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。     

铺设角度与铺层顺序对层合板稳定性的影响

以层合板结构的临界屈曲载荷系数最大化为优化目标,基于改进型模拟退火算法对层合板结构铺设角度和铺层顺序进行优化。由于层合板结构的铺层角度是离散变量,模拟退火算法适合求解离散变量的优化问题。利用模拟退火算法优化层合板铺层,在算法内采用并行计算、引入记忆功能同时设置双阈值终止准则,有效地提高了优化过程的收敛速度,同时避免优化过程中出现局部最优解。以临界屈曲载荷系数作为目标函数,选取复合材料层合板的铺设角度顺序为设计变量,采用改进的模拟退火算法得出复合材料层合板的最优铺设角度以及铺层顺序。     

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。      

Prediction of reliability analysis of composite tubular structure under hygro-thermo-mechanical loading

The present paper focuses on reliability prediction of composite structure under hygro-thermo-mechanical loading, conditioned by Tsai-Wu failure criterion, where the Monte–Carlo method is used to estimate the failure probability(Pf). This model was developed in two steps: first, the development of a deterministic model, based on an analytical and numerical approach, and then, a probabilistic computation. Using the hoop stress for each ply, a sensitivity analysis was performed for random design variables, such as materials properties, geometry, manufacturing, and loading, on composite cylindrical structure reliability. The probabilistic results show the very high increase of failure probability when all parameters are considered.     

Design and Manufacture of Conical Shell Structures Using Prepreg Laminates

The design and manufacture of unstiffened composite conical structures is very challenging, as the variation of the fiber orientations, lay-up and the geometry of the ply pieces have a significant influence on the thickness imperfections and ply angle deviations imprinted to the final part. This paper deals with the manufacture of laminated composite cones through the prepeg/autoclave process. The cones are designed to undergo repetitive buckling tests without accumulating permanent damage. The aim is to define a process that allows the control of fiber angle deviations and the removal of thickness imperfections generated from gaps and overlaps between ply pieces. Ultrasonic scan measurements are used to proof the effectiveness of the proposed method.     

Design of experiments for stacking sequence optimizations with genetic algorithm using response surface approximation

This study describes a new method of experimental design to obtain a response surface of buckling load of laminated composites. Many evaluations for genetic algorithms for stacking sequence optimizations require high computational cost. That evaluation cost can be reduced by an approximation using a response surface. For a response surface for stacking sequence optimizations, lamination parameters are adopted as variables of the approximation function of the entire design space instead of ply angles for each ply. This study presents, proposes and investigates a new method of experimental design in detail. For most analytical tools, stacking sequences is demand as input data and lamination parameters cannot be applied directly to the tools. Therefore, the present study proposes and applies a new D-optimal set of laminates to the stacking sequence optimizations of the problem of maximization of buckling load of a composite cylinder. The new experimental design is a set of stacking sequences selected from candidate stacks using D-optimality. Consequently, the D-optimal set of laminates is shown to be effective for design of experiments of response surfaces for maximization of the buckling load of composite structures.     

Stress distribution modeling for interference-fit area of each individual layer around composite laminates joint

The discussion about nonuniform stress distribution around interference-fit joint is particular significance in the design of composite laminates structures. In order to investigate the stress distribution of interference-fit area around composite laminates joint, an analytical model is developed for stress distribution based on the Lekhnitskii's complex potential theory. The normal and tangential stresses of contact are achieved by the relationship of deformation between pin and hole. The effects of ply orientation and interference percentage on stress components distributions of each individual layer around symmetrical laminates joint are discussed. In order to verify the validity of the analytical model, extensive 3D finite element models are established to simulate the stress components of laminates interference-fit joint. The results show that the analytical model is valid, and the laminate property and ply orientation have a significant effect on stress distribution trend while interference percentage mainly affects stress magnitude.     

