Yield behavior of porous nuclear fuel (UO2)

Uranium dioxide (UO₂) is one of the most common nuclear fuels. During burn-up, the fuel undergoes substantial microstructural changes including the formation of pressurized pores, thus becoming a porous material. These pores reduce the elastic modulus and alter the yield behavior of the material. In this work, a finite-element-based homogenization technique has been used to map the yield surface of UO₂ with pressurized pores. Two scenarios are considered; in the first, the fuel matrix is a ductile material with a Von-mises type behavior, while in the second, the matrix is quasi brittle, which is simulated using the concrete damaged plasticity (CDP) model available in ABAQUS. For both of the scenarios, it is found that the yield strength decreases with an increase in porosity for a given internal pore pressure. For a given porosity, the yield surface shifts towards the negative hydrostatic axis in the Haigh-Westergard stress space with an increase in pore pressure. When the matrix is quasi brittle, the decrease in tensile hydrostatic strength is less than the increase in compressive hydrostatic strength, whereas in the case of a ductile matrix, the changes in the hydrostatic strengths are same. Furthermore, the shape of the yield surface changes from one deviatoric plane to another in both scenarios. Analytical equations, which are functions of pore pressure and porosity, are developed to describe the yield surface of porous UO₂ while accounting for the changes in shape of the yield surface from one deviatoric plane to another. These yield functions can be used to predict the failure of porous UO₂ fuel.     

Computer simulation of effects of the pore size distribution on the kinetics of pressure-assisted final-stage densification

Most theoretical treatments of pressure-assisted densification of porous solids assume a single size for all pores. We remove this assumption and consider a distribution of pore sizes. Dissolution of intragranular pores by volume diffusion and dissolution of intergranular pores by grain-boundary diffusion are both treated. The evolution with time of pore size distributions is calculated for distributions that are initially described by log-normal and Weibull functions, and differences in predicted behaviours are discussed. The pore size distribution is then related to two important quantities: porosity and number of pores per unit volume. The assumption of a distribution of pore sizes is found to avoid certain unrealistic predictions obtained from models with a single pore size, such as abrupt disappearance of all pores and rapid approach to full density.     

Pore–crack orientation effects on fracture behavior of brittle porous materials

Mechanical behavior of two-dimensional microstructures containing circular pores were simulated under uniaxial and biaxial loading using the finite element method. Resulting stress distributions were combined with classical fracture mechanics to investigate fracture behavior of brittle porous materials assuming that randomly oriented cracks are present along pore surfaces. Multiple crack orientations were found to introduce a variability in Weibull modulus even for the same set of microstructures containing equal number and size of cracks. Also, the variability increases with increasing crack size to pore size ratio. Under uniaxial loading, angular distribution of fracture origin widens with increasing porosity.     

Preparation of bimodal porous mullite ceramics

Bimodal porous ceramics were prepared based on the needle-like mullite crystals, by leaching the glass matrix of the calcined body of the kaolin and transition metal oxide systems mixed with coal powder or polymethylmethacrylate beads as a pore-forming material. Small and large pores in the bimodal porous ceramics were in the range of 0.06–0.5 and 3–7 m, respectively. The small pores originated from gaps between the needle-like crystals, while the large ones originated from the pore formers. However, the large pores were suggested to be ink-bottle type in shape, because the measured size was much smaller by approximately an order of magnitude than the diameter of the pore former. In every case, the total and large pore volumes agreed well with those calculated based on the density and composition of the starting materials and the intrinsic porosity of the mullite ceramics.     

