间断级配吹填珊瑚砂剪切和颗粒破碎特性

岛礁地基通过水力吹填的方式建造而成。吹填地层中常存在大小颗粒共存的间断级配状态,颗粒级配的间断条件对吹填珊瑚砂的力学特性有显著影响。针对吹填珊瑚砂开展不同有效围压、相对密实度和含砾量的三轴排水剪切试验,探究静力荷载作用下间断级配珊瑚砂砾混合料的强度与变形特性,并分析珊瑚砂砾混合料颗粒破碎的演化规律。结果表明：试样的应力—应变关系曲线受到围压和密实度的显著影响,珊瑚砂砾混合料的峰值摩擦角和峰值剪胀角均随有效围压的增大而减小,随密实度的增大而增大;同一有效围压水平下,随着含砾量的增加,密实程度不同的珊瑚砂砾混合料强度表现出不同的增长规律,这可能与珊瑚砂砾混合料中粗细骨料接触状态和组成比例有关。另外,还研究了剪切过程中不同含砾量条件下珊瑚砂砾混合料的塑性功与相对破碎指数的关联性,建立了相对破碎指数B_r与塑性功W_p的相关函数。     

考虑颗粒破碎的堆石料动力变形特性模拟

颗粒破碎是影响堆石料强度和变形特性的重要因素,但目前针对颗粒破碎的模拟研究多在静力荷载条件下。为研究颗粒破碎对小应变条件下堆石料的动力变形特性的影响,采用多个等粒径小球按最密六方排列随机组合模拟不规则形状的堆石颗粒,通过碎片替换法模拟颗粒破碎,研究了花岗岩堆石料不同围压下的动力响应,探索了孔隙率对动弹性模量的影响,分析了振动过程中的颗粒破碎规律及配位数的频率分布。结果表明模拟的骨架曲线与室内试验结果具有较好的一致性,数值模拟可以较好地再现不同围压下堆石料的动力变形特征。在相同围压和动应力条件下,考虑颗粒破碎的试样会产生更多不可恢复的变形,动应变会明显增大,动弹性模量降低。振动过程中集合体的有效配位数会减小,与不考虑颗粒破碎的情况对比,考虑颗粒破碎的试样具有更多的力学不稳定颗粒,有效配位数的降低更显著。颗粒破碎对最大动弹性模量的影响较小,但会加快动模量随动应变增长而衰减的速率。孔隙率小的试样有效配位数高,且受力性能更好。在相同动应力条件下颗粒破碎较少,动弹性模量随动应变的增加而衰减的速率较慢,最大动弹性模量约为大孔隙率试样的1.2倍。最大动弹性模量主要与有效平均主应力和孔隙率相关,Hardin等提出的经验公式可以较好地描述最大动弹性模量与孔隙比和平均有效主应力的关系。该成果有助于认识粗粒料动荷载下的变形规律,为研究动荷载下的颗粒破碎行为提供参考。     

A constitutive model for granular soils

A simple constitutive model is presented to describe the mechanical behaviors of granular soils in a large stress range. A novel normal compression line(NCL) is first expressed by introducing a limit void ratio(e_L) in the double logarithmic scale.Subsequently, a state parameter(ξ) is defined to quantify the current state of granular soils, and a unified hardening parameter(H)that is a function of the state parameter(ξ) is developed to govern the hardening process of the drop-shaped yield surface.Combining with flow rule, a constitutive model for granular soils is proposed. Finally, the comparison between the predictions and the test results of Cambria sand and Coarse-grained material indicates that the model is able to describe the mechanical behaviors of granular soils in a large stress range.     