铺层角度对碳纤维/形状记忆环氧树脂层合板形状回复性能的影响

对不同铺层角度的碳纤维/环氧树脂形状记忆复合材料(SMC)层合板的弯曲回复性能进行了研究。结果表明,[±θ]_s铺层方式的SMC层合板的形状回复率、回复力均随着铺层角度增大而减小,回复时间随着铺层角度增大而增大,其中铺层角度增大至45°后,回复时间开始出现大幅的增加,铺层角度增大至60°后,回复率开始出现大幅的降低。对SMC层合板进行了15次的赋形-回复循环过程,发现不同铺层角度SMC层合板均能保持较稳定的形状记忆回复率和回复时间。但在铺层角度0~30°的范围内,层合板的形状回复力随着铺层角度增大而减小。最后分析了不同铺层角度SMC层合板的局部损伤,结果表明,[0]_4和[±15]_s铺层方式的SMC层合板基体已达到了其极限剪切应变,基体发生严重破坏,并且会随着赋形次数的增加而加剧。     

可剥布对T300/Cycom 970环氧树脂复合材料胶接性能的影响

在商用飞机复合材料结构制造过程中,可剥布在复合材料共胶接和二次胶接的表面处理过程中应用越来越广泛,逐步替代传统打磨方式。然而在实际应用中,可剥布与复合材料树脂体系之间存在一定的匹配性,经不同可剥布处理后的复合材料表面胶接性能存在较大差异。为研究其对胶接性能的影响,选用四种航空用可剥布对复合材料进行表面处理,采用热压罐工艺制备T300/Cycom 970环氧复合材料层压板,选取同一种航空用胶膜进行胶接。按照ASTM剥离测试和单搭接剪切测试标准,对T300/Cycom 970环氧复合材料胶接结构的性能进行测试,采用接触角测试、SEM和X射线能谱测试(XPS)分别对复合材料层压板制件和可剥布织物的表面润湿性、表面形貌和表面元素进行测试与分析。结果表明:选用聚酯湿可剥布处理后的T300/Cycom 970环氧复合材料胶接性能最佳;可剥布织物及涂层的残留会影响复合材料胶接性能;可剥布处理能改善复合材料表面润湿性,提高表面能,但并不能保障胶接质量;可剥布编织形式直接影响复合材料表面形貌,决定其表面粗糙度,对复合材料胶接性能也有较大的影响。     

复合材料壁板铺层参数对大展弦比机翼气动弹性优化的影响

开展了大展弦比复合材料机翼气动弹性综合优化设计研究,以复合材料层合板铺层厚度为设计变量,以多种气动弹性约束与强度/应变约束为限制条件对结构进行优化设计,从铺层比例和铺层非均衡两方面分析了蒙皮铺层参数的影响。研究表明: 在满足综合约束的条件下,随着0°铺层比例的增加,翼尖变形略微减小,颤振速度略有下降,副翼效率变化不大; 蒙皮铺层非均衡程度主要影响机翼静气动弹性能,随着蒙皮非均衡引起的机翼刚轴绕翼根向前缘逐渐偏转,翼尖垂直变形变化不明显,但翼尖负扭转变形的绝对值加大,副翼效率下降。     

碳纤维复合材料层合板低速冲击损伤应力分析

目的 为掌握碳纤维复合材料板在低速冲击载荷作用下的损伤规律,延缓失效破坏,对其冲击损伤的应力状态进行研究。方法 基于ABAQUS平台,建立碳纤维复合材料层合板低速冲击有限元模型,采用Hashin失效准则和VUMAT用户子程序,对碳纤维复合材料层合板的冲击过程进行数值模拟,同时考虑层合板层内与层间失效,以此来研究低速冲击条件下复合材料的损伤机理,分析冲击损伤过程中的应力变化趋势,讨论应力的分布状态。重点研究铺层角度及铺层距离冲头远近对应力的影响。结果 不同角度铺层的应力传播轨迹均沿着纤维方向和垂直于纤维方向同时扩展,应力均先增加至极限值而后迅速下降;铺层角度越大,板料的承载能力越弱,0°铺层的极限应力为1 432 MPa,而90°铺层的极限应力降至1 206 MPa;离冲头越远的铺层应力越小,达到峰值的时间更早且率先下降,说明远离冲头的铺层更早发生失效。结论 揭示了碳纤维层合板在低速冲击载荷作用下的应力状态及其对损伤的影响规律,能够为复合材料层合板零件设计提供参考。     

Reliability-based optimization of fibrous laminated composites

This paper is concerned with the optimum design of multiaxial fiber reinforced laminate systems under probabilistic conditions of loads and material properties. A multiaxially laminated composite is treated as a structural system with each ply contained in the composite as one element. The Tsai-Wu failure criterion is adopted as the limit state function of a unidirectional ply. It is assumed that the system failure occurs when any one of the plies in a laminate system fails. The multiple-check-point method is successfully applied to evaluate the system reliabilities of multiaxial laminates under probabilistic in-plane stresses. An optimization problem is defined to find the optimal number of fiber orientation axes, optimum orientation angles, and optimum ply ratios which yield the highest system reliability.     

