核形石灰岩单轴压裂试验设计和裂纹演化特征

船山组核形石灰岩是广泛分布于长江中下游及东南沿海各省的重要建筑材料。该文结合断裂力学理论,通过偏光显微镜观察和扫描电镜研究,探讨核形石灰岩在张应力作用下,微裂纹扩展形成宏观破裂的过程。研究显示,核形石颗粒界面、次生孔隙、粒间孔隙等处存在微裂纹,微裂纹扩展需要消耗能量,当裂纹扩至核形石附近时,有绕过核形石或穿过核形石扩展两种不同的路径,主要取决于核形石纹层类型、结构致密度、颗粒大小等结构因素。

天然岩石单轴压裂过程磁组构响应实验研究

首次以天然岩石样品为研究对象,对不同压力下岩石磁组构的变化特征及其与应力应变的关系进行了实验研究,监测了试件在压力不断增强的过程中内部微破裂事件及变形破坏过程。试验结果表明,以磁铁矿为主要载磁矿物的岩体(火成岩)样品,岩石磁组构对应力应变的响应较为明显,主要表现为在压力不断增强的过程中,磁化率各向异性度PJ的微弱增强,形状因子T值明显升高,磁化率主轴的快速旋转。本结果与前人利用人工合成的磁铁矿为载磁矿物的样品得出的研究结论基本一致。本研究结果可为天然岩石磁组构变化与应力应变关系的深入研究及磁组构构造意义的解释提供理论依据。

Fracture evolution around pre-existing cylindrical cavities in brittle rocks under uniaxial compression

The development of fracture around pre-existing cylindrical cavities in brittle rocks was examined using physical models and acoustic emission technique. The experimental results indicate that when granite blocks containing one pre-existing cylindrical cavity are loaded in uniaxial compression condition, the profiles of cracks around the cavity can be characterized by tensile cracking （splitting parallel to the axial compression direction） at the roof-floor, compressive crack at two side walls, and remote or secondary cracks at the perimeter of the cavity. Moreover, fracture around cavity is size-dependent. In granite blocks containing pre-existing half-length cylindrical cavities, compressive stress concentration is found to initiate at the two sidewalls and induce shear crack propagation and coalescence. In granite blocks containing multiple parallel cylindrical cavities, the adjacent cylindrical cavities can influence each other and the eventual failure mode is determined by the interaction of tensile, compressive and shear stresses. Experimental results show that both tensile and compressive stresses play an important role in fracture evolution process around cavities in brittle rocks.

原煤受载破坏形式的层理效应研究

为了研究层理对原煤受载破坏形式的影响,分别对平行层理煤样和垂直层理煤样进行单轴压裂实验和相同围压、不同瓦斯压力下的三轴压裂实验。结果表明:平行层理煤样的抗压强度略小于垂直层理煤样,但后者的峰值应变是前者的2.33倍;随着瓦斯压力增大,垂直层理煤样的峰值应力先增大后减小,变化的幅度为26.62%,平行层理煤样的峰值应力先减小后增大再减小,变化的幅度为35.14%;单轴压裂和三轴压裂时,平行层理煤样的破坏为整体纵向剪切破坏,煤样沿纵向裂开为几大块,垂直层理煤样的破坏为局部横向张拉破坏,煤样上半部分剧烈破坏,下半部分基本保持完整;三轴压裂与单轴压裂相比,垂直层理煤样破裂时保持完整的部分体积约增加73%,平行层理煤样劈裂的角度变化幅度约为42%。

Failure characteristics and its influencing factors of rock-like material with multi-fissures under uniaxial compression

The compression test on rock-like specimens with prefabricated closed multi-fissures made by pulling out the embedded metal inserts in the precured period was done on the servo control uniaxial loading instrument. The influence of fissure inclination angle and distribution density on the failure characteristics of fissure bodies was researched. It was found that, the fissure inclination angle was the major influencing factor on the failure modes of fissure bodies. The different developmental states of micro-cracks would appear on specimens under different fissure inclination angles. However, the influence of fissure distribution density on the failure mode of fissure bodies was achieved by influencing the transfixion pattern of fissures. It was shown by the sliding crack model that, the effective shear, which drove the relative sliding of the fissure, was a function of fissure inclination angle and friction coefficient of the fissure surface. The strain-softening model of fissure bodies was established based on the mechanical parameters that were obtained by the test of rock-like materials under the same experimental condition. And the reliability of experimental results was identified by using this model.

