孔隙形状及孔隙率对多孔材料弹性性能的影响

利用通用单胞法(GMC)计算了不同孔隙形状及孔隙率对多孔材料等效弹性参数的影响,计算中分别采用二维方形、圆形孔隙模型和三维立方体、球形孔隙模型模拟多孔材料。不同孔隙率下等效弹性参数的计算结果表明: 不同孔隙形状下,多孔材料等效弹性参数随孔隙率增大的退化程度不同;通过对比二维简化模型与三维模型的差异,发现二维简化模型对多孔材料等效弹性参数的估算值偏低。进一步将GMC计算结果和已有文献实验结果进行比较,发现两者具有较高的吻合度。最后将GMC模型与有限元、经验模型进行对比,得出GMC模型的局限性。综合计算结果,GMC具有一定的计算精度,可应用于工程实际分析中。     

Structural and mechanical properties and filtration in elastic fissured and porous material

Within the framework of the model of two coexisting homogeneous porous continua the article examines the influence of the state of stress of a material and of the liquid pressure on the effective parameters of the continua, and a system of equations of filtration is obtained that differs from the known system.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 46, No. 4, pp. 593–600, April, 1984.     

浮空器蒙皮膜复合材料单轴拉伸力学性能及弹性常数

对新型飞艇蒙皮材料在0°、15°、30°、45°、60°、75°、90°七个偏轴方向单轴拉伸循环试验结果进行了分析,给出了残余应变和弹性模量随循环次数的变化规律;得出了由单层板理论推导出的关于弹性模量的本构关系对各功能膜层压合成的平纹织物膜复合材料适用性较差。使用VIC-2D数字散斑测量系统测出膜材在拉伸过程中的位移场和应变场,通过位移场求膜材的泊松比和通过应变场验证分析膜材拉伸破坏机制,并可以预测断口形态和位置。采用两种不同规格的试样测试膜材的单轴拉伸强度,通过对比发现采用试样3更能反应材料Uretek5876实际强度。本文工作对该材料应用于飞艇结构设计和分析具有参考价值。     

Effect of nanotube geometry on the elastic properties of nanocomposites

Many analytical models replace carbon nanotubes with “effective fibers” to bridge the gap between the nano and micro-scales and allow for the calculation of the elastic properties of nanocomposites using micromechanics. Although curvature of nanotubes can have a direct impact on these properties, it is typically ignored. In this work, the nanotube geometry in 3D is included in the calculation of the elastic properties of a modified effective fiber. The strain energy of the nanotube and the effective fiber are calculated using Castligiano’s theorem and constraints imposed by the matrix on the deformation are taken into consideration. Model results are compared to results from archived literature, and a reasonable agreement is observed. Results show that the effect of nanotube curvature on reducing the modulus of the effective fiber is not limited to in-plane curvature but also to curvature in 3D. The impact of the nanotube curvature on the elastic properties of nanocomposites is studied utilizing the modified fiber model and the approach developed by Mori–Tanaka. Analytical results show that for a low weight fraction of nanotubes the effect of curvature seems to be minor and as the weight fraction increases, the effect of nanotube curvature becomes critical.     

Effects of randomness in steel mechanical properties on rotational capacity of RC beams

In this paper, an advanced model for predicting behaviour of RC beams, including effects of material nonlinearity and tension stiffening, has been adopted in order to investigate how randomness in mechanical weldable steel properties affects the estimate of the available rotational capacity. The mechanical properties characterising steel behaviour which are assumed to be random are: strain at the peak stress (A_gt), yield stress (f_sy) and the ratio of the peak stress (f_su) to the yield one (hardening ratio f_su/f_sy). Monte Carlo simulation has been carried out to analyse the influence of mechanical properties randomness on the evaluation of the inelastic response of RC beams; namely, statistical distribution of the available rotational capacity and its correlation, both with the distribution of each selected input random variable and with their multivariate distribution, has been studied. The investigation has shown that randomness in the rotational capacity induced by the steel deformation at the peak stress is the most significant. However, it was found that the low randomness in f_su/f_sy amplifies to a largest degree due to effects of material nonlinearity, which, in turn, affects development of the tension stiffening. Randomness in yield stress turns out to be strongly reduced by the element inelastic response, so that it can be taken as deterministic. Finally, the analysis has demonstrated that a characteristic value (k=5%) for the hardening ratio has to be assumed, being the assumption of the mena value for such parameter non conservative as far as the rotational capacity evaluation is concerned.

