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1.
Experimental and numerical study of hygro-thermo-mechanical properties of “Schilfsandstein” from Baden-Württemberg 总被引：1，自引：0，他引：1
J. Ožbolt G. Grassegger P. Van der Veken G. Periškić H.-W. Reinhardt 《Environmental Geology》2008,56(3-4):535-546
In the present paper the results of experimental and numerical investigations of hygro-thermo-mechanical properties of “Schilfsandstein” from Baden-Württemberg are presented and discussed. The aim of the study was to investigate the influence of extreme environmental conditions causing severe damage that can often be observed on various monuments during restoration and reconstruction process. In the experimental investigations the stone specimen was exposed to extreme hygro-thermal conditions, which caused significant damage (cracking). During the experiment the ultrasonic and acoustic emission analyses were performed. Subsequently, a 3D finite element analysis was carried out using a coupled hygro-thermo-mechanical model for stone. The experimental and numerical results indicate that wetting in combination with either decrease or increase of temperature lead to severe damage. The most critical locations for initialization of cracks are sharp spikes (notches), or acute angles that are inward or outward oriented (corners). Stiff but brittle layers, like late diagenetic iron cementations, might be a weak point for the initialization of crack. Furthermore, the bedding direction (anisotropy) seems to be a preferred direction for crack propagation. The results of the numerical analysis are in relatively good agreement with experimental observations. 相似文献
2.
Preexisting flaws and rock heterogeneity have important ramifications on the process of rock fracturing and on rock stability in many applications. Therefore, there is great interest in numerical modelling of rock fracture and the underlying mechanisms. We simulated damage evolution and fracture propagation in sandstone specimens containing a preexisting 3-D surface flaw under uniaxial compression. We applied the linear elastic damage model based on the unified strength theory following the rock failure process analysis code. However, in contrast to the rock failure process analysis code, we used the finite element method with tetrahedron elements on unstructured meshes. It provided higher geometrical flexibility and allowed for a more accurate representation of the disk-shaped flaw with various flaw depths, angles, and lengths through locally adapted meshes. The rock heterogeneity was modelled by sampling the initial local Young's modulus from a Weibull distribution over a cubic grid. The values were then interpolated to the computational finite element method mesh. This method introduced an additional length scale for the rock heterogeneity represented by the cell size in the sampling grid. The generation of three typical surface cracking patterns, called wing cracks, anti-wing cracks, and far-field cracks, were identified in the simulation results. These depend on the geometry of the preexisting surface flaw. The simulated fracture propagation, coalescence types, and failure modes for the specimens with preexisting surface flaw show good agreement with recent experimental studies. 相似文献
3.
Swelling clays in stone can generate damaging stresses during a wetting or a drying cycle, which lead to deterioration of building stones such as Portland Brownstone. There are two primary types of swelling identified for clays: short-range, ordered intracrystalline swelling, and long-range, continuous osmotic swelling. Identification of the swelling mode is important for understanding and ultimately preventing swelling damage. Through comparison of XRD and swelling experiments with cationic pretreatments and organic solvents, we demonstrate that intracrystalline swelling is the primary mode of swelling present in three different stones, including Portland Brownstone. The results highlight the importance of the counterbalancing cation to the swelling process, and a method for characterizing the intracrystalline swelling in sandstones is developed. Finally, the implications of long-term swelling behavior for stones are discussed. 相似文献
4.
对含预制裂纹的花岗岩进行单轴压缩试验研究预制裂纹倾角α对脆性岩石渐进性破坏过程的影响。首先埘破坏过程的轴向应力-横向应变曲线进行总结和讨论,然后分析预制裂纹与加载方向夹角α埘岩石的应应力门槛值：裂纹起始应力σci、裂纹扩展应力σed、峰值强度矿σf,由应变片记录的应力-应变曲线和试样的表嘶裂纹扩展情况的影响机制。结果表明,含有预制裂纹的岩石试样进行加载试验过程中,顶制裂纹倾角α的变化成了决定脆性岩石破裂办式的主要因素。故在对含节理、裂隙的脆性岩石的工程应用上,通过对岩休的轴向应力-横向应变向应变典线进行分析,可以对地下开挖工程起到指导设计开挖方式及支护形式的作用。 相似文献
5.
