Experiment and Study on Mechanical Property of Sandstone Post-peak Under the Cyclic Loading and Unloading

Engineering research has shown that the surrounding rock of deep roadways experienced many times of post-peak cyclic loading and unloading. So studying on rock mechanical characteristics of post-peak loading and unloading is helpful to control the deep surrounding rocks. The test using RMT-150B rock mechanics testing system, to experiment on the mechanical properties of sandstone of post-peak cyclic loading and unloading. The results show that: the stress–strain curves of post-peak cyclic loading and unloading have significant plastic hysteretic loops. The area of plastic hysteretic loops gloss back. The enveloping outer enclosure of cycle loading curve is the similar as the stress–strain curves of strain softening stage when the samples failure, which shows that post-peak failure of rocks have significant memory. With the increase of cycles, the cumulative deterioration parameters are gradually increased, and the ultimate bearing capacity, elastic of loading section of samples and cumulative deterioration parameters conform with the exponential attenuation function with constant term. With the increase of surrounding pressure, the total energy, dissipated energy and elastic energy of samples are gradually increased. With the increase of cycles, the total energy, dissipated energy and elastic energy of samples all are gradually decreasing, the rate of reduction decreases gradually. The samples exist in vertical splitting and transverse shear combination failure under the post-peak uniaxial cycles, or exist in shear failure under the post-peak triaxial cycles. The shear plane exists slip traces.     

不同应力路径下英安岩声发射b值特征及破坏前兆

对英安岩岩样开展单轴、常规三轴加荷和三轴卸荷试验,并对试验过程进行声发射监测,研究在3种应力路径下英安岩变形破坏过程的声发射b值特征差异,探索声发射b值在岩石破坏前兆方面的规律。研究结果表明:英安岩岩样的声发射b值特征与加载的应力路径关系密切,加荷路径即单轴路径和常规三轴路径下b值变化规律相似,加荷与卸荷路径下b值变化规律差异较大。单轴和常规三轴加荷试验中,围压对声发射b值特征有较大的影响。围压越大,岩样在破坏前,b值的整体水平越高。岩样在破坏时,声发射b值下降幅度越大、降低速率越快,且b值达到整个过程中的最小值。三轴卸荷试验中,围压对声发射b值特征影响不明显。与常规三轴试验相比,三轴卸荷试验在卸荷至一定水平后,其b值整体水平更低,最终破坏时b值的最小值更小。提出当岩样峰值强度后存在明显的应力跌落,此时声发射b值的快速下降可以作为破坏前兆。b值的临界值要通过大量室内岩石声发射试验的统计结果来确定。将声发射b值快速下降、临界值突破0.6作为试验英安岩岩样的破坏前兆。     

岩石摩擦滑动变形演化及声发射特征研究

进行岩石摩擦滑动变形演化过程的声发射特征试验研究。以数字散斑相关方法和声发射系统为试验监测手段,采用双面剪切模型试验方法,开展界面摩擦滑动速率与声发射振铃计数、界面摩擦滑动位移与声发射累计能量、试件变形能密度与声发射b值的对应关系研究。结果表明：（1）声发射振铃计数演化受岩石界面摩擦滑动速率影响,界面摩擦滑动速率的突变与声发射振铃计数激增有较好对应关系;（2）界面摩擦滑动位移与摩擦滑动过程中声发射累计能量演化趋势具有较好的对应关系,可以利用声发射累计能量对岩石摩擦滑动位移的变化趋势进行定性判断;（3）通过声发射b值演化分析可以得出,在剪应力缓慢增长阶段,界面摩擦滑动以随机分布的小尺度微破裂为主;在剪应力线性增长阶段后期,界面摩擦滑动以锁固段的大尺度微破裂为主;在剪应力波动增长阶段,岩石摩擦滑动失稳既可能是由小尺度微破裂为主引发的状态转化,又可能是由以锁固段大尺度微破裂为主引发的稳定状态转换。     

