Model for Granular Materials with Surface Energy Forces

In light of environmental differences (such as gravitational fields, surface temperatures, atmospheric pressures, etc.), the mechanical behavior of the subsurface soil on the Moon is expected to be different from that on the Earth. Before any construction on the Moon can be envisaged, a proper understanding of soil properties and its mechanical behavior in these different environmental conditions is essential. This paper investigates the possible effect of surface-energy forces on the shear strength of lunar soil. All materials, with or without a net surface charge, exhibit surface-energy forces, which act at a very short range. Although, these forces are negligible for usual sand or silty sand on Earth, they may be important for surface activated particles under extremely low lunar atmospheric pressure. This paper describes a constitutive modeling method for granular material considering particle level interactions. Comparisons of numerical simulations and experimental results on Hostun sand show that the model can accurately reproduce the overall mechanical behavior of soils under terrestrial conditions. The model is then extended to include surface-energy forces between particles in order to describe the possible behavior of lunar soil under extremely low atmospheric pressure conditions. Under these conditions, the model shows that soil has an increase of shear strength due to the effect of surface-energy forces. The magnitude of increased shear strength is in reasonable agreement with the observations of lunar soil made on the Moon’s surface.     

Influence of Water on the Compression Behavior of Decomposed Granite Soil

In order to investigate the influence of water on compression characteristics of decomposed granite soils, single-particle crushing and one-dimensional compression tests were carried out on three types of decomposed granite soils as well as on quartz-rich silica sand under both dry and wet conditions. Results showed that the initial crushing strength of a single particle was reduced and strength variability increased due to the weakening effects induced by the presence of water. Moreover, it was observed that the one-dimensional compression behavior of decomposed granite soil was related to the initial crushing strength. Finally, the magnitude of initial crushing strength was also affected by the degree of weathering of the soil.     

非均质岩石破裂及声发射演化数值模拟

在应变软化本构模型的基础上,考虑岩石材料非均质性和损伤过程中力学性质的弱化特性,建立了非均质岩石损伤软化本构模型,推导了损伤软化本构模型的差分格式,在VC++环境下实现了损伤软化本构模型在FLAC3D中的二次开发。研究了不同均质度对岩石变形强度等力学特性的影响,以及岩石破坏过程中的声发射演化特性。研究表明：随着岩石均质度的增高,岩石的破坏过程由延性向脆性转化,岩石峰值强度和峰值应变不断增大,而残余强度降低;当岩石均质度较低时,岩石破坏剪切带的形成会发生滞后,随着均质度的增加,单轴加载条件下岩石声发射体现出由强度低、频率高向强度高、频率低转化的特性,并表现出群震型、前震—主震—余震型和主震型3种典型模式。     

Residual Strength and Fracture Energy from Plane-Strain Testing

Field observations indicate that failure in soft rock is often associated with a slip surface or shear band, where deformation is concentrated in a narrow zone; displacements occur with decreasing stress within the shear band, whereas outside the band the material appears to be intact. In examining the propagation of the shear band, it is useful to establish the relation between shear stress and slip displacement. This was accomplished within a laboratory setting with a plane-strain compression apparatus, developed to study localized failure under controlled conditions. Tests on a soft rock, a sandstone with a uniaxial compressive strength of 10 MPa and a modulus of 2 GPa, were conducted to estimate fracture energy GIIC, a quantity used to evaluate energy dissipation of the failure process. GIIC was found to decrease by a factor of 3 when considering the actual displacements, rather than assuming tangential displacement only, that is, no displacement normal to the shear band. The experiments showed that the shear band was not completely formed until after peak strength and that sliding along the band during softening was associated with compaction; residual behavior exhibited virtually no volume change. The shear strength at peak stress was nonlinearly related to the normal stress, but the shear strength at the residual state displayed a linear relationship. For normal stresses less than the uniaxial strength, those typical of civil engineering practice, the response can be described as cohesion softening, with friction remaining constant in going from the peak to the residual stress states.     