堆石蠕变与颗粒破碎特性三轴试验

为分析堆石的蠕变规律和研究颗粒破碎与蠕变的关系,对某堆石料进行了不同应力水平和围压下的大型三轴排水蠕变试验。蠕变试验后通过筛分试验测量了堆石的颗粒破碎程度。结果表明堆石的蠕变变形与应力水平和围压有关;轴向蠕变和体积蠕变随应力水平的增大而增大,应力水平相同时则随围压增大而增大。堆石的应变一时间关系可用幂函数表示,轴向应变一应力关系和体积应变一应力关系分别满足双曲线函数和线性比例函数。随应力水平和围压的增大,堆石颗粒破碎增多。堆石颗粒破碎可用相对颗粒破碎指数度量,其与轴向蠕变和体积蠕变近似呈线性比例关系。     

A surface-based hiding function linking flume and field data

The quantification of the sheltering and exposure effects of non-uniform sediments has been widely achieved through hiding function models. Big challenge exists so far in the model parameter that is highly variable and differs greatly between laboratory flumes and field streams. This paper presents an improved surface-based hiding function. The force balance for particle inception was formulated and the allocation of the overall bed shear stress into each group of sediments was mimicked. The new hiding function was examined against and agrees well with the documented field and flume data. It was shown that the hiding function is closely related to the relative flow depth and the reference elevation in the velocity profile in addition to the bed material gradation. The power law of velocity profile that applies to both flume flows and natural streams can link the flume and field data together. The hiding function with b = 1/6 and b = 1/2 is applicable to natural streams and laboratory flumes, respectively. The value of b = 0.263 also works well for gravel bed rivers. The range of the reference elevation, namely z ₀ = 0.4D_m–1.4D_m, is recommended for either the flume or field data. The new hiding function contributes to addressing clearer physical meanings and a useful perspective for further improvement.     

A constitutive model for the state-dependent behaviors of rockfill material considering particle breakage

The particle breakage during specimen compaction had more significant influence on the position of the breakage critical-state line(BCSL)of Tacheng rockfill material(TRM)in the e-lnp’plane than the particle breakage during shearing,based on the large-scale triaxial compression tests on TRM in a wide range of densities and pressures.The state-dependent dilatancy and the plastic modulus were correlated to the breakage index,based on the formulations of the BCSL of TRM in the e-lnp’plane.The state-dependent model considering particle breakage was proposed for TRM within the framework of the generalized plasticity theory.The proposed model contained fourteen material constants.The test data of TRM from Group A were adopted to determine these material constants,while the test data from Group B were used independently to validate the model predictive capacity.The comparisons between model simulations and test data illustrated that the model with consideration of particle breakage could well represent the stress-strain behaviors of TRM,e.g.,the strain hardening and volumetric contraction behaviors at a loose state and the strain softening and volumetric expansion behaviors at a dense state,and also the particle breakage behaviors of TRM.     

An improved hypoplastic constitutive model of rockfill considering effect of stress path

An incrementally nonlinear hypoplastic constitutive model was introduced, which was developed without recourse to the concepts in elastoplasticity theory such as yield surface, plastic potential and the decomposition of the deformation into elastic and plastic parts. Triaxial drained tests on rockfill were conducted on a large scale triaxial apparatus under two types of stress paths, which were the stress paths of constant stress ratio and the complex stress paths with transitional features. Motivated by the effect of stress path, the Gudehus-Bauer hypoplastic model was improved by considering the parameter variations with different ratios of stress increment. Fitting parameter α presents a piecewise linear relationship with cosine of the slope angle θ determined by instantaneous stress path. The improved hypoplastic model can present peak stress increasing and volumetric strain changing from dilatancy to contractancy with the increase of transitional confining pressure σ _3t and the decrease of slope angle θ of stress path. Compared with the test data, it is shown that the model is capable of fully considering the effect of stress path on rockfill.     

Elimination of Voids at Interface of <Emphasis Type="Italic">β</Emphasis>-SiC Films and Si Substrate by Laser CVD

Void-free β-SiC films were deposited on Si(001) substrates by laser chemical vapor deposition using hexamethyldisilane (HMDS) as the precursor. The effect of the time of introducing HMDS, i e, the substrate temperature when HMDS introduced (T_in), on the preferred orientation, surface microstructure and void was investigated. The orientation of the deposited SiC films changed from <001> to random to <111> with increasing T_in. The surface showed a layer-by-layer microstructure with voids above T_in ? 773 K, and then transformed into mosaic structure without voids at T_in= 298 K. The mechanism of the elimination of voids was discussed. At T_in =298 K, Si surface can be covered by an ultrathin SiC film, which inhibits the out-diffusion of Si atoms from substrate and prohibites the formation of the voids.     

