纤维角度对变角度层合板减振性能的影响

自动铺丝技术可以实现复杂曲率曲线铺放,可极大提高角度设计的自由度。本文以改善复合材料层合板动态特性为目的,对变角度层合板的减振性能进行了研究分析。首先对不同角度变化变角度层合板进行自由衰减试验,研究了纤维角度变化与变角度层合板阻尼比的关系。然后对含相应角度变角度夹层板进行随机试验,研究了层合板随机激励条件下的振动响应,并采用共振峰处传递函数(Transition function,TF)和拾振点加速度总均方根(Root mean square,RMS)两种指标评价减振效果。结果表明:层合板阻尼比在纤维变化角度为±<45|60>时最大,纤维变化角度为±<73|88>时最小。基于RMS减振评价指标,±<45|60>夹层板较传统直线板减振性能提高27.13%;基于共振峰TF减振评价指标,纤维角度变化对不同共振峰减振效果规律差异明显。研究表明,变角度层合板减振性能明显优于传统直线层合板,相关实验结果将对变角度层合板减振设计及优化提供一定的参考意义。      

考虑纤维面外起伏的变刚度层合板优化策略研究

针对变刚度层合板在自动铺放制造过程中因间隙/重叠缺陷产生大量纤维面外起伏缺陷的问题,提出采用铺层偏移法与断送纱策略两种铺层优化策略来进行变刚度层合板的铺层设计,在研究过程中同时引入考虑间隙/重叠缺陷建模的方法。根据变刚度层合板铺层的特点提出缺陷重复单元的概念,通过对缺陷重复单元的分析来反映纤维面外起伏的影响,并提出通过纤维面外系数来表征变刚度层合板的纤维面外起伏尺度,最后对不同优化策略的变刚度层合板的屈曲性能进行分析。研究表明:基准设计方案、铺层偏移法与断送纱策略所对应的纤维面外起伏系数为0.83、0.95、0.93,所提出的优化策略对变刚度层合板的纤维面外起伏尺度有着明显的抑制作用。铺层偏移法优化后的[±<50/65>]_6s变刚度层合板最大厚度超差为33%,所对应的屈曲载荷为9117.1 N,屈曲载荷提升17.6%;断送纱策略优化后的[±<50/65>]_6s变刚度层合板最大厚度超差为50%,所对应的屈曲载荷为9716.3N,屈曲载荷提升25.3%。      

Delamination growth in composite laminates of variable stiffness

The production of new composite laminates with variable stiffness within the surface of plies was enabled by tow‐placement machines. Because of the variation of stiffness, these materials are called variable stiffness composite laminates (VSCL). Recently, many attempts were made to investigate their structural behaviour. In this contribution, a first‐order shear deformation theory is selected to model the multilayered composite laminates. The adopted theory is enhanced by the extended finite element method (XFEM) to describe discontinuities at element level of any interface of interest. To predict the location of the delamination onset, a traction–separation law is developed that is consistent with the XFEM topology. An exponential softening behaviour is implemented within the interface to model the delamination growth in a mixed‐mode direction. In order to solve the non‐linear equations of the delamination propagation, an arc‐length method is applied. The effect of the curvilinear fibre orientation on the location of the delamination onset is investigated. Subsequently, the structural response of the laminates is computed. According to the simplicity of the new approach using the XFEM; and based on the computational cost for calculating the stiffness of VSCL, the method is able to determine structural response of VSCL with less computational effort. Copyright © 2016 John Wiley & Sons, Ltd.     

