What physical phenomena can be neglected when modelling concrete at high temperature? A comparative study. Part 1: Physical phenomena and mathematical model

The paper deals with modelling of hygro-thermal performance and thermo-chemical degradation of concrete exposed to high temperature. Several possible simplifications in modelling of heat and mass transport phenomena in heated concrete are considered and their effect on the results of numerical simulations is analyzed.A mathematical model of concrete at high temperature, already extensively validated with respect to experiments, is used as the reference model. It is based on mechanics of multiphase porous media and considers all important couplings and material nonlinearities, as well as different properties of water above the critical point of water, i.e. 647.3 K (374.15 °C).In this part of the paper, first physical phenomena, as well as heat and mass flux and sources in a concrete element are studied, both during slow and fast heating process, to examine the relative importance of different flux components. Then, the mathematical model of concrete at high temperature, developed by Authors in the last 10 years, is briefly presented and for the first time all the constitutive relationships of the model are summarized and discussed in detail. Finally, the method of numerical solution of the model equations is thoroughly presented.In the companion paper (part II) a brief literature review of the existing mathematical models of concrete at high temperature and a summary of their main features and physical assumptions will be presented. Then, extensive numerical studies will be performed with several simplified models, neglecting chosen physical phenomenon or flux components, to evaluate the difference between the results obtained with the simplified models and with the reference model. The study will concern hygric, thermal and degradation performance of 1-D and 2-D axisymmetric concrete elements during fast and slow heating. The analysis will allow us to indicate which simplifications in modeling of concrete at high temperature are practically and physically possible, without generating excessive errors with respect to the full reference model.     

激光选区熔化增材制造中的粉体热动力学行为

激光选区熔化(SLM)可以直接成形近全致密、性能接近锻件的复杂结构金属零件, 是金属增材制造(3D打印)领域的热点技术之一. SLM成形过程中粉末颗粒的热/动力学行为复杂, 与零件成形缺陷及力学性能紧密相关. 本文介绍了离散单元法(DEM)与计算流体力学(CFD)联合建模在SLM中的创新应用, 结合粉末床原位测试及成形在线监测, 探索SLM粉末铺设和粉末床熔融两个工艺环节的复杂粉体热/动力学行为机制. 研究发现, 粉末铺设过程中: 粉体的黏结效应、壁面效应和渗流效应3种机制相互竞争、共同支配粉末动力学行为并最终决定粉末床铺设质量. 粉末床熔融过程中: 熔池喷发的高温金属蒸汽带动环境保护气体形成内旋涡流, 由此驱动散体粉末形成复杂流固耦合运动, 导致粉末床飞溅与剥蚀现象; 热浮力效应对粉末运动不起主导作用. 文中提出了DEM-CFD双向动态耦合模型, 可以充分考虑离散粉末与熔池蒸发气体之间的热力耦合作用, 为SLM粉体熔融热/动力学行为的仿真模拟提供了一种新途径.      

Protein-based bioplastics: effect of thermo-mechanical processing

Bioplastics based on glycerol and different proteins (wheat gluten, albumen, rice and albumen/gluten blends) have been manufactured to determine the effect that processing and further thermal treatments exert on different thermo-mechanical properties of the bioplastics obtained. Oscillatory shear, modulated differential scanning calorimetry, dynamic mechanical thermal analysis, thermo-gravimetric analysis and water absorption tests were carried out to study the effect of processing on the physical characteristics of the bioplastics. The protein-based bioplastics studied in this work present a high capacity for thermosetting modification because of protein denaturation that may favour the development of a wide variety of materials. The use of albumen or rice protein allows the reduction in both protein concentration and thermosetting temperature, leading to linear viscoelastic moduli values similar to those of synthetic polymers such as LDPE and HDPE. The hygroscopic characteristics of protein-glycerol bioplastics may lead to a decrease in the values of the linear viscoelasticity functions. However, hygroscopic properties depend on the protein nature and may be used for industrial applications where water absorption is required. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

