大、中等质量冲头冲击下支持跨度对复合材料层压板冲击的影响分析

基于ABAQUS软件建立三种支持跨度的层压板三维有限元模型,分别计算在两种质量冲头冲击下的响应和损伤。结果表明,在判断支持跨度对冲击结果的影响时,要考虑冲头质量与板质量的比;在大质量冲头冲击的情况下,层压板的响应呈现准静态特征,层压板支持跨度的变化对,最大接触力、冲击持续时间、分层损伤面积等参数的影响较为简单明确;对于中等质量冲头情况,冲击接触时间减小,层压板振动明显,导致冲头与层压板在冲击过程中会脱离接触。在这种情况下不同跨度下板的响应差异也较为明显,冲击力的大小和损伤情况要依据具体的分析。从冲击接触时间和层压板固有振动周期的角度分析了冲头质量水平对冲击响应的影响。     

含冲击损伤复合材料层压板压缩破坏机制的声发射特性研究

复合材料层压板在压缩破坏过程中包含了丰富的声发射信息.为了研究含冲击损伤的复合材料层压板的压缩破坏机制,采用声发射观察层压板的压缩破坏过程,通过分析声发射信号的特征规律,表征了在压缩载荷下材料损伤的形式及其演化过程.结果表明:通过对声发射参数(撞击计数、能量、幅值、事件位置)和载荷曲线进行综合分析,发现损伤的发展过程经过了初始阶段、平稳扩展期和断裂阶段,冲击造成的分层区域最先出现屈曲并最早破坏;在损伤初始阶段和平稳扩展期间,损伤是一种渐进式的增长,层压板具有一定的承载能力;在断裂阶段损伤快速扩展,层压板的承载能力迅速下降,在出现纤维密集断裂的现象后整体破坏.     

飞机复合材料层压板结构中的广义强度准则

通过对经典复合材料结构强度理论和准则在层压板上的适用性计算和试验对比研究,指出了在层压板上采用经典强度理论所得计算结果与试验结果的差异。并分析了产生这种差异的原因,主要是经典强度准则表达武中采用的无拉剪耦合假设和层与层之间理想粘接假设,只适用于单层板或正交各向异性板。本文提出了一种广义强度准则的思想,其核心是放松正交各向异性约束条件和通过试验测定强度参数,而在实际飞机结构中的准正交各向异性层压板结构设计上仍可使用经典层压板强度准则。     

复合材料层压板低速冲击响应尺度效应数值模拟研究

为了研究尺度效应对于复合材料层压板在低速冲击作用下的动态响应和冲击损伤的影响,基于相似理论,建立了三种不同尺寸的层压板受冲击的三维有限元模型。在该模型中,针对层压板的面内损伤,采用改进的Chang-Chang准则进行预测;针对层压板内层间分层损伤,则使用Cohesive界面单元进行模拟。一旦复合材料层压板在低速冲击作用下产生损伤,则对出现损伤的区域进行材料参数退化。采用该模型对三种不同尺寸的层压板的冲击过程进行有限元分析,并将不同冲击速度下的冲击响应进行比较,得出了如下结论：在层压板内未发生冲击损伤时,冲击产生的挠度和冲击力与相似理论解十分吻合,一旦出现冲击损伤,则冲击力的变化与相似理论解有所差别;如果两个缩放模型的冲击速度之比等于缩放比例的平方根,则两个模型中的相对分层尺寸基本是相同的,这个结果与已有的实验结果吻合;而对冲击后面内损伤的分析表明,其损伤尺寸不符合这一相似规律。     

孔隙率对复合材料层压板层间剪切力学性能影响研究

针对两种复合材料典型铺层,以试验的方式对4种不同孔隙率级差的层压板结构的层间剪切力学性能进行研究。分别对层压板结构的0°和90°方向取样,测试了层压板结构的0°和90°方向的层间剪切力学性能。基于试验结果量化分析了孔隙率与层压板结构层间剪切力学性能之间的关系。分析发现,在孔隙率为1.7%～2.3%之间,层间剪切强度下降最快;取样方向对试验件的层间剪切强度影响很小;厚度越大,层间剪切强度受孔隙率的影响越小。     

复合材料层压板低速冲击响应与损伤参数关系研究

论文针对复合材料标准冲击试样,用落锤试验手段研究了三种不同铺层层压板的低速冲击特性.分析了冲击响应接触力和能量的基本特性及变化规律,发现冲击能量相对于层压板阻抗水平的不同会导致接触力-时间曲线形状的差异.从冲击总能量、吸收能量和损伤耗散能量的角度阐释了能量和接触力与层压板分层损伤、凹坑变形的关系.发现损伤直接取决于吸收能量,与冲击总能量有较好的正相关关系.最大接触力是表征层压板损伤阻抗性能的一个门槛值.在一定能量条件下进行冲击,峰值接触力若达到层压板的接触力门槛值,则峰值接触力不再随损伤程度而增加,不能继续作为损伤的表征参数.同时,进行了冲击过程动态有限元模拟,得到的冲击响应参数曲线和损伤结果与试验结果有较好的对应关系.     

