Finite element modelling of the effect of non-metallic inclusions in metal forming processes

The presence of non-metallic inclusions can result in material failure during the metal forming process, or lead to a serious deterioration of the quality of the final product. Understanding the effects of inclusions during metal forming is therefore an important step towards predicting the behaviour of inclusions and subsequently minimising their consequences. To achieve this understanding the authors incorporated non-metallic inclusions into a finite element (FE) simulation of metal forming. The chosen metal forming process was rod drawing, the chosen inclusion material aluminium oxide (Al₂O₃) and the chosen inclusion shape spherical. Real rod drawing experiments were also designed with Al₂O₃ spheres embedded in a steel rod in order to compare experimental and simulated results. Specifically, from the experiments carried out the changes of the rod around the inclusion were investigated and the findings compared with the finite element simulation results of an equivalent model. The FE simulation of the experiments considered specifically the fracturing of brittle inclusions. A concept allowing the fracturing of brittle inclusions by means of finite element method is described. Experimental results from fourpoint bending tests, and tensile tests, for Al₂O₃ ceramic bodies were used to calibrate the simulation. Further experiments involved the crushing of Al₂O₃ spheres where the force necessary to achieve crushing was measured and compared with the results of the calibrated FE simulation. It is demonstrated that the intended FE method for the simulation of brittle fracture of inclusions can be used with good accuracy.     

Superconducting properties of diffusion processed niobium-Tin tape

The properties that make Nb3Sn an outstanding material for use in high-field superconducting solenoids are its high critical temperature (18.1°K), its high critical current density (in excess of 200 000 A/cm²at 100 kG and 4.2°K), and its high critical field (221 kG at 4.2°K). Since Nb3Sn is a brittle material, special techniques are required for its use. These techniques fall into two groups : 1) the conductor is initially fabricated from Nb and Sn in an unalloyed configuration, and this conductor is then wound into a solenoid and converted to Nb3Sn by heat treatment of the entire solenoid; 2) the alloyed Nb3Sn conductor is prepared in a very thin configuration. Thus, although it is brittle, it is still flexible enough to be wound directly into a finished solenoid. Extensive experimentation with the second technique has been undertaken and will be discussed with particular emphasis on the properties of laminated conductors as confirmed by performance of superconducting solenoids.     

Comparison of the developed thermal stresses in Al2O3–SG, ZrO2–12%Si+Al and ZrO2–SG coating systems subjected to thermal loading

In this investigation, thermal and structure finite element analysis has been employed to analyse the thermal stresses developed in Al₂O₃–SG, ZrO₂–12%Si+A1 and ZrO₂–SG.coatings subjected to thermal loading. Systems with 0.4 mm coating thickness and 4 mm substrate material thickness were modelled. Zirconia–spherical cast iron (SG) coatings with NiAl, NiCrAlY and NiCoCrAlY interlayers were also modelled. Nominal and shear stresses at the critical interface regions (film/interlayer/substrate) were obtained. The results showed that the lowest stress levels are in ZrO₂–SG coatings. Furthermore, the interlayer thickness and material combinations have a significant influence on the level of the developed thermal stresses. It is also concluded that the finite element technique can be used to optimise the design and the processing of ceramic coatings.     

ZnFe2O4纳米球的制备及其对丙酮的气敏性能研究

丙酮被广泛应用于工业和实验室,对丙酮浓度的检测十分重要。ZnFe₂O₄是一种尖晶石型三元金属氧化物,气敏性能优良,可广泛应用于气体传感器。本文采用简单的一步水热法制备了球状的ZnFe₂O₄气敏材料。通过XRD、XPS、SEM、TEM、N2吸附-解析仪对材料的形貌结构、化学组成、比表面积等进行分析,并以丙酮为目标气体对其气敏性能进行了综合研究。结果表明,ZnFe₂O₄纳米球是由纳米粒子自组装而成,有较大的比表面积;该ZnFe₂O₄基气体传感器在最佳工作温度150℃下对丙酮的灵敏度为65.74,并具有出色的选择性、稳定性、重复性,但随着湿度的增加其气敏性能逐渐降低。     

