周期性多孔材料等效剪切模量与尺寸效应研究

提出了以圆筒扭转力学模型为基础, 预测周期性多孔材料等效剪切模量及其研究尺寸效 应的一种简单有效计算方法. 以方形孔和圆形孔两种典型多孔材料为例进行了数值计算求解; 同时, 建立了几何参数随体胞尺寸缩放因子$n$的解析关系, 证明了两种构型的周期性多孔材 料的等效剪切模量均随尺寸缩放因子$n$的增大而减小. 当$n \to \infty $时, 即体胞尺寸相对整体结构无限小时, 多孔材料的等效剪切模量趋近收敛于一个恒定值; 当体胞的材料体分比增大时, 多孔材料等效剪切模量也随之增大. 此外, 依据周 期性多孔材料的结构对称特性, 使用体胞子结构有限元计算模型进行等效剪切模量及其尺寸 效应的预测, 极大地提高了计算效率.     

多孔材料/结构尺度关联的一体化拓扑优化技术

针对多孔材料的尺度效应和微结构构型的可设计性,提出以宏观结构最大刚度为目标,材料表征体胞构型为变量的材料／结构尺度关联一体化设计新方法．采用有限元超单元技术,验证了材料表征体胞尺度、边长比、平移、对称周期分布方式对构型设计结果的影响,实现了材料宏观布局设计、材料表征体胞构型精细设计以及多尺度均匀化设计的统一。基于凸规划对偶优化求解技术与二次型周长控制约束,完成了快速设计与材料分布棋盘格效应的控制。计算结果表明,在给定材料用量的情况下,该方法能有效地实现蜂窝结构的拓扑优化设计,设计结果充分反映了蜂窝夹层结构的尺度效应,为轻质结构设计提供了新的设计方法。     

自相似多孔材料的力学性能与吸能特性

通过对自相似多孔材料的研究,给出了该结构的代表体积单元和相对密度。采用有限元方法对多孔材料面外的准静态压缩以及动态压缩进行数值模拟,分析了其变形模式,提取了应力-应变响应曲线,讨论了相对密度对多孔材料的弹性模量、平台应力、密实应变以及单位质量吸能的影响。研究发现:多孔材料在面外准静态压缩时,弹性模量、平台应力随相对密度的增大而增大,密实应变随相对密度的增大而减小;相对密度主要影响多孔材料的平台应力和弹性模量。多孔材料在面外动态压缩时,在相同的冲击速度下,多孔材料相对密度越大,其单位质量吸收的能量越多。     

闭孔泡沫铝材料十四面体模型的改进

基于对闭孔泡沫铝发泡过程更为合理的假设,提出了描述胞体结构的改进的十四面体模型,使之可以反映密度增大时质量集中于支柱和顶点的情况。采用有限元方法及耦合边界条件,研究了闭孔泡沫铝的相对弹性模量、泊松比等弹性特征与胞体参数的关系,给出了拟合的弹性模量的计算公式,并对模型在弹性压缩变形下应力分布进行了分析。通过与已有模型的比较表明,改进模型可以较好地模拟闭孔泡沫铝材料的弹性性能。     

轻质波纹夹层结构的制备及其多功能应用研究进展

波纹夹层结构较其他轻质多孔材料具有结构简单、加工制造方便、制造成本低等优点,已得到广泛应用.本文概述了不同轻质波纹结构的构型及其表征方式,阐述了波纹夹层结构的制备及其无损检测技术,重点评述了波纹夹层结构的轻质高强、抗爆炸冲击、高效散热、吸声降噪、促动等多功能应用特性,总结了波纹夹层结构在工程应用上的关键进展,展望了波纹夹层结构在基础和应用研究方面的发展趋势.     

