Statistical Sensitivity Analysis of Lattice Space Structures

This paper presents a statistical sensitivity analysis for shape distortions of space structures. The approach is based on a statistical shape‐distortion analysis on the structural errors and an adjoint method of sensitivity analysis. The statistical shape‐distortion analysis allows the stochastic errors to be represented by member‐length tolerances. The sensitivity analysis is performed to predict the effects of member‐length errors for lattice space antennas on the surface accuracy. The formulas presented in this paper give an effective approach to predict the effects of member‐length errors on the shape distortions, to obtain effective structural elements to correct the shape distortions, and to design tolerance errors of the structural elements. Numerical examples for statically determinate and indeterminate two‐dimensional truss beams have been demonstrated to identify the members contributing most to the errors. These results show that the errors of the longitudinal elements of the structure are important for designing accurate truss structures. Moreover, the validity and effectiveness of the present approach have been investigated.     

Modeling Quench Sensitivity of Aluminum Alloys for Multiple Tempers and Properties: Application to AA2024

Quench factor analysis (QFA) was developed to predict the change in certain properties, such as yield strength, hardness, electrical conductivity, etc., with how the material cools from solution treatment temperature during quenching. This method necessitates that a set of coefficients be developed by laboratory testing for a particular alloy in a given temper. In the original QFA, these coefficients have been developed independently for each physical property of each temper that might be produced from the same alloy. However, properties used commonly to assess quench sensitivity, e.g., electrical resistivity, hardness tests, tensile tests in different directions, etc., all respond to the same physical change during quench: loss of solute caused by the precipitation of quench precipitates. The QFA method has been modified to handle multiple properties and multiple tempers. The new method allows multiple C-curves for multiple phases and/or nucleation mechanisms for quench precipitates. The advantages of this method include better statistical resolution when fitting multiple data concurrently and possible substitution of a cheaper test for the property and temper of interest. The use of this approach is demonstrated on data sets for aluminum alloy 2024 within this study.     

Modeling Early-Age Bridge Restraint Moments: Creep, Shrinkage, and Temperature Effects

An increasing number of bridges are being designed with continuous spans instead of simple spans. By reducing the number of joints in a bridge, the traveling public receives a better riding surface and corrosion caused by leaking joints can be reduced. Also, redundancy is created when the system is made continuous, producing a tougher structure. However, a continuous system is more complicated to design and secondary restraint moments due to creep, shrinkage, and thermal effects can develop at the connection. This paper presents results from an experimental study done to monitor the early age restraint moments that develop in a two-span continuous system made of full-depth precast concrete bulb tee girders. The restraint moments observed were compared to the predicted restraint moments using the RMCalc program . The observed restraint moments were significantly lower than predicted by the program. Expansion of the deck during curing, which is generally not considered in the predictions, significantly influenced the early age restraint moments. A simplified model to predict the restraint moments considering thermal effects is proposed.     

电解铝用高品质阴极炭块成型工艺的优化

分析了400 kA大型预焙槽的特性,对炭素成型工序的配方、粉料纯度、粘结剂的软化点及成型的方法进行了优化,确定最佳的工艺参数,生产出电解铝用高品质的阴极炭块。     

A New Method to Characterize and Model Stress-Relaxation Aging Behavior of Aluminum Alloys Under Age Forming Conditions

Metallurgical and Materials Transactions A - A new method that utilizes theories of thermally activated deformation and repeated transient stress-relaxation tests has been proposed and validated in...     

Cavitation During Creep Deformation in AA7075-T76: Cellular Automata Simulation and Experiments

Metallurgical and Materials Transactions A - In this work, cavitation during creep in AA7075-T76 was simulated employing a probabilistic cellular automata scheme. Two-dimensional cellular automata...     

Creep Modeling in Excavation Analysis of a High Rock Slope

Based on the distinct element method, a numerical procedure is presented for simulation of creep behavior of jointed rock slopes due to excavation unloading. The Kelvin model is used to simulate viscous deformation of joints. A numerical scheme is introduced to create incremental contact forces, which are equivalent to producing creep deformation of a rock-joint system. The corresponding displacement of discrete blocks due to creep deformation of contact joints can be calculated by equilibrium iteration. Comparisons of results between the numerical model and theoretical solutions of a benchmark example show that the presented model has excellent accuracy for analysis of creep deformation of rock-joint structures. As an application of the model, residual deformations of the high rock slopes of the Three Gorges shiplock due to excavation unloading and creep behavior are investigated. By simulating the actual excavation process, the deformation history of a shiplock slope is studied. Good agreement has been achieved between numerical prediction and field measurements. It demonstrates the effectiveness of the presented model in analysis of the creep deformation due to excavation unloading of high rock slopes.     

