基于可变微单元模型的三维编织复合材料管弹性性能预报

用可变微单元几何模型对管状三维编织复合材料的物理性能分析的结果，建立基于Voight等应变假设的混合率细观力学模型，对复合材料管的弹性力学性能进行预测。将纱线的正轴刚度通过转换得到其离轴刚度，用混合法得到材料的等效刚度矩阵和柔度矩阵，获得复合材料等效弹性性能参数是随坐标变化的函数。举例说明了三维编织复合材料管的弹性性能及其变化规律，为其用于承载结构设计研究提供理论依据。     

Influence of microstructure on elastic and viscoelastic properties of polyether ether ketone

Dynamic, creep and static tests were conducted to characterize experimentally the linear elastic and viscoelastic properties of polyether ether ketone (PEEK) as a function of the degree of crystallinity. The semicrystalline polymer was modeled as a two-phase composite material. Values for crystalline modulus and effective crystalline aspect ratios have been extracted from the model. These values together with experimental creep compliance values for amorphous PEEK have been used to predict the creep response for intermediate levels of crystallinity, which compare well with the experimental data. Time-temperature shifting parameters obtained from dynamic mechanical studies compare well with those from creep experiments. At temperatures below 140°C, the shifting parameters were found to be approximately equal for all levels of crystallinity. Prediction for long-term creep has been made from these tests by properly accounting for physical aging phenomenon.     

Morphologies and luminescence properties of SrZnO2 microstructure

The SrZnO2 of beardlike and sheetlike nanobundles, rod and treelike nanostructures have been synthesized by a citrate-gel combusting synthesis approach. As-prepared SrZnO2 shows orthorhombic structure with Pnma space group and unit cell with the lattice parameters: a = 5.830 A, b = 3.340 A and c = 11.348 A. By increasing the sintering temperature, the beardlike nanobundle gradually dissolved to form microrods and treelike microstructure. Citrate acid exerts a major influence in directing the formation of these unique SrZnO2 microstructures. These materials were analyzed for their use to luminescence materials. The as-made samples exhibit an efficient absorption and excitation band in the UV spectral region (centered at 380 nm). In the same time, the samples of with different morphologies showed a broad yellow emission peak centered at approximately 545 nm which should been associated with the composition and morphologies of sample or from the oxygen vacancies of semiconductor SrZnO2. The material may be used as novel conversion phosphors or host material of phosphor for white-light LEDs.     

Evaluation of elastic properties in 2-D non-homogeneous solids

 This paper is concerned with inverse evaluation of the material property in two-dimensional non-homogeneous elastic solids. It seeks to recover variable subsurface Lame's coefficients which are functions of depth. The method considers a temporal interval for which time dependent measurements are provided. It formulates an optimal estimation problem which seeks to minimize the error difference between the given data and the response from the system. The method leads to an iterative algorithm which, at every iteration, requires the solution to a two-point boundary value problem. Numerical results indicate that a close estimate of the unknown functions can be obtained based on the boundary measurements only. Received 18 October 2000     

Structural, elastic and thermodynamic properties of Ti2SC

The structural parameters, elastic constants and thermodynamic properties of Ti₂SC were investigated under pressure and temperature by using first-principles plane-wave pseudopotential density functional theory within the generalized gradient approximation. The obtained results are in agreement with the available experimental data. The bulk moduli along the a- and c-axes, B _a and B _c, almost linearly increase with pressure, and the former is always smaller than the latter. The ratio of B _c/B _a has a trend of gradual increase as the pressure increases. It is found that the elastic constants, anisotropy and Debye temperature of Ti₂SC increase with pressure, while axial compressibility along the a- and c-axes decreases with pressure. The thermal properties including the equation of state, the Grüneisen parameter γ, the anisotropies Δ_p, Δ _S1 and Δ _S2, and the heat capacity are estimated at various pressures and temperatures.     

