Random Field Modeling of Elastic Properties Using Homogenization

This paper addresses the random field characterization of elastic properties. Consistent random field properties of a continuum are derived from a simple stochastic micromechanical model of the continuum. Existing displacement-based techniques to homogenize the discrete microstructure are discussed and a new force-based technique is introduced. The stochastic features of a micromechanically consistent continuum can be considerably different from those based on a straightforward randomization of deterministic constitutive laws. Examples indicate the relevance for structural reliability. The impact of uncertainties associated with selecting micromechanical models is assessed.     

Mars Soil Mechanical Properties and Suitability of Mars Soil Simulants

Determination of Mars soil mechanical properties will improve future lander mission success and provide narrower constraints for geomorphological modeling. A soil mechanics investigation was conducted wherein soil mechanical properties were determined by computer reconstruction of mass wasting features observed in photographs of Mars Exploration Rover landing sites and analysis of natural slope stability. Mars soil mechanical properties were compared with thermal inertia measurements and a correlation is presented. Tests with rovers and equipment for Mars surface exploration and various past laboratory experiments have incorporated a number of different Mars soil simulants. Standard laboratory measurements were conducted to characterize the shear strength, grain size distribution, and densities of various Mars soil simulants. From these measurements, the ability of a given simulant to appropriately represent the mechanical properties of in situ Mars soils was judged. Specific simulants are recommended for certain regions of Mars.     

Field Verification of Two-Dimensional Surface Irrigation Model

A two-dimensional (2D) model of unsteady shallow-water flow in surface irrigation was developed to evaluate the influence of field-grading precision on surface irrigation performance. This paper presents field data for verification of this 2D model. No attempt was made here to evaluate irrigation performance. Verification of such models relies on independent estimates of parameters for infiltration and roughness. To accomplish this, water surface elevations were measured at 26 points within a 3 ha level basin. A double-bubbler system was used to obtain relative water depths. Field surveys were used to convert these to water surface elevations and field water depths, from which surface water volumes over time were computed. The infiltration function was determined by matching inflow minus surface volume over time with computed subsurface volume. A value of Manning n (0.05) was found for which advance and water depth hydrographs were both well predicted with the 2D model. Differences in advance for a plane versus undulating field surface were minor, except near the end of advance.     

连铸方坯直接轧制温度场及产品性能研究

直接轧制连铸坯切断后直接进入粗轧,没有加热炉加热过程,导致连铸坯温度场不均匀。通过有限元法对直接轧制连铸坯进粗轧机前的温度场以及直接轧制和传统轧制第一道次形变进行了数值模拟,结果表明:150 mm×150 mm连铸坯切断及之后的300 s内,连铸坯总体温度保持1 000℃以上的较高温度,可以实现直接轧制;直接轧制心部形变比传统轧制大,连铸坯心部比表面温度高,心表温差大有利于形变的渗透。进行了方坯直接轧制现场试验,验证了温度场模拟结果,同时对产品力学性能及时效进行试验,结果表明:方坯直接轧制连铸坯不均匀温度场导致了连铸坯各部位对应产品的性能差异;经时效试验,产品抗拉强度和屈服强度在20 MPa范围波动,变化的趋势不明显,断后延伸率在时效的前10天有明显升高。     

Random Field Modeling of CPT Data

An extensive set of cone penetration tests (CPT) soundings are analyzed statistically to produce an a priori 1D stochastic soil model for use at other similar sites. The data were collected by the Norwegian Geotechnical Institute at the site of a new airport just north of Oslo, Norway, and consists of 143 CPT soundings over an area of about 18 km2 in a reasonably homogeneous soil mass. The CPT data consist of cone tip resistance, side friction, and pore-water pressure measurements. Only the cone tip resistance is considered in this study, it being considered closest to a “point” property of the soil, and only the vertical variation is characterized. To perform the statistical analysis, the data sets are viewed as independent 1D realizations extracted from a statistically homogeneous 3D random field. Plots of various transformations of the data indicate that the cone tip resistance records are best represented using a fractal stochastic model corresponding to so-called fractional Brownian motion, and its parameters are estimated via maximum likelihood.     

