Dynamic Tensile Failure of the Rock Interface Between Tuff and Basalt

The dynamic tensile strength properties of the rock interface and its host rocks sampled from the Baihetan Hydropower Station from Western China were measured using a split Hopkinson pressure bar (SHPB). The results were compared with those for its two host rocks. The dynamic tensile strengths of the two host rocks, tuff and basalt have typical loading rate dependence. However, the dynamic response of the rock interface is much more complicated and at a given loading rate, varies between those of tuff and basalt. To explain the observation, numerical simulation using the discrete element method (DEM) was conducted to determine the detailed tensile failure process of the rock interface. The numerical simulation verifies that the variation of the dynamic tensile strength of the rock interface is a result of the variation of the interface geometry.     

Numerical Investigation of the Dynamic Compressive Behaviour of Rock Materials at High Strain Rate

The dynamic compressive strength of rock materials increases with the strain rate. They are usually obtained by conducting laboratory tests such as split Hopkinson pressure bar (SHPB) test or drop-weight test. It is commonly agreed now that the dynamic increase factor (DIF) obtained from impact test is affected by lateral inertia confinement, friction confinement between the specimen and impact materials and the specimen sizes and geometries. Therefore, those derived directly from testing data do not necessarily reflect the true dynamic material properties. The influences of these parameters, however, are not straightforward to be quantified in laboratory tests. Therefore, the empirical DIF relations of rock materials obtained directly from impact tests consist of contributions from lateral inertia and end friction confinements, which need be eliminated to reflect the true dynamic material properties. Moreover, different rocks, such as granite, limestone and tuff have different material parameters, e.g., equation of state (EOS) and strength, which may also affect the DIF of materials but are not explicitly studied in the open literature. In the present study, numerical models of granite, limestone and tuff materials with different EOS and strength under impact loads are developed to simulate SHPB tests and to study the influences of EOS and strength, lateral inertia confinement and end friction confinement effects on their respective DIFs in the strain rate range between 1 and 1,000 s^?1. The commercial software AUTODYN with user-provided subroutines is used to perform the numerical simulations of SHPB tests. Numerical simulation results indicate that the lateral inertia confinement, friction confinement and specimen aspect (L/D) ratio significantly influence DIF obtained from impact tests and the inertia confinement effect is different for different rocks. Based on the numerical results, quantifications on the relative contributions from the lateral inertia confinement and the material strain rate effect on DIF of granite, limestone and tuff material compressive strength are made. The effects of friction coefficient, L/D ratio and rock type on DIF are discussed. Empirical relations of DIF with strain rate for the three rock materials representing the true material strain rate effect are also proposed.     

酸性干湿循环下充填节理岩石劣化性能试验研究

为探究酸性干湿循环作用对充填节理岩石的损伤劣化规律,首先对预制充填节理岩石进行酸性和中性环境下不同次数的干湿循环前处理,再采用万能试验机和分离式Hopkinson杆（split Hopkinson pressure bar,简称SHPB）装置对其进行单轴压缩试验和动态冲击试验,对充填节理岩石纵波波速、静态抗压强度以及动态抗压强度的劣化效应进行分析,并进一步分析酸性和中性环境干湿循环作用下充填节理岩石动态冲击的破坏形态。结果表明,随着干湿循环次数的增加,无论是酸性环境还是中性环境,充填节理岩石的波速值、静态抗压强度以及动态抗压强度不断减小,劣化程度不断加剧,其中酸性干湿循环作用对静态抗压强度的劣化最显著;此外发现干湿循环作用下充填节理岩石静态、动态抗压强度与波速值之间均存在一致性较强的线性关系;酸性环境干湿循环作用下充填节理层粉化程度和两侧岩石破碎程度更严重。研究成果可为酸性环境干湿循环作用下工程岩体稳定性分析提供科学依据。     