基于铁粉还原的LiFePO4/C合成路径及其电化学性能研究

使用还原铁粉作为铁源, 通过超细球磨与喷雾干燥、高温煅烧技术制备了球形微纳米LiFePO₄/C复合材料。使用DSC/TG以及XRD对LiFePO₄/C复合材料的形成过程进行了分析; 使用SEM、穆斯堡谱仪等手段对复合材料进行分析; 使用电化学工作站、容量测试仪对其充放电行为进行分析。研究发现, 使用该合成技术路线, 在500~700℃下能够合成LiFePO₄/C复合材料。获得的LiFePO₄/C复合材料具有规则的球形外貌, 平均尺寸4~5 μm。该微米颗粒由200 nm左右细小颗粒组成, 颗粒间具有纳米尺寸微孔。穆斯堡谱仪测试结果表明, 复合材料中Fe处于+2价的价态。复合材料在1C倍率下表现出稳定的充放电行为, 平均比容量在156 mAh/g, 300次循环后, 容量保持率为92.8%。该技术制备的LiFePO₄/C复合材料具有潜在的应用价值。     

Fatigue of hybrid composites by a cohesive micromechanic model

A cohesive micromechanic fatigue model (CMFM) which identifies a nonconservative bonding reaction between a broken molecular chain and its neighbor as the main microscale source of fatigue damage accumulation has been recently developed for a unidirectional material constructed from a parallel set of chain-like elements. The successive breakages in each cycle are controlled by the statistical strength distribution of the elements and the probability and amount of interference which a broken element causes to its neighbors. The model gave a physically sound explanation to the fatigue power law S—N curve and the endurance limit phenomena by direct interpretations of microstructure parameters.

In this study the model is expanded by considering a material which have two components with different mechanical and statistical properties and are mixed to give a hybrid composite. The main target is to find the best combination for fatigue resistance, or more specifically, to explore the possibility of making a composite which is more fatigue resistant than each of its two components. It is found that mixing a brittle component (high modulus and low failure strain) with a soft one (low modulus and high failure strain) having a specific microstructure gives the desired effect if some requirements on the structural and mechanical properties are met.

The two materials are mixed in a form of bundles, so that the composite microstructure and fatigue resistance are controlled by their relative volume faction (a macro property) as well as the number of elements in each bundle (a micro property).     

锂离子电池正极材料Li1+xV3O8的合成及性能研究

研究了一种新型制备锂离子电池正极材料Li_1+xV₃O₈的工艺方法.以NH₄VO₃为原料,通过淬火法制备出V₂O₅溶胶,加入LiOH溶液后,通过喷雾干燥法制备球形前驱体,再通过一定的热处理即制得锂离子电池正极材料Li_1+xV₃O₈.试验中,进行了前驱体的DTA/TGA分析;对产物进行了XRD、SEM及电化学性能测试研究.结果表明,经过350℃热处理24h后得到的样品颗粒细小、呈球形、粒径分布均匀、结晶度好,并且还表现出很好的电化学性能,其首次放电比容量高达378mAh·g^-1,经过10次充放电循环后,其放电比容量为312mAh·g^-1.     

Y2O3对钛基激光熔覆层组织及性能的影响

针对Ti811钛合金硬度低、耐磨性差的问题,以TC4粉、Ni45A粉和Y₂O₃粉为原料,采用同轴送粉激光熔覆技术在Ti811钛合金表面进行了激光熔覆制备耐磨复合涂层的实验,分析了熔覆层的组织和相组成,测试了熔覆层的显微硬度和摩擦磨损等力学性能。研究表明:复合涂层组织由枝晶TiC、依附生长于枝晶TiC表面的纳米颗粒TiC、生长于基体表面的等轴球形（近球形）TiC、金属间化合物Ti₂Ni、增强相TiB、TiB₂及基体α-Ti组成,所有生成相呈均匀弥散分布状态;涂层中等轴球形（近球形）TiC和Y₂O₃构成了复合相结构,经二维点阵错配度计算表明,Y₂O₃的（111）晶面与TiC的（110）晶面的二维点阵错配度δ=6.54%,因此Y₂O₃可作为TiC的有效异质形核核心细化晶粒;涂层的显微硬度处于HV_0.5 655~700之间,较Ti811基材提高了约1.6~1.8倍;涂层的磨损机制主要为磨粒磨损,摩擦磨损性能较基材显著提升。      

Tensile strength of the brittle materials, probabilistic or deterministic approach?

