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1.
Estimation of uniaxial compressive strength (UCS) by P-wave velocity (VP) is of great interest to geotechnical engineers in various design projects. The specimen diameter size is one of the main factors that influence rock parameters such as UCS and VP. In this study, the diameter size of specimens that effect UCS and VP is investigated. Moreover, the correlation between UCS and VP are examined via empirical analysis. For this purpose, 15 travertine samples were collected and core specimens with a diameters size of 38, 44, 54, 64 and 74 mm were prepared. Then, uniaxial compressive strength and P-wave velocity tests were conducted according to the procedure suggested by ISRM (1981). It is concluded that the diameter size of the specimen has a significant effect on UCS and VP. Moreover, it was found that the best correlation between relevant parameters obtained for the specimen diameter of 38 mm.  相似文献   

2.
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.  相似文献   

3.
To deeply understand the cracking mechanical behavior of brittle rock materials, numerical simulations of a rock specimen containing a single preexisting crack were carried out by the expanded distinct element method (EDEM). Based on the analysis of crack tips and a comparison between stress- and strain-based methods, the strain strength criterion was adopted in the numerical models to simulate the crack initiation and propagation processes under uniaxial and biaxial compression. The simulation results indicated that the crack inclination angle and confining pressure had a great influence on the tensile and shear properties, peak strength, and failure behaviors, which also showed a good agreement with the experimental results. If the specimen was under uniaxial compression, it was found that the initiation stress and peak strength first decreased and then increased with an increasing inclination angle α. Regardless of the size of α, tensile cracks initiated prior to shear cracks. If α was small (such as α ≤ 30°), the tensile cracks dominated the specimen failure, the wing cracks propagated towards the direction of uniaxial compression, and the propagation of shear cracks was inhibited by the high concentration of tensile stress. In contrast, if α was large (such as α ≥ 45°), mixed cracks dominated the specimen failure, and the external loading favored the further propagation of shear cracks. Analyzing the numerical results of the specimen with a 45° inclination angle under biaxial compression, it was revealed that lateral confinement had a significant influence on the initiation sequence and the mechanical properties of new cracks.  相似文献   

4.
By applying the Griffith stress criterion of brittle failure, one can find that the uniaxial compressive strength (σc) of rocks is eight times the value of the uniaxial tensile strength (σt). The Griffith strength ratio is smaller than what is normally measured for rocks, even with the consideration of crack closure. The reason is that Griffith’s theories address only the initiation of failure. Under tensile conditions, the crack propagation is unstable so that the tensile crack propagation stress (σcd)t and the peak tensile strength σt are almost identical to the tensile crack initiation stress (σci)t. On the other hand, the crack growth after crack initiation is stable under a predominantly compressive condition. Additional loading is required in compression to bring the stress from the crack initiation stress σci to the peak strength σc. It is proposed to estimate the tensile strength of strong brittle rocks from the strength ratio of R = \fracs\textc | s\textt | = 8\fracs\textc s\textci . R = {\frac{{\sigma_{\text{c}} }}{{\left| {\sigma_{\text{t}} } \right|}}} = 8{\frac{{\sigma_{\text{c}} }}{{\sigma_{\text{ci}} }}}. The term \fracs\textc s\textci {\frac{{\sigma_{\text{c}} }}{{\sigma_{\text{ci}} }}} accounts for the difference of crack growth or propagation in tension and compression in uniaxial compression tests. \fracsc sci {\frac{{\sigma_{c} }}{{\sigma_{ci} }}} depends on rock heterogeneity and is larger for coarse grained rocks than for fine grained rocks. σci can be obtained from volumetric strain measurement or acoustic emission (AE) monitoring. With the strength ratio R determined, the tensile strength can be indirectly obtained from | s\textt | = \fracs\textc R = \fracs\textci 8. \left| {\sigma_{\text{t}} } \right| = {\frac{{\sigma_{\text{c}} }}{R}} = {\frac{{\sigma_{\text{ci}} }}{8}}. It is found that the predicted tensile strengths using this method are in good agreement with test data. Finally, a practical estimate of the Hoek–Brown strength parameter m i is presented and a bi-segmental or multi-segmental representation of the Hoek–Brown strength envelope is suggested for some brittle rocks. In this fashion, the rock strength parameters like σt and m i, which require specialty tests such as direct tensile (or Brazilian) and triaxial compression tests for their determination, can be reasonably estimated from uniaxial compression tests.  相似文献   

