双边非对称裂纹平板试样拉伸试验研究

采用自制的压–拉转换装置,配合RMT 150C岩石力学试验系统及数字散斑相关方法,对双边非对称裂纹类岩石平板试样进行直接拉伸试验,得到类岩石试样的拉应力–应变曲线、试样表面应变场演化过程和裂纹扩展模式。研究发现,类岩石试样直接拉伸的拉应力–应变曲线大致可以分为4个阶段：（1）近似线性阶段,预制裂纹基本不起作用,应力随应变增加较快,试样表面应变场的分布主要受试样内部的孔隙及颗粒的影响;（2）整体缓慢增加阶段,两预制裂纹和试样内部的孔隙及颗粒共同影响试样表面应变场的分布,整体上应力随应变呈增加趋势;（3）短暂峰值过渡阶段,试样中某个预制裂纹对试样表面应变场的分布起决定性作用;（4）破坏阶段,裂纹起裂位置在应变场相对集中区域,并扩展导致试样破坏。对于直接拉伸条件下的双边非对称裂纹平板试样,其中某条预制裂纹会率先扩展,先向远离前方裂纹的方向扩展,再向靠近前方裂纹的方向扩展,对采用数值模拟方法研究张拉应力状态下裂纹相互作用扩展规律具有重要意义。     

拉-压应力状态下脆性岩石强度及破坏机制颗粒流模拟

高地应力区地下岩体工程开挖常形成围岩拉-压应力状态,发生岩体张性破坏灾害。本文针对传统PFC平行黏结模型不能模拟脆性岩石高单轴压缩与拉伸强度比的问题,建立双抗拉强度参数的平行黏结强度准则,开展岩石拉-压数值模拟试验,得到了与物理试验接近的拉-压强度,实现了岩石高压拉强度比的模拟,并深入分析了破坏机制。研究结果表明随着围压的增加,破裂面倾角逐渐增大,由拉伸破裂转化为拉-剪破裂,发现了拉-压应力状态下破裂面处的雁行裂纹。根据细观颗粒位移场揭示了破裂面力学性质,随着围压的增加(破裂面倾角逐渐增大),破裂面张性逐渐减弱而剪性增强。可将拉-压应力状态下岩石损伤演化过程大致分为弹性变形阶段、稳定破裂发展阶段、不稳定破裂发展阶段和整体破裂阶段(峰后应力跌落及残余阶段)。围压较大时弹性变形和稳定破裂发展阶段相对较短,不稳定破裂发展阶段相对较长较剧烈,峰后残余阶段破裂面摩擦更强、应力波动较大。     

岩石倾斜裂隙与水平裂隙扩展贯通试验及数值模拟研究

采用最新配置的水泥砂浆材料,通过在试件中布置倾斜裂隙和水平裂隙,开展了单轴和双轴条件下的压缩试验,详细分析了不同加载条件下试件裂隙的扩展和贯通规律。试验结果表明:无裂隙内水压时,在单轴压缩条件下倾斜裂隙在扩展贯通水平裂隙后裂纹的扩展路径和方向发生明显改变,试样最终发生劈裂破坏。侧压的增大抑制了倾斜裂隙端部翼裂纹和次生裂纹的萌生,高侧压下翼裂纹没有贯穿水平裂隙,试样破坏模式变成了剪切破坏。水力耦合作用下,单轴压缩时倾斜裂隙翼裂纹尖端的最大主应力分布范围随着内水压的增大逐渐变小,翼裂纹的起裂应力、起裂角和峰值强度逐渐降低,试样发生劈裂破坏。通过数值模拟得到的裂纹扩展贯通规律与试验结果具有良好的一致性,开展了不同内水压时倾斜裂隙扩展贯通无水压的水平裂隙的数值分析,随着内水压的增加倾斜裂隙首先萌生共面裂纹随后产生翼裂纹,翼裂纹扩展贯通水平裂隙时扩展路径同样会发生改变。水力耦合作用下侧压也会抑制翼裂纹的萌生,随着内水压的增大减弱了侧压对倾斜裂隙翼裂纹的抑制作用。     