丝束变角度层合板屈曲性能的参数化研究

通过选择合适的纤维轨迹,丝束变角度(VAT)层合板相对于直纤维层合板能拥有更好的抗屈曲特性。为研究纤维轨迹特征长度和定向坐标系偏角对VAT层合板屈曲性能的影响,首先对原始的纤维角度线性变化方法进行改进,提出了一种纤维角度分段线性变化方法,拓展了纤维轨迹的设计空间;其次,采用改进后的纤维轨迹定义方法构建了一系列变刚度层合板;最后,基于有限元方法,从内力分布角度对变刚度层合板不同承载情况下的屈曲性能进行研究和探讨。数值结果表明:单向轴压工况下,采用半边长的特征长度和90°偏角的纤维轨迹,能使层合板的稳定性最好;双向轴压工况下,应将特征长度和定向坐标系偏角作为额外的设计变量,并通过优化获得最优的纤维轨迹。     

基于自动铺放技术的高精度变刚度复合材料层合板屈曲性能

基于自动铺放技术制备的曲线变刚度复合材料层合板,通过定制面内刚度,可有效提高结构的抗屈曲性能。在铺放过程中,铺放轨迹的路径规划是实现变刚度设计的关键技术之一。鉴于此,本文分别以纤维角度线性变化曲线、等曲率曲线及二次Bezier曲线构成的纤维轨迹为研究对象,对其压缩屈曲性能进行参数化分析。并利用有限元模型研究了铺丝头上丝带宽度对层合板型面精度和抗屈曲力学性能的影响。结果表明:在压缩工况下,二次Bezier曲线路径的抗屈曲性能最佳,等曲率曲线路径受曲率约束的影响最小。铺丝头丝束宽度一定,丝带宽度与重叠区域面积和抗屈曲性能呈负相关。使用最大的丝带宽度可最大程度地减小重叠区域面积,提高结构的型面精度,同时保证结构屈曲性能提高37.3%。      

Natural modes of vibration of variable stiffness composite laminates with curvilinear fibers

In this paper, natural frequencies and vibrational mode shapes of variable stiffness composite laminate (VSCL) plates with curvilinear fibers are studied. In each ply of this rectangular VSCL, the fiber-orientation angle changes linearly with respect to the horizontal coordinate. To define the modes of vibration of the laminates, a new p-version finite element, which follows third-order shear deformation theory (TSDT), is employed. The convergence properties of this new element are investigated. Taking manufacturing restrictions regarding the fiber curvatures into account, maps of natural frequencies as functions of tow-orientation angles are determined in demonstrative examples. It is verified that the use of curvilinear fibers instead of the traditional straight fibers introduces a greater degree of flexibility, which can be used to adjust frequencies and mode shapes.     

铺陈角度对芳纶/玻璃纤维层合板抗冲击性能的影响

采用Abaqus软件建立了圆锥形子弹正冲击芳纶/玻璃纤维复合材料层合板的有限元模型,将模拟结果与文献中的实验结果相比较验证了模型的可靠性,进而研究子弹以不同的速度对不同铺陈角度下的复合材料层合板冲击后初始速度与剩余速度的关系以及层合板的破坏特征。结果表明:当层合板铺陈角度不变且子弹击穿层合板时,子弹初始速度与剩余速度接近于线性关系;在子弹未穿透层合板时,[0/90°]铺陈角度的层合板抗弹性能最好,在子弹以600~900 m/s的较高速度穿透层合板时,[45/-45°]铺陈角度的层合板吸能效果最好;由破坏特征图表明铺陈角度对层合板的损伤面积和破坏机制影响不大。该研究可为防护装备的设计和优化提供参考。     

Optimum laminate design for strength and stiffness

A method is presented for minimum weight optimum design of symmetric fibre-composite laminates subject to multiple in-plane loading conditions which takes into account membrane stiffness requirements and strength limitations. The problem is cast as a non-linear mathematical programming problem in which the thicknesses of material placed at preassigned orientation angles are treated as the only design variables. The non-linear programming formulation is transformed into a sequence of linear programs employing an adaptation of the method of inscribed hyperspheres in which only critical and near critical constraints are considered at each stage in the procedure. Example applications illustrate that the method presented offers an efficient and practical optimum design procedure for the fundamental and recurring problem treated.     

