bridge between two cracks is normally failed in shear, tension–shear, and tension. The failure mode in which a crack rock or rock bridge behaves is found to be determined by the inclination angle of the original crack, initial stress state, and unloading condition. Another observation is that the secondary cracks are relatively easily created under high initial stress and quick unloading. 相似文献
18.
在研究双向压缩条件下压剪复合型裂纹应力分布特征及断裂破坏机制基础上,考虑渗透压对初始裂隙面上有效正应力的影响,提出高低渗透压环境的判定准则,并基于滑动裂纹模型理论及最大周向拉应力破坏准则,得到不同渗透压环境下初始裂隙尖端微裂纹起裂特征与规律。研究结果表明：压剪复合应力条件下,初始裂隙尖端发育微裂纹的最优倾角与裂隙面摩擦系数直接相关,随裂隙面摩擦系数的增大,最优初始裂隙倾角由45°起逐渐增大;低渗透压条件下,渗流场的存在使裂纹面摩擦系数发生弱化,进而使得最优初始裂隙倾角向45°靠近,而渗透压直接降低裂隙面上有效正应力且与裂隙倾角无关,其仅仅影响裂隙体材料的初裂强度;高渗透压条件下,初始裂隙面由压剪复合应力状态转化为拉剪复合应力状态,并在拉剪复合应力场作用下,尖端微裂纹起裂角随KI/KII的不断增大,由70.5°逐渐趋近于0°。 相似文献
19.
压缩作用下岩石内部细观裂纹扩展导致岩石产生损伤,其对岩石变形、强度等力学特性有着重要影响;然而,岩石内部裂纹扩展与剪切特性（黏聚力、内摩擦角及剪切应力）动态演化关系很少被研究。基于裂纹扩展机制推出的岩石应力-应变本构模型,并结合摩尔-库仑失效准则,推出了在岩石应力-应变关系峰值应力（对应岩石压缩强度）状态时,本构模型细观力学参数与岩石黏聚力、内摩擦角及剪切强度之间的状态关系。然后,引入岩石应力-应变本构关系塑性变形阶段服从摩尔-库仑屈服准则的力学流动规律,进而将已推出的应力-应变关系峰值状态点所满足的细观力学参数与黏聚力、内摩擦角关系,推广到岩石进入塑性变形后,岩石内部裂纹扩展（或应变）与黏聚力、内摩擦角及剪切应力动态演化的理论关系。随着裂纹扩展或应变增加,黏聚力、内摩擦角及剪切应力先增大,达到一个峰值点后减小,该结果与应力-应变本构曲线变化趋势相对应。通过试验结果验证了所提出理论结果的合理性。并讨论了初始裂纹之间摩擦系数对黏聚力、内摩擦角及剪切应力随裂纹扩展或应变演化规律的影响。 相似文献
20.
Two hundred observations of frictional behavior of seven low-porosity silicate rocks were made at temperatures to 700°C and pressures from 2.5 to 6 kbar. For all rocks except one, peridotite, stick-slip occurred at low temperature and gave way to stable sliding at some high temperature, different for each rock. These differences could be related to the presence or absence of minerals such as amphibole, mica, or serpentine. Up to some temperature, depending on rock type, the friction stress was relatively unaffected by temperature. The shear stress decreased at higher temperature, and in some cases such decrease was related to the coincidence of fracture and friction strength. While somewhat dependent on rock type, the friction stress for the seven rocks studied was about the same, within 10–15%. Up to 265°C, water had little effect on the frictional behavior of faulted granite at 3 kbar effective pressure. The frictional stresses measured in the laboratory were significantly higher than estimated for natural faults. This difference could be accounted for by high pore pressure or weak alteration materials in the natural fault zone. 相似文献