Rheological behaviors of fresh cement pastes with polycarboxylate superplasticizer

The rheologicalbehaviors of fresh cement paste with polycarboxylate superplasticizer were systematically investigated.Influentialfactors including superplasticizer to cement ratio(Sp/C),water to cement ratio(w/c),temperature,and time were discussed.Fresh cement pastes with Sp/Cs in the range of 0 to 2.0% and varied W/Cs from 0.25 to 0.5 were prepared and tested at 0,20 and 40 °C,respectively.Flowability and rheologicaltests on cement pastes were conducted to characterize the development of the rheologicalbehavior of fresh cement pastes over time.The exprimentalresults indicate that the initialflowability and flowability retention over shelf time increase with the growth in superplasticizer dosage due to the plasticizing effect and retardation effect of superplasticizer.Higher temperature usually leads to a sharper drop in initialflowability and flowability retention.However,for the cement paste with high Sp/C or w/c,the flowability is slightly affected by temperature.Yield stress and plastic viscosity show similar variation trends to the flowability under the abovementioned influentialfactors at low Sp/C.In the case of high Sp/C,yield stress and plastic viscosity start to decline over shelf time and the decreasing rate descends at elevated temperature.Moreover,two equations to roughly predict yield stress and plastic viscosity of the fresh cement pastes incorporating Sp/C,w/c,temperature and time are developed on the basis of the existing models,in which experimentalconstants can be extracted from a database created by the rheologicaltest results.     

Characterization of yield stress development of cement paste by electrical resistivity

Yield stress development of cement paste is potentially governed by percolation of 3-dimensional links formed by hydration products on the surface of the particles. It rises steadily at a gradual rate before a sudden increase in rate of growth. In this study, a method was proposed to predict the yield stress development based on the diameter of spread(D) of mini slump cone test and gradient from electrical resistivity measurement(K_m). To evaluate the significance of(D) and(K_m) in terms of yield stress, they were quantitatively compared to the initial yield stress(τ_0) and rate of yield stress growth(K) obtained from a rheometer. A mathematical relationship between the yield stress of cement paste, diameter of spread and electrical resistivity characteristic gradient was developed. The equation developed can be used as an alternative method to estimate yield stress of cement paste.     

Modified k-ω model using kinematic vorticity for corner separation in compressor cascades

A new method of modifying the conventional k-ω turbulence model for corner separation is proposed in this paper. The production term in the ω equation is modified using kinematic vorticity considering fluid rotation and deformation in complex geometric boundary conditions. The corner separation flow in linear compressor cascades is calculated using the original k-ω model, the modified k-ω model and the Reynolds stress model(RSM). The numerical results of the modified model are compared with the available experimental data, as well as the corresponding results of the original k-ω model and RSM. In terms of accuracy, the modified model, which significantly improves the performance of the original k-ω model for predicting corner separation, is quite competitive with the RSM. However, the modified model, which has considerably lower computational cost, is more robust than the RSM.     

Experimental study of the wake characteristics of a two-blade horizontal axis wind turbine by time-resolved PIV

Wind tunnel experiments of the wake characteristics of a two-blade wind turbine, in the downstream region of 0x/R10, have been carried out. With the help of the time resolved particle image velocimetry(TRPIV), flow properties such as the vortex structure,average velocity, fluctuations velocities and Reynolds stresses are obtained at different tip speed ratios(TSR). It is found that the wind turbine wake flow can be divided into velocity deficit region, velocity remained region and velocity increased region, with generally higher velocity deficit compared with a three-blade wind turbine wake. Once a blade rotates to the reference 0° plane,the tip vortices generate, shed and move downstream with the intensity gradually decreased. The leapfrogging phenomenon of tip vortices caused by the force interaction of adjacent vortices is found and more apparent in the far wake region. The axial fluctuation velocity is larger than radial fluctuation velocity at the blade root region, and the turbulent kinetic energy shares the similar trend as the axial fluctuation velocity. The axial normalized Reynolds normal stress is much larger than the radial normalized Reynolds normal stress and Reynolds shear stress at the blade root region. As the TSR increases, the radial location where the peak axial normalized Reynolds normal stress u'u'/U~2 and axial fluctuation velocity appear descends in the radial direction.     