5.
Rocks subjected to long-term loading have been known to suffer microcracking. The rate of cracking is sensitive to the type of the applied stress (tensile or compressive), and the magnitude of the stress relative to the instantaneous strength. In addition, crack growth is influenced by the environment (pressure and temperature) including the presence or absence of moisture.For tensile loading, the sensitivity of granite to time-dependent cracking is demonstrated through a fracture mechanics test known as double torsion. The crack velocity versus stress intensity function is established for two environments, room temperature and humidity and room temperature and 100 percent humidity.For compressive loading, time dependent cracking is evaluated from creep tests conducted in uniaxial compression in the same two environments. The rate of cracking is defined by finding the functional relationship between the rate of crack growth, expressed as the rate of crack volume strain, and uniaxial compressive stress.A variety of mathematical functions has been fitted to the obtained data. The traditionally-used power and exponential relationships give good correlation for both crack velocity and crack volume strain rate.The crack volume strain rate versus stress function can be integrated to obtain a lifetime estimate for Lac du Bonnet granite. After 1 000 years of loading in uniaxial compression at room temperature and 100 percent humidity, the strength of this granite could reduce from 225 MPa to 90–100 MPa.  相似文献   

6.
从康家湾铅锌金矿Ⅲ-1号矿体上盘围岩取大量岩样,分别加工制作了50个压缩和拉伸试验的试样。利用RMT-150B伺服试验系统对试样进行单轴抗压、抗拉试验,各获得了50个试验结果。采用假设检验法,分别对50个单轴抗压强度和50个抗拉强度进行检验,结果表明,它们分别服从正态分布和对数正态分布;对50个 、 和50个C、 ,进行不放回抽样,组成50组E、 、C、 。利用FLAC计算软件,对硐室围岩中的应力进行了计算,分别获得了50个最大主应力和50个最小主应力;采用同样假设检验法,证明它们分别服从对数正态和正态分布;根据单轴抗压、抗拉强度及围岩中的最大主应力、最小主应力概率密度函数,计算了硐室围岩不发生拉伸破坏和压缩破坏的可靠度;并对硐室围岩抗剪强度的校核,得出了该地下硐室围岩稳定的结论。  相似文献   

7.
黄星  李东庆  明锋  邴慧  彭万巍 《冰川冻土》2016,38(5):1346-1352
在寒区工程建筑物设计中,冻土的抗压、抗拉强度是两个重要的力学指标.在负温条件下,对粉质黏土、黄土和砂土进行单轴抗压和劈裂抗拉试验,研究冻土破坏时的破坏形态、破坏机理、应力-应变曲线和拉应力与径向位移关系曲线的形式,分析单轴抗压强度和劈裂抗拉强度的差异以及这两种强度随土质特性和温度的变化规律.试验结果表明：单轴载荷作用下试样破坏后呈鼓状,且表现为应变软化型塑性破坏特征;劈裂作用下产生沿直径向试样两侧延伸的裂缝,不同土质破坏后裂缝扩展的宽度和深度不同;冻土的抗压强度与抗拉强度均与负温存在很好的线性相关性,随温度的降低而增大;在相同温度条件下,冻土的抗压强度大于其抗拉强度;对于同一种冻土,其抗压强度的温度效应比抗拉强度的温度效应显著.本试验分析结果可为寒区工程的实际应用提供参考.  相似文献   

