SHPB压缩试验中红砂岩的力学与能量耗散特性研究

为了定量描述岩石在动态压缩过程中的耗能能力,采用SHPB装置对圆柱形红砂岩试样进行了单轴动态压缩试验,并采用高速摄像仪记录了试样的破坏过程。试验结果表明:随着入射能的增加,承受冲击加载后的试样呈现出完整、破裂和破碎3种不同状态,试样峰值应力呈现明显的应变率效应。在能耗特性方面,以临界入射能为间隔点,试样耗散能呈现出2个阶段的线性增长规律。当施加的入射能小于临界入射能时,试样在冲击后保持完整状态;当施加的入射能大于临界入射能时,试样在冲击后发生破碎,试样碎片飞出。基于线性耗能规律,分别定义了2个阶段的动态压缩耗能系数。当试样呈现完整阶段时,理想的动态压缩耗能系数为定值。在试样承受冲击后发生破碎阶段,动态压缩耗能系数随着入射能的增加而增加。     

水泥尾砂充填体劈裂拉伸破坏的能量耗散特征

应用INSTRON刚性伺服试验机对不同灰砂比的充填体进行了劈拉试验,测得荷载-位移和应力-应变全曲线.结合测得的力学参数计算出相关能量耗散特征参数,分析了不同情况下破坏过程的能量耗散变化规律,并通过数据统计回归建立了各种能量耗散指标与其影响因素的函数关系.实验结果显示:充填体只需吸收极少能量即可满足拉伸破坏所需,拉伸破坏是影响充填体断裂性质的主导因素.为防止充填体处于受拉环境,在矿房回采时应采用边孔控制爆破和不耦合装药,优化爆轰波破岩途径,减少欠挖超挖,保证矿柱形态规整,使充填体处于非拉区域以改善其受力状况;同时在矿房充填时应对需布设工程或应力集中区域等关键部位适当加大灰砂比,以增强充填体吸收破坏能量的能力.     

不同含水率红砂岩冲击过程中的能量耗散特性

地下岩体工程普遍处于不同水环境中,含水率成为影响工程岩体动态力学响应特性的重要因素,严重制约着其安全稳定性。选用红砂岩制备4种含水率试件,采用SHPB试验系统进行相同冲击速度的冲击试验。在分析红砂岩杨氏模量与含水率关系的基础上,基于能量计算方法,分别计算不同含水率红砂岩的总输入应变能、可释放弹性应变能和耗散能。研究3种能量随时间和应变的变化规律,并应用3种能量演化进行岩石微裂纹扩展和破坏判定以及损伤特性分析。结果表明,红砂岩的杨氏模量、起裂时间和破坏时间均随含水率的增加先减小后趋于平缓。随着含水率的增加,各能量演化阶段的起始应变先增大后减小,破坏（卸载）阶段起点处耗散能与总应变能的比值先上升后下降。红砂岩在时间尺度和应变尺度下的损伤累积速率随含水率的增加而增加。      

高压水射流破碎高围压岩石损伤场的数值模拟

利用ALE(arbitrary lagrange-euler)算法,考虑到一般情况下岩石处于高围压状态,建立了高压水射流冲击高围压岩石的数值模型.分析了高压水射流冲击下高围压岩石的损伤演化过程,指出岩石破碎过程呈阶跃式;通过对比无围压状态下岩石和高围压状态下岩石在高压水射流作用下破碎坑演化情况,指出处于高围压状态下的岩石损伤沿轴向的演化速率明显低于无围压状态下的岩石,沿径向的损伤演化受围压影响较小.通过分析4个典型单元在不同速度射流冲击下损伤演化情况,表明在提高射流速度可明显提高射流破岩效率,并在理论上对射流速度与射流破岩性能的关系进行了解释.     