Effect of Pulsed Magnetic Field on Microstructure and Mechanical Properties of Eutectic Al-Si Alloy

Pulsed magnetic field (PMF) processing has been employed for refining the microstructure of eutectic (Al-12.4Si) Al-Si alloy in the current study. The effect of PMF on microstructure and mechanical properties of eutectic Al-Si alloy was studied. The results show that the morphology of primary α-Al was refined from coarse columnar dendrites to fine equiaxed dendrites by PMF treatment. Fine short rod-like or rounded particle-like eutectic silicon was formed during solidification of eutectic Al-Si alloy treated by PMF. PMF treatment reduced the size of eutectic silicon from 49 to 2.3 μm in length, and the width from 3.1 to 0.6 μm. The aspect ratio of eutectic silicon was also reduced by PMF treatment from slightly less than 16 to slightly less than 4. The ultimate tensile strength and elongation of eutectic Al-Si alloy with PMF treatment at room temperature were about 201 MPa and 8.8 pct, respectively, which were increased by 47 and 73 pct, respectively, compared with the eutectic Al-Si alloy without PMF treatment.     

Influence of Electric Field on Mechanical Properties of Al-Li Alloy Containing Cerium and Electronic Mechanism

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Probabilistic Foundation Settlement on Spatially Random Soil

By modeling soils as spatially random media, estimates of the reliability of foundations against serviceability limit state failure, in the form of excessive differential settlements, can be made. The soil’s property of primary interest is its elastic modulus, E, which is represented here using a lognormal distribution and an isotropic correlation structure. Prediction of settlement below a foundation is then obtained using the finite element method. By generating and analyzing multiple realizations, the statistics and density functions of total and differential settlements are estimated. In this paper probabilistic measures of total settlement under a single spread footing and of differential settlement under a pair of spread footings using a two-dimensional model combined with Monte Carlo simulations are presented. For the cases considered, total settlement is found to be represented well by a lognormal distribution. Probabilities associated with differential settlement are conservatively estimated through the use of a normal distribution with parameters derived from the statistics of local averages of the elastic modulus field under each footing.     

Field Testing of Stabilized Soil

Remediation of a Superfund site in Stratford, Conn., involved stabilization of the subgrade with portland cement. Part of the remediation site was to be used as a parking area. The stabilized soil was to be covered with natural base∕subbase course materials and capped with an asphalt concrete cover. During the course of the remediation, a base-course layer could not be placed prior to the onset of winter. A field study was conducted to quantify any changes in the mechanical properties of the open stabilized subgrade subjected to freeze-thaw cycling during the winter of 1996–97. Field evaluation was conducted with pavement industry tools: the Clegg impact hammer and the dynamic cone penetrometer. Evaluation results show the viability of the Clegg hammer as an instrument for quality assurance and also show that there can be up to 50% loss in compressive strength of the subgrade within the uppermost layer of the material caused by freeze-thaw cycling.     

基于二维数值模拟的煤粉燃烧性能的研究

运用数值模拟的方法,建立燃烧炉内煤粉燃烧过程的二维数学模型,对两种混煤粉和新钢高炉现用喷吹煤粉在燃烧炉内的燃烧特性进行了模拟,分别得到了两种混煤及新钢高炉喷吹煤粉在燃烧炉内燃烧形成的温度场和焦炭分布规律。结果表明：两种混煤的燃烧性能明显好于新钢高炉喷吹煤粉。采用热重法测得3种煤的燃烧率和与CO2的反应性,得出了与模拟结...     

贝氏体区等温时间对低硅TRIP钢组织和力学性能的影响

研究了0.15C-1.5Mn-1.5Al-0.3Si TRIP钢820℃2 min加热后快冷至450℃盐浴中保温5～300s空冷的组织和力学性能。结果表明,随在贝氏体转变区450℃等温时间的增加,该钢的屈服强度和伸长率增加,抗拉强度降低,等温时间60s时强塑积最佳,为23 000MPa%;等温时间≤60s时随等温时间增加钢中残余奥氏体含量增加,>60s时随等温时间的增加钢中残余奥氏体含量降低,60s时钢中残余奥氏体达到最高值,为14%。     

Random Field Representation in a Biorthogonal Wavelet Basis

This paper develops a new representation scheme for random fields, which can be used in a variety of mechanics applications, based upon the projection onto a biorthogonal wavelet basis. The merits of such a scheme can be shown to result from the relaxation of the condition of orthonormality, while still requiring compact support of the basis functions. Earlier methods have relied on the diagonalization properties of wavelets to demonstrate how the Daubechies family of orthonormal wavelets is effective in weakening the correlation across scales for a large class of random processes. It is shown that biorthogonal processes achieve better decorrelation owing to the fact that fewer filter coefficients are needed to maintain the same support of basis functions when compared to the Daubechies family. Numerical examples of fields encountered in earthquake engineering and other applications are given.     