金属玻璃弹性性能与显微结构的分析与监测

利用高精度超声技术,研究Zr4lTil4Cul2．5Nil0．0Be22．5非晶合金的玻璃化转变过程,在不同退火温度下,特别是过冷液区,测量超声波在样品中传播的纵波和横波速度(Vl,Vs),进行X射线衍射结构的跟踪分析,监测样品密度（ρ）和显微硬度值（Hv),并观察纵波和横波衰减(αl,αs)的变化行为,测试结果为研究非晶成核、晶化过程中弹性性能与其微结构的内在关联提供了许多有益的实验数据和分析依据。     

A nanoindentation analysis of the effects of microstructure on elastic properties of Al2O3/SiC composites

Nanoindentation tests were carried out to investigate certain elastic properties of Al₂O₃/SiC_p composites at microscopic scales (nm up to μm) and under ultra-low loads from 3 mN to 250 mN, with special attention paid to effects caused by SiC particles and pores. The measured Young’s modulus depends on the volume fraction of SiC particles and on the composite porosity and it can compare with that of alumina. The Young’s modulus exhibits large scatters at small penetrations, but it tends to be constant with lesser dispersion as the indentation depth increases. Further analysis indicated that the scatter results from specific microstructural heterogeneities. The measured Young’s moduli are in agreement with predictions, provided the actual role of the microstructure is taken into account.     

Estimating mechanical properties of 2D triaxially braided textile composites based on microstructure properties

Textile composites manufactured using Resin Transfer Modeling (RTM) can offer advantages in some automotive applications including reduction in weight, while being relatively simpler to fabricate than standard laminated composites used for aerospace applications. However, one of the challenges that arise with these textile composite materials is that the mechanical properties are inherently dependent on the local and final (in-situ) architecture of the textile itself as a result of the molding and curing processes. While this provides additional latitude in the composite design process it also necessitates the development of analytical models that can estimate the mechanical properties of a textile composite based on the textile architecture and the properties of the manufactured component.In this paper, an analytical model is developed and its estimations are compared against experimental in-plane engineering properties for composites with various textile architectures. Results from the model are also compared against finite element (FE) based computational results. The microstructures of the 2D triaxially braided composite (2DTBC) studied were extensively characterized. The microstructure properties thus measured were used in the analytical model to estimate the mechanical properties. Uniaxial tension and V-notched rail shear tests were conducted on 2DTBC with different textile architectures. Good agreement between the analytical, computational, and experimental results were observed and are reported here. Furthermore, computational estimations of matrix mechanical properties are limited to the linear elastic range of a representative material volume (unit cell) and coupon data. Full mechanical response of larger 2DTBC structures, albeit of prime interest, is beyond the scope of this work and could be the focus of follow up studies.     

The elastic properties, elastic models and elastic perspectives of metallic glasses

Bulk metallic glass (BMG) provides plentiful precise knowledge of fundamental parameters of elastic moduli, which offer a benchmark reference point for understanding and applications of the glassy materials. This paper comprehensively reviews the current state of the art of the study of elastic properties, the establishments of correlations between elastic moduli and properties/features, and the elastic models and elastic perspectives of metallic glasses. The goal is to show the key roles of elastic moduli in study, formation, and understanding of metallic glasses, and to present a comprehensive elastic perspectives on the major fundamental issues from processing to structure to properties in the rapidly moving field.A plentiful of data and results involving in acoustic velocities, elastic constants and their response to aging, relaxation, applied press, pressure and temperature of the metallic glasses have been compiled. The thermodynamic and kinetic parameters, stability, mechanical and physical properties of various available metallic glasses especially BMGs have also been collected. A survey based on the plentiful experimental data reveals that the linear elastic constants have striking systematic correlations with the microstructural features, glass transition temperature, melting temperature, relaxation behavior, boson peak, strength, hardness, plastic yielding of the glass, and even rheological properties of the glass forming liquids. The elastic constants of BMGs also show a correlation with a weighted average of the elastic constants of the constituent elements. We show that the elastic moduli correlations can assist in selecting alloying components with suitable elastic moduli for controlling the elastic properties and glass-forming ability of the metallic glasses, and thus the results would enable the design, control and tuning of the formation and properties of metallic glasses.We demonstrate that the glass transition, the primary and secondary relaxations, plastic deformation and yield can be attributed to the free volume increase induced flow, and the flow can be modeled as the activated hopping between the inherent states in the potential energy landscape. We then propose an extended elastic model to understand flow in metallic glass and glass-forming supercooled liquid, and the model presents a simple and quantitative mathematic expression for flow activation energy of various glasses. The elastic perspectives, which consider all metallic glasses exhibit universal behavior based on a small number of readily measurable parameters of elastic moduli, are presented for understanding the nature and diverse properties of the metallic glasses.     