轻质金属点阵夹层板热屈曲临界温度分析

本文针对均匀温度场下四边简支和四边固支金属点阵夹层板的临界热屈曲温度进行了求解和参数影响分析。将点阵夹芯等效为均匀连续体,并且将夹层板的剪切刚度近似为点阵夹芯的抗剪切刚度,忽略夹芯的抗弯刚度且认为夹层板主要由面板来提供抗弯刚度。对于无法获得解析解的四边固支条件,通过对未知变量进行双傅里叶展开的方法求解了Ressiner夹层板模型的临界屈曲温度,理论分析结果与有限元计算结果吻合良好。进一步分析了不同边界条件、点阵胞元构型、点阵材料相对密度、面板厚度等对临界屈曲温度的影响规律。     

水中爆炸载荷下夹芯圆柱壳的结构优化

采用ANSYS/LS-DYNA有限元软件,研究了由三种不同相对密度的泡沫铝组合成的密度非均匀芯层泡沫金属夹芯圆柱壳的水下抗爆性能及均匀型泡沫金属夹芯结构的结构优化.结果表明:泡沫铝芯层相对密度配置顺序对夹芯圆柱壳的抗爆性能影响明显,可以通过调整芯层配置获得最佳吸能性能泡沫金属夹芯圆柱壳;采用Kriging多目标优化分析方法,获得了以夹芯壳单位面积重量最小、中心位移最小和能量吸收最大为目标,上、下面板的厚度的最优解.     

具有填充材料的金属格栅夹层板在高速冲击下动态响应的数值分析

利用大型非线性有限元程序ABAQUS和LS-DYNA,对具有填充材料的金属格栅结构的冲击问题进行数值模拟.研究了不同的填充材料(金属泡沫和陶瓷)分别填充到不同的格栅构型(波纹型、蜂窝型和加强六边形)夹层板后,各类夹层板受到金属泡沫子弹和不锈钢子弹冲击时变形与能量吸收特性,探讨了夹层板上下面层板、支撑格栅及填充材料等各部分的吸能比率.研究结果表明,泡沫填充夹层板在缓冲吸能方面具有优势,陶瓷填充夹层板则在抵抗冲击穿透方面更具有优势,不同构型的夹层板,性能略有不同.     

星型负泊松比多孔材料力学性能及应用研究

论文推导了星型负泊松比多孔材料的泊松比$弹性模量及相对密度的解析表达式"并通过有限元分 析验证了解析表达式的准确性:星型负泊松比多孔材料具有优异的吸能与隔振性能"论文设计了一种船用星型多 孔材料隔振基座:建立了不同层数$壁厚$泊松比的星型多孔材料隔振基座对应的数值有限元模型"探究了星型多 孔材料薄壁结构泊松比$层数以及壁厚对多孔材料隔振基座强度与减振性能的影响:研究表明"减少多孔材料薄壁 结构的层数和壁厚"选用胞元泊松比?%:$的星型多孔材料"可以获得低频隔振效果好$强度高的多孔材料隔振 基。     

金属蜂窝夹层结构抗水下爆炸特性

金属蜂窝夹层结构是一种新型的舰船防护结构,在舰船防护领域具有广阔的应用前景,但目前缺乏对其在实际水下爆炸载荷作用下动态响应的研究。为研究金属蜂窝夹层结构在水下爆炸载荷作用下的动态响应及防护性能,设计并制备了背板加筋蜂窝夹层结构样件以及相应的浮箱,在大型露天水池中进行了水下实爆 实验;通过声固耦合算法对结构响应进行模拟,实验结果与模拟结果吻合良好,随后分析了蜂窝夹层板的变形过程及能量吸收特性,量化了载荷参数（冲击因子）及结构参数（前后面板厚度比和芯体相对密度）对结构动态响应的影响;最后,以蜂窝夹层板的面密度和后面板中心点最大变形的无量纲量为目标函数,使用NSGA-Ⅱ遗传算法对结构进行了多目标优化,得到对应的Pareto前沿。结果表明,随着冲击因子的增大,蜂窝夹层板整体变形显著增大,蜂窝芯体始终是主要的吸能构件,但其吸能占比逐渐降低;随着前后面板厚度比或芯体相对密度的增加,蜂窝夹层结构的最大变形呈现先降低后升高的趋势,同时呈现不同的变形模式,芯体相对密度对结构变形的影响更为显著;对蜂窝夹层结构开展多目标优化可有效降低结构的面密度及最大变形,优化结果可为蜂窝夹层结构的设计选型提供参考。     