干湿交替作用后砂岩破裂过程实时观测与分析

采用岩石破裂全过程的细观力学试验系统进行了天然状态及干湿交替作用后的完整和含预制圆孔砂岩试件的单轴压缩破裂试验。利用显微镜观察了砂岩裂纹萌生、扩展以及试件的失稳破坏过程。经过干湿交替作用后岩样更容易在较低的应力状态下发生裂纹的扩展,并且裂纹扩展的初始阶段一般表现为拉破裂。完整试件的裂纹是随机性的三维扩展,而有预制圆孔试件一般是从预制圆孔周边开始扩展。天然状态下的预制圆孔试件在起裂时从圆孔的压应力集中区产生剪切裂纹;而干湿交替作用后,起裂时可能先从拉应力集中区产生拉裂纹,并且裂纹演化过程更为复杂一些,破裂形式多样化。与天然状态相比,经过干湿循环作用后的砂岩破坏脆性降低,强度等力学参数也随干湿交替次数增加而逐渐降低。     

花岗岩巴西劈裂渐进破坏特征与能量演化研究

既有的巴西劈裂试验与模拟研究多集中在宏观破坏模式与细观演化规律上,对劈裂渐进过程的能量演化特征的分析较少。结合数字图像相关技术(DIC)与声发射实时监测,开展花岗岩的巴西劈裂实验。首先分析实验过程中,岩石破裂模式与破裂尺度的演化规律。在此基础上,采用颗粒流程序(PFC2D)对劈裂试验过程进行数值模拟,分析实验过程中内部裂纹演化过程的能量特征。试验与数值模拟对比结果表明:加载过程中岩样的损伤变化一共经历了4个阶段,即裂隙压密阶段(Ⅰ)、裂纹萌生阶段(Ⅱ)、裂纹扩展阶段(Ⅲ)、峰后破坏阶段(Ⅳ)。试样在裂隙萌生和裂纹扩展阶段以拉张型微破裂为主,裂纹扩展阶段后期产生剪切型破裂,并在加载直径方向形成大尺度裂纹并贯穿整个圆盘形成宏观破坏;试样在裂隙压密和萌生阶段几乎无耗散能,外力所做功几乎都转为岩体内可释放应变能,在破裂扩展后期应变能快速释放,声发射能量在峰值应力附近时达到最大值,峰后破坏阶段试件的可释放应变能快速减小,能量通过形成大量新裂面被耗散掉。     

节理倾角对黄土力学特性影响试验研究

黄土节理种类多样、角度多变,是影响黄土力学性质的重要因素。为研究节理对黄土力学性质影响,开展含预制节理黄土试样单轴与三轴压缩强度试验,分析了非贯通节理对试样剪切破坏模式影响,探讨了节理倾角对黄土强度与变形特性影响规律。结果表明:节理试样剪切破坏均为压剪破坏,破坏模式可分为节理面与破裂面贯通型和节理面与破裂面斜交型,压力作用下非贯通节理尖端翼裂纹和次生裂纹不断萌生,易劣化扩展形成剪切破裂面;节理存在弱化了试样抗变形能力,单轴压缩条件下试样应力-应变曲线均呈应变软化型,预制节理降低屈服阶段变形模量,减小试样剪切破坏位移,加速试样剪切破坏;节理存在显著降低黄土强度,峰值强度与残余强度均随节理倾角呈现先减小后增大变化趋势,但变化幅度随围压增大而降低,黏聚力随节理倾角变化最为敏感,节理倾角60°时试样强度指标最低;预制节理倾角与黄土试样剪切破裂角越接近,节理面越易劣化贯通为剪切破裂面,试样抗变形能力越差,强度性质劣化越显著,试样越容易剪切破坏。研究成果为揭示黄土节理界面劣化对黄土边坡促滑机制提供参考。     

非贯通单节理岩体裂纹扩展方向研究

裂纹扩展方向的确定对分析岩桥破坏机制和岩体抗剪强度参数具有重要意义。首先以断裂力学观点推导了复杂应力条件下裂纹尖端应力分布函数的表达式,以节理岩体尖端的扩展裂纹可分为张拉裂纹和剪切裂纹为前提,基于Griffith破坏判据,提出了张拉裂纹扩展方向(张裂角)的计算公式;基于Mohr-Coulomb判据,提出了剪裂纹扩展方向(剪裂角)的计算公式。通过新判据与试验和其他判据的结果对比表明,该判据能准确判断张拉裂纹扩展方向,而剪裂角的扩展方向有待进一步试验验证。分析表明:在单向拉应力作用下,张裂纹扩展方向均有偏向于最大主应力方向的趋势,张裂纹与最大主应力夹角小于15°;双向拉应力作用下,随着微裂纹倾角变大,张裂纹有远离最大主应力方向的趋势;单轴压缩作用下,张裂角随裂纹倾角的增加而减小,而两者的和为先减小后增加。      