Two-dimension estimate of effective properties of solid with random voids

A discrete model for the estimation of the effect of random voids on the structural properties of a two-dimensional solid is presented. Triangular void cells are used to simulate randomly located micro-cracks.The proposed model is solved using Cell Method, a recent numerical method that allows a direct discrete formulation of balance equations. Both heterogeneities of the structure and stress concentrations due to voids distribution are taken into account by the model. Following an introduction, some aspects of Cell Method for plane elasticity that are relevant for this paper will be briefly recalled and results from simulations in the elastic field will be discussed. Next, the proposed model will be extended to plastic field and more simulations will be presented. Results show that this model can be successfully employed to assess the structural response of a 2D solid with randomly distributed voids.     

用固体润滑剂改善汽车刮水胶条使用性能的研究

作者将含有MoS_2和石墨的不同配方的固体润滑涂料刷涂在几种国产汽车刮水胶条上,并分别于干摩擦和水润滑的两种刮刷状态下,就固体润滑剂改善刮水胶条的擦拭性能进行了试验研究,指出这是使刮水胶条降低擦拭阻力、提高刮刷效果的可行途径,同时强调配制涂料时应当选用颗粒细小而均匀的固体润滑剂粉末。     

Constitutive Model Calibration for Powder Compaction Using Instrumented Die Testing

Calibration procedures for constitutive models for powder compaction are presented. A practical calibration method based on a die compaction experiment is presented. A newly developed apparatus consisting of a die instrumented with radial stress sensors is described. The paper proposes two contributions to account for errors present in instrumented die testing, which are due to 1) elastic compliance of the testing frame, influencing the measurement of axial strain and 2) the presence on non-homogeneous stress state in the test specimen. It is shown that system compliance is important for generating an accurate stress-strain curve for compression. The effect of different compliance correction methods is evaluated with regard to the accuracy of models predicting pressing forces. The system compliance becomes more significant during unloading in the die; this information is used to determine the elastic properties. A new compliance correction method is introduced following a detailed analysis of the forces and deformations of different parts of the loading frame. In instrumented die compaction the axial and radial stresses are measured at fixed locations and the specimen is subject to non-homogeneous stresses and strains due to the effect of friction between the powder and die wall. Starting from the Janssen-Walker method of differential slices a method to account for non-homogeneous stress and strain is developed.     

Determination of properties for the calculation of aqueous thin film drying

 This paper deals with the determination of various properties required for the numerical calculation of the thin film drying of a water based varnish applied on paper. Experimental and analytical methods which provide the activity of water in paper and in a water based varnish, and the diffusion coefficient of water in this varnish are presented. Received on 23 March 2001     

The Blasius and Sakiadis flow with variable fluid properties

A theoretical study of the effect of variable fluid properties on the classical Blasius and Sakiadis flow is presented in this paper. The investigation concerns engine oil, water and air taking into account the variation of their physical properties with temperature. The results are obtained with the numerical simulation of the governing equations and cover large temperature differences. Velocity and temperature profiles are presented, as well as values of wall shear stress and wall heat transfer, for a variety of temperatures between the plate and the ambient fluid. It is found that the variation of fluid properties and especially viscosity have a strong influence on the results. The results of oil and water are, in general, similar and are generalized to liquids whereas air results are different and are generalized to gases. Except of the new results found in the present work some inaccurate results existing in the literature have been identified.     

The Potential Scope of the Ultrasonic Surface Reflection Method Towards Mechanical Characterisation of Isotropic Materials. Part 1. A Theoretical Analysis

Background

This paper deals with the possible field of application of ultrasonic Surface Reflection Method (SRM) to achieve the mechanical characteristics of isotropic materials. This method is based on the measurement of the amplitude of the reflected wave at the interface between reference material and the material to be characterised. Objective: The purpose of Part 1 of this paper is to establish the theoretical conditions for the applicability of SRM.