Entransy dissipation minimization for generalized heat exchange processes

This paper investigates the MED(Minimum Entransy Dissipation) optimization of heat transfer processes with the generalized heat transfer law q ∝(△(T~n))~m T. For the fixed amount of heat transfer, the optimal temperature paths for the MED are obtained. The results show that the strategy of the MED with generalized convective law q ∝(△T)~m T is that the temperature difference keeps constant, which is in accordance with the famous temperature-difference-field uniformity principle, while the strategy of the MED with linear phenomenological law q ∝△(T~(-1)) is that the temperature ratio keeps constant. For special cases with Dulong-Petit law q ∝(△T)~(1.25) and an imaginary complex law q ∝(△(T~4))~(1.25), numerical examples are provided and further compared with the strategies of the MEG(Minimum Entropy Generation), CHF(Constant Heat Flux) and CRT(Constant Reservoir Temperature) operations. Besides, influences of the change of the heat transfer amount on the optimization results with various heat resistance models are discussed in detail.     

不同颗粒级配钙质砂剪切特性研究

为研究钙质砂剪切特性,对某岛钙质砂进行4组固结排水三轴试验,其围压分别是50、100、200、400 kPa。应力应变曲线表明钙质砂体变先剪缩后剪胀,在剪胀发展的过程中伴随应变软化现象。为进一步探究钙质砂应变软化的原因,筛除粒径小于0.5 mm,重复试验。研究结果表明大颗粒的钙质砂并没有出现应变软化现象。给出如下结论:(1)应变软化出现的位置比较偏后,且随着围压的增长,峰值强度会逐渐后移;(2)在低围压条件下钙质砂颗粒破碎诱发的应变软化可能性低;(3)细小颗粒诱发扁平状钙质砂出现定向滑移是试样表现应变软化的关键因素;(4)围压越高细小颗粒诱发应变软化的难度越大,峰值强度逐渐后移,并伴随着剪切带的出现。     

Thermodynamic assessment of solar-aided carbon dioxide conversion into fuels via Tin oxides

The conversion of CO_2 to liquid hydrocarbon fuels using solar energy is gaining attraction as a means to deal with climate change and energy depletion,and assessment for related thermochemical cycles has attracted great interests in recent years.Here,we perform the thermodynamical analysis on solar-aided CO_2 conversion reactions based on Tin oxides.The equilibrium compositions,production purity and CO_2 conversion are obtained.Also,the variations of conversion efficiency with respect to temperature,normal beam solar insolation,mean flux concentration ratio,initial CO_2 to SnO ratio and heat recuperation percentage are revealed.Our results indicate the initial CO_2 to SnO ratio,χ_(ini),has an evident impact on conversion efficiency andχ_(ini)=0.5,T=700 K andχ_(ini)=1,T=950 K,are favourable for solid C and gaseous CO production,respectively.The calculated maximum cycle efficiency with direct work production is 0.340 at T=950 K andχ_(ini)=1,demonstrating the high conversion efficiency of the proposed system.     

Reduction of residual stress in low alloy steel with magnetic treatment in different directions

The behavior that different magnetic treatment directions induce various amounts of welding residual stress reductions in low alloy steel was studied. Reductions of 26%–28% in the longitudinal stress σ _x were obtained when low frequency alternating magnetic treatment was applied perpendicularly to the welding bead, whereas reductions of 20%–21% in σ _x were measured by using the same treatment parameters except that the field direction was applied parallel to the bead. It is proposed that different extent of stress reductions caused by the above two treatment directions is attributed primarily to the alteration of the energy absorbed by domains from the external magnetic field, which part of energy can arouse plastic deformation in microstructures by the motion of domain walls.     

