整体壁板增量压弯试验与模拟

采用逐点增量弯曲的方法对网格式带筋整体壁板进行了试验研究,并进行了有限元模拟.研究表明:采用增量压弯成形工艺可以成形出具有一定曲率外形的整体壁板;压弯过程中蒙皮始终处于弹性变形阶段,筋条既有弹性变形产生,又有塑性变形产生;压下量达到一定值时,塑性变形的比率急剧增加.     

Z型材变曲率数控滚弯等圆弧逼近算法与实现

Z型框缘类零件是组成飞机骨架的主要受力零件,传统的加工方法无法保证零件的成形精度和表面质量.本文根据数控四轴滚弯成形的功能和工艺特点,在机床响应工步范围内,提出了变曲率零件外形轮廓等圆弧逼近算法,对变曲率零件外形轮廓简化成有限段等曲率弧段,并对各弧段曲率半径的回弹量做了补偿修正,完成了复杂变曲率Z型材连续滚弯成形和加工精度的控制.成形零件经标准检验样板检测发现:相比以往CAD手动划分滚弯加工,等圆弧逼近算法对零件的外形轮廓弧段的合理划分,有效提高了复杂变曲率Z型材零件的滚弯成形精度和加工效率.     

飞机蒙皮多道次滚弯加载轨迹设计方法研究

目的研究针对数控滚弯机的飞机蒙皮多道次滚弯加载轨迹设计方法。方法根据数控滚弯机机构的特点,提取零件截面线,根据截面线曲率进行分段和参数化方法设计加载轨迹,通过反向求解将加载轨迹转换为设备运动参数。结果飞机蒙皮多道次滚弯加载轨迹设计方法有效地提高了工艺参数设计的效率和准确度,并与数控设备相结合,输出了设备的控制参数。结论相比传统的以经验分析和通过反复试弯大致确定工艺程序及参数,并通过人工控制的飞机蒙皮滚弯成形方法具有突出的实质性特点和显著的进步。     

网格式整体壁板增量成形有限元模拟

将机加后的平面外形的壁板毛坯成形为具有复杂曲面外形的壁板零件具有很大难度,为此,采用有限元方法进行壁板成形工艺模拟,分析壁板加工过程中的应力应变分布与失稳开裂现象.研究表明:网格式整体壁板增量成形过程中,塑性变形主要集中于筋条处,成形后的壁板主要是由于筋条发生塑性变形而导致壁板外形的改变.在成形过程中,筋条容易出现失稳与开裂现象.采用分段逐点循环模拟的方法,实现了网格式整体壁板结构件增量成形过程的数值模拟.     

工艺参数对7075铝合金带筋壁板时效成形回弹的影响

目的 研究7075航空高强度铝合金带筋壁板时效成形过程中工艺参数对零件回弹的影响,以提高筋板类零件的产品质量。方法 利用正交试验开展不同工艺参数组合下的带筋壁板时效成形试验,并对带筋壁板时效成形后的回弹率进行极差分析和规律曲线分析。结果 带筋壁板时效成形后的回弹同时受到时效参数和筋板结构参数的影响,各因素按对回弹的影响程度由大到小的顺序依次为时效时间、筋条厚度、筋条高度和时效温度,筋条结构参数对调节构件回弹有重要作用,并且回弹率实测值基本处于回弹率随工艺参数变化的拟合曲线附近。结论 回弹率随时效时间和筋条高度的增加表现为非线性下降趋势,随筋条厚度的增加呈现线性上升趋势。7075铝合金带筋壁板时效成形后的回弹行为可以用回弹率回归方程进行较为合理的描述。     

冲击载荷作用下机身壁板破坏效应及结构优化EI北大核心CSCD

研究非包容抗爆结构约束下,冲击波和端头两种冲击载荷对机身壁板的破坏效应。以定向非包容抗爆结构冲击位置处机身壁板为研究对象,通过有限元数值模拟,分析了在非包容抗爆结构约束下冲击波和端头两种载荷作用于不同位置时,机身壁板的破坏模式和程度,并以限制机身壁板破坏范围为目的,对冲击位置处机身壁板进行了结构优化。结果表明:在冲击波作用下机身壁板的主要破坏模式为蒙皮剪切冲塞、沿筋和垂直于筋撕裂以及筋条断裂,在端头作用下则为蒙皮沿筋和垂直于筋撕裂以及筋条断裂;冲击位置对端头载荷作用下机身壁板的破坏程度影响较冲击波明显;端头冲击筋条位置对机身壁板的整体破坏程度较小,为较优的冲击方案;垂直加筋对两种冲击模式下机身壁板的破口扩展均有明显约束效果,但对端头作用下的约束效果更佳,端头冲击筋间或筋条位置时,增加垂直加筋壁板损伤面积分别减小了56%和39%。研究结果可为民机用非包容抗爆容器结构设计及冲击位置处机身壁板结构设计提供指导。     