For various loading rates we estimated the activated defect localization in Modified Brazilian Disk type glass specimens in comparison with standard spherical glass specimens. Specimen geometry can considerably affect the mechanical response of material, especially brittle ones, which are very sensitive to the distribution of defects. High and low loading rates of Modified Brazilian Disk lead glass specimens have been investigated using universal Instron test machine and compressive Hopkinson pressure bars. The experimental results obtained have been compared using the Weibull distribution for scatter strength variation. Stress distribution in the above specimens was calculated using the finite element method, which provided detailed analysis of the macromechanical brittle fracture mechanism. In static tests of spherical glass specimens, we observed generation of contact stresses, which result in activation of defects in the working parts of specimens, whereas no activated defects were observed in Modified Brazilian Disk specimens neither under static, nor under dynamic loading conditions. For specimens of various geometries and type of load application it is recommended to apply probabilistic approaches, e.g., the Weibull approach, insofar as contact stresses in brittle materials induce activation of defects, location of which depends on the specimen geometry and loading type. __________ Translated from Problemy Prochnosti, No. 1, pp. 100–115, January–February, 2006.     

Time-dependent interfacial failure in metallic alloys

Time-dependent intergranular brittle fracture has now been studied experimentally in a number of alloy systems, and the generic features are becoming clear. Mobile surface-adsorbed elements are caused to diffuse inward along grain boundaries under the influence of a tensile stress, and this can lead to sub-critical crack growth by decohesion. Oxygen is found to play this role in nickel-base superalloys and intermetallics, as well as in a precipitation-strengthened Cu–Be alloy. Crack-growth rates lie in the range 10⁻⁷–10⁻⁴ m sec⁻¹. The same kind of cracking is found in steels treated so that free sulfur is able to segregate to the surface, as well as in Cu-Sn alloys, in which the embrittling element is surface-segregated Sn. The latter has been studied in bicrystals, and the importance of the variation in diffusivity with grain boundary structure has been documented. Hydrogen-induced cracking is a special case of an extremely mobile embrittling element and is responsible for much of the brittleness found in intermetallics. The effect of boron in retarding brittle behavior in Ni₃Al has been shown to result partly from its interaction with hydrogen. This is a prime example of how segregated solutes can be used to ameliorate the tendency for diffusion-controlled brittle fracture.     

氨基功能化纳米纤维素气凝胶的制备及性能

选用3-（2-氨基乙氨基）丙基甲基二甲氧基硅烷（AEAPMDS）对球形纤维素纳米纤维（CNF）湿凝胶进行化学修饰,探讨了改性反应条件对氨基纳米纤维素中N含量的影响,选用叔丁醇溶液为置换溶剂,采用冷冻干燥制备了一种新型的生物质气凝胶。对所制备的3-（2-氨基乙氨基）丙基甲基二甲氧基硅烷-纤维素纳米纤维（AEAPMDS-CNF）气凝胶的微观形貌、结构特征、力学强度及CO₂的吸附等性能进行表征和分析。结果表明：反应时间为10 h、反应温度为90℃、AEAPMDS溶液的质量分数为12wt%时,是AEAPMDS-CNF气凝胶的最佳制备方案。改性后的纤维素气凝胶具有三维网络孔结构、质轻（ρ ≤ 0.0573 g·cm^-3）、高孔隙率（ε>90%）等特点,其压缩强度为0.46 MPa,CO₂吸附量高达1.54 mmol·g^-1,表现出优异的CO₂吸附性能,具有很大的应用前景。     

Numerical modeling of development of fracture in anisotropic composite materials at low-velocity loading

The problem of normal interaction of the steel isotropic compact cylindrical projectile with the orthotropic plate on ballistic limit in range of velocities of impact from 50 to 400 m/s is considered. Target material is as organoplastic with initial orientation of mechanical properties, and the material, which properties received by turn on 90° relative to the axis OY of an initial material. Fracture of targets is investigated; the comparative analysis of efficiency of their protective properties depending on orientation of elastic and strength properties of an anisotropic material is carried out. The task is solved numerically, using the method of finite elements in three-dimensional statement. The behavior of a material of the projectile is described by elastic–plastic model; the behavior of anisotropic material of the target is described by elastic–brittle model with various ultimate strength limits on pressure and tension.     

