Development of High Temperature Nanoindentation Methodology and its Application in the Nanoindentation of Polycrystalline Tungsten in Vacuum to 950 °C

The capability for high temperature nanoindentation measurements to 950 °C in high vacuum has been demonstrated on polycrystalline tungsten, a material of great importance for nuclear fusion and spallation applications and as a potential high temperature nanomechanics reference sample. It was possible to produce measurements with minimal thermal drift (typically ~0.05 nm/s at 750–950 °C) and no visible oxidative damage. The temperature dependence of the hardness, elastic modulus, plasticity index, creep, creep strain, and creep recovery were investigated over the temperature range, testing at 25, 750, 800, 850, 900 and 950 °C. The nanoindentation hardness measurements were found to be consistent with previous determinations by hot microhardness. Above 800 °C the hardness changes relatively little but more pronounced time-dependent deformation and plasticity were observed from 850 °C. Plasticity index, indentation creep and creep recovery all increase with temperature. The importance of increased time-dependent deformation and pile-up on the accuracy of the elastic modulus measurements are discussed. Elastic modulus measurements determined from elastic analysis of the unloading curves at 750–800 °C are close to literature bulk values (to within ~11 %). The high temperature modulus measurements deviate more from bulk values determined taking account of the high temperature properties of the indenter material at the point (850 °C) at which more significant time-dependent deformation is observed. This is thought to be due to the dual influence of increased time-dependency and pile-up that are not being accounted for in the elastic unloading analysis. Accounting for this time-dependency by applying a viscoelastic compliance correction developed by G. Feng and A.H.W. Ngan (J. Mater. Res. (2002) 17:660–668) greatly reduces the values of the elastic modulus, so they are agree to within 6 % of literature values at 950 °C.     

木材顺纹弹性模量的细观分析

 通过对10年生火炬松17号、50号家系的抗弯弹性模量(MOE)测试和 分析，以及速生材水杉、欧美杨的数据验证，说明，对于早晚材区别明 显的木材，单层复合材料细观力学的混合理论适用于其顺纹弹性模量的 分析，EL=E早+(E晚-E早)V晚, MOE随V晚的增大而增大. 另外还表明，早晚 材区别明显的木材本身可假定是一种由早、晚材复合而成的单层复合材 料. 短周期工业用速生材的MOE变异性较大，混合律不仅解释了这种变 异性，而且可以实现$MOE$的预测.     

The Effect of Grain Size on Deformation and Failure of Ti<Subscript>2</Subscript>AlC MAX Phase under Thermo-Mechanical Loading

An experimental investigation was performed to analyze the effects of grain size on the quasi-static and dynamic behavior of Ti₂AlC. High-density Ti₂AlC samples of three different grain sizes were densified using Spark Plasma Sintering and Pressureless sintering. A servo-hydraulic testing machine equipped with a vertical split furnace, and SiC pushrods, was used for the quasi-static experiments. Also, a Split Hopkinson Pressure Bar (SHPB) apparatus and an induction coil heating system were used for the dynamic experiments. A series of experiments were conducted at temperatures ranging from 25 °C to 1100 °C for strain rates of 10^?4 s^?1 and 400 s^?1. The results show that under quasi-static loading the specimens experience a brittle failure for temperatures below Brittle to Plastic Transition Temperature (BPTT) of 900–1000 °C and large deformation at temperatures above the BPTT. During dynamic experiments, the specimens exhibited brittle failure, with the failure transitioning from catastrophic failure at lower temperatures to graceful failure (softening while bearing load) at higher temperatures, and with the propensity for graceful failure increasing with increasing grain size. The compressive strengths of different grain sizes at a given temperature can be related to the grain length by a Hall-Petch type relation.     