8.
The diametrical compression of a circular disc (Brazilian test) or cylinder with a small eccentric hole is a simple but important test to determine the tensile strength of rocks. This paper studies the failure mechanism of circular disc with an eccentric hole by a 3D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. First, numerically simulated Brazilian tests are compared with experimental results. Special attention is given to the effect of the thickness to radius ratio on the failure modes and the peak stress of specimens. The effects of the compressive strength to tensile strength ratio (C/T), the loading arc angle (2α), and the homogeneity index (m) are also studied in the numerical simulations. Secondly, the failure process of a rock disc with a central hole is studied. The effects of the ratio of the internal hole radius (r) to the radius of the rock disc (R) on the failure mode and the peak stress are investigated. Thirdly, the influence of the vertical and horizontal eccentricity of an internal hole on the initiation and propagation of cracks inside a specimen are simulated. The effect of the radius of the eccentric hole and the homogeneity index (m) are also investigated.  相似文献   

9.
Shale usually exhibits strong anisotropy due to depositional environment and pre-existed microcracks caused by geological loading for a long time. Characterizing mechanical anisotropy properties of shale, especially the tensile strength anisotropy, plays an important role in the successful exploitation of shale gas. In this work, static and dynamic tests with semi-circular bending (SCB) specimen are conducted using hydraulic servo-control machine and modified split Hopkinson pressure bar (SHPB) system, respectively. To survey the tensile strength anisotropy of shale induced by stratification, samples are cored and cut into half by diametrical cutting along different angles relative to the stratification (0°, 30°, 45°, 60°, 90°, C0°). For dynamic tests, the utilization of pulse shaping technique ensures that the samples obtain dynamic equilibrium. The tensile strength values exhibit clear anisotropy under both static and dynamic loading conditions and show typical loading rate dependence at a given angle. An anisotropic index named αk is defined to describe the tensile strength anisotropy at a certain loading rate. The outcomes illustrate that the anisotropic index decreases as the loading rate increases. In addition, failure pattern owns different characteristic under different loading angles with respect to the stratification. These phenomena may be explained by the pre-existing microcracks, and cracks interaction during dynamic loading conditions.  相似文献   

10.
对陕西延安新宝塔山隧道Q2原状黄土进行了不同加载速率的无侧限抗压与贯入抗拉试验,研究了Q2原状黄土抗压与抗拉强度的影响因素。结果表明:加载速率对宝塔山Q2原状黄土抗压和抗拉强度影响较大,抗压强度与抗拉强度随加载速率增大而增大;高径比对抗拉强度也有较大影响,试验时当高径比控制在1.0左右时,高径比对宝塔山Q2原状黄土的抗拉强度影响相对较小,因此采用轴向压裂法测量黄土的抗拉强度时,高径比宜控制在1.0;在试验加载速率范围内,加载速率对宝塔山Q2原状黄土的压拉比影响不大,其压拉比在9.88~13.68范围内变化。  相似文献   

11.
The influence of mineral grain and grain boundary strength is investigated using a calibrated intact (non-jointed) brittle rock specimen subjected to direct shear with a particle-based distinct element method and its embedded grain-based method. The adopted numerical approach allows one to independently control the grain boundary and mineral grain strength. The investigation reveals that, in direct shear, the normal stress (σ n) applied to a rock specimen relative to its uniaxial compressive strength (UCS) determines the resulting rupture mechanism, the ultimate rupture zone geometry, and thus its shear stress versus horizontal displacement response. This allows one to develop a rupture matrix based on this controlling parameter (i.e., σ n/UCS). Mineral grain strength reductions result in the lowering of the apparent cohesion intercept of the peak linear Coulomb strength envelope, while grain boundary strength reductions change the peak linear Coulomb strength envelope to a bi-linear or curved shape. The impact of grain boundary strength is only relevant at σ n/UCS ratios <0.17 where tensile and dilatant rupture mechanisms dominate. Once shear rupture begins to be the dominant rupture mechanism in a brittle rock (i.e., at σ n/UCS ratios >0.17), the influence of weakened grain boundaries is minimized and strength is controlled by that of the mineral grains.  相似文献   

