功率型LED瞬态温度场及热应力分布的研究

针对功率型LED器件的热特性,以热应力理论为依据,采用有限元软件ANSYS进行热应力计算,得到了Lumileds的1 W LED瞬态温度场和应力场分布云图,基板顶面平行于X轴路径上的热应力,应变及剪应力的分布曲线.模拟结果表明最大应力集中在键合层边角处;轴向最大位移在透镜与热沉接触边缘;最大剪应力集中在键合层的边角区域...     

一种可适用于瞬态辐射换热问题的蜂窝夹芯壳体有限单元

提出一种三层32 结点温度壳单元, 对蜂窝夹芯复合材料壳体的瞬态温度场进行了有限元分析。分别假定上下面板与芯层沿厚度方向的温度分布规律, 事先近似满足上下表面的热边界条件及层间的温度连续条件, 以消减总的自由度数。壳体内外表面可以同时考虑热流、对流及辐射换热边界条件, 并通过温度的后处理使芯层温度场也满足层间的热流连续条件, 从而进一步提高温度场计算的准确性。两个实例分析验证了所提出单元的可靠性与有效性。     

6061Al／SiC层合复合材料在交变温度场作用下热应力的有限元分析

对 60 61Al/ Si C层合复合材料在交变温度场作用下的热应力进行数值分析。采用ANSYS有限元分析软件中的结构单元 ,将金属铝视为弹塑性材料 ,且采用 Mises随动强化塑性模型 ,同时计及温度对材料性能的影响 ,计算了不同温度下的残余塑性变形和热应力 ,并给出了2 0 5℃至 2 0℃交变温度场作用下的残余热应力循环曲线 ,数值计算结果与实验数据复合较好。本文的研究工作为该复合材料的疲劳寿命的预报提供良好的理论基础。     

复合材料浅球壳模态分析与结构优化设计

复合材料层合壳体在航空、航天、工程结构中得到了广泛应用.结合复合材料层合浅球壳的结构特点,基于有限元分析软件ANSYS,采用层合壳单元建立有限元模型,分析了几种壳体参数对浅球壳自振频率的影响,并采用函数逼近法和梯度寻优法相结合的方法对壳体参数进行优化设计,给出了壳体参数的最优组合,使壳体的一阶自振频率为最大,改善了壳体的动态特性,为复合材料层合浅球壳的结构设计提供了有价值的理论依据,也为进一步进行结构动力学分析奠定基础.     

6061Al／SiC层合复合材料在交变温度场作用下热应力的有限元分析

对６０６１Ａｌ／ＳｉＣ层合复合材料在交变温度场作用下的热应力进行数值分析。采用ＡＮＳＹＳ有限元分析软件中的结构单元，将金属铝视为弹塑性材料，且采用Ｍｉｓｅｓ随动强化塑性模型，同时计及温度对材料性能的影响，计算了不同温度下的残科塑性变形和热应力，并给出了２０５℃至２０℃交变温度场作用下的残余应力循环曲线，数值计算结果与实验数据复合材料好。本文的研究工作为该复合材料的疲劳寿命的预报提供良好的理论基础。     

剪切型压电层合圆柱壳的精确解

对剪切型压电层合圆柱壳在电场和机械力作用下的应力场、位移场和电势分布进行了分析。在每层的局部坐标系下对该层的应力场、位移场和电势进行傅立叶展开, 然后采用状态空间方法得到变系数的8 维一阶常微分方程组, 采用Frobenius 方法得到应力场、位移场和电势的解。数值算例显示了电场和机械力作用下, 壳体厚度对应力场和位移场分布的不同影响。得到的3 维精确解为其他剪切型压电弹性层合壳的壳体理论提供理论依据和验证。      

平板式SOFC结构热应力的有限元分析

采用有限元数值计算方法, 对平板式固体氧化物燃料电池(SOFC)的结构建立了三维有限元分析模型, 模拟计算了平板式SOFC单电池在均匀温度场中由于各层部件之间的热膨胀系数差异而产生的热应力, 并对模拟结果进行了分析和讨论, 为优化平板式SOFC的材料选择和结构设计提供了依据. 计算结果表明: 在阳极(或阴极)与电解质界面处出现热应力的最大值; 界面热应力的大小及分布与电极材料的热膨胀系数和温度载荷密切相关.     