粘土干湿循环中裂缝演变过程的数值模拟 总被引：6，自引：1，他引：6
在非饱和土简化固结理论基础上,对粘土在干湿过程中,表面裂缝从形成、发展到闭合的发展过程进行了数值模拟,模拟结果说明,本文采用的基于非饱和简化固结理论的计算方法是可行、实用的. 相似文献
6.
为研究温度冲击下煤的微观结构变化及其损伤断裂机制,以干燥颗粒煤为研究对象,分别开展了煤样的冷冲击和热冷冲击试验。利用扫描电镜（SEM）观测图像结果,分析对比了两种温度冲击前后煤样微观形貌、裂缝分布、开裂和延伸情况,结合断裂力学理论分析了煤样内部微裂缝的开裂机制和扩展方向,并通过ANSYS有限元软件模拟了微裂缝扩展时的应力场和位移场的分布情况,揭示了煤样的断裂机制。研究结果表明,两种温度冲击对煤的结构均造成了不同程度的破坏,温度冲击所形成的热应力最终导致了原始裂纹和新生裂纹的扩展和延伸;温度冲击下所产生的裂纹形式主要有沿晶裂纹、穿晶裂纹、翼型裂纹、交叉裂纹、枝须状裂纹和网状裂纹;分析结果表明,温差愈大,所产生的温度热应力愈大,热冷冲击所产生的裂缝的数量更多、扩展更充分,对煤样的破坏更严重,因此,热冷冲击的破煤效果更好。 相似文献
7.
本文运用伤力学和断裂力学理论,从能量的角度对结构性土体裂纹扩展进行了全过程分析,分析结果表明,结构性土体抗剪的断裂韧度小于其抗拉断裂韧度,在土体破损分析中,若能考虑运用损伤因素的影响来评价土体的工作状态,则比较安全。 相似文献
8.
《Geomechanics and Geoengineering》2013,8(1):27-41
Clays generally crack upon drying and the cracks gradually close up because of expansion of the clays induced by rainfall infiltration. Based on the concept of air drainage ratio, we introduce an improved simplified consolidation theory for unsaturated soils and apply it to solve the crack formation problem. We present initial conditions, special consideration, and finite-element (FEM) formulations for simulation of cracks under axisymmetric conditions. Similar to finding solutions for sand-well consolidation problems, a prism of clays surrounded by polygonal distributed cracks is simplified as a cylinder. Numerical simulations using the FEM formulations are performed on the processes of crack occurrence, propagation, and closure during drying and wetting. To investigate the influence of air drainage ratio distribution, three different schemes are adopted for computation. It is found that the behaviour of cracks in clays during drying and wetting can be well represented using the approach proposed in the paper. The simplified consolidation theory used in the paper for unsaturated soils is more suitable for crack analysis than the general consolidation theory currently applied. 相似文献
9.
S. McCabe B. J. Smith J. J. McAlister M. Gomez-Heras D. McAllister P. A. Warke J. M. Curran P. A. M. Basheer 《Environmental Earth Sciences》2013,69(4):1225-1235
Salt weathering is a crucial process that brings about a change in stone, from the scale of landscapes to stone outcrops and natural building stone façades. It is acknowledged that salt weathering is controlled by fluctuations in temperature and moisture, where repeated oscillations in these parameters can cause re-crystallisation, hydration/de-hydration of salts, bringing about stone surface loss in the form of, for example, granular disaggregation, scaling, and multiple flaking. However, this ‘traditional’ view of how salt weathering proceeds may need to be re-evaluated in the light of current and future climatic trends. Indeed, there is considerable scope for the investigation of consequences of climate change on geomorphological processes in general. Building on contemporary research on the ‘deep wetting’ of natural building stones, it is proposed that (as stone may be wetter for longer), ion diffusion may become a more prominent mechanism for the mixing of molecular constituents, and a shift in focus from physical damage to chemical change is suggested. Data from ion diffusion cell experiments are presented for three different sandstone types, demonstrating that salts may diffuse through porous stone relatively rapidly (in comparison to, for example, dense concrete). Pore water from stones undergoing diffusion experiments was extracted and analysed. Factors controlling ion diffusion relating to ‘time of wetness’ within stones are discussed, (continued saturation, connectivity of pores, mineralogy, behaviour of salts, sedimentary structure), and potential changes in system dynamics as a result of climate change are addressed. System inputs may change in terms of increased moisture input, translating into a greater depth of wetting front. Salts are likely to be ‘stored’ differently in stones, with salt being in solution for longer periods (during prolonged winter wetness). This has myriad implications in terms of the movement of ions by diffusion and the potential for chemical change in the stone (especially in more mobile constituents), leading to a weakening of the stone matrix/grain boundary cementing. The ‘output’ may be mobilisation and precipitation of elements leading to, for example, uneven cementing in the stone. This reduced strength of the stone, or compromised ability of the stone to absorb stress, is likely to make crystallisation a more efficacious mechanism of decay when it does occur. Thus, a delay in the onset of crystallisation while stonework is wet does not preclude exaggerated or accelerated material loss when it finally happens. 相似文献
10.