Griffith's theory of brittle fracture in three dimensions

The Griffith theory of brittle fracture is extended to the three-dimensional problem of a flat elliptical crack in an otherwise uniform field of tensile and shear stresses. A method for finding the correct expressions of the change in strain energy due to the elliptical crack is developed. This is done by expressing the stresses and displacements in terms of the radius R₀ of a large sphere around the crack and by imposing the condition of equilibrium that the stresses or displacements across the spherical surface should agree with the prescribed boundary conditions as R₀ → ∞. The strain energy due to the presence of the elliptical crack is found to be independent of the tension applied parallel to the crack plane at infinity. On the basis of the thermodynamic argument of Griffith, it is also observed that the critical tensile and shear stresses increase rapidly as the ratio of major to minor semi-axes of the ellipse approaches unity.     

Eu3+掺杂Lu3Al5O12球形荧光颗粒的合成及性能研究

以尿素为沉淀剂, 通过均匀沉淀技术制备前驱体颗粒, 经后续煅烧获得分散性能良好的球形(Lu_0.95Eu_0.05)₃Al₅O₁₂((Lu_0.95Eu_0.05)AG)荧光颗粒。通过调整尿素浓度实现了球形荧光颗粒尺寸的可控合成。在此基础上, 采用 FT-IR、TG/DTA、XRD、FE-SEM、TEM和PLE/PL对材料的合成、物相形成及荧光性能等进行一系列表征。分析结果表明: 前驱体经较低的温度1100℃煅烧即可获得(Lu_0.95Eu_0.05)AG球形荧光颗粒, 且该荧光颗粒具有高的理论密度, 适于闪烁体材料的应用。在235 nm电荷迁移带(CTB)的激发下, (Lu_0.95Eu_0.05)AG石榴石于592 nm (Eu³⁺的⁵D₀→⁷F₁磁偶极子跃迁)处呈现优异的橙红光发射, 其色坐标为(0.63, 0.37)。该荧光颗粒的发光强度随颗粒尺寸的增大而增强, 荧光寿命随颗粒尺寸的增大而缩短。球形(Lu_0.95Eu_0.05)AG石榴石颗粒有望成为一类新型荧光材料, 广泛应用于照明及显示领域。     

Fracture behavior of nano-layered coatings under tension

The fracture of brittle/ductile multilayers composed of equal thicknesses of Si and Ag layers evaporated on a thick substrate is studied with the aid of a four-point bending apparatus. The system variables include individual layer thickness (2.5 to 30 nm), total film thickness (0.5 to 3.5 μm) and substrate material (polycarbonate, aluminum alloy and hard steel). The fracture is characterized by transverse cracks that proliferate with load. The crack initiation strain ε_i is virtually independent of total film thickness and substrate material while increasing with decreasing layer thickness h, to a good approximation as ε_i ~ 1/h^1/2. At higher strains, film debonding and buckling are evident.The fracture conditions are determined with the aid of a 2D finite element analysis incorporating the inelastic response of the interlayer. A fracture scenario consisting of tunnel cracking in the brittle layers followed by cracking in the interlayers is shown to be capable of predicting the observed increase in crack initiation strain with decreasing layer thickness. To realize this benefit the interlayer must be compliant and tough to force tunnel cracking in the brittle layers. The explicit relation for the crack initiation strain obtained from the analysis can be used to assess fracture toughness and improve damage tolerance in nanoscale layered structures.     