旋转周期复合材料层合结构的有限元屈曲分析

针对空间结构中常见的复合材料层合壳体结构发展了一种多层相对自由度层合壳元。这种实体型壳元既可以用较粗的网格很好地模拟层合壳, 又易与三维实体单元相连接, 使变厚度、带有补强的复合材料层合壳体等复杂结构得以正确建模。同时运用旋转周期有限元技术对大规模的空间复合材料层合结构成功实施了屈曲分析。数值算例验证了本文计算策略的有效性。      

稳态温度场下轴对称梯度功能材料的热应力分析

本文基于热弹性理论及计算数学方法，对轴对称梯度功能材料内部各点的热应力进行了理论分析。提出了稳态温度场下，环状截面梯度功能柱体的温度分布与热应力计算公式。     

水冷壁管梯度陶瓷涂层的设计与性能研究

针对燃煤电厂锅炉水冷壁爆管问题,研究设计了一种热膨胀系数梯度变化的涂层结构.为了提高涂层与基体的结合强度,计算了涂层及管壁的温度场及热应力场的分布.结果表明:涂层与管壁的界面处热应力最大,并且热应力与涂层的热膨胀系数成正比,通过调整涂料各成分的含量来改变各层的热膨胀系数;采用双层结构梯度涂层,理论计算表明界面处的最大热应力为1.78 MPa;而通过拉伸力学性能测试得出涂层的平均抗剪切强度为5.60 MPa,是界面处最大热应力值的3倍左右.因此,在管壁表面喷涂功能梯度涂层,能够缓和涂层与基体界面间的热应力.     

基于三维有限元法的层合圆柱壳应力分析

针对空间结构中常见的蜂窝夹芯壳体提出了一种32节点相对自由度三层壳元,以及一种精确计算层间应力的后处理方案。这种32节点壳元可以更好地反映结构固有的特性,易与三维实体单元相连接,使变厚度、带有补强的蜂窝夹芯复合材料壳体等复杂结构问题得以正确建模。本文作者的后处理方案克服了位移有限元层间应力不连续的缺点,保证了应力精确满足边界条件。综合运用以上方法的典型算例表明:计算精度是令人满意的。     

Postbuckling of functionally graded fiber reinforced composite laminated cylindrical shells, Part I: Theory and solutions

Buckling and postbuckling behavior are presented for fiber reinforced composite (FRC) laminated cylindrical shells subjected to axial compression or a uniform external pressure in thermal environments. Two kinds of fiber reinforced composite laminated shells, namely, uniformly distributed (UD) and functionally graded (FG) reinforcements, are considered. The governing equations are based on a higher order shear deformation shell theory with von Kármán-type of kinematic non-linearity and including the extension-twist, extension-flexural and flexural-twist couplings. The thermal effects are also included, and the material properties of FRC laminated cylindrical shells are estimated through a micromechanical model and are assumed to be temperature dependent. The non-linear prebuckling deformations and the initial geometric imperfections of the shell are both taken into account. A singular perturbation technique is employed to determine the buckling loads and postbuckling equilibrium paths of FRC laminated cylindrical shells.     

A simulation study on the thermal buckling behavior of laminated composite shells with embedded shape memory alloy (SMA) wires

In this paper, numerical simulation analyses of the thermal buckling behavior of laminated composite shells with embedded shape memory alloy (SMA) wires were performed to investigate the effect of embedded SMA wires on the characteristics of thermal buckling. In order to simulate the thermomechanical behavior of SMA wires, the constitutive equation of the SMA wires was formulated in the form of an ABAQUS user subroutine. The computational program was verified by showing the response of the pseudoelasticity and shape memory effect (SME) at various temperatures and stress levels. Modeling of the laminated composite shells with embedded SMA wires and thermal buckling analyses were performed with the use of the ABAQUS code linked with the subroutine of the formulated SMA constitutive equations. The thermal buckling analyses of the composite shells with embedded SMA wires show that the critical buckling temperature can be increased and the thermal buckling deformation can be decreased by using the activation force of embedded SMA wire actuators.     

Interlaminar shear stresses in laminated composite plates under thermal and mechanical loading

The combined effects of thermal and mechanical loadings on the distribution of interlaminar shear stresses in composite laminated thin and moderately thick composite plates are investigated numerically using the commercially available software package MSC NASTRAN/PATRAN. The validity of the present finite element analysis is demonstrated by comparing the interlaminar shear stresses evaluated using the experimental measurement. Various parametric studies are also performed to investigate the effect of stacking sequences, length to thickness ratio, and boundary conditions on the interlaminar shear stresses with identical mechanical and thermal loadings. It is observed that the effect of thermal environment on the interlaminar shear stresses in carbon-epoxy fiber-reinforced composite laminated plates are much higher in asymmetric cross-ply laminate and anti-symmetric laminate compared to symmetric cross-ply laminate and unidirectional laminate under identical loadings and boundary conditions.     

Stress analysis of imperfect composite laminates with an interlaminar bonding theory

Delamination is a major damage mode in laminated composites since it can cause severe structural degradation. Based on an interlaminar shear stress continuity theory and a linear shear slip theory, a so-called Interlayer Shear Slip Theory was presented in a previous study. This theory was verified to be feasible for shearing-mode delamination analysis. However, in order to account for opening-mode delamination in laminated composites, the continuity of interlaminar normal stress and the modelling of normal separation on the composite interface should also be considered. The present study gives a complete discussion on the Interlaminar Bonding Theory. The effects of interlaminar bonding condition on the laminate deformation and stress distribution are also presented. It is concluded from numerical results that the present theory is suitable for analysis of composite laminates with imperfect interfaces.     