A computational study of elongated pore interactions in a low density porous copper

A finite element investigation of an array of high aspect ratio pores under tension deformation was performed to evaluate geometric features affecting bulk yield, strain hardening and strength in a ‘gasar’ porous copper material of 21.5% pore volume fraction. The analysis simulated a planar triagonal array representing a central pore and its six nearest neighbors. Typical pore dimensions of 18 μm in diameter, 108 μm in length and 60 μm transverse spacing between centers were used for the model. Local transverse constraint above and below groups of pores will vary due to the presence or absence of isolated pore free zones throughout the microstructure. Displacement boundary conditions were selected to evaluate the effect of zero, partial and full transverse constraint. The normalized load vs. normalized displacement results from the model are found to exceed that of the solid copper matrix by 89% for full transverse constraint and 16% for partial transverse constraint at peak normalized load, and 6% below that of the solid copper matrix for zero constraint at 0.20 normalized displacement. These results provide insight into the role of pore interactions and local geometric constraint on the comparatively high bulk strength observed in these materials. Full transverse constraint is also responsible for the deformation localization consistent with the appearance of the fracture surface.     

碳热还原-反应烧结法制备多孔氮化硅陶瓷

以廉价的二氧化硅、炭黑和硅粉为起始原料, 利用碳热还原-反应烧结法制备了高气孔率、孔结构均匀的多孔氮化硅陶瓷, 考察了原料中硅粉含量对多孔氮化硅陶瓷微观组织和力学性能的影响。XRD分析表明烧结后的试样成分除了少量的α-Si₃N₄相和晶间相Y₂Si₃O₃N₄外, 其余都是β-Si₃N₄相; SEM分析显示微观组织由棒状β-Si₃N₄晶粒和均匀的孔组成。通过改变硅粉的含量, 制备了不同气孔率, 力学性能优异的多孔氮化硅陶瓷。     

蒸压加气混凝土的孔结构及表征方法研究进展

蒸压加气混凝土因其质轻、保温性好、环保等优点而受到人们的重视。作为一种典型的宏观多孔建筑材料,蒸压加气混凝土的孔结构特征与其微观结构和性能有密切的关系。钙硅比、水料比、铝粉及工艺参数是影响蒸压加气混凝土孔结构的重要因素,孔隙率、孔径分布等孔结构特征与蒸压加气混凝土的强度、吸水性、干燥收缩、导热性能及耐久性等性能有紧密的联系。本文重点阐述了蒸压加气混凝土的孔结构特征及影响孔结构的主要因素,孔结构对蒸压加气混凝土性能的影响,并简述了蒸压加气混凝土孔结构现阶段主要采用的表征方法。     

Anisotropic compressive properties of porous copper produced by unidirectional solidification

Porous copper whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen and argon. The compressive yield strength of the porous copper with the cylindrical pores orientated parallel to the compression direction decreases linearly with increasing porosity. For the porous copper whose pore axes are perpendicular to the compressive direction, the compressive yield strength slightly decreases in the porosity range up to 30% and then decreases significantly with increasing porosity. The compressive stress–strain curves depend on the compressive direction with respect to the pore direction, which are due to the stress concentration around the pores and the buckling of the copper between the pores. From two different types of stress–strain curve, the energy absorption capacity of the porous copper with the pores parallel to the compressive direction is higher than that perpendicular to the compressive direction at a given porosity.     

三维织造预制体微观结构及致密化

碳纤维预制体是碳/碳材料的骨架,致密化过程中,基体碳填充预制体孔隙形成碳/碳复合材料。为探索碳纤维预制体微观结构与致密化的关系,以3种三维织造预制体为研究对象,通过金相显微镜观察预制体微观结构,建立预制体理论结构数字化模型,计算对比孔隙率情况后获得预制体中不同类型孔隙的体积含量,研究孔隙结构对化学气相沉积致密化效率的影响。研究结果表明:三维织造预制体中孔隙呈周期性重复排列,XY向纤维层间距、Z向纤维间距越小,预制体中纤维体积含量越高;预制体中孔隙主要由纤维束内孔隙和X,Y,Z向纤维束包围的束间孔隙构成;预制体中的孔隙大小和分布影响致密化效率,孔隙率高的预制体在致密化前期增密速率快,孔隙结构均匀的预制体在致密化后期增密速率快。     