不同围压下岩石峰后及残余变形特性试验研究

为使矿山能安全高效地开采,对深部矿山三向应力状态的岩石进行研究。试验采用RTS-500三轴流变仪,对较深部岩石进行常规三轴压缩试验,得到了应力-应变曲线。试验发现:随着围压的增大,峰值强度和残余强度都呈逐渐增大的趋势;峰值强度与残余强度的差值也逐渐降低;随着围压的增大,岩石峰值强度对应的岩石应变出现增大的现象;随着围压增大,岩石的弹性模量出现增大的现象。结论对于实际施工的工程稳定以及岩体的支护设计都具有指导和借鉴意义。     

围压对红砂岩应力波频谱和频带能量特性的影响

为研究围压对岩石应力波传播衰减特性影响, 对具有不同围压的红砂岩试件进行应力波传播试验.通过傅立叶变换得到反射波和透射波频谱数据, 分析频谱图形状和主频值随围压的变化规律.对反射波和透射波进行小波包分析, 研究不同围压工况下反射波和透射波频带能量分布的变化规律.结果表明, 随围压的增加, 反射波频谱图的幅值呈“减小—缓慢发展—增加”的趋势, 透射波频谱图的幅值逐渐增大后有略微减小, 且宽度也越来越窄.透射波主频值随围压的增加呈现“增大—稳定不变—减小”的趋势, 围压的两个临界值分别为单轴抗压强度的25 %和70 %, 反射波主频值变化与此相反.随围压的增加, 反射波主频带和透射波优势频带所占总能量百分比先增加后减小, 围压对低频带能量百分比的影响更大.      

基于耗散能演化的层状黄砂岩损伤本构模型及其验证

层理构造影响工程岩体力学性质及稳定性.为探究层理倾角对岩石变形损伤过程的影响,开展了0°,15°,30°,45°,60°,75°,90°共7种不同层理角度的黄砂岩纵波波速测试和单轴压缩试验,分析了层理角度对黄砂岩峰值强度、弹性模量及破裂模式的影响,基于弹性模量劣化程度和耗散能演化特征分别表征黄砂岩初始层理损伤变量和荷载损伤变量,并借助Logistic函数模拟了层理与荷载耦合损伤变量演化全过程,探讨了层理角度对黄砂岩损伤演化规律的影响,进一步结合损伤力学理论与有效介质理论,建立了能够模拟单轴压缩下层状黄砂岩变形全过程的分段本构模型.结果表明：随层理倾角增大,黄砂岩纵波波速逐渐增大,峰值抗压强度和弹性模量呈现先减小后增大再减小的倒N型变化趋势,各向异性特性明显;黄砂岩破裂模式与层理倾角密切相关,倾角在0°～60°范围内时,主要发生穿切层理弱面的劈裂型张拉破坏,倾角为75°和90°时,岩样发生沿层理弱面的剪切滑移和劈裂张拉破坏;基于耗散能表征的损伤演化曲线可分为初始无损伤、损伤开始、损伤加速及损伤减速终止4个过程,借助Logistic函数构建的理论损伤模型可以很好的模拟和预测层状黄砂岩损伤演...     

层状复合岩体损伤演化规律及分形特征

利用砂岩、大理岩、花岗岩制作6种不同组合方式的层状复合岩石,采用分离式霍普金森压杆试验系统,对不同组合方式的层状岩石进行动态冲击试验,利用高速相机记录其破坏形态,分析复合岩石材料的动态断裂模式、波阻抗效应以及能量耗散规律,探究不同复合岩石试件的动能及断裂能关系. 利用离散格子弹簧模型模拟复合岩石试件的动态断裂过程,分析复合试件的应力波传播特性及应力、损伤演化规律. 研究结果表明：复合岩石材料的动态断裂特征与上下层材料具有相关性,当下层材料动态起裂韧度较低时,裂纹从起裂至扩展到岩石胶结面历时较短. 上层材料对于复合岩石的应力传导作用具有较大的相关性,上层材料密度越大,更有利于透射波传递,应力传导效果越好,而下层材料与上层材料密度相差越大,胶结面上下端应力差越大;受波阻抗效应影响,复合岩石试件应力波的传播行为具有明显差异,波阻抗越大应力波传播速度越快,透射系数越大,产生更多的透射能;复合岩石试件的耗散能时密度、动能及断裂能与上下层岩石材料的密度有关,下层材料不变,上层材料密度越大时,耗散能时密度及断裂能更小,试件完全断裂时获得较大的动能.     