Structural,anisotropic and thermodynamic properties of Imm2-BCN

Structural, anisotropic, and thermodynamic properties of Imm2-BCN were studied based on density function theory with the ultrasoft psedopotential scheme in the frame of the generalized gradient approximation(GGA). The elastic constants were confirmed that the predicted Imm2-BCN is mechanically stable. The anisotropy of elastic properties were also studied systematically. The anisotropy studies of Young's modulus, shear modulus, linear compressibility, and Poisson's ratio show that the Imm2-BCN exhibits a large anisotropy. Through the quasi-harmonic Debye model, the relations between the equilibrium volume V, thermal expansion α, the heat capacity C_V and CP, the Grüneisen parameter γ, and the Debye temperature Θ_D with pressure P and temperature T were also studied systematically.     

Effects of electrode on resistance switching properties of ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> films deposited by magnetron sputtering

ZnMn₂O₄ films for resistance random access memory (RRAM) were fabricated with different device structures by magnetron sputtering. The effects of electrode on I-V characteristics, resistance switching behavior, endurance and retention characteristics of ZnMn₂O₄ films were investigated. The ZnMn₂O₄ films, using p-Si and Pt as bottom electrode, exhibit bipolar resistive switching (BRS) behavior dominated by the space-charge-limited conduction (SCLC) mechanism in the high resistance state (HRS) and the filament conduction mechanism in the low resistance state (LRS), but the ZnMn₂O₄ films using n-Si as bottom electrodes exhibit both bipolar and unipolar resistive switching behaviors controlled by the Poole-Frenkel (P-F) conduction mechanism in both HRS and LRS. Ag/ZnMn₂O₄/p-Si device possesses the best endurance and retention characteristics, in which the number of stable repetition switching cycle is over 1000 and the retention time is longer than 10⁶ seconds. However, the highest R _HRS/R _LRS ratio of 10⁴ and the lowest V _ON and V _OFF of 3.0 V have been observed in Ag/ZnMn₂O₄/Pt device. Though the Ag/ZnMn₂O₄/n-Si device also possesses the highest R _HRS/R _LRS ratio of 10⁴, but the highest values of V _ON,V _OFF, R _HRS and R _LRS, as well as the poor endurance and retention characteristics.     

Effect of Al in slag blended cement paste under sulfate attack

Unilateral sulfate attack of cementitious materials containing 40% slag with different water to binder ratios was investigated. The results showed that the degradation of slag blended cement pastes was nearly from the corners of paste surface with cracking and spallings, water-to-binder(w/b) ratio made a significant sense to the damage that low w/b ratio led to little weight loss, less cracking and spalling damage and vice versa. Microstructural experimental results demonstrated that in the three different stages of sulfate attack, degradation of pastes was primarily associated with the migration behavior and bonding configuration of aluminum, in the early ages Al was mostly present in C-(A)-S-H, and thus, the damage of pastes hardly appeared while at later ones Al had been largely transferred from C-(A)-S-H into AFt, leading to expansive damage.     

Composition dependence of phase structure and electrical properties of (1−y)Bi1−xNdxFeO3−yBiScO3 ceramics

In this work, we have studied a new lead-free ceramic of(1-y)Bi_(1-x)Nd_xFeO_(3-y)BiScO_3(0.05≤x≤0.15 and 0.05≤y≤0.15) prepared by a conventional solid-state method, and the influences of Nd and Sc content on their phase structure and electrical properties were investigated in detail. The ceramics with 0.05≤x≤0.10 and 0.05≤y≤0.15 belong to an R3 c phase, and the rhombohedral-like and orthorhombic multiphase coexistence is established in the composition range of 0.125≤x≤0.15 and y=0. The electrical properties of the ceramics can be enhanced by modifying x and y values. The highest piezoelectric coefficient(d33~51 p C/N) is obtained in the ceramics with x=0.075 and y=0.125, which is superior to that of a pure BiFeO_3 ceramic. In addition, a lowest dielectric loss(tan δ~0.095%, f=100 k Hz) is shown in the ceramics with x=0.15 and y=0 due to the involvement of low defect concentrations, and the improved thermal stability of piezoelectricity at 20–600oC is possessed in the ceramics. We believe that the ceramics can play a meaningful role in the high-temperature lead-free piezoelectric applications.