A multi-scale framework for effective elastic properties of porous materials

This paper presents a statistical micromechanics-based multi-scale material modeling framework to predict the effective elastic moduli of porous materials. The present formulation differs from most of the existing theoretical models in that the interaction effects among the pores are directly accounted for by considering the pair-wise interaction and the statistical information of pore distribution is included by applying the ensemble volume averaging process. The theory of average fields is employed to derive the stress and strain concentration factor tensors that relate the local average fields to the global averages. Closed-form and analytical explicit expressions for the effective elastic moduli of porous materials are obtained in terms of the mechanical properties of the matrix material and porosity. The dependence of effective elastic properties on the porosity is investigated. Comparison of our theoretical prediction with the results of the published experimental data and other existing theoretical models is performed to illustrate the predictive capability of the proposed framework for porous materials.     

叔丁醇基凝胶注模工艺制备轻质、高强莫来石多孔陶瓷

以叔丁醇为成型溶剂, 莫来石粉为起始原料, 采用凝胶注模成型方法制备出轻质、高强莫来石多孔陶瓷. 莫来石多孔陶瓷中的孔隙形成于干燥过程中叔丁醇的快速挥发, 孔隙分布均匀且相互连通. 随烧结温度升高, 气孔率、开气孔率和比表面积分别由77.8%、76.0%和10.39m²/g下降到67.6%、65.5%和4.26m²/g, 而抗压强度则由3.29MPa显著提高到32.36MPa, 材料孔径大小受烧结温度影响较小, 孔径尺寸呈单峰分布, 且几乎所有的气孔都为开口气孔, 透气度与孔径尺寸具有一致的变化关系. 莫来石多孔陶瓷在高气孔率条件下仍然保持高强度的主要原因是材料中均匀的孔隙结构、孔径尺寸小且相对集中、以及因烧结颈的形成在空间上所表现出的一种颗粒搭接骨架结构.     

金尾矿蒸压加气混凝土水化机理和微观结构分析

为明确金尾矿蒸压加气混凝土制品的水化机理和微观结构的内在关系,通过对比硬化坯体(NAC)、蒸压恒温养护0 h(AAC-0)和蒸压恒温养护8 h(AAC-8)的3组样品的XRD谱、IR谱图分析其水化产物种类的变化,并对比SEM照片分析了其微观结构变化.研究结果表明,随着蒸压养护过程的进行,坯体内的水化产物出现阶段性变化,最终由富钙型水化硅酸钙向托贝莫来石转化,由于托贝莫来石生成的局限性和同步性,导致微观孔壁结构出现明显的分层现象,后生成的托贝莫来石层使孔结构成为中空的刚性球,作为"骨料",起到骨架和支撑的作用.     

Topotactixity of the hydrogen reduction process in fused iron catalyst

Microstructural characterization of highly porous fused iron catalyst in the reduced and partially reduced states has been performed using electron microscopy techniques. The microstructure consists of 30–50nm iron crystallites in a network of highly defected iron, forming an interconnected porous structure. The size of the pores ranges from 10 to 20nm and they are aligned along a specific crystallographic direction of magnetite and/or iron. We found that the {111}Fe₃O₄ and the {011}Fe planes are preferentially reduced and preferentially exposed, respectively, during reduction. The magnetite--iron-pore channel chrystallographic orientation relationship can be expressed as ( 11)Fe₃O₄ // ( 10)Fe // pore channel and (0 2)Fe₃O₄ // (002)Fe pore channel. The former is known as the Nishiyama-Wassermann orientation relationship between face-centered and body-centered cubic materials. It is suggested that the high activity of this catalyst is due to the presence of alkali atoms on top of every other closely packed iron atom.     