Crack initiation stress and strain of jointed rock containing multi-cracks under uniaxial compressive loading: A particle flow code approach

The ratio of crack initiation stress to the uniaxial compressive strength(SCI,B/SUC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength(B,UCB,CI,A,A/SSSS) were studied by performing numerical stress analysis on blocks having multi flaws at close spacing's under uniaxial loading using PFC3 D. The following findings are obtained: SCI,B/SUC,B has an average value of about 0.5 with a variability of ± 0.1. This range agrees quite well with the values obtained by former research. For joint inclination angle, β=90°,B,UCB,CI,A,A/SSSS is found to be around 0.48 irrespective of the value of joint continuity factor, k. No particular relation is found betweenB,UCB,CI,A,A/SSSS and β; however, the average B,UCB,CI,A,A/SSSS seems to slightly decrease with increasing k. The variability ofB,UCB,CI,A,A/SSSS is found to increase with k.Based on the cases studied in this work,B,UCB,CI,A,A/SSSS ranges between 0.3 and 0.5. This range is quite close to the range of 0.4to 0.6 obtained for SCI,B/SUC,B. The highest variability of ± 0.12 forB,UCB,CI,A,A/SSSS is obtained for k=0.8. For the remaining k values the variability ofB,UCB,CI,A,A/SSSS can be expressed within ± 0.05. This finding is very similar to the finding obtained for the variability of SCI,B/SUC,B.

Stress-strain-acoustic responses in failure process of coal rock with different height to diameter ratios under uniaxial compression

Residual coal pillars play an important role in mining the adjacent coal seam safely,managing the gobs and maintaining the stability of abandoned coal mines.The height to diameter ratio(H/D)affects the stability of residual coal pillars.In this study,uniaxial compressive tests of coal specimens with five H/D(2.0,1.5,1.0,0.8 and 0.6)were performed,and the stress,strain and acoustic emission(AE)were monitored.Results show that the uniaxial compressive strength(UCS)and peak strain increase with H/D decreasing.An empirical equation is proposed to calculate the UCS based on the H/D.The AE activities during coal failure process can be separated into four periods.The span of quiet period and rapid decline period shorten with H/D decreasing.The smaller the H/D is,the more complicated the failure characteristics of coal will be.The failure form of coal with H/D of 2.0,1.5,and 1.0 is primarily shear failure,while splitting failure along the axial direction is the mainly mode when H/D is 0.8 or 0.6.The initiation,expansion,aggregation and connection of micro-cracks can be reflected by the real-time spatial evolution of AE event points.

Cracking process of rock mass models under uniaxial compression

Anisotropic strength and deformability of the rock mass with non-persistent joints are governed by cracking process of the rock bridges. The dependence of cracking process of jointed rock masses on the two important geometrical parameters, joint orientation and joint persistence, was studied systematically by carrying out a series of uniaxial compression tests on gypsum specimens with regularly arranged multiple parallel pre-existing joints. According to crack position, mechanism and temporal sequence, seven types of crack initiations and sixteen types of crack coalescences, were identified. It was observed that both tensile cracks and shear cracks can emanate from the pre-existing joints as well as the matrix. Vertical joints were included and coplanar tensile cracks initiation and coalescence were observed accordingly. For specimen with joint inclination angle ,8=75~, it was found that collinear joints can be linked not only by coplanar shear cracks but also by mixed tensile-shear cracks, and that a pair of them can form a small rotation block. Seven failure modes, including axial cleavage, crushing, crushing and rotation of new blocks, stepped failure, stepped failure and rotation of new blocks, shear failure along a single plane and shear failure along multiple planes, were observed. These modes shift gradually in accordance with the combined variation of joint orientation and joint persistence. It is concluded that cracking process and failure modes are more strongly affected by joint orientation than by joint persistence, especially when joint inclination angle is larger than 45~. Finally, variations of macroscopic mechanical behaviors with the two geometrical parameters, such as patterns of the complete axial stress-axial strain curves, peak strength and elastic modulus, are summarized and their mechanisms are successfully explained according to their different cracking process.