修正应变能密度因子准则及岩石裂纹扩展研究

由应变能密度因子理论得出裂纹沿着形状改变比能密度因子最小的方向扩展,但理论中所使用的应力强度因子是在拉应力作用下计算得出的,而自然界中的岩体通常处于压应力场中。因此,在修正的应变能密度因子理论的基础上,结合压缩荷载作用下的裂纹尖端应力强度因子,并考虑裂纹面之间的摩擦作用得出了针对压缩荷载作用下的岩石裂纹扩展的应变能密度因子理论,并运用该理论分析了裂纹倾角、围压以及裂面摩擦力对破裂角的影响,将分析结果与已有的试验和数值分析结果进行比较,取得了良好的一致性。分析得出,在单轴压缩荷载作用下临界破裂荷载随着裂纹倾角的增大而先减小、后增大,并且一个裂纹倾角对应多个破裂角,即裂纹朝多个方向发展;在三轴压缩荷载作用下,破裂角与围压大小有关。此研究成果可为压应力场中岩石裂纹扩展的数值模拟提供参考。     

透明类岩石内置三维裂纹扩展变形试验研究

采用力学试验手段探究岩石受压情况下内部裂纹扩展贯通机制是了解岩石破坏失稳机制的重要手段。由于无法观察真实岩体内部裂纹扩展过程且CT扫描实时性不足等原因,自行研制了一种各项性质与岩石接近的透明类岩石材料,以观察研究其内部三维裂纹的扩展贯通机制。这一方法克服了真实岩体不透明的特点,可更方便地观察岩体内部裂纹萌生、扩展不同阶段的形状。然后制作了一批内置单裂隙和双裂隙的试件,在RMT-150B多功能全自动刚性岩石伺服试验机上进行单轴压缩试验,详细观察研究了单裂隙和双裂隙试件在不同岩桥角和裂隙间距情况下的裂纹扩展贯通模式以及裂隙数量和间距对试件抗压强度的影响,并从理论方面对翼形裂纹的扩展过程进行了解释。试验表明,在不同的岩桥角和裂隙间距下,次生裂纹将呈现不同的扩展贯通模式,试验中观察到的次生裂纹有张拉翼裂纹、与翼裂纹反向生长的反翼裂纹和拉剪作用下的花瓣状裂纹等,试件最终破坏是各种形式的裂纹汇合贯通的结果。裂隙的存在极大地降低了试件的抗压强度,且随着裂隙数目增加,试件峰值强度呈降低趋势,同时裂隙间距也对试件的峰值强度产生一定的影响。试验成果对分析真实岩体的破坏失稳机制有着重要的参考价值。     

基于黄土结构损伤的地裂缝局部化破裂发展机制

针对西安地裂缝活动条件下黄土覆盖层裂缝的发展与形成,依据黄土的构度和应力比结构性参数变化规律,分析了黄土构度和抗拉强度之间的关系,建立了黄土结构性演变的数学模型,抗拉强度与抗剪强度随结构性参数变化强度准则,以及反映黄土结构性演变的Mohr-Coulomb屈服面弹塑性模型.通过本构模型嵌入FLAC3D软件,数值模拟西安地裂缝活动边界条件,以及结构性黄土地层结构,进行了黄土结构损伤变化和地层破裂带形成发展分析研究.表明地裂缝错动位移作用条件下,黄土地层塑形区域破裂带发展分为初始剪切破坏、初始拉伸破坏、剪切与拉伸破坏发展、破裂带贯通四个阶段;地层破裂带黄土的结构性损伤衰减变化突出,黄土应力比结构性参数等值线与地层塑性破裂带分布一致.揭示的黄土地层局部化破裂发展过程机制,对于认识地裂缝造成地面建筑物与地下工程灾害有重要的理论和实际意义.     

压实黏土的脆性断裂模型及有限元算法

利用弥散裂缝理论,提出了压实黏土拉伸状态下的脆性断裂模型.当压实黏土达到其极限抗拉强度后,通过建立单元的各向异性刚度矩阵,将土体裂缝弥散于实体单元,构造了平面应变条件下考虑压实黏土脆性开裂的有限元计算模式.通过对某压实黏土单轴拉伸试验成果的模拟计算,验证了构建的脆性断裂模型和有限元算法对土体拉伸破坏特性和裂缝发展过程的适用性.本文还进行了模拟软弱面水压“楔劈效应”的简单数值试验,表明压实黏土脆性开裂模型和算法可较好地模拟裂缝扩展行为.     