含瓦斯煤岩卸围压声发射特性及能量特征分析

利用自主研发的含瓦斯煤岩热流固耦合三轴伺服渗流装置配合声发射监测系统对不同围压作用下含瓦斯煤岩进行了卸围压试验,试验结果表明:不同围压作用下含瓦斯煤岩声发射事件率、累积振铃计数与应力曲线具有较好的对应关系,振幅总体分布在[42,60]dB之间,随着振幅的增加,声发射事件率呈现出递减的趋势;含瓦斯煤岩失稳破坏时声发射事件率与围压呈线性关系,而声发射累积振铃计数与围压呈指数函数关系;不同围压下含瓦斯煤岩卸围压实验中轴向应力加载阶段和围压卸载阶段能量特征是不同的,随着围压的增大,煤岩加载阶段吸收的能量明显增大,卸载阶段释放的能量也相应的增大,加载阶段和卸载阶段能量变化与初始围压均呈对数关系。      

基于微观尺度的页岩损伤破裂数值试验

为正确认识下寒武统页岩微观尺度破裂特征及声发射信号演化规律,基于数字图像处理技术对含石英充填的页岩进行非均匀性表征,采用岩石真实破裂分析系统RFPA2D-DIP建立数值模型,模拟页岩在不同围压作用下的破坏过程。试验结果表明:在微观尺度结构下,页岩破裂模式大致可归纳为3种形式,分别为倒V形（0、2 MPa）、V形（6 MPa）和倒Z形（10 MPa）。由于石英矿物颗粒分布不规律且具有非均匀性,导致应力分布对页岩微观结构产生显著差异。单轴时,累计声发射（简称AE）表现为“平缓-线性-平缓-稳定”模式;围压为2 MPa时,累计AE表现为“平缓-非线性-激增-平缓-稳定”模式;围压为6、10 MPa时,累计AE表现为“平缓-非线性-稳定”模式。页岩的破裂模式和微观结构破坏规律为页岩气的压裂开采提供重要的理论指导。      

An Experimental Study on the Pre-Peak Unloading Damage Evolution of Marble

The behavior of rock damage evolution under unloading conditions is of utmost importance for the analysis of the stress-induced failure of overstressed rock masses. In this paper, a new experimental approach, the incrementally cyclic loading–unloading pressure test (ICLUP test), is designed to quantify stress-induced micro-fracturing and fracturing under the condition of confining pressure reduction. The experimental results demonstrate that the pre-peak damage and deformation characteristics of marble specimens may be easily quantified by irreversible strains, and two damage stages, namely, the linear steady stage and the nonlinear unsteady stage, which are, respectively, represented as a linear steady rate and a nonlinear unsteady rate of damage evolution, occur along with the increase of unloading damage. The new model is proposed to describe the features of pre-peak unloading damage evolution, and the physical meanings and ranges of its material parameters are explained and analyzed. Furthermore, the evolution of volumetric dilation and elastic parameters which occurs along with the increase of unloading damage is revealed. Also discussed in this paper are the inhomogeneity and initial damage of specimens, as well as related research planned to be performed in the future.     

不同连通率断续节理岩体直剪破坏特征

在边坡稳定性评价中,不同形态、连通率及排列方式的岩桥起决定性作用。本文通过开展室内直剪实验并结合声发射特征参数的研究,揭示了不同连通率岩桥和不同法向应力状态下断续节理岩体的破坏规律。结果表明:在直剪条件下,随法向应力及连通率的变化,岩体破坏面呈现“一”、“X”、“锯齿”、“Z”以及复合型破坏,破坏面起伏度较小,且贯通破坏无时间先后性。法向压力一定时,随着断续岩桥连通率的提高,声发射事件累计计数峰值呈线性增长,计数峰值不断趋于增长。在岩桥连通率一定的情况下,随法向应力的增大,声发射事件计数峰值与累计计数在中等法向应力下最大。对断续节理岩桥变形破坏特征的研究,有利于分析节理岩体的力学特性,有助于边坡工程的稳定性评价与预测。     