Mechanical properties and cracking behaviors of limestone-like samples with two parallel fissures before and after grouting

In the present work,uniaxial compressive tests were carried out on limestone-like samples containing two parallel open fissures or cement-infilled fissures with different geometries.Mechanical property and crack behavior of limestone-like samples with two parallel open fissures or cement-infilled fissures were affected by bridge inclination angle and fissure inclination angle.Four types of coalescence of rock bridge for samples containing open fissures or cement-infilled fissures were summarized and classified.The closure of tensile crack was observed in the samples with small fissure inclination angle.This is a new phenomenon which is not mentioned in previous studies.Test results show that the peak strength,crack initiation stress,and coalescence type are different between open fissures and cement infilled fissures.The reason for this phenomenon is that grouting of cement can transfer stress and reduce stress concentration at the flaw tip and rock bridge area.

Sheet metal hardening curve determined by laminated sample and its adaptability to sheet forming processes

The hardening curve for sheet metal can be determined from the load-displacement curve of tensile specimen with rectangular cross-section. Therefore,uniaxial compression test on cylinder specimen made from laminated sample is put forward. Considering the influence of anisotropy on hardening properties and the stress state in popular forming process,plane strain compression test on cubic specimen made from laminated sample was advanced. Results show that the deformation range of hardening curves obtained from the presented methods is wide,which meets the need for the application in sheet metal forming processes. In view of the characteristics of methods presented in the paper and the stress strain state of various forming processes,the adaptability of the two methods presented in this paper is given.

Numerical investigation on the sensitivity of jointed rock mass strength to various factors

The mechanical properties of jointed rock masses, such as strength, deformation and the failure mechanism, can be understood only by studying the sensitivity of jointed rock mass strength (both the peak and residual strengths) to the factors that affect it. An orthogonal design of uniaxial compression tests was simulated on eighteen groups of jointed rock specimens having different geometric and mechanical properties using RFPA2D (Rock Failure Process Analysis) code. The results show that the peak strength is controlled by the geometric parameters of the joints, but that the residual strength is controlled by the mechanical prop- erties of the joint interfaces. The failure mode of jointed rock specimens is mainly shear failure. Joint quantity, or density, is the most important index that affects jointed rock mass strength and engineering quality.

Deformability characteristics of jointed rock masses under uniaxial compression

We investigated the combined influence of joint inclination angle and joint continuity factor on deforma- tion behavior of jointed rock mass for gypsum specimens with a set of non-persistent open flaws in uni- axial compression. Complete axial stress-strain curves were classified into four types, i.e., single peak, softening after multi-peak yield platform, hardening after multi-peak yield platform and multi-peak dur- ing softening. Observation of crack evolution on the specimen surface reveals that the deformation behavior is correlated to the closure of pre-existing joint, development of fractures in rock matrix and teeth shearing of the shear plane. To investigate the brittleness of the specimens, the ratio of the residual strength to the maximum peak strength as well as the first and last peak strains were studied. At the same joint inclination angle, the ratios between residual strength and the maximum peak strength and the last peak strains increased while the first peak strain decreased with the increase of joint continuity factor. At the same joint continuity factor, the curves of the three brittleness parameters vs. joint inclina- tion angle can either be concave or convex single-oeak or wave-shaoed.