Methods

First, the theoretical formulas necessary to obtain the mechanical properties of the material to be tested will be established. Then, on the basis of these analytical formulas, the validity of the results for the material to be studied will be discussed according to the choice of the mechanical properties of the reference material through uncertainty calculations. The measurand error of SRM is then compared to that of traditional methods (transmission, transmission in water bath, pulse-echo).

Results

The analytical solution to the inverse problem (the mechanical characteristics of the tested medium based on those of the reference medium and the waves’ amplitude) will be given. From this analytical solution, an analysis of the measurand error will be performed and a method for choosing the reference material will be proposed.

Conclusions

It appears that SRM is better suited than traditional methods in two specific cases: measurement of small deviations of mechanical properties from a reference material or characterisation of high damping materials. In Part 2 of this paper, the practical conditions of applicability of the method are described and then applied to different kinds of materials.

     

A new computational algorithm for contact friction modeling of large plastic deformation in powder compaction processes

In this paper, the large deformation frictional contact of powder forming process is modeled based on a new computational algorithm by imposing the contact constraints and modifying the contact properties of frictional slip. A simple and efficient numerical algorithm is presented for imposing the contact constraints and frictional contact properties based on the node-to-surface contact technique to simulate the large deformation contact problem in the compaction process of powder. The Coulomb friction law is used to simulate the friction between the rigid punch and the workpiece by the use of penalty approach. A double-surface cap plasticity model is employed together with the nonlinear contact friction algorithm within the framework of large FE deformation in order to predict the non-uniform relative density distribution during large deformation of powder die-pressing. Finally, the numerical schemes are examined for accuracy and efficiency in modeling of a set of powder components.     

New evaluations for multiaxial-stress properties of ceramic materials

This paper suggests some new evaluations for multiaxial-stress properties of ceramic materials. These evaluations include some that have been used for other kinds of materials, as well as others which have not been previously employed. In some cases, these methods represent modifications of existing evaluations. The paper is confined to macroscopic behavior based upon bulk laboratory specimens. The influences of volume, stress gradients and localized behavior are not considered here since considerable attention has recently been devoted to these questions. The important problem of fracture strength will not be considered since this property appears to be considerably influenced by localized microscopic behavior. However, new evaluations of remaining mechanical properties for states of combined stresses will be presented. These include elastic and plastic strength, stiffness, ductility, resilience and toughness. Emphasis on combined-stress properties was selected since recent critical reviews indicate the need for for such an evaluation. Part A of this paper outlines new experiments that are needed to evaluate the mechanical properties and to confirm theories proposed in Part B. In Part B of this paper, new macroscopic engineering-type theories for combined-stress behavior are presented for the first time. These theories attempt to predict combined-stress behavior from uniaxial tension and compression (or pure bending and compression) behavior. These theories provide for materials such as ceramics with different properties in tension and compression. A final section of this presentation is devoted to improvements in the evaluations of other mechanical properties of materials as related to high-temperature creep and fatigue properties.     

Propagation of elastic waves in water-saturated soils

A brief review of the literatures on the titled subject is given. A set of wave equations, taking the inertial coupling effect between soil skeleton and pore water into account, are established for saturated soils. The preliminary analysis shows that the nature of wave propagation is mainly influenced by permeability coefficient,k. There are three types of waves, two (P-and S-wave) propagating through soil skeleton and one(P-wave) through pore water. For a soil with large value ofk, compression wave velocity through pore water will be greater than that through single-phased water, and ask→∞, the former could be times as great as the latter. For a soil with extremely low permeability, the compression wave velocity could be either less or greater than that through single-phased water, depending on the rigidity of the soil passing through. Some phenomena observed from tests presented in the literature may be reasonably explained by the proposed theory herein, and thus more reliable parameters of soil could be obtained from wave velocity measurements. Further studies on this subject are still needed. This paper is a part of the dissertation of the first author for the Ph.D. degree, the second author is his advisor.     

