Elastic behavior of La0.67Ca0.33MnO3:ZrO2 composites

A series of Colossal Magneto Resistance materials, with compositional formula (1 − x) La_0.67Ca_0.33MnO₃ + xZrO₂ (where x = 0%, 10%, 20%, 40%, 60%, 80%) were prepared by sol–gel technique. When characterized structurally by X-ray diffraction they are found to have cubic structure. After measuring their bulk densities, the ultra sonic longitudinal (V_l) and shear velocities (V_s) were measured at room temperature using the pulse transmission technique. Using the ultrasonic data, the values of Young's and rigidity moduli along with Poisson's ratio and Debye temperatures have been calculated. As the materials are porous, zero porous elastic moduli have also been arrived at using a well-known model. The observed variation of elastic moduli with varying ZrO₂ concentration has been explained qualitatively.     

Surface Brillouin scattering studies on vanadium carbide

Vanadium carbides in a range of carbon to metal ratio were investigated by surface Brillouin scattering (SBS) on the (100) and (110) surfaces. The surface acoustic wave (SAW) velocity varies with the crystal surface and direction and displays an approximately linear increase with the carbon to metal ratio. The tensor elastic constants of the samples were extracted and, while they vary significantly from the different carbon to metal ratio, the value of anisotropy ratio remains close to 0.8 for all samples. The directional dependence of the Young's modulus and shear modulus are obtained from the tensor elastic constants. In the directions chosen, both Young's modulus and the shear modulus reduce with decreasing carbon to metal ratio, with this behaviour being directionally dependent.     

A Universal Analytic Potential-Energy Function Based on a Phase Factor

Using a field equation with a phase factor, a universal analytic potential-energy function applied to the interactions between diatoms or molecules is derived, and five kinds of potential curves of common shapes are obtained adjusting the phase factors. The linear thermal expansion coefficients and Young‘s moduli of eleven kinds of fuce-centered cubic (fcc) metals - Al, Cu, Ag, etc. Are calculated using the potential-energy function; the computational results are quite consistent with experimental values. Moreover, an analytic relation between the linear thermal expansion coefficients and Young‘s moduli of fcc metals is given using the potential-energy function. Finally, the force constants of fifty-five kinds of diatomic moleculars with low excitation state are computed using this theory, and they are quite consistent with RKR (Rydberg-Klein-Rees) experimental values.     

Determination of material parameters and texture of a polycrystalline aggregate from ultrasonic measurements

The propagation of ultrasonic waves in a polycrystalline aggregate is considered for a bulk sample with orthorhombic symmetry made of cubic crystals of the highest symmetry. The probability density function of the crystallite orientation and the stiffness moduli of a single grain (crystallite) of the polycrystalline aggregate are found from four ultrasonic velocities and the rules of orthorhombic symmetry. Moreover, Jaynes’ principle of maximum Shannon entropy and the averaging procedure proposed by Voigt are used. In this way, it is shown that on the basis of the measurements of four respectively chosen ultrasonic velocities it is possible to find simultaneously the maximum-entropy distribution function of the crystallite orientation and the crystallite stiffness moduli.     

Dynamic modulus of asphalt mixture by ultrasonic direct test

This paper describes the experimental procedure followed for direct determination of dynamic modulus of asphalt mixtures by ultrasonic direct test at a specified temperature. Tests were performed on ten cylindrical samples of dense and porous asphalt mixtures manufactured with dolerite and limestone aggregates. Dynamic moduli obtained by ultrasonic transmission, calculated at a frequency of 65 kHz, were compared with values directly determined by standard dynamic tests applied in Spain at frequencies of 2, 5, 8 and 10 Hz. The obtained results demonstrate that the magnitudes for moduli calculated by ultrasonics are higher than those obtained by standard dynamic tests. It is concluded that for asphalt mixtures tested ultrasonically the increase of moduli magnitude can be associated with an increase in the frequency used but may also be due to the different testing methods. Nevertheless, these values can be used as a reference value for dynamic modulus of asphalt pavements at low strain, being necessary to apply a correction factor to replace the low frequency standard dynamic test, which is more expensive, difficult and time consuming.     