不同应变率下砂岩动态强度准则的试验研究

分别利用RMT－150C和多功能分离式霍普金森压杆（SHPB）试验系统对砂岩进行了不同应变率下的单轴压缩、三轴压缩和拉伸试验。结果表明,在围压为0或围压一定的情况下,应变率低于102 s-1时,抗压强度的增加与应变率的量级呈正比;在应变率高于102 s-1时,岩石的抗压强度增加趋势与应变率的1/3次方呈正比。拉伸强度也存在类似规律。针对101～102 s-1应变率范围内缺少岩石动态强度准则这一现状,根据上述试验规律,结合Mohr-Coulomb准则、Hoek-Brown准则和Griffith准则的原理,给出了不同应变率范围内动态Mohr-Coulomb准则和动态Hoek-Brown准则的具体表达形式。研究结果表明,在低应变率情况下,动态Mohr-Coulomb准则和动态Hoek-Brown准则均适用,但Griffith准则判别结果误差很大。在高应变率情况下,动态Mohr-Coulomb准则比较适用,Griffith准则仅适用于评估高应变率单轴抗压强度和抗拉强度之间的关系。     

A Review of the Tensile Strength of Rock: Concepts and Testing

A review of the tensile strength of rock was conducted to determine the relationship between direct tensile strength (DTS) and Brazilian tensile strength (BTS) and to examine the validity of estimating tensile strength from other measured properties, such as the crack initiation (CI) threshold. A data set was gathered from the existing literature where tensile values could be reliably correlated with unconfined compressive strength or CI values. It was determined that the BTS obtained in standard testing is generally greater than the equivalent DTS and that this relationship is rock type dependent. CI yields a reasonable estimate of tensile strength and this correlation is improved when the BTS values are reduced to DTS values by rock type specific correlations. The factor f, in DTS = f BTS, can be considered to be approximately 0.9 for metamorphic, 0.8 for igneous and 0.7 for sedimentary rocks. The relationships presented demonstrate that there is wide scatter in the available data for estimating tensile strength likely due to both specimen variability and testing configuration, including platen geometry and relative stiffness. Estimates of tensile strength should only be used for preliminary design purposes and measurements should be made to confirm preliminary assumptions for each design.     

An Example of Realistic Modelling of Rock Dynamics Problems: FEM/DEM Simulation of Dynamic Brazilian Test on Barre Granite

The scope of this study is to numerically simulate the behaviour of Brazilian disc specimens as observed in laboratory during dynamic, high-strain rate, indirect tensile tests using an innovative combined finite-discrete element method (FEM/DEM) research code. Laboratory experiments using a split Hopkinson pressure bar (SHPB) apparatus were conducted by the authors and the measured indirect tensile strength values were used to verify the FEM/DEM models. In the models the applied boundary conditions, related to the loading rate of the specimen, were matched with the experimental observations. The results of the numerical simulations, including tensile strength and failure time, are in agreement with the laboratory findings. The main failure mechanisms, i.e. tensile splitting along loading axis and shear failure close to loading platens are captured by the numerical model. A linear relationship between tensile strength and loading rate is found for the range of dynamic strain rates tested and simulated. The simulation results are in good agreement with laboratory observations and demonstrate the potential for using FEM/DEM to realistically model dynamic response of rocks.     

Dynamic Brazilian Tests of Granite Under Coupled Static and Dynamic Loads

Rocks in underground projects at great depth, which are under high static stresses, may be subjected to dynamic disturbance at the same time. In our previous work (Li et al. Int J Rock Mech Min Sci 45(5):739–748, 2008), the dynamic compressive behaviour of pre-stressed rocks was investigated using coupled-load equipment. The current work is devoted to the investigation of the dynamic tensile behaviour of granite rocks under coupled loads using the Brazilian disc (BD) method with the aid of a high-speed camera. Through wave analyses, stress measurements and crack photography, the fundamental problems of BD tests, such as stress equilibrium and crack initiation, were investigated by the consideration of different loading stresses with abruptly or slowly rising stress waves. The specially shaped striker method was used for the coupled-load test; this generates a slowly rising stress wave, which allows gradual stress accumulation in the specimen, whilst maintaining the load at both ends of the specimen in an equilibrium state. The test results showed that the tensile strength of the granite under coupled loads decreases with increases in the static pre-stresses, which might lead to modifications of the blasting design or support design in deep underground projects. Furthermore, the failure patterns of specimens under coupled loads have been investigated.     