The failure mechanisms induced by a wedge‐shaped tool indenting normally against a rock surface are investigated using the discrete element method (DEM). The main focus of this study is to explore the conditions controlling the transition from a ductile to a brittle mode of failure. The development of a damage zone and the initiation and propagation of a brittle fracture is well captured by the DEM simulations. The numerical results support the conjecture that initiation of brittle fractures is governed by a scaled flaw length Λ, a ratio between the flaw size λ and the characteristic length (where KIc is the toughness and σc the uniaxial compressive strength). The size of the damage zone agrees well with analytical predictions based on the cavity expansion model. The effects of a far‐field confining stress and the existence of a relief surface near the indenter are also examined.Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
11.
页岩储层天然裂缝、水平层理发育,水力压裂过程中可能形成复杂的体积裂缝。针对页岩储层体积裂缝扩展问题,基于流-固耦合基本方程和损伤力学原理,建立了页岩储层水力压裂体积裂缝扩展的三维有限元模型。将数值模型的模拟结果与页岩储层裂缝扩展室内试验结果进行对比,二者吻合较好,从而证明了数值模型的可靠性。通过一系列数值模拟发现：（1）水力压裂过程中水平层理可能张开,形成水平缝,水平与垂直缝相互交错,形成复杂的体积裂缝网络;（2）水平主应力差增大,体积裂缝的分布长度（水平最大主应力方向压裂裂缝的展布距离）增加、分布宽度（水平最小主应力方向压裂裂缝的展布距离）减小,体积裂缝的长宽比增加;（3）压裂施工排量增大,体积裂缝的分布长度减小、宽度增加,压裂裂缝的长宽比降低;（4）天然裂缝的残余抗张强度增大,体积裂缝分布宽度减小、分布长度增加,体积裂缝的长宽比增加。研究成果可以为国内的页岩气的压裂设计和施工提供一定的参考和借鉴。 相似文献
12.
The safety and stability of core wall rockfill dams during impoundment are threatened by the wetting deformation of up-stream shell materials. The serious wetting deformation not only aggravates the collapse settlement of upstream rockfill but also intensifies differential settlement of the dam crest during impoundment, and then causes cracks on the dam crest. On the basis of the proposed wetting deformation model and its simulation method of wetting deformation, this paper simulated the impoundment process of Guanyinyan core wall rockfill dam and studied the deformation characteristic of the dam during impoundment. In addition, the smeared cracking model was used to simulate the crack propagation on the dam crest, and the crack develop and spread mechanism was analysed. The results show that the simulated deformation can fit the in-situ data well, and the simulated crack propagation is in good agreement with the actual situation. Once watered, the upstream rockfill and core wall have significant settlement, and the whole dam crest has obvious horizontal displacement towards the upstream. It is on the same order of magnitude that the increment of horizontal displacement and settlement at the top of the dam during impoundment. In the process of impoundment, the upper part of the dam tends to deform towards the reservoir area, which will lead to tensile cracks appearing in the rockfill areas on both upstream and downstream sides of the core wall of the dam crest, and the propagation direction of the cracks is basically parallel to the adjacent core wall surface. With the water level rising, the cracks on the downstream side of the dam crest mainly extend vertically, and the cracks on the upstream side of the dam crest not only extend vertically, but also extend horizontally. 相似文献
13.