Cr、Mo和W对FeAl金属间化合物电子结构和力学性能影响的第一性原理研究

采用基于密度泛函理论的第一性原理方法,计算了B2型FeAl金属间化合物的Fe8Al8和Fe8XAl7(X=Cr,Mo和W)超晶胞系统总能量、结合能、晶格常数、弹性常数、态密度和差分电荷密度,研究了合金元素对B2型FeAl金属间化合物晶体结构、电子结构和力学性能的影响。根据系统驰豫和几何优化确定了合金系统的稳定晶体结构;计算结果表明:随着加入元素原子半径的增大,合金的晶格常数相应增大,Fe8WAl7的晶格常数最大,Fe8CrAl7的晶格常数最小。Cr、Mo和W的加入均提升了FeAl的体模量、剪切模量和弹性模量以及改善了FeAl的脆性,其中Mo的加入对FeAl的脆性改善作用最大。根据电子结构和Cauchy压力参数计算结果的分析,FeAl金属间化合物为脆性相,主要原因是其电子结构中Fe的s、p、d态与Al的s、p态存在电子轨道杂化,呈明显的共价键特征。合金元素改善FeAl脆性的微观机理为:合金元素原子以d轨道电子为主参与了FeAl金属间化合物的电子杂化,增强了FeAl合金的结合能力;合金元素原子的加入使电荷转移量增加,增强了原子间离子键成分的作用,提高了FeAl合金的稳定性。     

Miniaturized bend tests on partially stabilized EB-PVD ZrO2 thermal barrier coatings

The elastic properties of thermal barrier coatings (TBCs) are important for modelling the lifetime of these coatings. A new test setup has been developed to measure the system modulus of electron-beam enhanced physical vapour deposited (EB-PVD) TBC coatings by miniaturized bend tests.Due to the brittleness, low stiffness and small thickness of the top coat and its complex microstructure, it is difficult to measure its Young's modulus by standard mechanical testing. For this reason, a special sample material has been prepared which consists of a 1 mm thick layer of EB-PVD TBC. This material was isothermally heat treated for different times at 950 °C, 1100 °C and 1200 °C and then tested in a specially developed miniaturized bend test. The bend test setup permits mechanical tests with a high resolution in stress and strain, where the strain is measured by digital image correlation. So the stiffness of the free-standing TBC samples could be measured with a high accuracy and the sintering behaviour of the EB-PVD TBC and the consequent rise of Young's modulus could be determined. The results show a significant increase of the system modulus with heat treatment time and temperature caused by sintering of the coating. An activation energy of 220 kJ/mol for the process has been determined.In addition, the material was tested by nanoindentation in order to measure Young's modulus on a local scale, and the porosity of the samples was determined by quantitative image analysis.     

Relating tensile, bending, and shear test data of asphalt binders to pavement performance

Eight different asphalt binders representing a wide range of applications for pavement construction were tested in uniaxial tension, bending, and shear stresses. Theoretical analyses were performed in this study to convert the data from the three engineering tests to stiffness moduli for predicting pavement performance. At low temperatures, high asphalt stiffness may induce pavement thermal cracking; thus, the allowable maximum stiffness was set at 1,000 MPa. At high temperatures, low asphalt stiffness may lead to pavement rutting (ruts in the road); master curves were constructed to rank the potential for rutting in the asphalts. All three viscoelastic functions were shown to be interchangeable within the linear viscoelastic region. When subjected to large deformation in the direct tension test, asphalt binders behaved nonlinear viscoelastic in which the data under bending, shear and tension modes were not comparable. The asphalts were, however, found to exhibit linear viscoelasticity up to the failure point in the steady-state strain region.     

Nb-Si-N 纳米复合薄膜中的界面力学性能研究

基于密度泛函理论的第一性原理方法,研究了Nb-Si-N纳米复合薄膜间隙型界面结构和置换型界面结构的力学性能。采用第一性原理方法计算了不同应变下的系统能量,通过分析得到其力学常数。计算出间隙型界面和置换型界面结构的体积模量分别为233.595 GPa和280.204 GPa,剪切模量分别为70.716 GPa和125.677 GPa,杨氏模量分别为192.702 GPa和327.994 GPa。这表明Nb-Si-N纳米复合薄膜置换型界面结构的抗剪切形变与抗压缩形变都优于间隙型界面结构。杨氏模量各向异性分析显示,两种界面结构各方向弹性较为一致,NbN各方向弹性差别较大。     

First-principles density functional calculations of physical properties of orthorhombic Au2Al crystal