Numerical estimation of mixed convection heat transfer in vertical helically coiled tube heat exchangers

A numerical investigation of the mixed convection heat transfer from vertical helically coiled tubes in a cylindrical shell at various Reynolds and Rayleigh numbers, various coil‐to‐tube diameter ratios and non‐dimensional coil pitches was carried out. The particular difference in this study compared with other similar studies is the boundary conditions for the helical coil. Most studies focus on constant wall temperature or constant heat flux, whereas in this study it was a fluid‐to‐fluid heat exchanger. The purpose of this article is to assess the influence of the tube diameter, coil pitch and shell‐side mass flow rate on shell‐side heat transfer coefficient of the heat exchanger. Different characteristic lengths were used in the Nusselt number calculations to determine which length best fits the data and finally it has been shown that the normalized length of the shell‐side of the heat exchanger reasonably demonstrates the desired relation. Copyright © 2010 John Wiley & Sons, Ltd.     

New matrix method for analyzing vibration and damping effect of sandwich circular cylindrical shell with viscoelastic core

Based on the linear theories of thin cylindrical shells and viscoelastic materials, a governing equation describing vibration of a sandwich circular cylindrical shell with a viscoelastic core under harmonic excitation is derived. The equation can be written as a matrix differential equation of the first order, and is obtained by considering the energy dissipation due to the shear deformation of the viscoelastic core layer and the interaction between all layers. A new matrix method for solving the governing equation is then presented With an extended homogeneous capacity precision integration approach. Having obtained these, vibration characteristics and damping effect of the sandwich cylindrical shell can be studied. The method differs from a recently published work as the state vector in the governing equation is composed of displacements and internal forces of the sandwich shell rather than displacements and their derivatives. So the present method can be applied to solve dynamic problems of the kind of sandwich shells with various boundary conditions and partially constrained layer damping. Numerical examples show that the proposed approach is effective and reliable compared with the existing methods.     

Bending and vibration analyses of a rotating sandwich cylindrical shell considering nanocomposite core and piezoelectric layers subjected to thermal and magnetic fields

The bending and free vibration of a rotating sandwich cylindrical shell are analyzed with the consideration of the nanocomposite core and piezoelectric layers subjected to thermal and magnetic fields by use of the first-order shear deformation theory (FSDT) of shells. The governing equations of motion and the corresponding boundary conditions are established through the variational method and the Maxwell equation. The closed-form solutions of the rotating sandwich cylindrical shell are obtained. The effects of geometrical parameters, volume fractions of carbon nanotubes, applied voltages on the inner and outer piezoelectric layers, and magnetic and thermal fields on the natural frequency, critical angular velocity, and deflection of the sandwich cylindrical shell are investigated. The critical angular velocity of the nanocomposite sandwich cylindrical shell is obtained. The results show that the mechanical properties, e.g., Young’s modulus and thermal expansion coefficient, for the carbon nanotube and matrix are functions of temperature, and the magnitude of the critical angular velocity can be adjusted by changing the applied voltage.     

Skin friction and heat transfer in rapidly oscillating boundary layers

The influence of rapid oscillations in the outer part of a boundary layer upon the time-averaged skin friction and heat transfer is investigated analytically. The oscillations are taken to be harmonic. The only restriction on the oscillation amplitude is that it should be sufficiently small to permit the use of the boundary layer equations. The derived asymptotic formulae show the explicit dependence of momentum transfer on the frequency and the time-averaged boundary layer flow. For the heat transfer similar formulae can be derived in a number of limiting cases, viz. when the Prandtl number is either large or small.     