铝及镁合金的半固态成形

半固态加工技术（SSP）起源于20世纪70年代，因其具有许多独特的优点，现已成功地应用于多种有色金属及黑色金属。本文主要介绍铝合金及镁合金半固态成形的基本原理、生产工艺及工业生产概况。     

Modeling of the Effect of Temperature, Frequency, and Phase Transformations on the Viscoelastic Properties of AA 7075-T6 and AA 2024-T3 Aluminum Alloys

The viscoelastic response of commercial aluminum alloys 7075-T6 and 2024-T3 as a function of temperature is presented. Experimental data are obtained with a dynamic-mechanical analyzer (DMA) at different loading frequencies and compared with the available transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) data. The effect of successive microstructural transformations (particle precipitation and redissolution) is revealed. An analytical model is developed, which fits the mechanical response up to 573?K (300?°C). The model takes into account the concentration of Guinier-Preston Zones (GPZ) and metastable precipitates (???? in AA 7075-T6 and ????/S?? in AA 2024-T3), allowing us to determine the kinetic parameters of these transformations. The activation energies were previously obtained by several authors from DSC measurements and other techniques, showing considerable dispersion. The presented data, obtained with a completely different technique, allow us to reduce the uncertainty on these data and show the potential of DMA measurements in the study of microstructural transformations.     

拉深件毛料外形的计算机辅助模拟与实验验证

汇场法已被证明是一种可行的理论方法。本文进一步研究了它对平面异形拉深零件外形的自动处理,绘出了自动绘制出的异形件毛料外形,用拉深实验检验了汇场法自动确定的几件典型零件毛料外形。在此基础上,提出了汇场法对设置拉深凸埂的指导作用,同时指出汇疡法确定毛料外形的精度可通过汇强的调整来改善。     

Modeling of Bolt Joint Behavior of Cast Aluminum Alloy (A380-T5) by Coupling Creep and Plasticity in Finite Element Analysis

Aluminum casting alloys exhibit creep behavior when the materials are exposed to high temperature and load. In this article, the stress- and temperature-dependent creep behavior of a die casting A380-T5 aluminum alloy was simulated using a classical constitutive model. The bolt-load retention behavior of the material was analyzed in a head bolt joint in an aluminum engine under thermal cycle condition using the finite element method. In this simulation, transient thermal analysis was performed first to calculate the metal temperature at the head bolt joint as a function of time during engine thermal cycling. This temperature was then input as the thermal loading in the subsequent structural analysis to calculate its effect on the bolt-load retention. The finite element analysis (FEA) model for the bolt-load retention simulation includes not only the plasticity in all metal components but also the creep properties of head bolt threads in the cast aluminum engine block. The FEA model was validated by good correlation between the predicted head bolt-load loss and the experimental measurement during engine thermal cycling. The simulation results also indicated that creep in the head bolt threads of cast aluminum engine block was mainly responsible for the load loss in the head bolt joint. This article is based on a presentation given in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties” which occurred March 12–16, 2006 during the TMS Annual Meeting in San Antonio, Texas under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

形变热处理对7075铝合金组织和性能的影响

研究了形变热处理对7075合金组织和力学性能的影响。结果表明：形变热处理工艺可大幅提高合金板材的强度,且合金板材保持了较好的塑性。金相观察发现,预时效析出的沉淀相可为合金板材冷变形后的终时效强化相均匀析出的优先成核提供条件;形变处理引入大量的位错,促进了终时效时高密度均匀细小过渡强化相η/的析出。     

7075铝合金累积叠轧焊组织与性能

为了提高7075铝合金的力学性能,7075铝合金在350℃无润滑条件下进行了5道次的累积叠轧焊实验,通过X射线衍射(XRD)与透射电镜(TEM)分析,研究了7075铝合金在叠轧过程中微观组织的演化规律,利用室温拉伸实验,研究了叠轧道次对7075铝合金力学性能的影响规律,并且采用扫描电镜(SEM)对拉伸断口形貌进行了分析。结果表明:7075铝合金在叠轧过程中材料的组成相η相发生回溶,数量减少;微观组织经历由位错缠结/位错胞状结构向形变亚晶结构转变的过程,5道次后,形成了尺寸小于1μm的亚晶组织;材料的强度随道次的增加而增加,5道次后,其抗拉强度与屈服强度分别达到373.52,315.84 MPa,约为原始合金的1.8倍和3.2倍,同时,延伸率则随着叠轧道次的增加而下降,5道次后,延伸率仅为原始合金的1/3,并且拉伸断裂由韧性断裂转变为脆性断裂。     

Process Modeling and Optimization in Sheet Metal Forming – Selected Applications and Challenges

This paper presents an overview on the application of FE simulation as a virtual manufacturing tool in designing process for manufacturing sheet metal parts. Input parameters to simulate a process are key elements in successful simulation for process design. In this paper several methods to determine input parameters for process simulation are discussed. Practical examples of application of FE simulation are presented for improvement of existing and/or designing new forming processes for manufacturing sheet metal parts.     