缝合增强复合材料帽型加筋壁板界面拉脱性能

为了防止复合材料帽型加筋壁板结构在服役过程中发生筋条与蒙皮的脱粘失效,引入了缝合技术来提高筋条-蒙皮界面拉脱性能.采用单线弯针缝合设备缝合纤维编织布,通过真空辅助树脂灌注技术(VARl)固化成型,制备缝合帽型加筋壁板试样.通过对试样进行筋条拉脱试验和有限元数值模拟,研究界面的失效机制及缝合参数对帽型加筋壁板界面结合性能的影响规律.结果表明:在拉脱载荷作用下,缝合试样的峰值载荷比未缝合试样明显增大.帽型试样的拉脱承载力随缝合密度的增大先增加后减小,在缝合密度(注:缝合密度表示缝合针距(单位mm)×行距(单位mm),下文同)为5×10时,相比未缝合试样,最高增加了26.7％;帽型接头的拉脱承载力随缝线细度的增加而增加,在缝线细度为1500 D时增加了39.7％.蒙皮/筋条厚度比为2时,结构拉脱承载力增加了27.35％,缝合的增强效果最明显.     

缝合复合材料T型加筋壁板的抗剪与抗弯性能

为了防止复合材料T型加筋壁板在服役过程中因剪切和弯矩作用而发生筋条与蒙皮的脱粘失效,引入了缝合技术来提高筋条-蒙皮界面的结合性能.采用自主研发的单线弯针缝合设备来缝合干纤维,通过真空辅助树脂灌注技术(VARI)固化成型,脱模后制成缝合T型加筋壁板试样.通过对试样进行剪切和弯曲试验,研究缝合的增强机理以及缝线细度对T型接头性能的影响规律.结果表明:在剪切应力作用下,缝合试样的峰值载荷比未缝合试样有明显提高,随着缝线细度增大,T型接头的峰值载荷升高,缝线细度增大到1 500 D时,峰值载荷提高55. 0% .在弯曲应力作用下,随着缝线细度的增大,T型接头的峰值载荷先升高后降低.缝合对T型接头在两种不同应力下的初始损伤载荷均无明显影响.     

U形高筋整体壁板激光诱导精密成形实验研究

目的 提高U形高筋整体壁板激光诱导成形精度。方法 根据U形高筋整体壁板特点,首先初步确定成形扫描策略,然后采用几何分析方法计算不同扫描位置成形后的理论变形量,并根据理论变形量对扫描策略和成形工艺参数进行优化。随后开展试验研究。同时利用在线可视化测量手段,对U形高筋壁板的变形量进行测量。结果 扫描道次数和激光器功率大小对U形高筋整体壁板成形精度影响很大,随着扫描次数和激光器功率的增加,变形量随之增大。结论 成形后板材的尺寸误差不大于0.4%,实现了U形高筋整体壁板的高精密成形。     

复合材料加筋壁板结构选型设计

为确定某型飞机平尾壁板结构截面尺寸参数,验证计算分析方法是否合理、有效,通过有限元屈曲分析与试验相结合的手段,对复合材料加筋壁板轴压稳定性进行研究。研究了复合材料加筋壁板在相同支持条件下,提高加筋壁板筋条截面积占总截面积的比例对复合材料加筋壁板初始屈曲和承载能力的影响。试验结果表明:试验结果与计算结果较为接近,说明计算中采用的分析方法和分析模型准确有效;在筋条截面积比例提高3.0%的情况下,屈曲载荷提高10.0%左右。所得结论可指导复合材料加筋壁板设计在型号研制中的应用,为初步确定平尾壁板结构截面形式和尺寸提供参考。      

Mechanical properties and design of sandwich materials

Sandwich materials consisting of a low density core with stiff skins offer considerable potential for weight saving in panel applications, where the main loads are flexural. Sandwich materials of interest for car and van body panels, seat shells, etc, include steel/plastic laminates, integral skinned plastic foams and glass fibre-reinforced polyester skins with foamed plastic cores. In this paper, basic design formulae for the flexural stiffness and strength of such sandwich materials are reviewed and a method for designing optimum sandwich structures for least weight or cost is given. Mechanical property data are presented on a range of sandwich materials of potential interest for vehicle panel applications. It is then shown how use of the least-weight design method enables core and skin thicknesses to be determined and gives a means of improving the flexural properties of existing sandwich constructions.     