PS溶液渗透对非饱和土的增强效应研究

特定模数硅酸钾溶液(PS)已广泛的用于我国西北干旱地区土遗址加固保护工程中,并且取得了显著的效果。但是PS对潮湿地区遗址土加固效果,目前尚缺乏成熟的理论和实验验证。本文从室内试验的资料出发,研究PS对潮湿地区土遗址非饱和土的增强效应。本次研究土样取自位于潮湿环境中的浙江杭州良渚土遗址(3000 B C.),在实验室内对高含水量(13％ ~25％)的重塑土样进行了PS渗透加固。通过研究对比加固前后重塑土样的固结特性和抗剪强度,发现 PS对潮湿地区遗址土也有一定的加固效果。加固效果随初始饱和度的不同而不同,但初始含水量达25%的土在PS作用下仍可能得到增强。经PS渗透的土样不仅提高了初始固结性能和抗剪强度,而且加固土样而且呈现出一定的弹脆性特征,呈现出塑脆性转化,表明PS渗透可能产生对土初始结构的稳固作用。这一探索研究将为扩大PS在土遗址加固保护工程的应用提供一定的理论支持。     

库岸再造对雅泸高速公路岗子上隧道进口岸坡的影响

雅泸高速公路青杠咀特大桥泸沽岸桥台与岗子上隧道进口衔接段位于大渡河右岸上部冰积扇上,由于瀑布沟水电站的蓄水将使岸坡前缘约1/4部分没于水下。对整个岸坡影响较大的岩土体主要是由上-中更新统冰积、冰水沉积层组成的冰水堆积物,这类岩土体在水的作用下,其物理力学性质和强度将会发生显著变化,弱化工程性状,从而影响岸坡的稳定性及桥台布置。可以预见,对岸坡起主要影响作用的外部条件就是水的作用,库岸再造将是影响岸坡稳定性的控制性因素。本文以岗子上隧道进口岸坡为研究对象,通过对岸坡工程地质条件、岩土体结构及物理力学特性、岸坡影响因素的分析,深入研究了瀑布沟电站水库蓄水后引起的库岸再造对岸坡稳定性的影响。并采用卡丘金法、两段法(多段法)、极限平衡分析法对库岸再造影响宽度进行预测。3种方法的预测结果表明:瀑布沟电站蓄水后该段岸坡各个不同部位均会产生不同程度的塌岸破坏。根据该库岸岸坡的结构特点和3种方法的适用条件,提出对库岸岸坡不同梯段需要采用不同的预测方法综合预测,即对于水下岸坡可按两段法预测,而对于水上岸坡可综合考虑卡丘金法和极限平衡法的计算结果。对于类似库岸岸坡的稳定性分析具有一定的指导意义。     

Asymptotic suction profiles for the Blasius and Sakiadis flow with constant and variable fluid properties

A theoretical study of the effect of variable fluid properties on the Blasius and Sakiadis flow with uniform suction at the asymptotic state is presented in this paper. The investigation concerns air and water taking into account the variation of their physical properties with temperature. Velocity and temperature profiles are presented as well as values of the displacement thickness, momentum thickness, shape factor, wall shear stress and Nusselt number for different temperatures of the plate and the ambient fluid. It is found that the nondimensional displacement thickness, momentum thickness, shape factor, absolute wall shear stress and Nusselt number are identical in both Blasius and Sakiadib flow at the asymptotic state for a fluid with constant properties. The same is valid for any fluid with variable properties if the temperature boundary conditions are the same in Blasius and Sakiadis flow.     