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Résumé Dans cette étude, on a adopté un modèle très élaboré de poutre en béton armé capable de représenter les effets principaux de la non-linéarité des matériaux et du phénomène de raideur due à la traction. Ceci nous permet de rechercher avec soin l'influence du caractère aléatoire des caractéristiques mécaniques de l'acier soudable sur l'estimation de la capacité rotationnelle des éléments structurels. Les propriétés mécaniques aléatoires permettent de caractériser le comportement de l'acier sont: la déformation associée à la tension maximale (A_gt), la tension à la limite d'élasticité (f_sy) et le rapport entre les tensions maximum (f_su) et à la limite élastique (rapport de recrudescence: f_su/f_sy). L'influence des caractéristiques mécaniques sur l'évaluation de la réponse inélastique des poutres en béton armé a été évaluée à partir d'une simulation Monte Carlo. On a donc pu évaluer la relation entre la distribution statistique de la capacité rotationnelle de la poutre et les distributions caractérisant, les différentes grandeurs prises au hazard, qu'elles soient considérées séparément ou conjointement. L'étude a montré que l'aspect aléatoire impliqué par la déformation maximale sur la capacité de rotation est le plus significatif parmi toutes les grandeurs considérées. On se rend compte que le caractère aléatoire réduit qui caractérise la grandeur f_su/f_sy devient plus amplifié à cause de la forte non-linearité des matériaux et à cause du phénomène de raideur. La casualité de la tension à la limite d'élsticité s'avère par contre fortement réduite par la réponse inélastique de l'élément, ce qui rend possible l'hypothèse de l'aspect déterministe de cette grandeur. Enfin, l'analyse a démontré l'importance d'attribuer, pour le rapport f_su/f_sy, la valeur caractéristique (k=5%) et non pas sa valeur moyenne, qui reste peu conseillée en ce qui concerne l'évaluation de la capacité de rotation de l'élément.

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Editorial Note The University of Florence is a RILEM Titular Member.     

Effect of sintering temperature on the density, piezodielectric response, and mechanical and elastic properties of materials of different functional groups

This paper describes the synthesis of Ba-doped PbZn_1/3Nb_2/3O₃-PbMg_1/3Nb_2/3O₃-PbNi_1/3Nb_2/3O₃-PbTiO₃ multicomponent solid solutions rich in PbNi_1/3Nb_2/3O₃, which include ferroelectric relaxors, ferroelectrics with a diffuse phase transition, and classic ferroelectrics, and presents the phase diagrams along three cuts through a section near PbNi_1/3Nb_2/3O₃. We examine the effect of sintering temperature on the density, piezodielectric response, and mechanical and elastic properties of ceramics of different functional groups; optimize the conditions for the fabrication of such ceramics, and compare the performance parameters of the materials obtained with those of their commercially available analogs.     

Influence of the geometry and material properties on the dynamic stress field in the matrix containing a spheroidal particle reinforcement

The fracture and fatigue properties of particle reinforced composites are greatly influenced by stress concentration around the reinforcements. This study investigates the stress concentration around a prolate spheroidal particle that is embedded in an elastic matrix and subjected to asymmetric dynamic loading. A hybrid technique that combines the finite element method with spherical wave functions is used to study the problem. The stress concentrations within the matrix at the matrix–particle interface are found to be dependent on the frequency of excitation, aspect ratio of the particle and the material properties of both matrix and particle. The study reveals that the dynamic stress concentration can reach much higher values than the corresponding static case.     

Influence of the nanocellulose raw material characteristics on the electrochemical and mechanical properties of conductive paper electrodes

Paper-based conductive electrode materials of polypyrrole (PPy) and nanocellulose (NC) have received much attention lately for applications in non-metal-based energy storage devices, ion exchange, etc. The aim of this study was to study how the primary characteristics of NC raw materials impact and electrochemical properties of conductive NC–PPy composite sheets. Three NC raw materials were used: Cladophora cellulose (NC_UU) produced at Uppsala University, Cladophora cellulose (NC_FMC) produced at FMC Biopolymer, and microfibrillated cellulose (NC_INN) produced at Innventia AB. Composite paper sheets of PPy coated on the substrate NC material were produced. The NC raw materials and the composites were characterized with a battery of techniques to derive their degree of crystallinity, degree of polymerization, specific surface area, pore size distribution, porosity, electron conductivity, charge capacity and tensile properties. It was found that the pore size distribution and overall porosity increase upon coating of NC fibres for all the samples. The charge capacity of the composites was found to decrease with the porosity of the samples. It was further found that the mechanical strength of the pristine NC sheets was largely dependent on the overall porosity, with NC_INN having the highest mechanical strength and lowest porosity in the series. The mechanical properties of the composite NC–PPy sheets were significantly diminished as compared with pristine NC sheets because of the impaired H-bonding between fibres and PPy-coated nanofibres. It was concluded that to improve the mechanical properties of PPy–NC sheets, a fraction of additive bare NC fibres is beneficial. Future study may include the effect of both soluble and insoluble additives to improve the mechanical strength of PPy–NC sheets.     

The effect of cementation on the elastic properties of granular material

A simple analytical model is developed to describe the mechanics of cemented granular material. The two-dimensional problem of the deformation of an arbitrarily shaped cement layer has been reduced to an ordinary differential equation of the second order. A simple analytical solution is obtained to model the normal and shear deformation of a cement layer with straight boundaries. The problem of the normal compression of two contacting circular elastic grains and an elastic cement layer between them has been reduced to a linear integral equation. This problem has been solved both for two-dimensional (cylindrical grains) and three-dimensional (spherical grains) cases. The elastic modulus of such a combination is strongly increased by cementation compared to the case of a classic Hertzian contact.     