基于混合分层理论的粘弹层合圆柱壳的阻尼振动和层间应力研究

应用混合分层理论,并在壳厚方向采用位移和应力插值函数推导出粘弹层合圆柱壳的动力学方程。计算了两层粘弹层合圆柱壳的振动频率和结构损耗因子,与Okazaki A的结果吻合良好。针对不同模量和厚度的粘弹性材料,计算出层合圆柱壳的层间横向应力的幅值。结果表明:过大的层间法向正应力是导致自由阻尼层合壳脱层破坏的主要因素,采用环向加强的粘弹性材料将有效地降低层间法向正应力的幅值。     

Effects of layup sequences on stresses of thick composite cantilever tubes

In this study, a new simple-input displacement-based method is used to study effects of layup sequences on stresses, strains, and deformations of thick laminated orthotropic cantilever straight tubes under transverse loading. Three-dimensional stress distributions are obtained based on the most general displacement field of elasticity. A layer-wise theory which includes the full three-dimensional constitutive relations is used. A non-dimensional simple coefficient is introduced to compare interlaminar radial stresses of different layup sequences. Finally, some design guidelines are proposed to consider effects of layup sequences of laminated thick composite tubes subjected to shearing load.     

Evaluation of different advanced finite element concepts for detailed stress analysis of laminated composite structures

Despite their high specific stiffness and strength, laminated composite materials, e.g. fibre-reinforced plastic plies stacked at different fibre orientations, are susceptible to damage. Damage can be divided into interalaminar damage and interlaminar damage. Delamination is a typical kind of interlaminar damage which occurs in laminated composite materials, often accompanied with intralaminar damage, and may lead to a catastrophic structural collapse. The first and most crucial step in the prediction of failure of Laminated Composite Structures (LCS) is to accurately determine the stresses, particularly the three transverse stress components, also called the interlaminar stresses. It is proposed in the present paper that the integration of a displacement based solid-shell formulation and partial-hybrid stress formulation will lead to an accurate and robust solid-shell element, suitable for the efficient and detailed interlaminar stress calculation.     

Effect of damage levels and curing stresses on delamination growth behaviour emanating from circular holes in laminated FRP composites

This paper deals with the study of the effect of drilling induced delamination damage levels and residual thermal stresses (developed during manufacturing process of cooling the laminate form curing temperature to room temperature) on delamination growth behaviour emanating form circular holes in graphite/epoxy laminated FRP composites. Two sets of full three dimensional finite element analyses (one with the residual thermal stresses developed while curing the laminate and the other without residual thermal stresses i.e. with mechanical loading only) have been performed to calculate the displacements and interlaminar stresses along the delaminated interfaces responsible for the delamination onset and propagation. Modified crack closure integral (MCCI) techniques based on the concepts of linear elastic fracture mechanics (LEFM) have been used to calculate the distribution of individual modes of strain energy release rates (SERR) to investigate the interlaminar delamination initiation and propagation characteristics. Asymmetric variations of SERR obtained along the delamination front are caused by the overlapping stress fields due to the coupling effect of thermal and mechanical loadings. It is found that parameters such as ply orientation, drilling induced damage levels and material heterogeneity at the delaminated interface dictate the interlaminar fracture behaviour of laminated FRP composites.     

A mixed solid‐shell element for the analysis of laminated composites

This paper presents a versatile low order locking‐free mixed solid‐shell element that can be readily employed for a wide range of linear elastic structural analyses, that is, from thick isotropic structures to multilayer anisotropic composites. This solid‐shell element has eight nodes with only displacement degrees of freedom and few assumed stress parameters that provide very accurate interlaminar stress calculations through the element thickness. These elements can be stacked on top of each other to model multilayer structures, fulfilling the interlaminar stress continuity at the interlayer surfaces and zero traction conditions on the top and bottom surfaces of the laminate. The element formulation is based on the well‐known Fraeijs de Veubeke–Hu–Washizu mixed variational principle with enhanced assumed strains formulation and assumed natural strains formulation to alleviate the different types of locking phenomena in solid‐shell elements. The distinct feature of the present formulation is its ability to accurately calculate the interlaminar stress field in multilayer structures, which is achieved by the introduction of a constraint equation on the interlaminar stresses in the Fraeijs de Veubeke–Hu–Washizu principle‐based enhanced assumed strains formulation. The intelligent computer coding of the present formulation makes the present element appropriate for a wide range of structural analyses. To assess the present formulation's accuracy, a variety of popular numerical benchmark examples related to element convergence, mesh distortion, and shell and laminated composite analyses are investigated and the results are compared with those available in the literature. These benchmark examples reveal that the proposed formulation provides very good results for the structural analysis of shells and multilayer composites. Copyright © 2011 John Wiley & Sons, Ltd.