The study attempts to perform a systematical investigation of the thermodynamic, mechanical and electronic properties of orthorhombic Au₂Al crystal by using first-principles calculations incorporated with a quasi-harmonic Debye model. In addition, their temperature, hydrostatic pressure and direction dependences are also addressed. The investigation begins with evaluation of the equilibrated lattice constants and elastic constants of Au₂Al single crystal. Next, the mechanical features of the single crystal, such as ductile-brittle characteristic and elastic anisotropy, are assessed based on the Cauchy pressures, shear anisotropy factors and directional Young's modulus. Alternatively, the pressure-dependence of polycrystalline mechanical properties of Au₂Al, including bulk, shear and Young's moduli, and ductility, brittleness and microhardness characteristics are also estimated. Furthermore, the study also characterizes the temperature-dependence of thermodynamic properties of Au₂Al single crystal, namely, Debye temperature and heat capacity. At last, electronic characteristic analysis is carried out to predict the electronic band structures and density of states profiles of the crystal.The calculation results indicate that Au₂Al crystal is an elastically anisotropic material at zero pressure and a highly ductile material with low stiffness. In addition, the Young's moduli of the crystal would be markedly enhanced with the increase of the hydrostatic pressure. It is also found that the heat capacity of Au₂Al at low temperature strictly sticks to the Debye T³ law.     

考虑接头几何参数的I-core全钢三明治板的刚度分析

激光焊接I-core三明治板常用的刚度设计公式,没有考虑接头几何参数的影响,在实际应用中其准确性受到影响. 通过外伸三点弯曲试验,对六种接头几何参数的I-core全钢三明治板的弯曲刚度和剪切刚度进行测量,并结合有限元计算,分析了接头几何参数对弯曲刚度和剪切刚度的影响. 结果表明,增加焊缝宽度可显著提高弯曲刚度和剪切刚度,而焊缝宽度大于60%芯板厚度后,焊缝宽度对刚度的影响减小. 间隙对刚度的影响可忽略.基于此结果,对三明治板设计时常用的刚度公式进行了修正.     

Modelling of deformation and failure of crash toughened adhesive bonded lap shear joints

Abstract

With an increase in the use of advanced high strength steels in vehicle architectures, materials joining issues have become increasingly important. Among the various joining methods, adhesive bonding is increasingly used in automobile manufacturing. Successful implementation of adhesive bonding to improve structural crashworthiness and reduce vehicle weight requires the knowledge of issues related not only to processing but also to joint performance. In this study, a new anisotropic yield criterion which is determined from tensile and shear tests, is developed and incorporated into a finite element model to predict the static load displacement curves of various adhesive bonded steel joints. In the developed model, when the calculated plastic strain of bonded steel joint reaches the equivalent plastic strain of the adhesive the joint is regarded as failed. Modelling results have been validated by the experimental data. Since the model covers different steel grades, gages, and overlap distances, the model can be used to predict the static strengths of adhesive bonded assemblies. Finally, the model is employed to evaluate the effect dissimilar steel grades and sheet gages on the joint strength of lap shear bonded steel joints. The results show that for the joints made of dissimilar steel grade and sheet gage, the stiffness (i.e. strength and thickness) of the two adherends should be balanced to obtain the optimum joint strength.     

Velocities of elastic waves and the elasticity moduli of nickel-base superalloys and of the 60N21 alloy

Measurements of velocities of elastic waves have been performed in polycrystalline samples of an intermetallic compound Ni₃Al, this intermetallic compound alloyed with niobium or cobalt, superalloys ZhS36 and VKNA-4U, and the 60N21 alloy. Elasticity moduli have been calculated, namely, Young’s modulus, shear modulus, bulk modulus, and the Poisson ratio. The results obtained are compared with those calculated from the tensor of the elasticity moduli obtained for single crystals. The calculations have been performed in the Voigt and Reuss approximations.     

Determination of Young's modulus and Poisson's ratio for coatings

Designing thin-film coatings to meet engineering needs requires the knowledge of accurate mechanical properties of the coatings. Young's modulus and Poisson's ratio are two basic mechanical properties of materials, which should be conveniently measured. This paper reports a direct and non-destructive method for the measurement of the Young's modulus and Poisson's ratio of a thin-film coating and its substrate based on the extended Hertz theory for the contact of coated bodies. The theory is used to analyze load-displacement data from a spherical indentation in the elastic range, in which the substrate effect is intrinsically modeled. The Young's modulus and Poisson's ratio are determined at the same time through minimizing the difference between the measured and specially defined modified Young's moduli. Two sets of validation experiments are also reported. This new method does not require any assumptions on pressure distribution and Poisson's ratio and can be easily incorporated into current indentation analysis systems.     