A study of elastic aftereffect in injection casting of thermoplastic feedstocks based on SiC,AlN, WC powders and its influence on mechanical properties of the workpiece material

The paper addresses the changes in dimensions of products prepared from thermoplastic feedstocks based on SiC, AlN, WC powders and paraffin by injection molding. The authors study the influence of the process pressure, temperature, and powder particle size on the elastic aftereffect in the product material as well as the influence of the injection casting variables (pressure and temperature) on the ultimate bending strength of the product material.     

Influence of air and material damping on dynamic elastic modulus measurement

In this paper, air and material damping effects on the dynamic elastic modulus measurement of a flexural vibrating beam with free ends are evaluated, according to the Bernoulli-Euler beam equation. The theoretical analysis indicates that the measured elastic modulus is not substantially influenced by material damping. However, the measured modulus decreases with an increasing extent of air damping. In addition to theoretical analysis, experimental results for glass and alumina specimens also show that air damping decreases the measured modulus.     

管道修复用管状纺织复合材料的力学性能分析

管状纺织复合材料作内衬管修复受损管道,是通过气压或水压翻转衬于管道中的,在翻衬过程中,由于翻衬压强的作用,管状纺织复合材料要承受复杂的应力与应变。本文中通过有限元方法对管状纺织复合材料翻衬时的应力和应变情况进行了分析,而且有限元模拟值分别与理论值和试验值进行了比较。结果表明：管状纺织复合材料在翻转时,翻转的头端,应力和应变不稳定,有应力集中的情况,因此在翻转头端最容易发生管状纺织复合材料的破裂;在管状纺织复合材料几何参数与组分材料一定的情况下,可通过有限元方法,给出不同翻衬压强值下其可修复管道的管径范围。     

Investigating the effects of loading,mechanical properties and layers geometry on fatigue life of asphalt pavements

In this paper, fatigue life, crack growth trajectory, and stress intensity factors of top‐down and bottom‐up cracks have been investigated in a multilayered asphalt pavement using numerical method. Finite element models, Paris law, and maximum tangential stress criteria were used to determine the effects of vehicle position, thickness, and stiffness of layers on fatigue life of pavements. The results show that the minimum fatigue life of pavements with top‐down crack occurs when the wheels are symmetrically located relative to the crack plane. On the other side, in pavements with bottom‐up crack, it occurs when the front tire located on top of the crack. Moreover, the result reveals that the top‐down crack grows slower than bottom‐up crack. Also, the surface layer's stiffness has the most effect on fatigue life of pavement.     

Geometry,mechanical and ballistic properties of ADI material perforated plates

In this paper, austempered ductile iron has been evaluated as an alternative to steel for perforated plates applied in the ballistic protection of military vehicles. The austempering was performed in lower and higher austempering ranges in order to obtain two types of austempered ductile iron: one with a higher strength, and the other with a higher ductility. Perforated plates having two different thicknesses of 7 and 9 mm were mounted in front of basic armour and 12.7 × 99 mm armour – piercing incendiary ammunition was fired from 100 m. It was shown that the austempered ductile iron material austempered at a lower temperature has superior ballistic resistance, providing a full (five out of five armour – piercing incendiary shots stopped) ballistic resistance if combined with 13 mm basic armour plate. The thicker austempered ductile iron perforated plate provides more significant penetrating core damage, and therefore, lower basic plate damage. On the other hand, the thinner austempered ductile iron material perforated plate can be considered optimal due to its lower weight and higher mass effectiveness. In austempered ductile iron material austempered at a higher temperature, besides a lower hardness, bulk retained low-carbon metastable austenite transforms into martensite through strain induced mechanism, causing a partial brittle fracture.     

Influence of pore geometry on the effective response of porous media

The generalized method of cells (GMC) is used to study the influence of pore geometry on the effective elastic properties and inelastic response of porous materials. Periodic microstructures with four distinct pore geometries are studied and the results for effective elastic properties are compared with several other available models and experimental results. Predictions for the inelastic response of porous alumina are presented for tensile loading, as a function of pore geometry and pore volume fraction, with the inelastic behavior of the bulk material modeled using a unified visco-plasticity theory. All results are presented for discrete pore shape and discrete porosity. It is shown that pore geometry can have a significant influence on both elastic and inelastic response, that pore geometry can be associated with parameters from other models, and that the generalized method of cells is an efficient, flexible and reliable method of analysis for such problems.     

材料力学性能对液力胀接过程的影响分析

为寻求装配式空心凸轮轴结构设计和材料选择的理论依据,应用弹塑性力学分析了弹性模量和流动应力对液力胀接的影响.将内外管的变形过程简化为平面应变问题,根据典型点应力-应变曲线分析了液力胀接过程,并给出了不同材质套管的连接条件.研究结果表明:影响液力胀接的主要因素是内外管弹性模量和流动应力,这两个材料参数决定胀后内外管弹性回复量的大小;实现液力胀接内外管应满足的条件是:内外管弹性模量的比值大于内外管流动应力的比值.