考虑尺寸效应的双材料轴对称界面端应力奇异性

提出了一种分析双材料轴对称界面端的应力奇异行为的特征值法.基于弹性力学空间轴对称问题的基本方程和一阶近似假设,利用分离变量形式的位移函数和无网格算法,导出了关于应力奇异性指数的离散形式的奇异性特征方程.由奇异性特征方程的特征值和特征向量,即可确定应力奇异性指数、位移角函数和应力角函数.数值求解了纤维/基体轴对称界面端模型的奇异性特征方程, 结果表明:尺寸效应参数δ(奇异点与轴对称轴的距离和应力奇异性支配区域大小的比值)影响着应力奇异性的强弱与阶次, 准一阶近似解析解只是δ>>1时的一个特例.      

基于近场动力学理论的层压板损伤分析方法

提出了一种基于近场动力学理论 的纤维增强复合材料层压板的渐进损伤分析方法.在弹性力学和复合材料力学的基础上, 推导了适用于近场动力学建模的微模量和临界伸长率等基本参量, 结合经典层压板理论中的偏轴模量, 构建了适用于各向异性材料的对点力函数, 可分析3种形式的损伤:纤维断裂, 基体开裂和分层破坏.分析了含圆孔层压板在拉伸载荷作用下的破坏过程, 预测结果与试验结果吻合良好.     

影响双材料界面端三维应力奇异性的几何因素研究

应用常规有限单元分析技术,对几种典型接头形式的三维双材料结构界面端点附近应力奇异性进行了研究,重点分析了棱角(两自由平面的夹角)大小对界面端点附近应力奇异性指数的影响。数值分析结果表明:棱角大小对界面端应力奇异性指数有明显影响,棱角越大,奇异性指数越小;当棱角趋于180°时,端点附近的应力奇异性指数收敛于界面端线上的值(等于平面应变条件下的理论值)。研究发现,如果采用圆弧对三维双材料结构的棱边进行倒角,使相应的界面端线变成光滑连续曲线,则原界面端点附近的应力奇异性会完全退化为界面端线附近的应力奇异性,即界面端点独特的应力奇异性现象消失。     

电致伸缩材料内电极附近应力奇异性研究

应用复变函数的方法,研究电致伸缩材料内置电极附近的应力奇异性.基于精确的电边界条件,采用Hilbert理论以及复变函数中的Cauchy积分与留数定理,首先分别给出了柔性电极和刚性电极的复势函数解,然后就这两种极限情况,讨论了电极刚度对应力场奇异性的影响.研究结果表明:无论对柔性电极还是刚性电极,Max-well应力的应力场均呈现r-1阶的奇异性,但对于前者总应力奇异性系数为零,而对于后者总应力奇异性系数与基体的材料常数有关.     

含椭圆孔非对称层合板的Green函数

运用作者建立的复合材料层合板问题的求解方法，得到了含椭圆孔和裂纹的无限大非对称层合板在广义集中载荷作用下的Ｇｒｅｅｎ函数．利用这些结果可研究含孔或裂纹的非对称层合板在集中载荷作用下的力学行为，还可结合边界元方法对复杂层合板结构建立有效的数值分析方法     

Stress analysis of finite composite laminate with multiple loaded holes

Based on the classical laminated plate theory, a finite composite plate weakened by multiple elliptical holes is treated as an anisotropic multiple connected plate. Using the complex potential method in the plane theory of elasticity of an anisotropic body, an analytical study concerned with the stress distributions around multiple loaded holes in finite composite laminated plates subjected to arbitrary loads was performed. The analysis makes use of the Faber series expansion, conformal mapping and the least squares boundary collocation techniques. The effects of plate and hole sizes, layups, the relative distance between holes, the total number of holes and their locations on the stress distribution are studied in detail. Some conclusions are drawn.     

Alleviation of the stress concentration with analogue reinforcement

In order to reduce the stress concentration around a hole in a plate, new, “analogue” reinforcements instead of reinforcing rings were used in this investigation. In two of these specimens, reinforcements with different volume fractions were arranged to coincide with the stress trajectories for an infinite plate with a hole under uniaxial tension. Two other specimens containing straight rectangular-grid-type reinforcements were made by using a photofabrication method. Specimens were then prepared by sandwiching these reinforcements between two epoxy-resin plates. Plane specimens, i.e., without reinforcement, were also made of the same epoxy resin for comparison. The stress concentrations at the edge of the hole under uniaxial tension were determined by photoelastic techniques. The measured stress-concentration factors were compared with well-known values for an infinite, isotropic, homogeneous plate containing a hole. Results were also compared with published data on [90/0/90/0]_s 7-ply laminated composite plates, and on plates strengthened with reinforcing rings. A definite reduction in stress concentration was observed on specimens containing analogue reinforcement.     