Nonlinear stability of a shallow unsymmetrical heated orthotropic sandwich shell of double curvature with orthotropic core

Equations which govern the behavior of an elastic unsymmetrical, orthotropic sandwich shell of double curvature with orthotropic core having different elastic characteristics under uniform heating are derived. The face sheet may be of unequal thickness of different materials. However, a restriction that the radii of curvature of the shell elements be large compared with the overall thickness of the sandwich shell is imposed. The variational procedure has been used to obtain the five equations which govern the behavior of the heated orthotropic sandwich shell for the stability. In case of symmetry the equations resemble with those of Grigolyuk. Finally, the numerical results of a square or a rectangular simply supported curved plate section of a cylindrical shell under thermal loading have been computed and compared with other known results. The graphs have been drawn to show the effects of different sandwich material for immovable and movable edge conditions.     

Free vibration of circular cylindrical shell with constrained layer damping

Free vibration characteristics of circular cylindrical shell with passive constrained layer damping (PCLD) are presented. Wave propagation approach rather than finite element method, transfer matrix method, and Rayleigh-Ritz method is used to solve the problem of vibration of PCLD circular cylindrical shell under a simply supported boundary condition at two ends. The governing equations of motion for the orthotropic cylindrical shell with PCLD are derived on the base of Sanders’ thin shell theory. Numerical results show that the present method is more effective in comparison with other methods. The effects of the thickness of viscoelastic core and constrained layer, the elastic modulus ratio of orthotropic constrained layer, the complex shear modulus of viscoelastic core on frequency parameter, and the loss factor are discussed.     

Thermomechanical response of metallic sandwich tubes with prismatic cores considering active cooling

In this paper, the transient temperature distribution and the thermomechanical response of sandwich tubes with prismatic cores are analyzed considering active cooling. The effective thermal conductivities of prismatic cores with active cooling are derived. By using the effective thermal conductivities, the transient temperature fields of the tubes are predicted and are found to be very close to the results of finite element simulations, which confirms the correctness of the effective thermal conductivity. Based on the high-order sandwich shell theory, the thermal structural responses of sandwich structures are studied and compared with the results of finite element simulations. The reduction of the thermal structural responses as a result of active cooling is studied to demonstrate the advantages of prismatic cellular materials. The design of replacing the solid metal with cellular materials which has the capability of active cooling can reduce the temperature and the thermal structural response of the structure.     

Exact solution of conductive heat transfer in cylindrical composite laminate

This paper presents an exact solution for steady-state conduction heat transfer in cylindrical composite laminates. This laminate is cylindrical shape and in each lamina, fibers have been wound around the cylinder. In this article heat transfer in composite laminates is being investigated, by using separation of variables method and an analytical relation for temperature distribution in these laminates has been obtained under specific boundary conditions. Also Fourier coefficients in each layer obtain by solving set of equations that related to thermal boundary layer conditions at inside and outside of the cylinder also thermal continuity and heat flux continuity between each layer is considered. In this research LU factorization method has been used to solve the set of equations.     

含轴对称初始缺陷的具有正交各向异性表层的夹层圆柱壳的非线性屈曲

夹层圆柱壳具有很高的结构效能。在许多工程结构中被广泛采用。本文研究分析了含有轴对称初始缺陷的夹层圆柱壳在轴压下的非线性屈曲问题。该夹层壳具有正交各向异性表层和各向同性可承剪的夹心．利用Stein的前屈曲一致理论得出了前屈曲挠度随轴向载荷及缺陷参数的变化情况，运用Galerkin法导出了屈曲控制方程，并进行了数值计算，得到了屈曲载荷、缺陷幅值、缺陷波数、夹心模量等参量之间的关系．结果表明与壳体实际屈曲模态相同的初始缺陷具有很大的危害性，可以通过增加壳体表层的轴向弹性模量或优化夹心的有关参数等途径来提高屈曲载荷，改善壳体屈曲性能。     

下降液膜层流换热发展段中的积分分析

对等温竖直壁面上层流液膜发展段的流动换热进行了理论分析 ,用积分法得到并求解了速度和温度边界层的积分方程式 ,求得了发展段的流动和换热特性的变化规律 ,讨论了进口流态和普朗特数对换热的影响。