Identifying Two-Dimensional Characteristics of Surface Water Hydraulics in an Irrigated Paddy Field

To efficiently manage the water supply for paddy rice and to reduce the inundation damage from floods, it is necessary to enhance our understanding of the characteristics of the surface water hydraulics in an irrigated paddy field. In this study, a two-dimensional numerical model is used, and the model’s applicability for simulating surface irrigation and drainage is evaluated. The model’s Manning roughness coefficient is estimated, taking into account the growth stage of the paddy rice; the simulation results are in good agreement with the observations. Model simulations reveal that the initial ponding depth affects the wave propagation and the increase in water depth during irrigation. There are few differences in piezometric heads among the positions except for those close to the outlets during drainage. Constructing drainage ditches in a paddy field reduces the drainage time to 24 h. Also, the study results reveal that it is preferable to determine the appropriate water management measures by considering the characteristics of the surface water hydraulics of paddy fields to reduce costs and labor input.     

Two-Dimensional Transient Temperature Field of Finish Rolling Section in Hot Tandem Rolling

Comprehensively considering the factors such as descaling cooling, air cooling, watercooling, frictional heat and deformation heat in gap of every stand, heat conduction betweenwork roll and strip etc, a model of two-dimensional transient temperature field of finish rollingsection in hot tandem rolling was built with finite difference method to calculate the temperaturefields of strip and work roll. So two-dimensional accurate analysis and calculation of strip tem-perature were realized, and the theoretical basis for predicting and controlling strip temperaturewas provided. The simulated results show that the model is practical and reliable.     

Mechanical Properties of Ausformed Carbide-Free Bainite

Metallurgical and Materials Transactions A - In this study, the effect of ausforming temperature and strain on the bainite transformation in a model alloy containing 0.39 pct C, 2.0 pct Mn, 1.9 pct...     

Effects of Soil Water Salinity on Field Soil Hydraulic Functions

Unsaturated soil hydraulic parameters and functions used in numerical models to simulate water flow and solute transport in the unsaturated zone are generally considered invariant of soil water salinity levels. This study uses 5 years of field soil water salinity levels at three observation sites from the Land Retirement Demonstration Project (LRDP) (20069) located in western Fresno County, California, to test the hypothesis that field unsaturated soil hydraulic properties are also a function of soil water salinity level. The HYDRUS-1D software package for simulating one-dimensional (1D) movement of water, heat, and multiple solutes in variably saturated media, and Parameter Estimation (PEST), a model-independent parameter optimizer, is used to optimize the soil hydraulic parameters and downward bottom flux corresponding to three different average soil salinity levels at each site. The results show that at the same pressure head, soil water content is less with higher soil water salinity as compared with lower soil water salinity. It is thus concluded that the use of soil water salinity invariant soil water hydraulic parameters in numerical modeling can seriously compromise predictions, especially for a variable soil water salinity environment.     

Empirically Based Gamma-Distributed Random Wall Pressure Field in Silo

Measurements show that silo wall pressures exhibit large fluctuations in time and space during discharge of the silo. This observation is important for the design of the silo wall because spatial pressure variations may impose substantial bending moments in the silo wall that otherwise may be small or vanishing due to the carrying ability of the membrane forces in the silo wall. Information about the stochastic properties of this pressure variation cannot be obtained from any existing continuum model for the silo medium flowing within the confinement of the silo. Therefore the modeling must presently be tied to statistical analyses of the empirical evidence combined with simple mechanical principles. This paper analyzes the same data set as that analyzed by Munch-Andersen et al., but on the basis of a completely different and physically less disputable stochastic pressure field model than the one considered by them. It is shown that an explicitly constructed gamma distribution type of field in equilibrium with itself fits well to the measurements made in the Swedish Karpalund silo.     

Undrained Bearing Capacity of Two-Strip Footings on Spatially Random Soil

A probabilistic study on the interference of two parallel rough rigid strip footings on a weightless soil with a randomly distributed undrained shear strength performed. The problem is studied using the random finite element method, where nonlinear finite element analysis is merged with random field theory within a Monte Carlo framework. The variability of undrained shear strength is characterized by a lognormal distribution and an exponentially decaying spatial correlation length. The estimated bearing capacity statistics of isolated and two footings cases are compared and the effect of footing interference discussed. Although interference between footings on frictionless materials is not very great, the effect is shown to be increased by soil variability and spatial correlation length.     

Stochastic Finite Elements with Multiple Random Non-Gaussian Properties

The spectral formulation of the stochastic finite-element method is applied to the problem of heat conduction in a random medium. Specifically, the conductivity of the medium, as well as its heat capacity are treated as uncorrelated random processes with spatial random fluctuations. Using the spectral stochastic finite-element method, this paper analyzes the sensitivity of heat conduction problems to probabilistic models of random data. In particular, both the thermal conductivity and the heat capacity of the medium are assumed to be uncertain. The implementation of the method is demonstrated for both Gaussian and lognormal material properties, modeled either as random variables or random processes.