Stochastic finite element analysis of the free vibration of laminated composite plates

Using the Stochastic Finite Element Method (SFEM) to perform reliability analysis of the free vibration of composite plates with material and fabrication uncertainties has received much attention lately. In this work the stochastic analysis is performed using the First-Order Reliability Method (FORM-method 2) and the Second-Order Reliability Method (SORM). The basic random variables include laminae stiffness properties and material density, as well as the randomness in ply orientation angles. Modeling of the composite behavior utilizes a nine-noded isoparametric Lagrangian element based on the third-order shear deformation theory. Calculating the eigenvectors at the mean values of the variables proves to be a reasonable simplification which significantly increases solution speed. The stochastic finite element code is validated using comparisons with results of Monte Carlo simulation technique with importance sampling. Results show that SORM is an excellent rapid tool in the stochastic analysis of free vibration of composite plates, when compared to the slower Monte Carlo simulation techniques.     

Progressive damage analyses of angle‐ply laminates exhibiting free edge effects using continuum damage mechanics with layer‐wise finite element method

Capability of continuum damage mechanics (CDM) to predict the damage mechanism evolution of composite laminates has rarely been carried out, and most of the previous CDM works mainly focused on the overall response of the laminates. In this paper, progressive damage and overall response of the composite laminates under quasi‐static, monotonic increasing loading are investigated using three‐dimensional (3D) CDM implementation in a finite element method that is based on the layer‐wise laminate plate theory. In the damage formulation, each composite ply is treated as a homogeneous orthotropic material exhibiting orthotropic damage in the form of distributed microscopic cracks that are normal to the three principal material directions. The progressive damage of different angle‐ply composite laminates under quasi‐static loading that exhibit free edge effects is investigated. It is shown that using CDM global behaviour and various damage mechanisms affected by the complex nature of free edges can be qualitatively well predicted.     

Optimal design of a composite structure

This paper presents a design methodology for a laminated composite stiffened panel, subjected to multiple in-plane loads and bending moments. Design variables include the skin and stiffener ply orientation angles and stiffener geometry variables. Optimum designs are sought which minimize structural weight and satisfy mechanical performance requirements. Two types of mechanical performance requirements are placed on the panel, maximum strain and minimum strength. Minimum weight designs are presented which document that the choice of mechanical performance requirements cause changes in the optimum design. The effects of lay-up constraints which limit the ply angles to user specified values, such as symmetric or quasi-isotropic laminates, are also investigated.     

Effect of ply angle misalignment on out-of-plane deformation of symmetrical cross-ply CFRP laminates: Accuracy of the ply angle alignment

This paper discusses the accuracy of ply angle alignment and how it relates to out-of-plane deformation in carbon fiber reinforced plastics (CFRP) laminates. We investigated the deformation of symmetrical cross-ply laminates under hot and humid conditions. In spite of the symmetrically stacked laminates, unpredictable out-of-plane deformation occurred over time due to ply angle misalignment. The deformation was unstable and disproportionate to the absorbed moisture. A Monte Carlo simulation based on laminate theory was performed to quantify the deformation induced by the ply angle misalignment. Symmetrical cross-ply laminates were found to twist as they absorbed water when they underwent ply angle misalignments. By comparing the analytical results with experimental results, we concluded that a standard deviation of approximately 0.4° exists as ply angle misalignment in the laminates used in this study and that this slight ply angle misalignment can be a significant factor in out-of-plane deformation of cross-ply laminates.     