Static and Dynamic Flexural Strength Anisotropy of Barre Granite

Granite exhibits anisotropy due to pre-existing microcracks under tectonic loadings; and the mechanical property anisotropy such as flexural/tensile strength is vital to many rock engineering applications. In this paper, Barre Granite is studied to understand the flexural strength anisotropy under a wide range of loading rates using newly proposed semi-circular bend tests. Static tests are conducted with a MTS hydraulic servo-control testing machine and dynamic tests with a split Hopkinson pressure bar (SHPB) system. Six samples groups are fabricated with respect to the three principle directions of Barre granite. Pulse shaping technique is used in all dynamic SHPB tests to facilitate dynamic stress equilibrium. Finite element method is utilized to build up equations calculating the flexural tensile strength. For samples in the same orientation group, a loading rate dependence of the flexural tensile strength is observed. The measured flexural tensile strength is higher than the tensile strength measured using Brazilian disc method at given loading rate and this scenario has been rationalized using a non-local failure theory. The flexural tensile strength anisotropy features obvious dependence on the loading rates, the higher the loading rate, the less the anisotropy and this phenomenon may be explained considering the interaction of the preferentially oriented microcracks.     

Discrete element modelling of rock cutting: from ductile to brittle transition

It is well accepted that there is a transition of failure mode from ductile to brittle with increasing depth of cut during rock cutting process. Rock failure modes affect cutting efficiency, and knowledge of the failure transition is essential to the determination of optimum cutting parameters. The critical transition depth can be linked with rock properties. In this study, an attempt was made to model rock cutting process and to check the dependence of the critical failure mode transition depth on the brittleness of rock. For this purpose, dimensional analysis was first performed to establish the correlations between rock macro‐properties and micro‐parameters for discrete element simulations. Following the specimen calibration procedure, two types of synthetic rocks having approximately the same uniaxial compressive strength were generated as the synthetic specimens for simulating the rock cutting process. The first specimen was created using conventional model construction method with identical bond strengths between particles, giving rise to undesirably high indirect tensile strength. The second specimen was created using a proposed clustering algorithm such that the ratio between the tensile and compressive strength matches reasonably well with that of real rocks. The results of rock cutting simulations demonstrate that failure mode transition took place in both models, but for the clustered model the transition emerged at a shallower cutting depth. A further exploration was made to derive the critical depth for this transition based on the simulations performed on the clustered models. The derived relationship indicates that the critical transition depth decreases as strength ratio or brittleness of the rock increases. This provides a very useful tool for predicting the critical depth which can be used to help cutting tool design and cutting parameter optimisations. Copyright © 2015 John Wiley & Sons, Ltd.     

高温后大理岩动态劈裂拉伸试验研究

为了研究高温处理后的岩石材料在冲击加载速率作用下的动态劈裂拉伸性能,利用大直径分离式霍普金森压杆试验设备对经历不同高温作用冷却后的“大理岩”平台巴西圆盘试样进行不同加载速率作用下的径向冲击试验。研究了高温后大理岩的动态劈裂拉伸强度及动态劈裂破坏形式随温度和冲击加载速率的变化规律。试验结果表明,与静态劈裂拉伸强度相比,经历不同高温作用冷却后,大理岩的动态劈裂拉伸强度有明显的提高,表现出显著的冲击加载速率强化效应,同一冲击加载速率作用下,随着温度的升高,动态劈裂拉伸强度表现出先增大后减小的变化趋势;高温后大理岩的动态劈裂破坏形式受到冲击加载速率和温度的共同影响。     

苏南宁镇地区震旦纪至白垩纪的岩石力学性质的系统测试数据浅析

苏南宁镇地区,震旦系至第四系出露完整,研究程度颇深,诸地层的不同特征,以作为华南各地区地层对比之标准而著称。近几年来,随着大规模勘探工作的进行,宁镇地区地层的物性参数、电性参数和力学参数等各种资料,均在不断完善之中。本文试以该区震旦系至白垩系地层的岩石单轴抗压强度等的系统测试数据,对其有关的岩石力学特性进行粗浅地分析。     

Indirect tensile test assessments for rock materials using 3-D disc-type specimens

Indirect test methods are usually the preferred approach for determining the tensile strength of brittle and quasi-brittle materials such as ceramics, rocks, and concrete. In this paper, the tensile strength of a rock material (Iranian Harsin marble) was obtained indirectly by means of two disc type samples. The test samples were the well-known Brazilian disc and semi-disc specimen under three-point bend loading. Since the existing formula for determining the indirect tensile strength is based on 2-D stress assumption which ignores the effects of specimen thickness, a general 3-D tensile strength formulation was derived for each sample by employing finite element analysis. A series of tensile strength tests were conducted on Harsin white marble using disc and semi-disc specimens with different thicknesses in parallel to finite element analysis. The test results using 3-D formulation showed that the tensile strength of the tested marble depends on the thickness/diameter ratio of the specimen and generally decreases from 11.5 to 7 MPa on increasing the thickness of test samples. The average values of tensile strength were about 8.8 MPa for the Brazilian disc specimen and 9.8 MPa for the semi-disc specimen. The good agreement existing between the results of the two specimens suggests that the semi-disc specimen can also be used for determining the rock tensile strength in addition to using the conventional Brazilian test.     