12.
李露露  高永涛  周喻  金爱兵 《岩土力学》2018,39(10):3668-3676
三叉裂隙是自然界普遍存在的一种岩体缺陷形式,其对岩体的力学特性有重要影响。对含预制三叉裂隙的水泥砂浆试样进行室内单轴压缩试验,配合使用摄像机拍摄裂纹的起裂、扩展、贯通过程,通过数字图像技术处理获取试样的应变场云图,并结合PFC2D程序研究不同?、? 条件下试样的强度特征、裂纹模式和裂纹演化扩展规律。研究表明：三叉裂隙对试样单轴抗压强度有明显的削弱作用。当? 恒定为120°时,试样在? = 30°时达到最大抗压强度;当? 恒定为90°时,随?增大,试样抗压强度呈先减小后增大的趋势,且当? = 45°时达到最大抗压强度。试样产生的裂纹可分为3类,分别是张拉型裂纹(Ⅰ型裂纹)、剪切型裂纹(Ⅱ型裂纹)、混合型裂纹(Ⅲ型裂纹)。这3类裂纹通常从裂隙尖端开始产生,并且Ⅰ型裂纹沿加载方向扩展,通常未扩展至试样边界;Ⅱ型和Ⅲ型裂纹通常与加载方向呈一定角度扩展至试样边界。通过对裂纹的几何形态和组成宏观裂纹的微裂纹成分的分析,得知导致含三叉裂隙试样在单轴压缩条件下失效的是张拉破坏。数字图像技术得到的应变云图表明,当载荷达到一定阶段,裂隙尖端出现应力集中,微破裂开始发育并聚集成微破裂区,微破裂区扩大产生宏观裂纹。通过对主应变和剪应变云图分析,发现导致试样失效的是张拉破坏,剪应变在裂纹扩展过程中的影响较小。  相似文献   

13.
This paper presents the results of a photoelastic and numerical study of stress distributions (contours and trajectories) around fault models of various geometries, submitted to a biaxial compressive load. It aims to describe typical biaxial stress behaviours and emphasize the existing differences with the well-known uniaxial compressive load case. Stress trajectories are sometimes shown by joint sets acting as markers of a paleostress field, and they can be interpreted by particular shallow tension–compression situations. At depth fractures can be reactivated, or can dilate under conditions of triaxial compression, and behaviour is essentially controlled by a high stress ratio (high σ3/σ1 ratio). In spite of the potential importance of such stress states on fracture permeability and fluid flow, and although they are frequently found at depth in a reservoir context, such stress conditions are poorly investigated, particularly in terms of stress perturbations.The presented analogue experiments consisted of compression tests done on polymethylmethacrylate (PMMA) plates; this material has mechanical properties comparable to those of brittle rocks in the upper crust, and presents birefringence. The samples contained open defects acting as faults, and the stress trajectories around these faults were investigated using a photoelastic device. Comparable numerical experiments were realised with a finite-element code (Franc 2D), using frictionless fault models.First, the effect of an increasing biaxial compressive load ratio σ3/σ1 on stress trajectories around an isolated open defect was explored. It was shown that the stress trajectories were drastically modified when σ3/σ1 increased from 0.2 to 0.4, this result being consistent with previous studies. In particular, when σ3/σ1 was superior or equal to 0.4, external isotropic points around where trajectories diverged (called repulsive isotropic points) appeared near the tips, but away from the defects. They tended to move away from the defects towards the main load direction when σ3/σ1 increased. The described isotropic points were points of stress trajectory divergence, i.e. points where stress decreased, implying that zones around them were strongly unfavourable to shear reactivation.Second, stress trajectories around fault models of various geometries (oblique isolated defect, dilational jog, compressive jog, and complex patterns) were studied, the applied biaxial compressive load ratio being 0.7. These biaxial stress trajectories were compared with similar uniaxial stress trajectories in order to explore the existing differences between the two regimes. It enabled new stress trajectory geometries to be described and interpreted. In particular, numerous external repulsive isotropic points were observed, and defect tips were shown to be zones of high convergence of stress trajectories. Furthermore, in contrast to the uniaxial compressive load case, stress trajectories and stress contours were geometrically similar within dilational and compressive jogs under biaxial compressive load. For both jog types, the centre of the overlapping zone, and the areas along the wall of a jog-defining fault and facing the overlapping tip of the other fault, proved to be zones of low mean stress, implying that fluids may migrate towards them from the tips of the faults, in response to mean stress gradients. Furthermore, the centre of the jogs exhibiting a high differential stress was a favourable area in terms of fracture reactivation, which may facilitate fluids transfer and storage.It was also observed that for both the uniaxial and biaxial compressive loads, isotropic zones were localised at the acute angle between branching defects.  相似文献   