Analysis of Soil Strength Degradation during Episodes of Cyclic Loading, Illustrated by the T-Bar Penetration Test

Pipelines and risers form an essential part of the infrastructure associated with offshore oil and gas facilities. During installation and operation, these structures are subjected to repetitive motions which can cause the surrounding seabed soil to be remolded and soften. This disturbance leads to significant changes in the operative shear strength, which must be assessed in design. This paper presents an analytical framework that aims to quantify the degradation in undrained shear strength as a result of gross disturbance—in this case through repeated vertical movement of a cylindrical object embedded in undrained soil. The parameters of the framework were calibrated using data obtained in a geotechnical centrifuge test. In this test a T-bar penetrometer, which is a cylindrical tool used to characterize the strength of soft soil, was cycled vertically in soil with strength characteristics typical of a deep water seabed. Using simple assumptions regarding the spatial distribution of “damage” resulting from movement of the cylinder, and by linking this damage to the changing undrained shear strength via a simple degradation model, the framework is shown to simulate well the behavior observed in a cyclic T-bar test. This framework can potentially be extended to the similar near-surface behavior associated with seabed pipelines and risers.     

岩石材料脆性剪切破坏模式下的强度分析

使用声发射法与三轴试验相结合的方法对岩石试样微裂隙产生和发展进行监测,以获取岩石脆性剪切混合破坏模式的特点.采用断裂力学与岩石力学理论相结合的方式进行理论分析和试验数据处理,得到了试样三阶段特征强度随应力状态变化的规律,并提出了一种适用于岩石脆性、剪切混合型破坏强度分析方法,据此建立了Mohr-Coulomb、Griffith和Hoek-Brown等强度准则与脆、剪混合强度模型的关系.采用此模型对水厂边坡混合花岗岩的全应力-应变试验数据进行脆剪混合强度分析,理论值与试验值具有良好的一致性.      

Comparison between Models of Rock Discontinuity Strength and Deformation

One important component in the design of tunnels in urban areas is a correct assessment of the interaction between the underground excavation with other structures in the vicinity. In this sense a correct stress-strain response by the model representing the rock mass behavior is essential. The shear and normal displacement of rock discontinuities and their shear and normal stiffness control the distribution of stress and displacement within a discontinuous rock mass. In conditions where an equivalent continuum based approach is not applicable, the joint material model should be able to describe important mechanisms such as asperity sliding and shearing, post-peak behavior, asperity deformation, and the effect of soft infilling. The distinct element code UDEC was used to simulate the direct shear tests on a natural joint profile, and the prediction of two existing models of discontinuity strength and deformation were then compared with a new soil-infilled joint model and with experimental data for clean and soil-infilled rock joints. A numerical modeling of a cavern excavated in a jointed medium is also presented to illustrate the response of different models. The proposed soil-infilled joint model described more comprehensively the occurrence of dilation and compression with lateral displacements and also better represented the double peak shearing in relation to the adopted squeezing mechanism that could not be captured by the two existing models.     

Engineering Properties of Lunar Soil Simulant JSC-1A

This study was carried out to assess the tensile and shear strength in lunar soil, and to examine the variation as a function of density and confinement. Geotechnical engineering properties of the lunar soil simulant designated Johnson Space Center Number One-A lunar soil simulant (JSC-1A) have been investigated experimentally. To better understand these soil properties, a variety of conventional and unconventional experiments were conducted on JSC-1A to determine its grain-size distribution, cohesion, friction angle, dilatancy angle, tensile strength, and appropriate low strain elastic constants. These experiments were conducted on JSC-1A at a variety of densities prepared through tamping densification to quantify the response of the soil over a range of conditions. To simulate lunar conditions, the samples were prepared at medium to very high relative densities. Grain-size distribution, shear strength, tensile strength, dilatancy angles, and elasticity modulus of the JSC-1A were compared with lunar soil and other simulants.     