三轴作用下的小型围岩试件在开挖卸荷过程中的声发射特征

为了获得巷道围岩开挖卸荷过程中的声发射特征与开挖卸荷状态之间的关系,使用水泥砂浆厚壁圆筒围岩试样,开展开挖卸荷条件下3种不同卸荷速度的声发射试验研究。利用自主研发的小型巷道围岩开挖卸荷模型试验系统,声发射系统及变形监测系统采集试样卸荷过程中的声发射时频信号及变形信息,并对声发射b值进行研究。研究发现：（1）试验过程具有4个特征阶段：加载段―维持段Ⅰ―卸荷段―维持段Ⅱ,这4个阶段与振铃累计曲线有良好对应关系。（2）加载段,高频高幅信号较多,分布范围广,试样内部原始裂隙在三向应力作用下压密明显;维持段Ⅰ,高频高幅信号明显减小,分布范围变窄,试样三向持续压密,变形微小;卸荷段,高、低频信号增多,幅度增加,应变率较高,变形增加速度快,卸荷对试样内侧影响效应更显著;维持段Ⅱ,较慢速卸荷的高幅信号持续增多,内外侧以较低应变率持续产生变形。（3）卸荷速度越大,卸荷段振铃累计曲线出现“陡升”的幅度越高,维持段Ⅱ“陡升”出现越早,幅度越小。（4）卸荷速度增大,b值整体量值减小且越趋于小波幅平稳变化,相对声发射大事件占比提高,破裂尺度更大。     

岩桥直剪细观破坏特征与剪胀效应研究

节理岩体的剪切贯通机制影响着边坡的稳定性。为揭示锁固段型非贯通节理岩体在不同连通率和法向应力下的破坏特征,在室内直剪试验中结合高速摄影与AE特征参数分析其剪切全过程及剪胀效应。结果表明:节理岩体直剪试验中,法向应力的增大及节理连通率的下降会致使峰值剪切应力及峰值剪切位移增大;节理连通率与法向应力对其破坏特征具显著影响,表现为节理连通率较高且法向应力较小时呈直接剪断的特性,节理连通率降低后呈拉剪复合破坏,出现剪胀现象,而法向应力的增大使得剪胀效应呈波动现象;AE特征与岩桥贯通过程一致,事件数峰值随节理连通率的降低及法向应力的增大而增大且位于峰后。试验得到的岩桥细观破坏特征与剪胀效应对研究锁固段型岩质边坡的贯通破坏机制具指导意义。     

Evolution of Rock Cracks Under Unloading Condition

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.     

不同岩石破裂全过程的声发射序列分形特征试验研究

通过对不同岩性的岩石进行单轴压缩声发射试验,获取岩石破裂全过程中的载荷-轴向变形曲线及声发射参数,观察试件破裂失稳时的破坏情况,分析破坏过程的载荷变化关系。着重对比了不同岩石的不同力学性质、岩石声发射序列的时域特征和声发射序列的分形特征。研究结果表明,采用声发射率、能率可以很好地描述岩石破裂损伤的整个阶段;计算岩石声发射率、声发射能率的关联维数,可得出岩石破裂过程的声发射序列具有分形特征;岩石破裂过程的声发射分维值D反映了岩石内部微裂隙的统计演化规律;不同岩性的岩石破裂过程的声发射参数序列的分形特征具有一定的共性;归纳总结出岩体声发射序列分维曲线的演化模式,即波动→持续下降演化模式,提出可以将分维值的持续下降作为岩体破裂失稳的前兆。     

含预制单裂隙花岗岩的真三轴单面临空岩爆试验研究

通过开展含预制单裂隙花岗岩的真三轴单面临空岩爆试验,并利用高速摄像系统和声发射（acoustic emission,简称AE）系统监测岩爆过程,探究了不同产状裂隙岩石的破坏模式、强度变形和声发射演化特性,分析了裂隙产状与岩爆过程及弹射动能之间的关系,对比了含裂隙岩石岩爆发生机制与无裂隙完整岩石的差异。有关力学特性的分析表明,随着裂隙倾角的减小,岩样破坏模式大体呈现由“内剪外劈”向“Z型斜剪”变化的趋势,裂隙对岩石强度的削弱作用不断增大。当裂隙倾角小于30°时,岩石峰值应力普遍仅为完整岩样的一半左右;小倾角裂隙的长度越大,岩样岩板劈裂现象变得显著,形成岩爆坑略微变大,且强度折减幅度越大,峰值轴向应变相应变小;裂隙位置向临空面靠近会加剧岩板的劈裂效应,塑性阶段普遍会产生较大变形并萌生大量裂纹,当裂隙已出露且切断临空面将不易形成岩爆坑。有关岩爆过程及弹射动能的分析表明,随着裂隙倾角的减小,岩样弹射动能呈现先显著降低后小幅回升的变化规律,30°倾角为转变拐点;岩样内部裂隙距临空面越近,裂隙岩样的弹射动能越小;树脂填充裂隙使得岩样弹射动能极大提升,而水泥填充的则无明显提升。有关声发射特征的分析表...     