Strength failure and crack coalescence behavior of sandstone containing single pre-cut fissure under coupled stress, fluid flow and changing chemical environment

In order to study the strength failure and crack coalescence characteristics of cracked rocks, uniaxial compression experiments were conducted on cylindrical sandstone specimens, sampled from Longyou Grottoes of Zhejiang Province, China, with a single pre-cut crack soaking in different chemical solutions. Based on the results of uniaxial compressive test under different chemical solutions and velocities of flow, the effect of strength and deformation characteristics and main modes of crack coalescence for cracked rocks under chemical corrosion were analyzed. The results show that the pH value and velocity of the chemical solutions both have great influence on the sandstone sample's uniaxial compressive strength and deformation characteristics. Cracked sandstone samples are tension-destructed under uniaxial compression, and the crack propagation directions are consistent with the loading direction. The phenomena of crack initiation, propagation and coalescence of sandstone are well observed. Four different crack types are identified based on the crack propagation mechanism by analyzing the ultimate failure modes of sandstone containing a single pre-cut fissure. The failure process of specimen in air is similar with the specimen under chemical solutions, however, the initial time of crack occuring in specimen under chemical solutions is generally earlier than that in the natural specimen, and the crack propagation and coalescence process of specimen under chemical solutions are longer than those of the natural specimen due to softening of structure of rock caused by hydro-chemical action. Immersion velocity of flow and chemical solutions does not have influence on the ultimate modes of crack coalescence.

Correlations between direct and indirect strength test methods

The difficulties associated with performing direct compression strength tests on rocks lead to the development of indirect test methods for the rock strength assessment. Indirect test methods are simple, more economical, less time-consuming, and easily adaptable to the field. The main aim of this study was to derive correlations between direct and indirect test methods for basalt and rhyolite rock types from Carlin trend deposits in Nevada. In the destructive methods, point load index, block punch index, and splitting tensile strength tests are performed. In the non-destructive methods, Schmidt hammer and ultrasonic pulse velocity tests are performed. Correlations between the direct and indirect compression strength tests are developed using linear and nonlinear regression analysis methods. The results show that the splitting tensile strength has the best correlation with the uniaxial compression strength.Furthermore, the Poisson's ratio has no correlation with any of the direct and indirect test results.

“固-气”耦合相似材料峰值能量特性试验研究

为了研究骨胶比和阻气胶结剂质量比2个因素对“固-气”耦合相似材料试件单轴压裂过程中峰值能量的影响规律,运用YYW-II-EX无侧限压力仪和SAEU2S多通道声发射系统,对“固-气”耦合相似材料试件进行了单轴压裂的能量耗散试验,对峰值能量进行了线性拟合及敏感性分析。试验结果表明:随着试件抗压强度的增加,“固.气”耦合相似材料试件单轴压裂过程的峰值能量变化呈现3个阶段:剧烈变化阶段、过渡阶段、平缓阶段。峰值能量与试件的骨胶比成对数关系,而与阻气胶结剂质量比成正相关的指数关系;试件在单轴压裂过程中的峰值能量变化的显著区处于骨胶比0.02~0.05和阻气胶结剂质量比0.048~0.063的区域;其平缓区处于骨胶比0.05~0.1和阻气胶结剂质量比0.015~0.035和0.063~0.077的范围;相似材料试件的阻气胶结剂质量比对峰值能量的敏感性更明显,研究结果为二维裂隙演化区域的定量化判别提供了理论依据。

Numerical Simulation of Rock Fracturing under Uniaxial Compression Using Virtual Internal Bond Model

A multi-scale virtual internal bond (VIB) model for the isotropic materials has been recently proposed to describe the material deformation and fracturing. During the simulation process of material fracturing using VIB, the fracture criterion is directly built into the constitutive formulation of the material using the cohesive force law. Enlightened by the similarity of the damage constitutive model of rock under uniaxial compression and the cohesive force law of VIB, a VIB density function of rock under uniaxial compression is suggested. The elastic modulus tensor is formulated on the basis of the density function. Thus the complete deformation process of rock under the uniaxial compression is simulated.