MgCu2,Mg2Ca和MgZn2 Laves相力学性质和电子结构的第一性原理计算

通过基于密度泛函理论的第一性原理计算方法,对MgCu2,Mg2Ca和MgZn2的力学性能和电子结构进行计算,计算所得晶格参数与实验值和文献值相吻合。合金形成热和结合能的计算结果表明,MgCu2具有最强的合金形成能力和结构稳定性。计算了MgCu2,Mg2Ca和MgZn2的弹性常数,推导了体模量、剪切模量、弹性模量和泊松比。结果表明,MgCu2、Mg2Ca和MgZn2均为延性相,MgCu2的刚度最大,MgZn2的塑性最好。通过对结合能和弹性常数的计算,预测了MgCu2、Mg2Ca和MgZn2的熔点。通过对态密度（DOS）、Mulliken布居数、电子占据数和差分电荷密度的计算,分析了MgCu2、Mg2Ca和MgZn2的结构稳定性和力学性能机制。最后,计算和讨论了3种金属间化合物的Debye温度。     

Elastic properties of double layered manganites R_（1.2）Sr_（1.8）Mn_2O_7（R=La,Pr, Nd,Sm）

The polycrystalline colossal magnetoresistive double-layered manganite samples R1.2Sr1.8Mn2O7(R = La Pr, Nd, Sm) were prepared by the sol–gel method and their room temperature elastic behavior was investigated by ultrasonic pulse transmission technique at 1 MHz. The values of elastic constants were calculated from longitudinal and shear velocities and they were corrected to zero porosity using Hasselman and Fulrath's formulae. The elastic constants of the samples were also estimated by Modi's heterogeneous metal-mixture rule which is based on the metal ions present in the samples. The measured,corrected, and estimated values of elastic moduli are found to increase with decreasing rare earth ion size. The variation of elastic moduli with the size of the rare earth ion is interpreted in terms of strength of interatomic bonding.     

Measurements of deformations and strains of a Si-epoxy-FR4 structure during thermal testing

Deformations and strains of a Si-epoxy-FR4 structure were evaluated during thermal testing using high sensitivity, real-time Moiré interferometry. The specimen studied was a sandwich structure consisting of a silicon chip, epoxy underfill and FR4 substrate. The deformations and strains of the FR4–underfill and silicon–underfill interfaces of the specimen under certain thermal loading were examined. The results show that the shear strain increases significantly along the interfaces, with the maximum shear strain occurring at the intersection of the specimen edge and the silicon–underfill interface. The shear strain at the silicon–underfill interface experienced a 2% increase after heated for two hours at 100 °C, but the shear strain at the FR4–underfill interface showed a 12% increase. This is an indication that the creep effect is more dominant in the FR4–underfill interface. The interfaces of the specimen experience partial strain recovery after one hour of the holding time at 20 °C.     

基于有限元的闸式剪板机刀架结构优化

刀架是闸式剪板机中的重要部件,其结构设计的合理性与剪切质量紧密相关。本文在对闸式剪板机刀架刚性以及变形规律研究分析的基础上,结合实践经验提出了5种刀架结构的改进方案,采用有限元方法对改进结构的刚性进行了分析比较,确定方案5为最终的优化结构。试制样机的剪切质量较原结构机床的剪切质量有较大提高。本文提出的结构优化方法对剪板机刀架结构的优化与改进具有实际的指导意义。     

Characterizing the temperature dependence of electronic packaging-material properties

A computer-controlled, temperature-dependent material characterization system has been developed for thermal deformation analysis in electronic packaging applications, especially for printed wiring assembly warpage study. For fiberglass-reinforced epoxy (FR-4 type) material, the Young's moduli decrease to as low as 20–30% of the room-temperature values, while the shear moduli decrease to as low as 60–70% of the room-temperature values. The electrical resistance strain gage technique was used in this research. The test results produced overestimated values in property measurements, and this was shown in a case study. A noncontact strau]n measurement technique (laser extensometer) is now being used to measure these properties. Discrepancies of finite-element warpage predictions using different property values increase as the temperature increases from the stress-free temperature.     

Functionally Graded Stainless Steel Fabricated by Direct Laser Deposition: Anisotropy of Mechanical Properties and Hardness

Direct laser metal deposition is a kind of advanced rapid manufacturing technology, which can produce near net shape parts by depositing metal powders layer by layer. This study demonstrates fabrication, the anisotropy of mechanical properties and hardness of a graded steel. The characteristics of constituent phases, microstructure, mechanical anisotropy, and microhardness were investigated using electron backscatter diffraction, optical microscopy, tensile test machine, and microhardness tester. It was found that the graded steel is dense and free of cracks. The crystal structures of the as-built samples evolved in three grades from fcc structures to fcc+bcc structures and then to bcc+fcc structures. Samples in x and z directions showed obvious mechanical anisotropy. The samples machined in x direction showed higher strength and lower elongation than those machined in z direction due to the presence of lack-of-fusion pores and the higher metallurgical bonding between layers in the x direction. The microhardness of the as-built samples increased along the cross section from the substrate(159.7 HV) to the top surface(545.4 HV).