Structure–property relations for balsa wood as a function of density: modelling approach

In order to find an alternative core material to balsa wood in composite sandwich structures, it is important to understand balsa’s elastic properties in relation to its complex microstructural organisation. In the present work, experimental data on the elastic constants and microstructural features of balsa wood were collected for different porosities (densities) and processed into structure–property relations. An inverse problem was solved to predict variation of the cell wall properties with density, such that the collected experimental structure–property relations were satisfied. The Young’s modulus of the cell wall material in the longitudinal direction was found to increase with balsa’s density, which is consistent with the knowledge that the cell wall material stiffens during tree maturation. The value reported in the literature falls in the middle of the predicted range. The proposed micromechanical model also accurately calculated elastic properties of balsa wood at the mesolevel including longitudinal, radial, and tangential directions. The model took into account the presence of ray cells. It was shown that the addition of 15 % of rays increased the radial Young’s modulus up to 4 times with only slight decrease in the longitudinal modulus.     

介电弹性材料的机电耦合性能研究

作为一种新型的电活性聚合物,介电弹性材料可被用作柔性致动器。其中材料的介电性能和机械性能是影响其机电耦合致动性能的关键因素。通过实验方法研究了一种典型的介电弹性材料VHB4910在不同温度和频率下的介电常数和弹性模量,基于实验结果分析了该材料的机电耦合性能。结果表明:依赖于频率和温度的弹性模量是影响该介电弹性材料致动变形的主要因素,对致动性能的影响最大可达4个数量级,材料的介电常数对其致动性能的影响相对较小。     

缝纫复合材料层合板面内弹性模量分析

基于对缝纫孔附近局部细观结构的分析,提出了一种预测缝纫复合材料层合板面内力学性能的理论模型．从分析缝孔单胞的纤维弯曲几何特征入手,最终得到单向板及层合板的弹性常数．通过有限元分析研究了缝纫参数对复合材料层合板面内等效模量的影响．研究结果表明,缝纫造成单向板及层合板面内材料性质的不均匀,随着缝纫密度和缝纫线直径的增加,层合板的等效模量逐渐降低．     

The effect of pre-thermal history on shear and uniaxial elongational viscosity of a tetrafluoroethylene/hexafluoropropylene copolymer near the crystal melting transition

The melt rheology of a commercially available tetrafluoroethylene/hexafluoropropylene copolymer, which is known as Teflon FEP copolymer, was studied to examine the effect of pre-thermal history during sample preparation conditions on dynamic shear and uniaxial elongational measurements. The first experimental series includes the sample preparation under hot press at 320 °C followed by a rapid cooling. The master curves were successfully obtained at 300 °C from the time-temperature superposition principle. The loss modulus G″ was found to be proportional to the angular frequency in a double-logarithmic plot toward 0.01 (rad/s), while the slope of the storage modulus G′ did not become 2. The elongational viscosity as a function of time under constant strain rates showed weak strain-hardening, which was enhanced with larger strain rates. The second experimental series contain three kinds of samples with different pre-thermal history to control rheological properties. All samples were hot-pressed at 320 °C followed by a rapid cooling to room temperature for the sample A and a slow cooling for the sample B and C. The dynamic shear and elongational measurements were performed at 270 °C for all samples, which were heated from room temperature for the sample A and B, but heated up to 280 °C and cooled down to 270 °C for the sample C. The distance between G″ and G′ become narrower in the order of the sample C, B, and A. In the same order, unexpectedly, the strain-hardening in the elongational viscosity measurements became the strain-softening. These unusual properties were discussed from a residual crystallinity.     

平面织物复合材料机械性能的数值细观力学分析

本文根据应变能等效原理,利用有限元分析方法,处理了单层平面织物复合材料弹性性质的细观力学估算问题.建立了平面织物增强树脂的单方向波纹模型,对于织物的结构,组分性质与复合材料的宏观性能的关系作了系统处理.给出了一组较完整的平面织物复合材料单层弹性常数的估算结果,数值予告与实验结果比较,表明文中所提出的方法是有效的,并且可以进一步用它来考虑更为完善的平面织物复合材料二维波纹模型的分析.     