丝束变角度层合板屈曲性能的有限元分析

连续丝束剪切（CTS）作为一种新的丝束变角度（VAT）铺放技术,能克服传统的自动铺丝技术（AFP）的诸多缺陷。鉴于此,分别采用CTS和AFP这2种技术铺设而成的VAF层合板为研究对象,对其屈曲性能进行了有限元分析;在施加相同端部轴向位移的条件下,以铺层顺序[90±<0|75>]_4s为例,对比分析了2种层合板的内力分布;最后,讨论了丝束最小转弯半径对2种铺放技术的影响。结果表明：CTS层合板因其变厚度的特性,其屈曲性能优于AFP层合板;与AFP技术受制于最小转弯半径,导致其在小尺寸结构中的应用受到限制相比,CTS技术适用于各种尺寸的层合板。     

Bezier曲线变角度层合板设计及屈曲特性分析

基于二次Bezier曲线法,开展纤维变角度层合板的屈曲特性研究。首先,对原始的二次Bezier曲线进行了扩展,提出了分段式二次Bezier曲线变化方法,重新定义了纤维变角度铺放参考路径,确定了变角度层合板的表示形式。其次,以[0±20(β)65]_(2s)和[90±20(β)65]_(2s)为例对变角度层合板的屈曲特性进行了有限元分析,并与定角度层合板对比。最后,研究了不同终止角α和连接点参数β对层合板屈曲性能的影响,结果表明:连接点参数β的增大使得两种变刚度层合板的线性屈曲载荷先增大后减小,终止角大于起始角时,层合板的一阶屈曲载荷逐渐增大,铺放角度的增大有利于层合板屈曲载荷的提高。     

基于自动铺放技术的变刚度复合材料层合板固化变形特性

准确预测和控制变刚度结构的固化变形是合理进行变刚度设计的关键环节,复合材料结构的固化变形不仅会影响结构的刚度强度特性、同时会对结构的装配性能产生影响。基于自动铺放技术,提出面向过程的丝束-路径-面板多级三维变刚度有限元模型算法。结合Kamal自催化固化动力学模型和广义Maxwell黏弹性本构模型进行热-化学-力多场耦合分析,计算固化过程中结构内部温度场、固化度场和残余应力场的变化,最终得到变刚度层合板的固化变形特性。通过参数化分析,研究了自动铺放过程中轨迹控制参数T₀、T₁和覆盖法则对结构固化变形产生的影响。研究结果表明：T₀=45°时,当T₁0变刚度结构的固化变形随着T₁的增大而增大,当T₁>T₀,变刚度结构的固化变形随着T₁的增大而减小。100%覆盖法则有效减小结构的固化变形,而0%覆盖法则使固化变形增大。本文提出的方法可以有效预测工艺参数对变刚度结构固化变形的影响。     

Optimization of course locations in fiber-placed panels for general fiber angle distributions

Fiber-reinforced composites are usually designed using constant fiber orientation in each ply. In certain cases, however, a varying fiber angle might be favorable for structural performance. This possibility can be fully utilized using tow placement technology. Because of the fiber angle variation, tow-placed courses may overlap and ply thickness will build-up on the surface. This thickness buildup affects manufacturing time, structural response, and surface quality of the finished product.     

Adjoint-based optimal variable stiffness mesh deformation strategy based on bi-elliptic equations

There are many recent advances in mesh deformation methods for computational fluid dynamics simulation in deforming geometries. We present a method of constructing dynamic mesh around deforming objects by solving the bi-elliptic equation, an extension of the biharmonic equation. We show that introducing a stiffness coefficient field a(x) in the bi-elliptic equation can enable mesh deformation for very large boundary movements. An indicator of the mesh quality is constructed as an objective function of a numerical optimization procedure to find the best stiffness coefficient field a(x). The optimization is efficiently solved using steepest descent along adjoint-based, integrated Sobolev gradients. A multiscenario optimization procedure is performed to calculate the optimal stiffness coefficient field a^蜧(x) for a priori unpredictable boundary movements. Copyright © 2011 John Wiley & Sons, Ltd.     