Calorimetric Studies of Precipitation and Dissolution Kinetics in Aluminum Alloys 2219 and 7075

Differential scanning calorimetry (DSC) was used to study the kinetics of precipitation and dissolution of metastable and stable phases in aluminum alloys 2219 and 7075. A comparison of DSC scans obtained at heating rates of 1, 5, 10, and 20 K per minute showed that, during a DSC scan, the rates of precipitation of θ′ and θ in 2219 and η′ and η in 7075 were limited by their reaction kinetics. Likewise, the rates of dissolution of GP zones, θ′ and η′, were found to be dominated by kinetics. In contrast, the dissolution of θ and η was dominated by the thermodynamic equilibrium between these phases and the matrix. Analysis of the kinetically dominated reaction peaks and their dependence on heating rate and particle size showed that the GP zone dissolution reaction could best be described by a three-dimensional volume diffusion limited rate expression with an activation energy equal to that for diffusion. The rate of formation of θ′ was best described by an Avrami expression withn = 1.1, indicating that nucleation was not the rate controlling step. A pronounced dependence of the θ′ formation rate on prior plastic deformation was observed and ascribed to the influence of the matrix dislocation density on diffusivity.     

Effect of Operation Parameters on Diffusion Bonded AA5083, AA6082 and AA7075 Aluminum Alloys

Rolled plates of 5 mm thick AA5083, AA6082 and AA7075 aluminum alloys Joints were fabricated by diffusion bonding at different temperatures. The microstructure evolution of AA5083, AA6082 and AA7075 aluminium alloys were characterized by transmission electron microscopy. Metallurgical investigations and mechanical tests were also performed to correlate the microstructural investigations with the mechanical properties of the produced diffusion bonded joints. It was observed that the bonding and shear strength increased with the increase in bonding temperature due to the diffusion of micro-constituents in the interface. Higher temperature enhanced the uniform distribution of secondary phase particles, which further improved the reduction in pores/defects in the bonded joints.     

Sensitivity Analysis of Nonequilibrium Adaptation Parameters for Modeling Mining-Pit Migration

The nonequilibrium adaptation parameters of a depth-averaged two-dimensional hydrodynamic and sediment transport model were examined in the study. Calculated results were compared to data measured in two sets of published laboratory experiments that investigated mining-pit migration under well-controlled boundary conditions including steady flow and uniform rectangular cross sections along the flume except in the vicinity of the experimental mining area. The two sets of experiments were chosen as representatives of bed-load-dominated and suspended-load-dominated cases, respectively. A sensitivity analysis was conducted to estimate the influence of the nonequilibrium adaptation parameters on mining-pit migration simulation. Calculated results indicate that appropriate selection of the adaptation parameters is critical in order to close the nonequilibrium sediment transport formulas when modeling mining-pit migration.     

Parameter Optimization and Sensitivity Analysis for Disturbed State Constitutive Model

Constitutive models for geologic materials and interfaces involve a number of parameters that need to be determined from appropriate laboratory tests. Because the test behavior is influenced by a number of factors such as material variability in test specimens, initial density, mean pressure, and stress paths, the parameters determined from such tests need to be averaged or optimized. The averaging procedure is often used. However, in view of the importance of the parameters in analysis and design, it is desirable and necessary to use advanced procedures such as optimization methods so as to find their improved and realistic values. This paper presents an optimization procedure for the determination of parameters in the unified disturbed state concept constitutive models. A series of multiaxial laboratory tests on a sand under different initial mean pressures, density, and stress paths are used to evaluate the optimized parameters. The stress-strain and volume change behavior is then back-predicted using the parameters from the conventional averaging procedure and the proposed optimization procedure. The results show that the optimized parameters provide improved predictions of the test data. The optimized parameters are used in a finite element procedure to predict cyclic behavior in a boundary value problem involving a shake table test. The proposed procedure can provide a useful methodology for the optimization of parameters for a wide range of available (plasticity, creep, damage, etc.) constitutive models. It can lead to improved analysis and design of geotechnical problems, particularly while using computer (finite element) procedures.     

Discrete-Element Modeling: Impacts of Aggregate Sphericity, Orientation, and Angularity on Creep Stiffness of Idealized Asphalt Mixtures

Hot-mix asphalt (HMA) contains a significant amount of mineral aggregate, approximately 95% by weight and 85% by volume. The aggregate sphericity, orientation, and angularity are very important in determining HMA mechanical behaviors. The objective of this study is to investigate the isolated effects of the aggregate sphericity index, fractured faces, and orientation angles on the creep stiffness of HMA mixtures. The discrete-element method was employed to simulate creep compliance tests on idealized HMA mixtures. Two user-defined models were used to build 102 idealized asphalt-mix digital specimens. They were the R-model and the A-model, short for a user-defined rounded aggregate model and a user-defined angular aggregate model, respectively. Of the 102 digital specimens, 84 were prepared with the R-model to investigate the effects of aggregate sphericity and orientation, whereas the remaining 18 were built with the A-model to address the effect of aggregate angularity. A viscoelastic model was used to capture the interactions within the mix specimens. It was observed that (1) as the sphericity increased, the creep stiffness of the HMA mixture increased or decreased, depending on the angles of aggregate orientation; (2) as the angle of aggregate orientation increased, the creep stiffness of the HMA mixture increased, with the rate depending on the sphericity index values; and (3) compared with the sphericity index and orientation angles, the influence of aggregate fractured faces was insignificant.