Plane strain extension of a strip made of a material with stress state type dependent properties and weakened by cuts with circular base

The limit properties of many heterogeneous materials such as grounds, concrete, ceramics, cast-iron alloys, and various heat-resistant and powder materials, as well as the properties of many composite materials, depend on the loading conditions. Neglecting the effects exhibited by such materials may result in nonconservative limit load analysis for some types of loading and possibly in an overly increased end product weight by failing to take into account stronger material properties for other types of loading. This paper presents a possible approach to modeling the behavior of such materials under plastic deformation, which is demonstrated for the sample problem on the extension of a strip weakened by cuts with circular base. An analytic solution on the basis of a rigid-plastic model of the material and a numerical solution by the finite elementmethod with elastic strains and small strengthening taken into account are presented.     

Size-dependent sharp indentation—I: a closed-form expression of the indentation loading curve

In this paper, a closed-form expression of the size-dependent sharp indentation loading curve has been proposed based on dimensional analysis and the finite deformation Taylor-based nonlocal theory (TNT) of plasticity (Int. J. Plasticity 20 (2004) 831). The key issue is to link the results of FEM based on TNT plasticity with those obtained using conventional FEM by taking as the effective strain gradient, η, that presented in the work of Nix and Gao (J. Mech. Phys. Solids 46 (1998) 411), thus avoiding large-scale finite element computations using strain gradient plasticity theories. Two experiments carried out on 316 stainless-steel and pure titanium have been used to verify the effectiveness of the present analytical model; the results demonstrate that the present analytical expression of the size-dependent indentation loading curve corresponds very well to the experimental indentation loading curve. The empirical constant, α, in the Taylor model estimated from the experimental data has the correct order of magnitude. Also, the results presented in this part can be further applied to establish an analytical framework to extract the plastic properties of metallic materials with sharp indentation on a small scale where the size effect caused by geometrically necessary dislocations is significant. This will be discussed in detail in the second part of the paper.     

Effect of Gas Diffusion Layer Properties on Breakthrough Time and Pressure

The gas diffusion layer (GDL) plays an important role in the removal of product water from the catalyst layer to the flow plate in a fuel cell. Numerous studies have reported water management, especially in the GDL, as the limiting factor hindering convective and diffusive transport of reactants which results in lowering power density. In this paper, an experimental technique is presented to study the GDL water transport properties associated with the breakthrough conditions which are critical to overall water management. Fluorescence microscopy technique is used to measure the pressure and time required for water to penetrate and break through the surface of the GDL. The results obtained for GDLs produced by different manufacturers confirm that the breakthrough time and pressure are larger for PTFE treated hydrophobic GDLs. The results are analyzed in terms of the contact angle, thickness, and SEM images to see the effects of different structural properties. The changes in morphology due to compression are also presented. In addition, the changes in breakthrough conditions when samples are reused are presented. The results provide basic insights into the water transport properties of the GDL, leading to the design of new materials with enhanced water management.     

Numerical investigation of powder compaction of gear wheels

A method to judge the porosity distribution within complex powder compacted 3D structures using a dynamic 3D dilatant finite strain finite element program is presented. The method is demonstrated for a gear wheel, using a combined FKM Gurson model with parameters calibrated from experiments to model a ferrous powder. Compaction is pursued until a final average porosity of 3% in the gear. The method is shown successful in judging the influence on local as well as average properties from change in geometrical parameters and compaction speed.     

Correlation mechanism of friction behavior and topological properties of the contact network during powder compaction

The effect of friction behavior on the compacted density is significant, but the relationship between the topological properties of the contact network and friction behavior during powder compaction remains unclear. Based on the discrete element method (DEM), a DEM model for die compaction was established, and the Hertz contact model was modified into an elastoplastic contact model that was more suitable for metal-powder compaction. The evolution of the topological properties of the contact network and its mechanism during powder compaction was explored using the elastoplastic contact model. The results demonstrate that the friction behavior between the particles is closely related to the topological properties of the contact network. Side wall friction results in smaller clustering coefficient (CC) and excess contact (EC) in the lower region near the side wall. Corresponding to this phenomenon, the upper region near the side wall has more high-stress particles when the major principal stress threshold was considered, and the CC and EC are significantly higher than those in the other regions. This study provides a theoretical basis for improving powder compaction behavior.