砂卵石土的剪切流变特性研究

应用RYL-600岩石剪切流变仪对长沙市某边坡砂卵石土试样进行剪切流变试验,分析砂卵石土的剪切流变特性。试验表明正应力越高时,能够引起砂卵石土试件发生剪切流变破坏的剪切应力也随之增高。砂卵石土剪切流变在低于长期抗剪切强度的应力作用下,表现出黏弹特性;在高于长期强度的应力作用下,表现出黏弹塑性。应用五元黏弹性模型与VR黏塑性模型串联得到的黏弹塑性模型对砂卵石土全程流变曲线进行模拟,将拟合结果与试验数据进行分析比较,验证了新模型具有正确性和合理性,这对砂卵石土工程具有重要的指导意义和参考价值。     

Application of Weibull Statistics to the Tensile Strength of Rock Aggregates

Rock aggregates forming part of engineering structures such as embankments, rock fills, pile foundations, and pavements are subjected to static and dynamic loads. As a result of these loads crushing occurs in two different forms: abrasion and particle fragmentation. Particle fragmentation takes place when a particle is subjected to a tensile stress greater than its tensile strength. This tensile strength is a highly variable parameter. The work presented in this study shows that Weibull statistics can be used to characterize the variability in tensile strength of rock aggregates by only using one constant called the Weibull modulus. This parameter can be used not only to describe variability in tensile strength but also to describe the effect of size specimen in tensile strength. The higher the modulus, the lower is the influence of the variability of tensile strength and sample size. More than 390 different specimens coming from a red Biotite Gneiss and a grey Quartzite were tested to validate the use of Weibull statistics. The Weibull modulus was found to be equal to 2.75 and 4.23 for the red Biotite Gneiss and the grey Quartzsite respectively.     

Analysis of Shear Banding in True Triaxial Tests on Sand

A series of true triaxial tests have been performed on rectangular prismatic specimens of Santa Monica Beach sand at three different relative densities to study the occurrence of failure, mechanisms that create failure, and soil behavior in the vicinity of failure. One mechanism is smooth peak failure, in which the soil continues to behave as a continuum with uniform strains, and smooth peak failure is followed by strain softening. Another mechanism is shear banding, whose occurrence in the plastic hardening regime limits the strength of the soil. Presented here are analyses based on theoretical conditions for localization and subsequent shear banding and on the results of the true triaxial tests. Thus, the strength increases as b [=(σ2 ? σ3)/(σ1 ? σ3)] increases from 0 to about 0.18, remains almost constant until b reaches 0.85, and then decreases slightly at b = 1.0. Shear banding initiates in the hardening regime for b-values of 0.18–0.85. Thus, peak failure is caused by shear banding in this middle range of b-values, and a smooth, continuous 3D failure surface is therefore not generally obtained for soils. The experimental results agree with the theoretical conditions for the occurrence of shear banding and its consequent effect on the 3D failure stress states for soil.     

低温条件下边坡岩石动态力学特性实验研究

我国多年冻结区和季节性冻结区面积广泛,在这些地区进行工程建设和矿产资源开采必须考虑特殊的地质和气候条件,其中寒区边坡的稳定性问题值得研究.以位于西藏自治区的玉龙铜矿为例,矿区平均海拔约4000 m,最冷月日平均最低气温约-20℃,冻结期长,边坡稳定性受冻融作用显著,冻结岩层给爆破开挖带来诸多困难,制约了矿山生产效率.为研究低温条件下边坡岩石的动态力学特性,从西藏玉龙铜矿边坡钻取了大理岩试样,借助含低温控制系统的分离式霍普金森压杆(SHPB)实验系统,对常温干燥、常温饱水和低温冻结三种状态的岩样进行了动态拉压力学实验,以探究温度、含水量对岩石动态力学性质的影响.试验结果表明:(1)受低温水冰相变和岩石基质冷缩的共同影响,-20℃冻结岩样的平均单轴动态压缩、拉伸强度较常温下有所增大.其中,岩石基质的冷缩现象是造成冻结岩石强度显著提高的主要原因.四种应变率下,压缩应力分别增大1.30、1.62、1.41、1.43倍,拉伸应力分别增大1.36、1.28、1.22和1.29倍;(2)受孔隙水软化影响,饱水岩样动态强度小于干燥岩样,因此同一应变率下的实验数据满足规律,即冻结岩样强度最高,干燥次之,...     