基于热声发射技术的陕北火烧岩烧变温度识别

开展火烧岩烧变温度的识别对于火烧岩的形成及演化具有重要的指导意义。基于热Kaiser效应,通过火烧岩在热处理过程中的声发射累计能量以及参数RA(上升时间/振幅)和AF(平均频率)值的变化,对陕北神木张家峁火烧岩的阈值温度进行了预测。结果表明:火烧岩热声发射特征能有效识别火烧岩烧变温度,与火烧岩经历温度的岩相学分析基本一致;研究区火烧岩存在显著的地层烧变温度梯度,第1—第5层岩石烧变阈值温度逐渐升高,对于第6层和第7层岩石,最高温度超过700℃,对于第8层岩石,阈值温度为245℃;当热处理温度较低时,岩石内部沿晶拉张裂纹发育,超过600℃后,穿晶剪切裂纹所占比例开始增加,声发射信号出现二次增长,同时声发射出现平静期,累计能量曲线出现多个平台。火烧岩烧变温度的识别,对于煤矿防治水及围岩稳定性研究等具有重要的现实意义。      

冻融及加卸荷条件下川藏交通廊道典型岩石力学特性的劣化规律

为了探究川藏交通廊道沿线典型岩石冻融循环条件下的劣化规律,选取昌都-林芝段的花岗岩、片麻岩和砂岩为试验对象,开展冻融循环条件下岩石加卸荷试验,结果表明:（1）随着冻融循环次数的增加,岩石抗压强度损失率达30%,粘聚力降幅达18.4%,内摩擦角降幅达10.5%,弹性模量逐渐下降,泊松比逐渐增加;（2）三轴压缩试验中,岩样的变形模量呈现与抗压强度类似的劣化趋势,但是变形模量的劣化幅度比抗压强度劣化幅度大;冻融循环作用下岩石抗压强度越大劣化程度越低,对砂岩的劣化最明显,片麻岩次之,花岗岩最小;（3）与三轴压缩试验相比,在卸围压试验中,冻融循环作用对岩石的卸荷量同样有劣化作用,卸荷程度较小时岩石劣化并不明显,随着卸荷量的逐渐增加,卸荷量大于80%时,岩石的变形模量呈指数型下降,泊松比呈指数型增加;（4）随着冻融循环次数的增加,三轴压缩试验中由拉张和剪切破坏造成的裂纹数量增多;卸围压试验中岩石以拉张破坏为主;岩石微裂纹数量增加的同时,不平整度增加,矿物颗粒之间的胶结状态变差;（5）综合试验结果分析,冻融作用对岩石劣化作用最强的为砂岩,其次是片麻岩,最弱为花岗岩.      

Investigation of the acoustic emission characteristics of artificial saw-tooth joints under shearing condition

Shear failure of rockmass along a weak plane occurs frequently in rock slopes and in underground tunnels. To study the shear behaviour and acoustic emission (AE) characteristics of joints under different experimental conditions (asperity height, shear rate, and normal load), irregular artificial saw-tooth joints with different asperity heights were sheared in the laboratory, and the AE signals were detected and analysed. The results demonstrated that the strength of the joints increased with increases in normal load and asperity height, while the strength of the joints first increased and then decreased when the shear rate was elevated. The ideal curve of the cumulative hits could be divided into a quiet period, a slow rise period, and a sharp growth period, which could be used to monitor and predict the potential shear failure of the joints. The higher the asperity was, the higher the peak energy rate and the lower the peak hit rate, and cumulative hits at failure were because of differences in asperity size and number. The peak hit rate and cumulative energy tended to increase gradually with the decrease in shear rate, and the minimum peak hit rate and energy rate at failure were both attained at the maximum shear rate. In addition, curves of the energy rate and hit rate showed large fluctuations at higher shear rates. The peak energy rate and cumulative energy under low normal stress could be greater than the peak energy rate and cumulative energy under high normal stress, and the peak hit rate and cumulative hit number under high normal stress could be larger than that of under low normal stress.     