节理岩体动态破坏的SHPB相似材料试验研究

采用相似材料模型试验对不同节理倾角、节理贯通度、节理条数、载荷应变率、节理充填物厚度、节理充填物类型及试件长径比等7种工况下的节理岩体动态强度及破坏模式进行了分离式Hopkinson压杆(SHPB)试验研究。结果表明：节理岩体动态破坏模式及强度与节理构造形态密切相关。对于单节理岩体,其强度及破坏特征在很大程度上受节理倾角控制,节理倾角0°、90°试件动强度分别为完整试件的90%和71%,且其破坏形式均为张拉破坏;倾角60°试件动强度几乎为0;倾角30°、45°试件的动强度分别为完整试件的50%和18%,且其破坏以剪切破坏为主,兼有张拉破坏。中心1/4、1/2、4/5及全贯通节理试件的峰值强度分别为完整试件的95%、74% 、28%和17%,即随节理贯通度增加,试件动强度逐渐降低。含1~3条节理的试件动强度分别为完整试件的54%、23%和10%,即随节理条数增加,试件动强度随之有较大幅度降低,但节理条数的增加并没有改变其破坏模式。随着节理充填物厚度增加及节理充填物强度降低,试件强度依次递减,但破坏模式并没有改变。完整试件和节理试件的动强度均随着载荷应变率的增加而变大,且前者对载荷应变率的敏感性要远远高于后者,相应地试件的破坏模式也变得更加复杂。两类试件的动强度均随着试件长径比的增加先增大后减小,即存在一个最佳长径比。     

加锚岩石抗弯特性试验研究

采用煤层顶板岩石作为加锚基体,用钢丝模拟锚杆,薄钢片模拟钢带,对加锚体进行了三点弯试验,并与常规试件进行对比分析。结果表明,因岩石抗压与抗拉性能的差异,试件在弯曲过程中,下表面拉应变的增长速度大于上表面压应变的增长速度,裂纹最先在下表面产生并逐渐向上发展。加锚试件,因锚杆改善了锚固区域岩石的力学性能,且钢带与锚杆共同承担了一定的拉应力,锚固试件抗弯能力有所增强。试件截面应力状态可以结合破坏过程分为3个阶段,第1阶段为弹性阶段;第2阶段为裂纹产生与扩展阶段,岩石承载能力逐渐劣化,拉应力逐渐向钢带转移,岩石裂纹发展受限,试件抗弯能力有较大提升;第3阶段为破坏阶段,锚杆失黏,挠度持续增加而试件承载能力趋于稳定。     

平台巴西劈裂试验确定岩石抗拉强度的理论分析

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。     

干湿循环条件下煤矿砂岩分离式霍普金森压杆试验研究

由于降雨、季节引起地下水位升降等原因,地壳中岩体常处于干湿循环状态。为研究干湿循环对岩石动态力学性能的影响,以安徽恒源煤矿北风井-259 m处砂岩为研究对象,采用?50 mm变截面分离式霍普金森压杆试验装置,对长径比为0.5的煤矿砂岩试件经干湿循环作用后实施单轴冲击压缩试验,获得了砂岩动态单轴压缩应力-应变曲线。研究发现,由于自由水的Stefan效应,干湿循环1次对砂岩具有增强作用,单轴动态抗压强度最高;其后,随着干湿循环次数的增加,砂岩受水侵蚀物理弱化作用,砂岩动态单轴抗压强度呈乘幂关系降低,砂岩试件的冲击破碎块度逐渐变小。试验结果表明,干湿循环12次时砂岩动态抗压强度比干湿循环1次降低约24%,表现出较强的劣化效应。     