Scaling effects in notched carbon fibre/epoxy composites loaded in compression

The aim of the work was to develop an understanding of the failure mechanisms controlling the strength of composites of different dimensions and hence to be able to predict size effects in composite structures without resorting to empirical laws. Adequate models do not currently exist, and extensive testing is necessary, which is very costly. The ability to predict the effect of size on strength would be a major step forward, which would reduce costs and encourage the more widespread usage of these materials in the aerospace and other industries. In this article the effects of scaling (specimen size) on the strength of notched laminates are presented. The most important variables have been identified as hole (notch) size, ply and laminate thickness. Manufacturing defects and specimen design can also lead to premature failures, especially in unnotched laminates, but in laminates with an open hole are of less significance, since the notch dominates the fracture. The compressive strength results are compared to data obtained for the same composite system and laminate stacking sequences loaded in uniaxial tension.     

The effect of the measurement frequency on the elastic anisotropy of fibre laminates

Static tests and ultrasonic measurements (2.25 MHz) have been carried out on a series of composite laminates of glass fibres in a polypropylene matrix. A range of angle ply laminates were prepared for this study, with laminate angles θ of ±0, 10, 20, 30 and 40^∘. The high frequency measurements were made using the ultrasonic immersion technique, which allows the determination of a complete set of the elastic constants of a material. The relationship between the ultrasonically determined elastic constants of the angle ply laminates was found to be in excellent agreement with theoretical predictions, as previously validated for carbon fibre/epoxy angle ply laminates. A comparison between the ultrasonic and statically measured values was made for two of the angle ply laminates (θ = 0 and 20^∘). It was found that the static values were lower than those measured at ultrasonic frequency, particularly those constants that were more matrix dominated (for example the transverse moduli of the laminates). Measurements on a pure polypropylene sample at both testing frequencies confirmed that the change in matrix properties with frequency was the cause of this difference. The change in properties with test frequency is likely to be much larger in this system than in other composite materials because the glass transition temperature of polypropylene is close to ambient temperature. Dynamic mechanical tests (1 Hz) were carried out on a sample of pure polypropylene to assess this effect. We also give an appropriate method of estimating the dependence of glass transition temperature on frequency. The results for polypropylene are compared with those for other commonly used polymer matrix materials: epoxy resin, nylon and polyetheretherketone (PEEK): DMTA measurements were also made on these samples. The effect of test frequency on matrix properties, for the glass/PP laminates, was further investigated by examining the relationship of the Poisson's ratios with laminate angle using a mixture of ultrasonic experiments and theoretical predictions. Previously we have shown that the degree of anisotropy between the reinforcing fibre and the matrix phase is paramount in determining whether the material will show a negative Poisson's ratio at a critical laminate angle. The ultrasonic measurements carried out in this study on the glass/PP laminates showed a minimum in one of the Poisson's ratio at a laminate angle of 32°, but the value did not become negative. However, theoretical predictions showed that for a static frequency measurement (1 Hz), where the matrix is softer and hence the anisotropy of each laminate ply is higher, the laminate will show a negative Poisson's ratio with a minimum at a laminate angle of around 28°.     

Lamb wave-based quantitative identification of delamination in CF/EP composite structures using artificial neural algorithm

Delamination in composite structures plays a major role in lowering structural strength and stiffness, consequently downgrading system integrity and reliability. A Lamb wave-based quantitative identification technique for delamination in CF/EP composite structures was established. Propagation of Lamb waves in a series of composite laminates, individually bearing a delamination, was evaluated using dynamic FEM analyses. Taking advantage of wavelet transform and artificial neural algorithms, an Intelligent Signal Processing and Pattern Recognition (ISPPR) package was developed, by which the spectrographic characteristics of simulated Lamb wave signals in the time-frequency domain were extracted and digitised as Digital Damage Fingerprints (DDF), to construct a Damage Parameters Database (DPD). The DPD was then used offline to train a multi-layer feedforward artificial neural network (ANN) under supervision of an error-backpropagation (BP) algorithm. Assisted by an active online structural health monitoring (AO-SHM) system with an active piezoelectric actuator/sensor network, the proposed methodology was validated online by identifying actual delaminations in CF/EP (T650/F584) quasi-isotropic composite laminates.     

无矩层板的能量优化准则

本文论证和给出了任意薄膜力场作用下复合材料无矩层板的最小应变能铺层优化准则,并讨论了Ⅱ、Ⅳ内力象限优化铺层的性质。