A STUDY OF RESIDUAL STRENGTH OF SISAL TEXTILE REINFORCED VINYL ESTER

In this study, the residual strength of sisal textile reinforced vinyl ester resin is studied using specimens with a central hole. Two kinds of chemicals, silane and permanganate, are used to treat sisal fibre surfaces. The effects of fibre surface treatments on the residual strength of sisal fibre reinforced composites with different central hole sizes are analysed. Optical microscopy （OM） surveys provide sound evidence for the relationship between residual strength properties and fracture morphologies of sisal textile reinforced vinyl ester. Several theoretical models used to predict the residual strength of laminated composites are briefly reviewed. Point stress criterion （PSC） and average stress criterion （ASC） models are used to analyze the mechanical properties of sisal textile reinforced polymers with a central hole in this research. Material constant, characteristic length （d0 or lc）, is obtained and used to analyze the mechanical behavior of the composites. The residual strength of the composites predicted by PSC and ASC models is in good agreement with the experimental results..     

Micromechanics-based constitutive modeling for unidirectional laminated composites

A micromechanics-based constitutive model is developed to predict the effective mechanical behavior of unidirectional laminated composites. A newly developed Eshelby’s tensor for an infinite circular cylindrical inclusion [Cheng, Z.Q., Batra, R.C., 1999. Exact Eshelby tensor for a dynamic circular cylindrical inclusion. J. Appl. Mech. 66, 563–565] is adopted to model the unidirectional fibers and is incorporated into the micromechanical framework. The progressive loss of strength resulting from the partial fiber debonding and the nucleation of microcracks is incorporated into the constitutive model. To validate the proposed model, the predicted effective stiffness of transversely isotropic composites under far field loading conditions is compared with analytical solutions. The constitutive model incorporating the damage models is then implemented into a finite element code to numerically characterize the elastic behavior of laminated composites. Finally, the present predictions on the stress–strain behavior of laminated composite plate containing an open hole is compared with experimental data to verify the predictive capability of the model.     

Stiffened Composite Panels with a Stress Concentrator Under in-Plane Compression

The in-plane compressive strength of a stiffened thin-skinned composite panel with a stress concentrator is examined. Two possible in-plane failure mechanisms are investigated. The first one is near-surface instability at the edge of the cutout, where high stress gradients are expected because of the stress concentration and the thickness heterogeneity of the laminated skin. Analytical 3D formulas allowing simple parametrical investigation of the phenomenon under consideration are derived. The second failure mechanism is fiber microbuckling in 0°-plies. An equivalent crack model is used to predict the compressive strength of a multidirectional composite laminate. How a stiffener affects the compressive strength of the thin-skinned panel with a hole is studied for both mechanisms. Experimental and predicted values of the critical load are in good agreement.     

新修正偶应力理论Reddy型层合板稳定分析

基于新修正偶应力理论建立了一个Reddy型复合材料层合板稳定性模型。该理论中曲率张量不对称,而偶应力矩张量对称。Reddy型层合板模型能够满足横向剪切应力为0的自由表面条件,而且横向剪切为二次函数,避免了常剪力一阶理论需要引入的剪力修正系数。为了便于工程应用,通过虚功原理推导了只含纤维材料尺度参数正交铺设的Reddy型层合板偶应力模型的稳定性方程,并以微尺度正交铺设四边简支层合方板为例,分析了不同铺设角和轴向载荷作用时临界载荷的细观尺度效应,并且与一阶剪切变形和Kirchhoff板理论结果对比。结果表明,本文建立的新修正偶应力Reddy型层合板模型更适合分析较厚的复合材料层合板稳定性的尺度效应。     

含圆孔复合材料层板在拉伸载荷下损伤破坏过程的模拟

本文应用有限元法模拟受拉含孔夏合材料层板损伤起始、累积直至破坏的过程。在模拟中引入了作者早些时建立的包含基体开裂层非对称约束影响的损伤本构关系，以及裂纹密度与损伤区应力关系函数等。本文给出损伤累积模拟方法并编制了程序，用该程序可以获得层板加载过程中各层的损伤状态、计算其刚度衰减和应力重分布以及最终破坏载荷。给出了数值模拟算例并与现有研究结果进行了比较。     

基于新修正偶应力理论的Mindlin层合板稳定性分析

基于新的各向异性修正偶应力理论提出一个Mindlin复合材料层合板稳定性模型。该理论包含纤维和基体两个不同的材料长度尺度参数。不同于忽略横向剪切应力的修正偶应力Kirchhoff薄板理论,Mindlin层合板考虑横向剪切变形引入两个转角变量。进一步建立了只含一个材料细观参数的偶应力Mindlin层合板工程理论的稳定性模型。计算了正交铺设简支方板Mindlin层合板的临界载荷。计算结果表明该模型可以用于分析细观尺度层合板稳定性的尺寸效应。     

Thickness dependence of viscoelastic stress relaxation of laminated microbeams due to mismatch strain

Time-dependent behaviors due to various mismatch strains are very important to the reliability of micro-/nano-devices. This paper aims at presenting an analytical model to study the viscoelastic stress relaxation of the laminated microbeam caused by mismatch strain. Firstly, Zhang’s two-variable method is used to establish a mechanical model for predicting the quasi-static stress relaxation of the laminated microbeam. Secondly, the related analytical solutions are obtained by combining the diffe...