围压与料浆浓度对尾砂充填体峰后损伤演化研究

对于矿山地下深部开采存在的地压问题,选取3组料浆浓度充填试件在RMT-150C压力机上经过不同围压三轴压缩后再进行单轴压缩试验,获得经过3组围压压缩后不同料浆浓度试件的应力—应变曲线数据,并结合Lemaitre应变等价原理,得到全尾砂胶结充填体的峰前后损伤值公式和损伤本构方程。从损伤—应变曲线可知：当料浆浓度一定时,增大围压,充填体弹性变形阶段缩短,更早达到屈服极限且峰值损伤值增大,继续增大围压,充填材料峰值损伤值降低;在低围压条件下,增大料浆浓度抑制了峰前损伤增长,但促进峰后损伤增长;在高围压条件下,随着料浆浓度的增大,充填材料损伤值先降低后升高,促进了试件破坏。     

Numerical Simulation on Damage and Failure of Brittle Rocks under Different Confining Pressures

A numerical analysis has been conducted to evaluate the effect of confining pressure on the damage evolution and the fracture process of brittle rock materials by using the rock failure process analysis code (RFPA2D). A fractal dimension D associated with material damage and a damage variable ω are defined to describe the damage process of the specimen subjected to tensile or compressive load. We find that the spatial distribution of microfractures is fractal and has very good statistical self-similarity in the specimen under different confined conditions. The fractal dimension D increases progressively with loading. Meanwhile, the damage variable ω develops from 0 to 1, and the specimen failure is characterized by the critical damage variable ωc with 0.4 ? ωc ? 0.8. The initial damage is delayed by the confining pressure. The confining pressure plays an important role in the mode of rock failure in numerical simulations. The tensile microfractures dominate near the brittle splitting of the specimens under no and low confining pressure. Conversely, a few tensile microfractures appear in the ductile shear failures of the specimens under immediate or high confining pressures. The tensile microfractures run through all failure processes in the specimen under uniaxial tensile loading. The stress-strain curves are also obtained in numerical simulation. The results show that the specimen is strengthed by the confinement and fails according to Mohr-Coulomb failure criterion.     

Inelastic Damage Analysis of Reinforced Concrete Bridge Columns Based on Degraded Monotonic Energy

This paper summarizes the prediction of seismic damage of two existing bridges. The objective is to apply a damage index definition for reinforced concrete bridge columns under cyclic loading to existing bridge columns that might experience real seismic loading in the future, and to evaluate the ability of the damage index in describing the damage progression of the bridge columns during real seismic loading. Two existing bridges were selected from the Greater Vancouver Area in Canada. The first bridge, the Garneau Flyover, was designed in 1985 to ATC-6-1981 and is expected to have sufficient resistance to lateral earthquake loading. The second bridge, the Clydesdale Street Underpass, was designed long before ATC-6-1981 and is expected to show little or no lateral earthquake resistance. The damage index is applied to each of these structures, with columns modeled using the CANNY nonlinear structural analysis program. Shear and bond slip deformations were considered by making a simple modification to the column flexural properties. A series of nonlinear dynamic analyses were performed using records from the 1971 San Fernando, 1978 Miyaki-Oki (Japan), 1989 Loma Prieta, and 1999 Taiwan earthquakes fitted to the Vancouver firm ground spectrum. The calculated damage index provides a simple numerical indicator of the damage during an earthquake, easily computed from the results of a nonlinear dynamic analysis.     