A numerical investigation of porous titanium as orthopedic implant material

Porous titanium is being developed as an alternative orthopedic implant material to alleviate the inherent problems of bulk metallic implants by reducing the stiffness to be comparable to bone stiffness and allowing complete bone ingrowth. However, a porous microstructure is susceptible to local permanent plastic strain and residual stress under cyclic loading which reduces damage tolerance and therefore limits their application as orthopedic implants. The mechanical properties of porous titanium are governed by the microstructural configurations such as pore morphology, porosity, and bone ingrowth. To understand the influence of these features on performance, the macroscopic and microscopic responses of porous Ti are studied using three-dimensional finite element models. The models are generated based on simulated microstructures of experimental materials at porosities of 15%, 32% and 50%. The results show the effect of porosity and bone ingrowth on Young’s modulus, yield stress, and microscopic stress and strain distribution. Importantly, simulations predict that the bone ingrowth reduces the stress and strain localization under cyclic loading so significantly that it counteracts the concentration condition caused by the increased porosity of the structure.     

Modélisation thermique des milieux poreux et prise en compte du couplage des paramètres de porosité pour l'étude de l'influence de la forme des pores

The aim of the first part of this work is to show the dependence of porous media on geometrical parameters: form and distribution of pores and maximal porosity. It also proves the necessity to consider this dependence when building correct patterns of porous media and more particularly when comparing materials which have different forms and distributions of pores. These points are not generally well-studied in existing patterns both from a thermal standpoint as from a mechanical one. The second part of this work concerns implementing thermal patterns that allow to compare a material constituted by spherical pores distributed in a cubic system with one constituted by ellipsoidal pores uniaxially oriented and distributed in a parallelepipedic system with variable dimensions. Thermal hypotheses are specified and geometrical simplifications compatible with the dependence of porosity parameters are introduced to simplify calculations. This leads to directly usable results on the influence of form and distribution of pores on the thermal characteristics of diphasic porous materials. Established patterns can be transposed for diphasic non-porous materials, which could be developed for porous materials by taking into account a third liquid stage in addition to solid and gaseous stages.     

Effect of pore shape on the thermal conductivity of partially saturated cement-based porous composites

Increasing demands of new cement-based porous composites for thermal insulation required by sustainable development are crying out for clarifying the key factors affecting their thermal properties. In this study, we focused on the effect of pore shape, a factor that was often slighted or ignored before, on the thermal conductivity of composite materials. We established a predicting model for a simple two-phase inclusion-matrix system based on effective medium and mean-field theories. The model can be easily extended to assess the effective thermal conductivity of multi-phase and multi-scale composites. The generalized model involves both homogenization schemes and material structures including the shapes and orientations of inclusions. We showed that the shapes of inclusions affect the effective thermal conductivity of composites significantly. The established model were used to predict the thermal conductivities of cracked ordinary concrete and partially saturated autoclaved aerated concrete reported in the literature with in-depth discussion.     

仿竹结构单丝玻璃纤维增强多孔聚醚砜基复合材料

竹子是一种以竹纤维为增强体、多孔木质素为基体而组成的天然复合材料.本文借鉴竹子的结构特征,采用高性能热塑性聚合物浸没沉淀相转化法在玻璃纤维(GF)表面沉积梯度孔径分布的多孔聚醚砜(PES)基体,制备仿竹结构单丝玻璃纤维增强多孔聚醚砜基复合材料(GF/PES),并对其微观形貌、拉伸力学性能和“温度-模量”智能响应性进行了...     

Wet-milling disposal of autoclaved aerated concrete demolition waste – A comparison with classical supplementary cementitious materials

The raw autoclaved aerated concrete waste (RA) is an important construction and demolition waste. The present work investigated the possibility of utilizing ultrafine autoclaved aerated concrete waste (WA) as a supplementary cementitious material (SCM) after wet-milling disposal. Results indicated that it is possible to realize the utilization of RA as a SCM by wet-milling treatment. The ultrafine WA blended mixture presented earlier hydration heat flow than the other alternatives. WA blended mixture showed almost the highest reactivity index (almost 100%) and compressive strength at each curing age, whereas coarse RA and SS blended systems presented the lowest values. WA blended mixture showed relatively low initial and secondary sorptivity coefficients. It was concluded that the high secondary sorptivity coefficients of FA and RA was due to the formation of connected voids by porous RA particles and partially reacted hollow FA spheres.