Compressive response and failure of braided textile composites: Part 2—computations

In this, the second part of a two part paper, results obtained by using the finite element (FE) method in conjunction with micromechanics to predict the effective elastic stiffness and strength of a carbon 2D triaxially braided composite (2DTBC), are presented. The 3D FE based micromechanics study was carried out on one representative unit cell (RUC) of the carbon 2DTBC (the “micromodel”). The FE models were first used to determine the macroscopic elastic orthotropic stiffnesses of the 2DTBC. The micromodel was deemed acceptable (in terms of the number of elements used in the mesh of the micromodel) if the elastic stiffnesses it displayed were within 5% of the elastic properties found experimentally. Subsequently, buckling eigenmodes were determined for the FE RUC under uniaxial and biaxial loading states, corresponding to the experimental investigation reported in part I of this two part paper. The lowest symmetric modes were identified and these mode shapes were used as imperfections to the FE model for a subsequent nonlinear response analysis using an arc-length method in conjunction with the ABAQUS commercial FE code. The magnitude of the imperfections was left as a parameter and its effect on the predicted response was quantified. The present micromechanics computational model provides a means to assess the compressive and compressive/tensile biaxial strength of the braided composites and its dependence on various microstructural parameters. It also serves as a tool to assess the most significant parameter that affects compressive strength.     

Rheological behavior of suspensions of modified and unmodified cellulose nanocrystals in dimethyl sulfoxide

The rheological behavior of cellulose nanocrystal (CNC) and modified CNC (mCNC) suspensions in dimethyl sulfoxide (DMSO) was investigated. The efficiency of the surface modification of CNCs by grafting an organic acid chloride to produce hydrophobic CNCs has been verified by X-ray photoelectron spectroscopy (XPS). The thermal degradation temperature of the mCNCs was found to be 165 versus 275 °C for CNCs. The CNC suspensions in DMSO at 70 °C underwent gelation at very low concentration (1 wt%) after 1 day. The network formation was temperature sensitive and did not occur at room temperature. For gels containing 3 wt% CNCs, the complex viscosity at 70 °C increased by almost four decades after 1 day. For the mCNCs in DMSO, a weak gel was formed from the first day and temperature did not affect the gelation. Finally, the effect of adding 10 wt% of polylactide (PLA) to the solvent on the rheological properties of CNC and mCNC suspensions was investigated. The properties of suspensions containing 1.9 wt% CNCs and mCNCs increased during the first and second days, and PLA did not prevent gel formation. However, the reduced viscosity and storage modulus of the CNC and mCNC gels with PLA were lower than those of samples without PLA.     

An experimental and analytical investigation of the rail shear-test method as applied to composite materials

This report presents the results from an experimental and analytical investigation of the stress distributions occurring in a rail shear test. The effects of nonuniform stresses induced by differential thermal expansion, rail flexibility and specimen aspect ratio on measured shear modulus and ultimate strength of composite laminates are shown. A two-dimensional linearly elastic finite-element model was used to analytically determine how various geometric parameters influenced the magnitude and distribution of inplane normal and shear stresses in a tensile-rail-shear specimen. Rail shear tests were conducted at room temperature and 589 K (600°F) on selected graphite-polyimide composite laminates using two titanium rail configurations. The analysis and test methods are discussed, and the results of the effects of the various parameters on shear modulus and ultimate strength are presented.     

复合材料模量细观分析的一般性模型

基于细观力学理论,提出了横观各向异性复合材料弹性常数的分析模型,并给出了纵向(纤维长度方向) 与横向弹性模量、横截面内剪切模量和泊松比等材料性能参数的理论推导和计算公式. 此方法简洁准确地反映了材料横观各向异性性能,改进了常规细观力学模型,也为工程分析提供了理论依据;同时在复合材料力学课程教学中,有助于深化学生对复合材料细观分析的理解,具有一定的启发性.     