充填节理破坏机理及实验

充填节理的破坏主要有充填物破坏和充填物与节理接触面破坏两种形式,其抗剪强度与最小抗剪部分的强度相同.当充填物破坏时,单独对充填物部分进行受力分析,得到此时的节理抗剪强度公式.讨论了充填节理强度随法向压应力及充填厚度的变化规律:随着法向压应力的增加先增大后减小,但随充填厚度的变化并不十分明显.对不同厚度的砂浆充填节理进行直剪试验.结果表明:其剪切强度与破坏模式有关;在同一破坏模式下,不同充填厚度的节理其剪切强度变化不大.      

Pile Bearing Capacity Factors and Soil Crushabiity

The existence of large magnitude stresses at the tip of a bearing pile is a well known phenomenon leading to crushing of soil grains and thus affecting pile behavior. Classical foundation design calculations which assume that the soil fails in shear and neglect volume change can be safely used where stress levels or particle strengths prevent crushing, however in the case of weak grains or high foundation stresses consideration should be given to the effects of grain crushing and the resulting volumetric compression. Model pile tests have been carried out in two skeletal carbonate sands and a standard silica sand with the aim of examining the variation of skin friction and end bearing capacities with degree of penetration. The mobilization of the strength of crushable soils requires a much higher strain level while at the same time the end bearing pressure on the model piles exceeded 10?MPa inducing considerable particle breakage. The peak skin friction for all sands occurred at a settlement normalized by pile diameter, S/D, of less than 0.1. At this point the carbonate sands generally had lower skin friction values than the silica sand. Further displacement caused a rapid decrease in skin friction for all three materials. At higher lateral stresses the less crushable Toyoura silica sand generated higher skin frictions. Samples of Chiibishi sand were sectioned and photographed. It was observed that a spherical plastic zone was formed at the base of the pile which expanded with increasing S/D and a degraded layer of broken particles developed around the pile as S/D increased. Large values of the Marsal particle breakage factor were restricted to a zone extending outwards to one pile radius. An end bearing capacity modification factor has been proposed to adapt the conventional bearing capacity equation for soil crushability. This modification factor is a function of soil compressibility and degree of penetration. The factor was shown to decrease with increasing soil compressibility and increase with normalized penetration S/D.     

Characterization of Cemented Sand in Triaxial Compression

This work aims at studying the stress-strain-strength behavior of an artificially cemented sandy soil produced through the addition of portland cement. An analysis of the mechanical behavior of the soil is performed from the interpretation of results from unconfined compression tests, drained triaxial compression tests with local strain measurements, and scanning electron microscopy, in which the influence of both the degree of cementation and the initial mean effective stress was investigated. For cemented sandy soils, it was concluded that the unconfined compression resistance is a direct measurement of the degree of cementation. Consequently, the triaxial shear strength can be expressed as a function of only two variables: (1) the internal shear angle of the nonstructured material; and (2) the unconfined compression resistance. In addition, a logarithmic formulation is adopted to express the relationship between static deformation moduli and axial strain amplitude in axisymmetric conditions. Data from other reported investigation programs give to the proposed correlations a broader acceptance to general geotechnical applications.     

Soil Uncertainty and Its Influence on Simulated G/Gmax and Damping Behavior

In this paper, recently developed probabilistic elastoplasticity was applied in simulating cyclic behavior of clay. A simple von Mises elastic–perfectly plastic material model was used for simulation. Probabilistic soil parameters, elastic shear modulus (Gmax) and undrained shear strength (su), needed for the simulation were obtained from correlations with the standard penetration test (SPT) N-value. It has been shown that the probabilistic approach to geo-material modeling captures some of the important aspects—the modulus reduction, material damping ratio, and modulus degradation—of cyclic behavior of clay reasonably well, even with the simple elastic–perfectly plastic material model.     