Numerical simulation of the fracture process in cutting heterogeneous brittle material

The process of cutting homogeneous soft material has been investigated extensively. However, there are not so many studies on cutting heterogeneous brittle material. In this paper, R‐T^2D (Rock and Tool interaction), based on the rock failure process analysis model, is developed to simulate the fracture process in cutting heterogeneous brittle material. The simulated results reproduce the process involved in the fragmentation of rock or rock‐like material under mechanical tools: the build‐up of the stress field, the formation of the crushed zone, surface chipping, and the formation of the crater and subsurface cracks. Due to the inclusion of heterogeneity in the model, some new features in cutting brittle material are revealed. Firstly, macroscopic cracks sprout at the two edges of the cutter in a tensile mode. Then with the tensile cracks releasing the confining pressure, the rock in the initially high confining pressure zone is compressed into failure and the crushed zone gradually comes into being. The cracked zone near the crushed zone is always available, which makes the boundary of the crushed zone vague. Some cracks propagate to form chipping cracks and some dip into the rock to form subsurface cracks. The chipping cracks are mainly driven to propagate in a tensile mode or a mixed tensile and shear mode, following curvilinear paths, and finally intersect with the free surface to form chips. According to the simulated results, some qualitative and quantitative analyses are performed. It is found that the back rake angle of the cutter has an important effect on the cutting efficiency. Although the quantitative analysis needs more research work, it is not difficult to see the promise that the numerical method holds. It can be utilized to improve our understanding of tool–rock interaction and rock failure mechanisms under the action of mechanical tools, which, in turn, will be useful in assisting the design of fragmentation equipment and fragmentation operations. Copyright © 2002 John Wiley & Sons, Ltd.     

Influence of inclination angles and confining pressures on mechanical behavior of rock materials containing a preexisting crack

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.     

单轴循环荷载下含天然裂隙脆性页岩变形及破裂特征试验研究

利用RMT–150C岩石力学测试系统,对重庆彭水含天然裂隙脆性页岩在单轴循环荷载作用下的变形及破裂特征进行了试验研究。研究结果表明：（1）在循环加卸载和裂隙的共同影响下页岩所含天然裂隙使页岩性质局部劣化、加剧裂隙扩展和破坏提前,导致屈服应力、破裂压力和峰值强度等减小,其中峰值强度降低了13.7%～58.3%;（2）轴向应变形成封闭的“尖叶”状滞回环,并呈疏-密-疏排列,而横向应变形成上开口“8”字形滞回环,并呈密-疏排列,横向应变-循环次数曲线可分为初始变形阶段、小速率等速变形阶段、大速率等速变形阶段和失稳破坏阶段等四阶段演化规律,前期横向应变突变现象可作为天然裂隙和新裂隙扩展、交汇完成,进入大速率等速变形阶段的标志,后期突变可作为整体失稳破坏的前兆;（3）含天然裂隙页岩的破坏模式主要呈拉剪贯通模式和拉贯通模式,两种贯通模式均至少包含一条贯通天然裂隙的拉裂隙;（4）在弹性阶段,有效弹性模量与损伤面积系数呈线性关系,损伤面积系数越大,有效弹性模量越小;（5）在低应力水平内循环,不可逆变形缓慢增加,轴向应变-循环次数曲线始终处于初始变形阶段,试样不发生破坏;在高应力水平内循环,经历3个变形阶段后试样发生破坏;在接近峰值应力的应力水平内循环,曲线直接进入加速变形阶段,几次循环后试样发生破坏。该研究为认清页岩的裂隙扩展形成复杂裂隙网的发展机制提供了有益参考。     