On the propagation of elastic waves through composite media II

Summary The propagation of elastic waves (both longitudinal and transverse) through polyurethane rubbers filled with different amounts of sodium chloride particles was studied at 0.8 MHz and 5 MHz. At a constant filler concentration (∼10% by volume), the velocity of these waves appeared to be independent of filler size. On the other hand, both velocities were found to increase with filler content. From the wave velocities, the effective modulus for longitudinal waves, L, bulk modulus, K, and shear modulus, G, were calculated according to the relations for a homogeneous isotropic material. All three moduli appear to be monotonically increasing functions of filler content, c, over the whole experimentally accessible temperature range (−80°C to +80°C for L and K; −80°C to about −30°C for G) and they, moreover, reflect the glass-rubber transition of the binder. Poissons ratio, μ, was found to decrease with increasing filler content and shows a rise at about −30°C as a result of the approach of the glass-rubber transition. The attenuation of the elastic waves was also measured in the temperature ranges mentioned. For filler particles beyond a critical size both tan δ_L and tan δ_G in the hard region are independent of the filler content within the accuracy of the measurements. The critical size depends on the type of wave and on its frequency. In the rubbery region, however, tan δ_L increases with particle size (at a constant content of 10% by volume) and even shows an enhancement with the smallest particles (1–5 μ) at 0.8 MHz. Moreover, it is found that for the same filler size tan δ_L increases with filler content. In some cases an anomalous damping behaviour was found, such that in the rubbery region the attenuation rises indefinitely with temperature. For filler particles larger than the above-mentioned critical size, tan δ_G and tan δ_L increase in the hard region as well. Finally, the experimental results are compared with existing theories on the elastic properties of and wave propagation through composite media.     

Elastic constants characterization on graphite at 500°C by the virtual fields method

In this paper the elastic constants of graphite at elevated temperature were experimentally investigated by using the virtual fields method (VFM). A new method was presented for the characterization of mechanical properties at elevated temperature. The three-point bending tests were performed on graphite materials by an universal testing machine equipped with heating furnace. Based on the heterogeneous deformation fields measured by the digital image correlation (DIC) technique, the elastic constants were then extracted by using VFM. The measurement results of the elastic constants at 500°C were obtained. The effect on the experimental results was also analyzed. The successful results verify the feasibility of using the proposed method to measure the properties of graphite at high temperature, and the proposed method is believed to have a good potential for further applications.     

油酸钡和钛酸酯复合添加剂在液体石蜡中的协同减摩抗磨效应

在四球摩擦磨损机上考察了润溶性钡盐添加剂与月桂酸钛酸酯复配时的摩擦磨损性能,并对多元氧化物的表面微观物理性质及作用机理进行了讨论。研究结果表明,油溶性钡盐添加剂与月桂酸酸钛酸酯之间具有协同减摩抗磨效应,从而提高无卡咬负荷和改善抗磨性能。钢球磨损表面Ｘ射线光电子能谱分析结果表明,表面生成了Ｔｉ和Ｂａ等的多元氧化物。     

Evaluation of the Effect of Thermal Oxidation and Moisture on the Interfacial Shear Strength of Unidirectional IM7/BMI Composite by Fiber Push-in Nanoindentation

Fiber push-in nanoindentation is conducted on a unidirectional carbon fiber reinforced bismaleimide resin composite (IM7/BMI) after thermal oxidation to determine the interfacial shear strength. A unidirectional IM7/BMI laminated plate is isothermally oxidized under various conditions: in air for 2 months at 195 °C and 245 °C, and immersed in water for 2 years at room temperature to reach a moisture-saturated state. The water-immersed specimens are subsequently placed in a preheated environment at 260 °C to receive sudden heating, or are gradually heated at a rate of approximately 6 °C/min. A flat punch tip of 3 μm in diameter is used to push the fiber into the matrix while the resulting load-displacement data is recorded. From the load-displacement data, the interfacial shear strength is determined using a shear-lag model, which is verified by finite element method simulations. It is found that thermal oxidation at 245 °C in air leads to a significant reduction in interfacial shear strength of the IM7/BMI unidirectional composite, while thermal oxidation at 195 °C and moisture concentration have a negligible effect on the interfacial shear strength. For moisture-saturated specimens under a slow heating rate, there is no detectable reduction in the interfacial shear strength. In contrast, the moisture-saturated specimens under sudden heating show a significant reduction in interfacial shear strength. Scanning electron micrographs of IM7/BMI composite reveal that both thermal oxidation at 245 °C in air and sudden heating induced microcracks and debonding along the fiber/matrix interface, thereby weakening the interface, which is the origin of failure mechanism.     