8YSZ 团聚粉体压溃强度 及其对 PS-PVD 沉积行为的影响研究

等离子物理气相沉积 (PS-PVD) 作为一种热障涂层新型制备技术, 受到国内外的广泛关注。 PS-PVD 工艺 采用细小球形团聚粉体为原料, 原料在高温高速等离子射流中瞬时加热 - 熔融 - 气化, 这一复杂过程中粉体 - 等 离子体射流相互作用机理尚未完全揭示, 特别是粉体颗粒在高温高速射流中快速溃散气化过程研究较少。 本文研 究了细小球形团聚粉体的压溃强度测试方法, 进而对不同粘结剂类型、 含量、 不同颗粒尺寸的粉体压溃强度进行 表征, 研究了粉体特性对压溃强度影响规律, 并系统分析了粉体沉积效果。 结果表明, 随着粉体颗粒尺寸的增大, 其压溃强度呈现逐步降低趋势, 不同特性粉体趋势不同; 粘结剂类型变化、 特别是一定含量纳米尺度颗粒的存在, 会增大粉体压溃强度, 一定范围内粉体压溃强度越高, 涂层沉积速率越高。     

Shear Band Analysis in the Biaxial Test

Simulation of a biaxial test by finite-element calculations may lead to unexpected localized deformations even for an elastic-perfect plastic Mohr–Coulomb model. The effects of localization lead to difficulties in computations. This paper describes a simple model for the strength development after occurrence of localization. The localization model describes the postfailure behavior assuming elastoplastic behavior inside the shear band and elastic behavior of the material outside the shear band. The resulting two nonlinear equilibrium equations are analyzed with a rigorous nonlinear scheme. The nonlinear equations show snap-through and snap-back behavior. Snap-back occurs due to energy release of unloading of the material outside the shear band. The simulations show that localization in shear bands is possible for orientations between the Roscoe and Mohr–Coulomb direction. Shear bands with orientations close to the Roscoe direction show a more gradual decrease of strength in comparison to bands with orientations close to the Coulomb direction. The flexibility of the shear band is shown to be important for postbifurcation behavior.     

Normal Transmission of S-Wave Across Parallel Fractures with Coulomb Slip Behavior

When an elastic wave propagates through a rock mass, its amplitude is attenuated and velocity is slowed due to the presence of fractures. During wave propagation, if the shear stress at a fracture interface reaches the fracture shear strength, the fracture will experience a large shear displacement. This paper presents a study of the normal transmission of S-waves across parallel fractures with Coulomb slip behavior. In our theoretical formulation, the method of characteristics combined with the Coulomb slip model is used to develop a set of recurrence equations with respect to particle velocities and shear stress. These equations are then solved numerically. In a comparison with the theoretical study, numerical modeling using the universal distinct element code (UDEC) has been conducted. A general agreement between UDEC modeling and theoretical analysis is achieved. The magnitude of the transmission coefficient is calculated as a function of shear stress ratio, nondimensional fracture spacing, normalized shear stiffness, and number of fractures. The study shows that the shear stress ratio is the most important factor influencing wave transmission, and the influence of other factors becomes more apparent when the shear stress ratio is small.     

基于扩展离散元法的超声振动粉碎装置颗粒粉碎效果分析

根据超声振动原理设计了一种高效颗粒超细粉碎装置,在扩展离散元分析软件中对物料在变幅杆高频冲击下的粉碎过程进行仿真模拟,并与实验结果相比较,分析超声频率、粒径大小、颗粒材料对粉碎效率的影响。研究结果表明,随着超声频率的增加,装置粉碎效率先增大后趋于稳定,考虑到系统稳定性,选取38 kHz为最优超声振动频率;装置对粒径为200~500μm的颗粒具有良好的粉碎效果;装置对高强度、高硬度的天然、人工合成材料均有良好的粉碎效果,随着硬度的下降,粉碎效果变优,尤其适用于颗粒内部具有微缺陷和微裂纹的材料;通过对比实验验证了仿真结果的可靠性。