变刚度复合材料开孔板拉伸行为数值模拟及试验验证

纤维曲线铺放是提高复合材料构件力学性能的有效方法之一。本文针对复合材料开孔板铺放轨迹进行了研究,利用B样条曲线插值拟合获取了开孔板最大主应力铺放轨迹,并通过离散网格法建立了变刚度开孔板模型,通过引入Tsai-Wu损伤失效判据以及常刚度退化准则,进行了拉伸失效数值模拟及损伤失效分析,并分别铺放了两组常刚度和变刚度开孔板试验样件,进行了拉伸对比试验。结果表明：数值模拟与实验数据吻合较好,变刚度开孔板相比常刚度开孔板,拉伸强度提升了26.92%,且两者损伤失效演化过程显著不同。     

含纤维波纹缺陷复合材料层合板的损伤分析

纤维波纹是复合材料层合板制备过程中的一种常见缺陷,会导致其刚度和强度显著下降,有效地预测含波纹缺陷复合材料层合板的失效强度具有显著的意义。基于此,本文采用解析的方式分别构造了纤维波纹呈正弦起伏与余弦起伏状的复合材料层合板模型。利用该模型,以Tsai-Wu准则作为失效判据,研究了一种含纤维波纹的碳纤维/环氧树脂复合材料层合板在受压情况下的损伤演化过程,得到了碳纤维/环氧树脂复合材料层合板的初始损伤强度。与有限元方法计算得到的损伤位置和损伤强度非常吻合,验证了本文算法的正确性。另外,相比于有限元方法,本文所述计算方法具有模型构造简单、计算效率高等优点,便于快速分析和确定含纤维波纹缺陷复合材料层合板的损伤位置与损伤强度。      

Difference operator for variable stiffness plates

A symmetric central finite difference operator is derived for the small deflection analysis of transversely loaded, ‘thin’, elastic plates, whose stiffnesses vary as functions of the in-plane co-ordinates. The operator is written here in Cartesian co-ordinates for a square mesh, but the same technique can also be used to construct an operator for a rectangular mesh.     

变刚度橡胶球铰的刚度特性与疲劳寿命分析

进行变刚度橡胶球铰的承载特性研究,有限元数值分析与实验结果均表明其刚度曲线表现为明显的非线性特性。正常载荷工况刚度值小、平稳,极限载荷刚度曲线出现拐点,刚度值显著增大。改变橡胶球铰的止挡高度,会直接影响刚度曲线的拐点位置。可据载荷工况,通过调整止挡高度改变刚度特性。据橡胶超弹特性,用有限元分析数据计算疲劳载荷工况下球铰危险点的等效应力范围,结合S-N曲线对橡胶球铰的疲劳寿命分析预测,并通过台架疲劳实验验证。结果显示橡胶球铰经150万次疲劳试验后未失效,与寿命预测值基本吻合。     

Interface affected zone for optimal strength and ductility in heterogeneous laminate

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含分层损伤复合材料层压板低速冲击模型刚度计算方法

针对基于弹簧质量模型的层压板低速冲击模拟方法,研究产生分层损伤后层压板整体刚度的计算方法。首先将板分为中心含分层的圆形区域与四周未分层环形区域。根据薄板弯曲理论,利用边界条件及两区域交界处的转角协调条件得到两个区域的挠度,进而计算刚度。基于ABAQUS软件建立了含损伤层压板的有限元模型进行该计算方法的验证,结果表明,该方法的计算结果与有限元模型结果吻合较好。进行了层压板落重冲击试验,根据无损检测得到的分层信息利用本方法进行了弯曲刚度的折减,再利用弹簧-质量模型模拟有损伤产生过程的冲击接触力,模拟结果和实验与有限元结果吻合良好,证明所提出的层压板低速冲击弹簧-质量模型分层刚度折减算法是有效的。