Hardness and elastic properties of Bi2O3-based glasses

The hardness and elastic properties of 20PbO · xBi₂O₃ · (80 – x)B₂O₃ glasses with x = 20–60 were evaluated through usual Vickers indentation and nanoindentation tests. The glass transition temperature (T _g = 295–421°C), Vickers hardness (H _v = 2.9–4.5 GPa), true hardness (H = 1.5–3.8 GPa) and Young's modulus (E = 24.4–72.6 GPa) decreased monotonously with increasing Bi₂O₃ content. This compositional trend demonstrates that the strength of Bi–O chemical bonds in these glasses is considerably weak compared with B–O bonds and plastic deformations under indentation loading occur easily. The elastic recovery after unloading was about 45% for the glasses with x = 20–50, and the Poisson's ratio was 0.27 for the glass with x = 20. The fracture toughness was evaluated to be 0.37–0.88 MPam^1/2 from the values of H _v and E, and it was proposed that not only weak Bi–O bonds but also boron coordination polyhedra (BO₃ or BO₄) and their arrangements affect on crack formation. From the temperature dependence of Vickers hardness up to the glass transition region, it was suggested that the glasses with high Bi₂O₃ contents belong to the category to fragile glass-forming liquids.     

Mechanical and elastic properties of transparent nanocrystalline TeO2-based glass-ceramics

Mechanical and elastic properties of transparent TeO₂-based glass-ceramics (15K₂O · 15Nb₂O₅ · 70TeO₂) consisting of nanocrystalline particles (each particle size: 40–50 nm) and showing optical second harmonic generation were evaluated by means of usual Vickers indentation and nanoindentation tests. The precursor glass has Vickers hardness H _v of 2.9 GPa, Young's modulus E of 54.7 GPa, the fracture toughness K _c of 0.25 MPam^1/2 and Poisson's ratio of 0.24. The transparent nanocrystalline glass-ceramic heat-treated at 420°C for 1 h has H _v = 3.8 GPa, E = 75.9 GPa and K _c = 0.34 MPam^1/2, and the opaque glass-ceramic heat-treated at 475°C for 1 h has H _v = 4.5 GPa, E = 82.9 GPa and K _c = 0.68 MPam^1/2, demonstrating that poor mechanical and elastic properties of the precursor TeO₂-based glass are improved through sufficient crystallization. The fracture surface energy, brittleness and elastic recoveries (about 44%) after unloading (the maximum load: 30 mN) of transparent nanocrystalline glass-ceramics are almost the same as those of the precursor glass, implying that the interaction among nanocrystalline particles is not so strong.     

Effective ply and constituent elastic properties for cracked laminates

Symmetric laminates with cracked plies were studied utilizing finite element unit cell models. The laminate unit cell models were developed based on periodicity which assumes uniformly spaced cracks. A volume-averaging scheme was developed for evaluating the volume-averaged stresses, strains and effective residual properties in the cracked plies. Sensitivities of residual properties to several material parameters were examined. The predictions of the finite element model were found to be in good agreement with published experimental results. Finally, an approach is described for computing the effective fiber and matrix properties, from the effective lamina properties, as required for progressive failure analysis using multicontinuum theory. It was found that, even though the fibers are undamaged, their effective continuum properties in the transverse direction must be degraded to achieve constituent properties that coincide with the damage mode and the residual effective ply properties.     

Underlying mechanisms for unique elastic properties of nanocrystalline Au

In order to get an insight into the grain boundaries (GBs) in nanocrystalline (n-) metal, we prepared the high-density n-Au with ρ/ρ₀>99% by the gas-deposition method and carried out the vibrating reed measurements, where ρ/ρ₀ is the relative density referring to the bulk density. The strain amplitude dependence (SAMD) of the resonant frequency (f) and the internal friction (Q⁻¹) was measured for the strain () amplitude between 10⁻⁶ and 2×10⁻³ and for temperature between 5 and 300 K. No plastic deformations are detected for the present strain range, where f decreases for up to 10⁻⁴ and then turns to increase, showing saturation for between 10⁻⁴ and 2×10⁻³. The low temperature irradiation by 2 MeV electrons or 20 MeV protons causes an increase in the Young’s modulus at 6 K, which is surmised to reflect a modification of the anelastic process in the GB regions. In contrast, the SAMD of f is hardly modified by irradiation, suggesting that it is indicative of a collective motion of atoms in n-Au.     