新型超硬材料z-BC2N的弹性、硬度与热导率研究

采用第一性原理计算研究了超硬材料z-BC₂N的弹性各向异性性质、应力-应变关系、硬度及最小热导率性质。计算得到的晶体力学行为判据B/G为0.87, 泊松比为0.084, 普适弹性各向异性指数为0.09992。[100]晶向上最大拉伸强度达到180 GPa, 应变方向上最大剪切强度达到160 GPa, 维氏硬度值为77.07 GPa。基于Cahill模型得到的最小热导率为6.811 W/(m·K)。结果表明: z-BC₂N是脆性材料且力学稳定性良好, 有非常高的拉伸强度、剪切强度, 体弹模量为各向同性, 杨氏模量各向异性程度不大。z-BC₂N的最小热导率低于金刚石的最小热导率。     

Effect of Ti/C additions on the formation of Al3Ti of in situ TiC/Al composites

A novel technique for fabricating TiC particulate-reinforced commercial purity Al composites was introduced. The mechanism of formation of brittle Al₃Ti up to 30 μm in size produced in the composites was studied and a method of eliminating them was put forward. The results show that: (1) the brittle Al₃Ti phase is always present in the composites when the Ti:C molar ratio is 1:1. In this case, the tensile elongation of the composite was only 4%, much lower than the value of unreinforced aluminum (20%); and (2) the formation of the brittle Al₃Ti phase can be eliminated entirely from the final product by using a proper Ti:C molar ratio of 1:1.3 in the Ti–C–Al preforms. In this case, the tensile elongation of this composite was 10%, higher than the value of the composite with a lot of Al₃Ti (4%). Moreover, improvement in the tensile elongation of the composite was accompanied by an increase of the ultimate tensile strength.     

Framework for a generalized four-stage fracture model of cement-based materials

In cement-based materials the full range from brittle to ductile fracture can be achieved by changing the material structure, the loading conditions, the specimen size and/or the boundary conditions. Considering just the material, at one side of the spectrum hardened cement paste behaves brittle, whereas at the other side, new fibre reinforced cements may behave ductile. Structural conditions affect the brittleness/ductility as well, and by simply changing the loading (uniaxial tension, uniaxial and confined compression, etc.), the specimen/structure size or by changing the boundary conditions the full range from brittle to ductile response can be observed. Basically there is no difference in behaviour between the various loading cases and the same four-stage fracture process can always be identified. The four ‘universal’ stages are the linear elastic regime, the microcrack regime (before the maximum load is reached), the macrocrack regime (viz. the first, usually steep part of the softening curve), and the bridging stage. Microcracks are defined as cracks that can be arrested by elements in the material structure, whereas macrocracks can only be delayed/ arrested by means or structural measures at a larger scale than the material structure. In this paper it is tried to develop a unified view on fracture of materials belonging to this broad class, which may be seen as conceptual framework for an all encompassing fracture model for cementitious materials.     

A model of brittle fracture propagation part I—continuum aspects

The micromechanism of crack propagation in steel is described and analyzed in continuum terms and related to the macroscopic fracture behavior. It is proposed that propagation of cleavage microcracks through favorably oriented grains ahead of the main crack tip is the principal weakening mode in brittle fracture. This easy cleavage process proceeds in the Griffith manner and follows a continuous, multiply connected, nearly planar path with a very irregular front which spreads both forward and laterally and leaves behind disconnected links which span the prospective fracture surface. A discrete crack zone which extends over many grains thus exists at the tip of a running brittle crack. Final separation of the links is preceeded by plastic straining within the crack zone and occurs gradually with the increasing crack opening displacement. It is suggested that in low stress fracture, straining of the links is the only deformation mode. However, it is recognized that under certain conditions plastic enclaves may adjoin the crack zone. This deformation mode is associated with high stress fracture, energy transition and eventually with crack arrest.

Energy dissipation resulting from the two deformation mechanisms is related to crack velocity, applied load and temperature and the crack velocity in a given material is expressed as a function of the external conditions. Fracture initiation and crack arrest are then discussed in terms of the conditions which are necessary to maintain the propagation process. Finally, the dimensions of a small scale crack tip zone for a steady state, plane strain crack are evaluated as functions of material properties and the elastic stress intensity factor.

The microstructural aspects of brittle fracture will be discussed in a separate Part 2 [1].