We report on the crystallization behavior and the salt weathering potential of Na2SO4, MgSO4 and an equimolar mixture of these salts in natural rock and porous stone. Geochemical modeling of the phase diagram of the ternary Na2SO4–MgSO4–H2O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Model calculations include stable and metastable solubilities of the various hydrated states of the single salts and the double salts Na2Mg(SO4)2·4H2O (bloedite), Na2Mg(SO4)2·5H2O (konyaite), Na12Mg7(SO4)13·15H2O (loeweite) and Na6Mg(SO4)4 (vanthoffite). In situ Raman spectroscopy was used to study the phase transformations during wetting of pure MgSO4·H2O (kieserite) and of the incongruently soluble salts bloedite and konyaite. Dissolution of kieserite leads to high supersaturation resulting in crystallization of higher hydrated phases, i.e. MgSO4·7H2O (epsomite) and MgSO4·6H2O (hexahydrite). This confirms the high damage potential of magnesium sulfate in salt damage of building materials. The dissolution of the incongruently soluble double salts leads to supersaturation with respect to Na2SO4·10H2O (mirabilite). However, the supersaturation was insufficient for mirabilite nucleation. The damage potential of the two single salts and an equimolar salt mixture was tested in wetting–drying experiments with porous sandstone. While the high damage potential of the single salts is confirmed, it appears that the supersaturation achieved during wetting of the double salts at room temperature is not sufficient to generate high crystallization pressures. In contrast, very high damage potentials of the double salts were found in experiments at low temperature under high salt load.1 相似文献
14.
单轴压缩下含倾斜单裂纹砂岩试件裂纹扩展量测研究 总被引：3，自引：0，他引：3
选用砂岩作为试验材料,进行不同倾斜角度单预制裂纹的单轴压缩试验研究,透过裂纹扩展量测技术以及配合相关断裂准则,得到：(1) 理论与实验的裂纹起裂角随裂纹倾斜角增加而增大;(2) 透过相关断裂准则,砂岩试件在裂纹尖端产生反翼裂纹是因为剪力破坏;(3) 在单轴压缩作用下砂岩试件裂纹扩展受单裂纹的倾斜角影响,当裂纹倾斜角较小时,裂纹不易起裂;当裂纹倾斜角较大时,裂纹起裂比较容易;(4) 在单轴压缩作用下砂岩试件则受单预制裂纹的倾斜角影响,当裂纹倾斜角较小时,裂纹扩展属于非稳定扩展;当裂纹倾斜角较大时,裂纹扩展属于稳定扩展;(5) 在单轴压缩作用下砂岩试件裂纹尖端A侧与裂纹尖端 B侧的裂纹扩展平均速度随裂纹倾斜角增加而降低。 相似文献
15.
The crystallisation of soluble salts within the pores of the stone is widely recognised as a major mechanism causing the deterioration
of the stone-built architectural heritage. Temperature, in turn, is one of the main controls on this process, including salt
precipitation, the pressure of crystallisation and the thermal expansion of salts. Most laboratory experiments on decay generated
by salts are just carried out with convective heating regimes, while in natural environments building stones can undergo radiative
and convective heating regimes. The thermal response of stone to these different heating regimes is noticeably different and
might influence the crystallisation patterns of a salt within a stone. The aim of this work is to raise awareness on the different
patterns of crystallisation of NaCl within a porous stone tested with different heating regimes (convection and radiation)
and the implications that this could have on the design of experimental modelling of natural weathering conditions in laboratory
simulations. Results show that heating regime affects the sodium chloride distribution within a stone with high percentage
of microporosity. In this case, radiation heating facilitates the generation of subefflorescences, while convection heating
promotes efflorescences. This has a clear implication both on the stone decay in natural environments and on the methodologies
for testing salt decay, as subefflorescences are more destructive than efflorescences. In this sense, the use of convective
heating in laboratory experimentation might underestimate the potential damage that sodium chloride may generate. This counsels
the use of radiation heating test methods in addition to convection for the laboratory study of salt crystallisation. 相似文献
16.