14.
The prediction of production rate in ornamental stones sawing is very important in cost estimation and process planning of the rock sawing plants. The main aim of this paper is finding a mathematical correlation between production rate and rock brittleness indexes. The utilized data have been collected from several stone factories in Iran. Seventeen different granite and carbonate rocks have been experienced sawing conditions with large-diameter circular saws. The laboratory tests such as uniaxial compressive strength and tensile strength, were carried out on the rock samples which were collected from these factories. The ratio of compressive strength to tensile strength (B 1), the ratio of compressive strength minus tensile strength to compressive strength plus tensile strength (B 2), and the area under the line of compressive strength and the line of tensile strength (B 3) were used as brittleness indexes in this study. Then, the relationships between these brittleness indexes and production rate were investigated by using a regression analysis. The results show that, there is not a good relationship between the production rate and B 1 and B 2. However, the production rate correlates with B 3 very well. Therefore, there is a reliable prediction for ornamental stones production based on B 3 as a brittleness indexes.  相似文献   

15.
理论上土体的抗拉强度与抗压和抗剪强度一样是描述土体力学性质的重要指标之一,也是研究土体张拉破坏特性的基础。由于土体抗拉强度在数值上相对较小,且难以准确测量,在岩土工程领域常常被忽视。随着工程中的张拉破坏问题越来越突出,土体抗拉强度特性引起许多学者的关注,相关研究成果也越来越多。文中对土体抗拉强度试验研究方法进行了系统的归纳和总结,对比分析了各种方法的优缺点,认识到,（1）土体抗拉强度试验方法总体上可分为直接法和间接法两大类,直接法是在试样两端直接施加拉力直到试样发生张拉破坏,根据破坏时的最大拉力及对应的破裂面面积计算出土体的抗拉强度。间接法主要通过一些理论假设,把压应力转换成相应的拉应力并基于一些理论公式计算土体抗拉强度;（2）按试样受力条件,直接法可分为单轴拉伸和三轴拉伸,一般都需要开发专门的拉伸试验设备,以实现拉力荷载的施加及其在试样内的有效传递。常用的方式有粘结、锚固、模具夹持及摩擦力传递等,都各有优缺点,但模具夹持法相对而言更具操作性。间接法中比较有代表性的有巴西劈裂试验、土梁弯曲试验和轴向压裂试验等,一般较适应于刚度较大的土体如化学固化土。最后,笔者提出了今后该课题的研究重点,包括制定土体抗拉强度试验方法规范及标准,研发简单易操作的土体拉伸试验设备,拉伸试验过程中土吸力的测量及控制方法,土体拉伸过程中应变场的准确获取方法及土体张拉特性的数值模拟研究等。  相似文献   

16.
This paper presents the results of laboratory experiments during the investigation of the stress–strain characteristics of Brisbane tuff disc specimens under diametral compressive cyclic loading. Two different cyclic loading methods were used: namely, sinusoidal cyclic loading and cyclic loading with increasing mean level. The first method applied the S–N curve approach to the indirect tensile strength (ITS) of rock specimens for the first time in the literature, and the second method investigated the effect of increasing cyclic loading on the ITS of rock specimens. The ITS of Brisbane tuff disc specimens was measured using the Brazilian tensile strength test. The reduction in ITS was found to be 33% with sinusoidal loading tests, whereas increasing cyclic loading caused a maximum reduction of 37%. It is believed that the fracturing under cyclic loading starts at contact points between strong grains and weak matrices, and that contact points at grain boundaries are the regions of stress concentration (i.e., indenters). Transgranular cracks emanate from these regions and intergranular cracks sometimes pass through the contact points. Once cracking begins, there is a steady progression of damage and a general ‘loosening’ of the rock, which is a precursor to the formation of intergranular cracks.  相似文献   