Velocity Distribution and Energy Dissipation along Stepped Chutes Lined with Wedge-Shaped Concrete Blocks

Stepped channels lined with wedge-shaped concrete blocks may constitute a low-cost alternative to provide overtopping protection of embankment dams if the discharge capacity of existing spillways is not adequate or even to be used as the main spillway of newly built embankment dams. This paper addresses the velocity distribution and the energy dissipation, downstream of the inception point, on stepped chutes lined with wedge-shaped concrete blocks. An experimental setup was developed with two flumes designed with a relative scale of 1∶2.5. Air concentration was measured with an optical probe in several cross sections of both flumes. The velocity profiles along chutes lined with wedge-shaped blocks with the upper face sloping downstream were analyzed. The measurements’ accuracy was checked by comparing discharges indicated by a facility flowmeter and obtained by the integration of velocity and air concentration profiles. The effect of the steps-slope in the energy dissipation is studied. Values of the Darcy-Weissbach friction factor are proposed for this type of chute lining, for transition flows, and for skimming flows.     

Dynamic Testing of Prestressed Concrete Bridges and Numerical Verification

This paper describes different dynamic tests carried out on three bridges, B14, B15, and B13, over highway E19, which connects Brussels and Antwerpen in Belgium. Different excitation types are considered: a drop weight, a heavy truck on the bridge, and ambient vibrations mainly due to the traffic under the bridge. Finite-element models are constructed to support and verify the dynamic measurements. The modal parameters are extracted from the response time series using the data dependent system approach. A vector autoregressive model is developed and successfully applied to the measured responses in time domain. Good correlation between the finite-element simulations and the experiments is obtained.     

Nonlinear Creep Damage Model for Concrete under Uniaxial Compression

An isotropic model for creep damage of concrete under uniaxial compression is proposed, where the combined effect of nonlinear viscous strain evolution and crack nucleation and propagation at high stress levels is considered. Strain splitting assumption is used for creep and damage contributions. Creep is modeled by a modified version of solidification theory. As usual in the modeling of damage of concrete, a damage index based on positive strains is introduced. As particular cases, the proposed model reduces to linear viscoelasticity for long time low stress levels whereas, for very high stresses, tertiary creep causing failure at a finite time can be described. The effect of strength variation with time is also included. The model is numerically implemented to perform time integration of nonlinear equations by means of a modified version of exponential algorithm. The model is validated through comparison with experimental results. Some numerical examples are also presented, where the roles of concrete ageing and strength variation with time are investigated.     

Improved Image Analysis for Evaluating Concrete Damage

The use of images, whether in routine maintenance, or postearthquake reconnaissance, has quickly become the preferred approach to record and archive the exterior damage of existing infrastructure. Postsurvey analysis of these images, coupled with careful record keeping, provide invaluable data regarding the health of a structure. However, often significant amounts of data are obtained, especially for large structures, such as bridges. Therefore an automated procedure, which reliably and robustly reports on damage observed from these images, with minimal human intervention, is desirable. To this end, in this work, we present a statistical-based method for conducting image analysis, specifically for the purpose of evaluating concrete damage (cracks, spalling, etc.). We illustrate the derivation of the method, which is grounded in Bayesian decision theory and subsequently present results of the analysis of images with discrete cracks to illustrate its promise.     

动静组合荷载下岩石特性的三维数值模拟研究

针对深部岩石工程中,围岩在动荷载压力作用之前就已经承受高静应力或地应力,因此对三维动静荷载联合作用下的岩石变形及破坏特性展开研究具有重要意义。为了探究三维动静组合加载下岩石的力学特性,采用FLAC3D数值模拟方法对岩样进行模拟,以岩石单轴抗压强度的0%、20%、50%和80%作为轴压值,分别设置0 MPa、2 MPa、4 MPa、6 MPa、8 MPa为围压进行加载。得出以下结论:在轴向静压固定、围压变化下,试件的组合强度逐渐升高;当对岩石试件的径向进行加载,增强岩石的整体强度,表现出等效应力逐渐变大;当围压不变,轴向静压变化的情况下,轴向静压值成为岩石试件毁坏程度和破碎速度的主要条件,当压力值增大到一定程度,试件将发生破碎。