Jaime Horta Eduardo Rojas María L. Pérez‐Rea Teresa López Juan B. Zaragoza 《国际地质力学数值与分析法杂志》2013,37(8):932-944
This paper describes a random solid‐porous model capable of simulating the structure of porous materials. To this purpose, the grain and pore size distributions as well as the void ratio of the material are required. Solids and pores are distributed at random in the model's space according to a size strategy. Herein, the model is used to simulate the retention curves of soils. The Laplace equation is used to determine the size of the pores able to saturate or dry during a wetting or drying process, respectively. The continuous path principle is used to define those elements that effectively saturate or dry during these processes. With this procedure, it is possible to simulate the main retention curves as well as the scanning curves during wetting–drying cycles. Some experimental results reported in the international literature have been used to test the model. This model can be enhanced to study the mechanical behavior of unsaturated soils. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
17.
共面非贯通裂隙的贯通机制对于确定外载下岩质边坡的滑动面位置及滑动面综合抗剪强度至关重要。采用预制共面非贯通裂隙石膏模型试样单轴静动载对比试验,对不同裂隙倾角共面排列裂隙的扩展、贯通过程进行了观测,研究显示：共面非贯通裂隙不同倾角下的贯通模式存在较大差异,裂隙倾角为0o～35o时以裂隙面发生闭合变形为主;45o～65o时裂隙间较易出现剪切型破坏模式;75o～90o预制裂隙面较难产生滑动,裂隙试样主要产生劈裂形式的破坏。动载下预制裂隙试样裂尖翼裂纹及次生共面裂纹起裂后易朝原起裂方向快速发展;易在两预制裂隙内端部产生直接贯通,这与静载下岩桥处的贯通常通过分支裂纹拐折扩展、相连不同。含共面非贯通裂隙试样在裂隙倾角为35o左右时强度呈现最小值,这与贯通性裂隙试样裂隙倾角为60o左右呈现最小值相差较大,这是因为裂隙面摩擦强度没来得及发挥作用所致。因此,含非贯通节理裂隙岩体的综合抗剪强度公式应引入强度发挥系数,以充分考虑岩桥胶结强度与裂隙面摩擦强度不能同步发挥作用的破坏本质。 相似文献
18.
脆性岩石的损伤与裂隙扩展 总被引：7，自引：2，他引：7
岩石内部各种微裂隙的产生及发展,导致岩石的物理力学性质的劣化,此即损伤对岩石作用的结果。本文根据大理岩破坏过程中裂隙扩展的作用,定义了视损伤变量D=ε_v~k/ε_v~m,并导出了岩石的应力-应变关系和损伤变量D与D的关系。通过损伤试验,得出与理论值相吻合的结果。 相似文献
19.
含双裂隙岩石裂纹演化机理的离散元数值分析 总被引：2，自引：0，他引：2
采用离散单元法探讨了预制双裂隙岩石的裂纹演化机理。用近期从试验资料提取的无胶结厚度含抗转动能力的岩石微观力学模型和相应的离散单元法商业软件,模拟了含不同预制倾角的双裂隙岩石试样在单轴压缩作用下裂纹的扩展与贯通规律,揭示了裂纹演化的宏微观机理。同时,将离散元法DEM岩石试样的裂纹的扩展和贯通规律以及强度特性与室内试验结果进行了比较分析。结果表明,预制裂隙之间以及端点处的拉应力集中是导致裂隙岩石破坏的主要原因,且DEM数值试验得到裂纹的演化规律与室内试验结果较为一致。含30°的预制裂隙的岩石试样最容易起裂,含75°的预制裂隙的岩石试样最困难起裂,造成此种现象的原因可能是裂纹在垂直于主应力方向上的长度不同导致试样受拉区域大小不同。 相似文献
20.
Clay-containing stones such as Portland Brownstone (USA), Villarlod Molasse (Switzerland) and Tarifa Sandstone (Spain), are expected to weather as a result of wetting and drying cycles. During drying events, contraction of the drying surface leads to stresses approaching the tensile strength of the stone. However, we have found that the magnitude of these stresses is reduced by the ability of the stone to undergo stress relaxation. In this paper we describe novel methods to determine the magnitude of the stresses and the rate at which they develop and relax. We also discuss the influence of swelling inhibitors on the magnitude of swelling and the rate of the stress relaxation of these stones. The implications of our findings for the understanding of damage due to swelling of clays are discussed.Special Issue: Stone decay hazards 相似文献