溯源侵蚀引发的拉裂-倾倒型黄土崩塌形成机制

溯源侵蚀是黄土丘陵区最强烈的动力地质作用。常导致沟头后退、坡脚掏蚀,进而引发崩塌灾害。文章通过对研究区崩塌灾害的统计分析,厘定出溯源侵蚀作用引发崩塌的典型类型为拉裂-倾倒型。该类型崩塌具有后壁陡直、粗糙、规模大、分布广、危害范围广,潜在危险性大等特点。分析认为溯源侵蚀引发拉裂-倾倒型崩塌是水力侵蚀和重力侵蚀耦合作用的结果。该类型崩塌的形成,主要是由于斜坡在地表水力侵蚀和重力作用下,坡体内拉应力分布区、剪应力分布区和强烈水力侵蚀区三个区发生了转化,经历了斜坡应力重新分布、拉应力的增大与范围扩大,剪应力状态改变与范围集中、坡顶拉张与孔隙水压力耦合作用下的崩塌体倾倒破坏三个阶段。控制和影响拉裂-倾倒型黄土崩塌稳定性主要因素包括:斜坡几何特征、土体物理力学参数及裂隙中的静水压力。随着土体含水率的上升,坡顶拉张裂缝深度加深,侵蚀凹槽深度加深,崩塌体稳定性逐渐降低。     

爆炸气体驱动下岩体破裂的有限元-离散元模拟

在原有有限元/离散元（FEM/DEM）耦合分析方法中,实现了一种新的爆破计算模型。该模型考虑了在爆生气体的作用下,随着裂隙的扩展,气体占据的体积不断增大,气体压力逐渐减小这一问题。同时考虑了气体嵌入与爆腔联通的裂隙对裂隙的作用力。克服了原有FEM/DEM方法中的爆破模型仅仅将压力施加于爆腔四周的岩壁上,无法考虑爆生气体嵌入生成的裂隙对裂隙的作用。提出了一种新颖的贯通裂隙网络形成的递归搜索算法,只需通过编写一个简单的递归函数,即可实现复杂裂隙网络的搜索,采用一种简洁的方法完成了对复杂问题的处理。最后通过一个爆破算例,结果表明FEM/DEM方法可以对爆炸过程中应力波的传播及岩体中裂纹的萌生、扩展进行全程捕捉,展现了该方法用于爆破模拟的潜力。     

Life-time prediction for rocks under static compressive and tensile loads: a new simulation approach

The life time or time to failure of rocks under load is governed by microstructural defects, like microcracks, voids etc. The life time can be predicted either by empirical exponential laws or physical laws based on damage and fracture mechanics. The proposed numerical model is based on subcritical crack growth using the linear elastic fracture mechanical approach and is implemented as a numerical cellular automate. The algorithm considers both tensile and shear fracturing. Each cell contains a microcrack of random length according to a given probability function. Fracture growth is controlled by the Charles equation. Macroscopic cracks are the results of the coalescence of growing microcracks. Within the numerical approach elasto-plastic stress redistributions take place. If the stress intensity factors have reached the critical values or the microcrack has reached the zone dimension, the zone is considered as fractured and residual strength values are assigned. The proposed approach was applied to rock samples under uniaxial compressive and tensile loads (creep tests). Successful results were obtained in respect to the predicted life time, damage evolution and the fracture pattern. Conclusions for further improvements and extensions of this methodology were drawn.     

Polygonal grain‐based distinct element modeling for mechanical behavior of brittle rock

The microstructure of rock was numerically reproduced by a polygonal grain‐based model, and its mechanical behavior was examined by performing the uniaxial compression test and Brazilian tests via the Universal Distinct Element Code. The numerical results of the model demonstrated good agreement with the experimental results obtained with rock specimens in terms of the stress–strain behavior, strength characteristics, and brittle fracture phenomenon. An encouraging result is that the grain‐based model‐Universal Distinct Element Code model can reproduce a low ratio of tensile to compressive strength of 1/20 to 1/10 without the need for an additional process. This finding is ascribed to the fact that the geometrical features of polygons can effectively capture the effects of angularity, finite rotation, and interlocking of grains that exist in reality. A numerical methodology to monitor the evolution of micro‐cracks was developed, which enabled us to examine the progressive process of the failure and distinguish the contribution of tensile cracking to the process from that of shear cracking. From the observations of the micro‐cracking process in reference to the stress–strain relation, crack initiation stress, and crack damage stress, it can be concluded that the failure process of the model closely resembles the microscopic observations of rock. We also carried out a parametric study to examine the relationships between the microscopic properties and the macroscopic behavior of the model. Depending on the micro‐properties, the model exhibited a variety of responses to the external load in terms of the strength and deformation characteristics, the evolution of micro‐cracks, and the post‐peak behavior. Copyright © 2016 John Wiley & Sons, Ltd.