Linear elastic constitutive relation for multiphase porous media using microstructure superposition: Freeze-thaw soils

The constitutive relation derived in our previous work based on the microstructure superposition technique is implemented here for three-phase microstructure configuration to study the mechanical behavior of freeze-thaw soils. Three scenarios were considered for the frozen soils: frozen soil consisting of only two solids, soil skeleton and ice; frozen soil with soil skeleton, ice, and void; and frozen soil consisting soil skeleton, ice and pores saturated with fluid. The frozen soil studied is Alaska frozen soil mainly consisting of clay and silt particles at temperature about −10 °C. The effective elastic constants were calculated for the media under each scenario using two sets of elastic constants of soil skeleton (clay mineral). The modeled results were compared with Hashin-Shtrikman's upper bound solution and the experimentally measured data. In addition to be able to model the mechanical behavior of freeze-thaw soils, the derived constitutive relation, as indicated in the results of this study, could also be used as a tool in determining the microstructure of freeze-thaw soils by measuring the elastic constants of soil skeleton, the elastic properties and unfrozen water content of the media.     

一种改进的基于两单胞模型的三维角联锁机织复合材料弹性性能数值预测方法及实验验证

为准确预测三维角联锁机织复合材料的宏观弹性性能,对基于CT图像几何参数实测数据建立的内单胞和面单胞细观实体模型进行数值分析,其中面单胞模型采用组合面单胞形式,并开展了三维角联锁机织超高分子量聚乙烯(UHMWPE)纤维/聚氨酯复合材料的经向拉伸实验。结果表明:基于两单胞模型预测该复合材料的宏观弹性模量与实验结果吻合较好,组合面单胞的经向拉伸模量小于内单胞;经向拉伸时复合材料在经纱间接触面处、纬纱沿宽度方向的端部和经纱与基体的交界面处易出现应力集中现象;当纬纱层数小于30层时,应该考虑表面区域对复合材料整体力学性能的影响。      

Investigation of the elastic/crystallographic anisotropy of welds for improved ultrasonic inspections

Ultrasonic inspection is an effective way of ensuring the initial and continued integrity of welded joints non-destructively. The accuracy of the technique can be compromised due to spatial variations in the anisotropy of the material stiffness in the weld region. Predicted in-plane weld stiffness maps can be used to correct the ultrasound paths for improved results, but these are based on several assumptions about the weld material. This study has examined the validity of these assumptions and provided detailed weld metal grain orientation maps from which a stiffness map has been calculated for an Inconel 600 weld. Good agreement was found except near the boundaries of the weld. Further it was found that the crystal growth (most compliant) direction was typically oriented around 14.5° out of plane towards the welding direction. Having validated the model, a comparison of predicted and calculated stiffness maps was made. The predicted map was found to be satisfactory over the majority of the weld area.     

二维二轴编织复合材料几何模型及弹性性能预测

提出了二维二轴1×1和2×2编织复合材料的几何模型,模型考虑了纤维束的相互挤压及横截面的变化。基于细观分析和体积平均法,建立了预测二维二轴编织复合材料弹性性能的理论分析方法。数值结果与试验结果吻合,表明该方法行之有效,且具有运算快、精度高、适合工程分析等优点。分析了编织角、纤维体积含量和纤维束横截面形状对材料弹性常数的影响。研究表明,编织角对弹性常数的影响具有互补性,材料弹性模量与纤维体积含量成正比,纤维束截面形状变化对材料弹性常数影响不大。     

制备工艺对BaO-TiO2-B2O3-SiO2体系LTCC性能和微观结构的影响

研究了溶胶-凝胶法、高温熔融法、预煅烧法分别制备的BaO-TiO2-B2O3-SiO2体系LTCC的介电性能、烧结性能以及微观结构.研究发现,溶胶-凝胶法制备的LTCC材料烧结温度较低,坯体在880℃烧结即可达到最大收缩,其烧结收缩率大于高温固相法制备的LTCC;高温熔融法制备的LTCC介电性能稳定,但烧结温度偏高,大约950℃才能达到最大收缩;预煅烧法制备粉体简单、有效,烧结温度在900℃左右,但LTCC中活性成分较多,容易造成气孔率偏高,致密化难度增大且介电常数稳定性不好等缺点;在总结这3种制备工艺对LTCC玻璃陶瓷材料性能和微观结构的基础上,利用适量掺杂Al2O3等添加剂的方法,在保证介电性能满足应用的前提下,大大提高了预煅烧法制备LTCC的烧结体致密度.