卸荷条件下岩石变形特征及本构模型研究

基于岩石试件的卸荷试验,研究了卸荷条件下岩石的应力—应变全过程曲线和破裂特征,研究表明：卸荷过程中岩石向卸荷方向回弹变形强烈、扩容显著、脆性破坏特征明显;卸荷条件下岩石破坏具有较强的张性破裂特征,各种级别的张裂隙发育,其剪性破裂面追随张拉裂隙发展。基于试验认识,将卸荷条件下岩石的本构模型分为4段：弹性段、卸荷屈服段、峰后脆性段及残余理想塑性段;通过体积应变εv将Griffith和Mohr Coulomb屈服准则结合起来,建立卸荷条件下岩石的屈服准则,并在应变空间建立相应的卸荷非线性屈服段本构方程;运用应变软化理论,建立卸荷岩石峰后脆性段本构方程。     

岩体在加,卸荷条件下破坏效应的对比分析

在岩体加、卸荷试验研究的基础上，从应力－应变关系、强度特性、声发射及破坏特征等方面，对岩体在加、卸荷条件下的破坏特性进行对比分析。结果表明：同类型的岩体在相同的主差应力作用下，卸荷产生的扩容量比加荷的更大；卸荷破坏的强度比加荷的更低；卸荷产生的声发射率比加荷的更高；加荷下岩体易呈剪切破坏特征，而卸荷下岩体常呈张剪复合型破坏特征。     

Mechanical Behaviour of Reservoir Rock Under Brine Saturation

Acoustic emissions (AE) and stress–strain curve analysis are well accepted ways of analysing crack propagation and monitoring the various failure stages (such as crack closure, crack initiation level during rock failure under compression) of rocks and rock-like materials. This paper presents details and results of experimental investigations conducted for characterizing the brittle failure processes induced in a rock due to monocyclic uniaxial compression on loading of two types of sandstone core samples saturated in NaCl brines of varying concentration (0, 2, 5, 10 and 15 % NaCl by weight). The two types of sandstone samples were saturated under vacuum for more than 45 days with the respective pore fluid to allow them to interact with the rocks. It was observed that the uniaxial compressive strength and stress–strain behaviour of the rock specimens changed with increasing NaCl concentration in the saturating fluid. The acoustic emission patterns also varied considerably for increasing ionic strength of the saturating brines. These observations can be attributed to the deposition of NaCl crystals in the rock’s pore spaces as well some minor geo-chemical interactions between the rock minerals and the brine. The AE pattern variations could also be partly related to the higher conductivity of the ionic strength of the high-NaCl concentration brine as it is able to transfer more acoustic energy from the cracks to the AE sensors.     

Characteristics of the unloading process of rocks under high initial stress

The unloading process of rocks under high initial stress is complex, and verifying the mechanism of the unloading process in the field or in a laboratory is not straightforward. In this study, the unloading process of rocks under high initial stress was characterised by a mathematical physics model, which was then implemented in the finite element program LS-DYNA for analysis. In particular, the implicit and explicit methods were performed in sequence in the finite element simulation of rocks with initial stresses. In the numerical simulation, the characteristics of the dynamic unloading process of rocks were investigated for various peak initial stresses, initial stress release paths and initial stress release rates (ISRRs). The numerical results indicated that the rock failure could be induced by the release of the initial stress; furthermore, there is a relationship between the magnitude of the unloading failure and the peak initial stresses, the initial stress release paths and the ISRRs. When the initial stresses were at the same level, the equivalent initial stress release rate (EISRR) was introduced to quantitatively describe the characteristics of the unloading process. Using the numerical results, the unloading failure process was characterised, and a method for the static stress initialisation-dynamic unloading of rock was developed.     

高地应力区砂岩在卸荷条件下的变形参数劣化试验研究

以锦屏某高边坡砂岩所赋存的高地应力环境为基础,开展峰前卸围压力学特性试验,研究了应力-应变全过程曲线、变形特征及参数劣化效应。试验结果表明,相对于加载试验,岩样卸荷破坏表现出明显的脆性特征,相同初始围压下卸荷破坏所需偏应力较加载试验少;卸荷过程横向应变和体积应变急剧增大,卸荷方向表现出明显的扩容特征;当初始围压小于某一值（30 MPa）,岩样破坏所需卸荷量随初始围压增大而增大,当初始围压达到一定值（40 MPa时）,脆性特征更加明显,很少的卸荷量即可引起岩样破坏。以表征岩石卸荷程度的卸荷量H为参变量,分别对高、低围压下卸荷过程中变形参数（变形模量和泊松比）随卸荷量变化关系进行拟合,得到高、低地应力区卸荷过程中变形劣化参数的关系式。试验结果和计算方法对高应力区边坡开挖的稳定性评价具有一定指导意义。