17.
Cyclic loading tests of rock salt were performed to investigate the characteristics of damage evolution of the surrounding rock in gas caverns. In this experiment, the cyclic loading process was carried out on seven stress levels from 20 to 86 % of the uniaxial compressive strength. The sine wave with a frequency of 1 Hz was adopted in the cyclic loading test. Experimental results show that at the first stress level, the damage evolution is rather limited or negligible under cyclic loading, which is controlled within the elastic limit. With the increase in stress level, the damage evolution becomes more evident. An increasing tendency of damage variable can be observed if the stress level surpasses 40 % of the uniaxial compressive strength. A maximum damage value of around 0.95 is recorded when the stress level is over 85 % of the uniaxial strength. In this paper, a damage evolution equation is also introduced and good agreement is obtained with the experimental data. Based on the experimental data, it is shown that the design practice of gas cavern concerning the degree of strength utilization of the surrounding rock is rather conservative considering the reducing effect of the minimal gas pressure on the damage evolution.  相似文献   

18.
为了完善平台巴西劈裂试验测定岩石抗拉强度的理论基础,基于二维弹性理论,建立对弦载荷下的平台巴西劈裂力学模型,采用应力场叠加法求得圆盘内的应力近似解析解。该应力理论解与有限元数值解一致,证明理论求解的合理性。在此基础上,对比研究了平台加载角对圆盘内应力大小及应力集中程度的影响,其结果表明：平台加载角越大,圆盘加载处的应力集中程度和压拉应力比都急剧降低,而圆盘内的拉应力值和拉伸区却只有一定程度减小;过大或过小的平台加载角都不利于平台巴西试样发生中心拉伸劈裂破坏,其最优平台加载角在20°~30°之间。最后,依据Griffith强度破坏准则,推得岩石抗拉强度的理论计算公式,其结果与已有抗拉强度经验公式及试验所得抗拉强度相符较好。  相似文献   

19.
This study investigates the effect of a heat‐treatment upon the thermo‐mechanical behaviour of a model cement‐based material, i.e. a normalized mortar, with a (w/c) ratio of 0.5. First, a whole set of varied experimental results is provided, in order to either identify or validate a thermo‐mechanical constitutive model, presented in the second paper part. Experimental responses of both hydraulic and mechanical behaviour are given after different heating/cooling cycling levels (105, 200, 300, 400°C). The reference state, used for comparison purposes, is taken after mass stabilization at 60°C. Typical uniaxial compression tests are provided, and original triaxial deviatoric compressive test responses are also given. Hydraulic behaviour is identified simultaneously to triaxial deviatoric compressive loading through gas permeability Kgas assessment. Kgas is well correlated with volumetric strain evolution: gas permeability increases hugely when εv testifies of a dilatant material behaviour, instead of contractile from the test start. Finally, the thermo‐mechanical model, based on a thermodynamics approach, is identified using the experimental results on uniaxial and triaxial deviatoric compression. It is also positively validated at residual state for triaxial deviatoric compression, but also by using a different stress path in lateral extension, which is at the origin of noticeable plasticity. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

20.
This paper presents a new analytical criterion for brittle failure of rocks and heavily over-consolidated soils. Griffith’s model of a randomly oriented defect under a biaxial stress state is used to keep the criterion simple. The Griffith’s criterion is improved because the maximum tensile strength is not evaluated at the boundary of the defect but at a certain distance from the boundary, known as half of the critical distance. This fracture criterion is known as the point method, and is part of the theory of critical distances, which is utilised in fracture mechanics. The proposed failure criterion has two parameters: the inherent tensile strength, σ 0, and the ratio of the half-length of the initial crack/flaw to the critical distance, a/L. These parameters are difficult to measure but they may be correlated with the uniaxial compressive and tensile strengths, σ c and σ t. The proposed criterion is able to reproduce the common range of strength ratios for rocks and heavily overconsolidated soils (σ c/σ t = 3–50) and the influence of several microstructural rock properties, such as texture and porosity. Good agreement with laboratory tests reported in the literature is found for tensile and low-confining stresses.  相似文献   

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