基于开挖卸荷劣化的边坡岩体宏观力学参数USMR分析评价方法及验证

为研究锦屏某边坡岩体在开挖卸荷条件下的岩体宏观力学参数,对该区砂岩进行室内卸荷试验研究,结果表明：相对于加载试验,卸荷过程中岩样的强度及变形参数发生了劣化,低、高围压下岩石变形及破裂形式也有所不同。为此,在对SMR及CSMR修正的基础上,提出了考虑开挖方式、边坡形态及卸荷损伤的边坡卸荷岩体的评价体系USMR法。基于USMR法对研究区边坡开挖岩体宏观参数（变形模量Em、岩体抗压强度 、岩体抗拉强度 、岩体抗剪强度参数cm和 ）进行了动态分析,并分别考虑了不同围压 及卸荷损伤De条件下岩体参数的劣化规律。分析表明：开挖卸荷过程中岩体宏观参数发生了不同程度的劣化,拉应力区、低应力区和高应力区岩体参数的劣化规律有所不同,拉应力区及低应力区岩体力学参数对开挖卸荷更加敏感。研究结果对边坡开挖卸荷岩体的稳定性分析与评价具有一定指导意义。     

等效岩体技术在断续双节理岩石试件破裂机制细观分析中的应用

采用等效岩体技术,将岩体中的岩块和节理分别用颗粒体模型和光滑节理模型描述,从细观角度开展了人工预制含两条断续节理岩石试件力学特性及破裂机制的研究。同时,结合室内试验研究成果,通过计算与试验结果的对比分析,验证了等效岩体技术在断续节理岩体力学特性研究中的适宜性和可靠性。主要研究结果如下：（1）随着断续节理产状的不同,虽可将宏观破裂划分为剪切模式、翼型张拉模式和混合模式等3种类型,但从细观角度看,颗粒间张拉型微破裂均是引起各类宏观破裂模式的主要诱因。（2）在试样达到抗压强度前,破裂声发射事件次数较少,强度均较低,主要孕育于断续节理尖端处,并沿两断续节理尖端的连线在岩桥区域累积贯通。（3）在试样达到抗压强度后,破裂声发射事件次数迅速增加,且破裂强度有所提高。（4）各类试样中,声发射事件累积数和破裂强度之间均近似满足Doseresp生长函数关系。     

An Experimental Study of the Rate Dependence of Tensile Strength Softening of Longyou Sandstone

It is well-known that the strengths of sandstones measured under fully saturated conditions are smaller than those measured under nominally dry conditions. This strength softening phenomenon has profound implications to rock engineering. In this work we investigate the tensile strength softening of Longyou sandstone from China. Defining the strength softening factor as the ratio of the strength under nominally dry conditions over that under saturated conditions, the static compressive strength softening factor of Longyou sandstone is close to 2 and the static tensile strength softening factor is about 7.9. To further address the applications, where the load may be dynamic, we examine the rate dependence of the tensile strength softening of this sandstone. The dynamic tensile strength is measured using the split Hopkinson pressure bar system in combination with the Brazilian disc sample geometry. The results show that the tensile strength softening factor decreases with the loading rate. Because the saturated sample shows stronger loading rate sensitivity than the dry sample, the softening factor decreases with the loading rate.     

昔格达组半成岩微观结构与力学性质研究

以金沙江寨子村昔格达组半成岩为研究对象,通过X射线衍射、电镜扫描,测定了矿物成分、天然与饱和状态矿物颗粒微观结构;通过三轴压缩试验,研究了昔格达组半成岩受水和围压影响的强度及变形变化规律,并探讨了微观机制;通过对昔格达组半成岩、土、软岩强度指标与含水率的关系进行统计,分析了昔格达组半成岩不同于土和软岩的强度特性,并给出了针对此类岩土体的工程分级建议。研究表明：（1）微观结构显示昔格达组半成岩有明显不同于土和岩石的弱胶结结构特征,在饱和后胶结结构易遭破坏;（2）昔格达组半成岩黏聚力、摩擦角均随含水率增加而减小,平均模量在高含水率下随围压增加而增大,围压一定时随含水率增加而减小;（3）昔格达组半成岩、土、软岩的黏聚力大小为软岩>昔格达组半成岩>土,黏聚力对含水率的敏感性为软岩>昔格达组半成岩>土,摩擦角对含水率的敏感性为土>昔格达组半成岩>软岩;（4）将Φ50 mm×100 mm标准试件的单轴抗压强度在0.2～3 MPa,黏聚力在30～200 kPa的岩土体归类为硬土?软岩,建议在工程实际应用中将其与岩石和土进行区分。