纤维增强复合材料弹性性能预测的域分解方法及应用

提出了新的有限元建模方法,即域分解方法,用于预测纤维增强复合材料单向带T300/BSL914C(环氧树脂)和AS4/3501-6(环氧树脂)的弹性性能。域分解方法基于区域叠合技术,分别建立单胞的整体域与纤维域模型用于代替传统有限元建模方法中单胞的基体域与纤维域模型。整体域是真实基体体积与纤维体积的叠加,两区域网格独立划分,互不影响。采用MSC.Nastran中的多节点约束Explicit单元,在整体域与纤维域节点之间建立位移连接属性模拟单胞基体域与纤维域之间的位移约束关系,从而实现两区域的耦合计算。计算结果表明:域分解方法单胞模型纤维增强方向弹性模量Ez预测值与试验值误差在7%以内,其余弹性常数也都与试验值吻合较好。域分解方法不仅可以大大简化纤维增强复合材料的细观力学建模,而且可以准确地预测纤维增强复合材料的弹性性能。     

基于等效特征应变原理的复合材料有效模量细观力学分析

基于等效特征应变原理,提出了一种新的复合材料有效模量细观力学分析方法。首先,在等效特征应变原理基础上提出平均等效特征应变原理,它可用于解决有限体下任意形状(无论是凸或凹形)的单个夹杂或多个夹杂的弹性变形问题。其次,将平均等效特征应变原理与细观力学直接均匀法相结合,来分析确定复合材料的有效模量。最后利用复合材料纤维与基体的力学性能参数及纤维的体分比,借助MATLAB编程方法,预测其有效模量。通过将理论预测值与已有的的试验值、其它理论预测值进行对比,验证了新分析方法的合理性和分析精度。     

A Continuum Theory of Deformable,Semiconducting Ferroelectrics

Ferroelectric solids, especially ferroelectric perovskites, are widely used as sensors, actuators, filters, memory devices, and optical components. While these have traditionally been treated as insulators, they are in reality wide-band-gap semiconductors. This semiconducting behavior affects the microstructures or domain patterns of the ferroelectric material and the interaction of ferroelectrics with electrodes, and is affected significantly by defects and dopants. In this paper, we develop a continuum theory of deformable, semiconducting ferroelectrics. A key idea is to introduce space charges and dopant density as field (state) variables in addition to polarization and deformation. We demonstrate the theory by studying oxygen vacancies in barium titanate. We find the formation of depletion layers, regions of depleted electrons, and a large electric field at the ferroelectric–electrode boundary. We also find the formation of a charge double layer and a large electric field across 90° domain walls but not across 180° domain walls. We show that these internal electric fields can give rise to a redistribution or forced diffusion of oxygen vacancies, which provides a mechanism for aging of ferroelectric materials.     

On the thermal buckling of simply supported rectangular plates made of a sigmoid functionally graded Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> based material

We study the thermal buckling of a simply supported sigmoid functionally graded (SFGM) rectangular plate using first-order shear deformation theory. The S-FGM system consists of ceramic (Al₂O₃) and metal (Al) phases varying across the plate thickness according to a law described by two power-law functions. The effective properties of the composite are determined by the rule of mixtures, whose implementation is simpler than that of methods of micromechanics. The thermal heating is characterized by a uniform, linear, or sinusoidal temperature distribution across the plate thickness. The effects of the plate aspect ratio, the relative thickness, the gradient index, and the transverse shear on the buckling temperature difference are studied.