An analytical model for composite sandwich panels with honeycomb core subjected to high-velocity impact

In this paper, an analytical model for perforation of composite sandwich panels with honeycomb core subjected to high-velocity impact has been developed. The sandwich panel consists of a aluminium honeycomb core sandwiched between two thin composite skins. The solution involves a three-stage, perforation process including perforation of the front composite skin, honeycomb core, and bottom composite skin. The strain and kinetic energy of the front and back-up composite skins and the absorbed energy of honeycomb core has been estimated. In addition, based on the energy balance and equation of motion the absorbed energy of sandwich panel, residual velocity of projectile, perforation time and projectile velocity have been obtained and compared with the available experimental tests and numerical model. Furthermore, effects of composite skins and aluminium honeycomb core on perforation resistance and ballistic performance of sandwich panels has been investigated.     

A resin film infusion process for manufacture of advanced composite structures

This paper reports work conducted at the CRC-ACS for the manufacture of advanced composite structural demonstrators using a resin film infusion (RFI) process. The influences of a number of processing parameters on the quality of cured parts are discussed. A number of demonstrators are presented including an aileron skin panel, swaged wing rib and 3-bay aft-box structure. These demonstrators indicate the cost-effectiveness of the RFI process for manufacture of advanced composite structures.     

Finite element modelling of fibre reinforced polymer sandwich panels exposed to heat

A finite element model that predicts temperature distribution in a composite panel exposed to a heat source, such as fire, is described. The panel is assumed to be composed of skins consisting of polymer matrix reinforced with fibres and a lightweight core (the paper concentrates on the crucial aspect of the problem, i.e. the behaviour of the ‘hot’ skin of the panel. The core is assumed not to decompose, and the ‘cold’ skin is treated exactly as the ‘hot’ skin.) It is assumed that the polymer matrix undergoes chemical decomposition. Such a model results in a set of coupled non‐linear transient partial differential equations. A Galerkin finite element framework is formulated to yield a fully implicit time stepping scheme. The crucial input parameters for the model are carefully identified for subsequent experimental determination. Copyright © 2004 John Wiley & Sons, Ltd.     

Hybrid processing of thick skins for honeycomb sandwich structures

Carbon-epoxy prepregs are generally used to form the skins of honeycomb sandwich structures used in aerospace or racing yachts. For some applications, it is desirable to increase the thickness of the skins. In order to achieve an ideal core pressure level during cure for maximal skin-core bonding, the issue of air extraction from the honeycomb cells through the skin during processing thus becomes critical, in particular if vacuum only processing is used. In the present work, partially impregnated prepregs, called semipregs, having high initial transverse permeability to air, are combined with traditional prepregs to form a hybrid skin. Results are presented on the pressure change inside the honeycomb cells and the skin permeability to air during cure, as well as on skin-core adhesion. The final sandwich quality is assessed and found to be comparable to that obtained with prepreg skins.     

LOW VELOCITY PERFORATION BEHAVIOUR OF POLYMER COMPOSITE SANDWICH PANELS

The paper describes low-velocity impact tests on square panels made from two polymer composite sandwich constructions, namely woven glass vinyl ester skins with Coremat core and woven glass epoxy pre-preg skins with honeycomb core. The impact velocity was up to 8 m s^-1 with an impact mass of up to 30 kg giving a maximum impact energy of 882 J. This maximum energy gives full perforation of the panels. The panels were 0.5 m by 0.5 m with clamped but free to pull in boundary conditions. The impactor geometry considered was a 50 mm diameter hemisphere. Results are expressed in the form of energy and failure mode plots and it is shown that the energy absorbing capabilities of the panels increase with the velocity of impact. The increase in energy absorption is attributed to an increase in the core crush stress and skin failure stress at high strain rates. Some discussion is given on the influence of the energy absorbing capabilities of constituent materials on the overall energy absorption behaviour of the panel. Suggestions have also been made for increasing panel perforation energy.     

SANDWICH STRUCTURE DELAMINATION OF RESIN TRANSFER MOLDING

One of the apparent advantages of sandwich structures is that after the core is made, the sandwich is produced in one process by resin transfer molding (RTM) and no adhesive is used between the core and skins. The bond between the core and skins is therefore likely to depend upon the core material, the type of matrix and the core surface roughness. This is of great importance, because the stiffness of the sandwich structure is likely to be reduced by even partial delamination of the core and skins. The objective of this study was to ascertain the effects of manufacturing parameters such as injection pressure, mold temperature, core thickness and core surface roughness on skin/core adhesion using the direct tensile adhesion and peel test methods. Polyurethane foam was used as the core material throughout the work. The major objective was to examine different surface treatment methods by which the strength of the skin-core bond could be improved. The influence of the core surface roughness on the adhesive fracture energy and the delamination between core and skin were also measured. The fracture energy release rate equation was used as the basis for comparison and for measurements of the adhesion. For this purpose a double-cantilever beam was used to characterize the delamination. Critical energy release rate (G_IC) and fracture toughness (K_IC) were calculated using several alternative methods based on linear elastic fracture mechanics.     

Light-weight honeycomb core sandwich panels containing biofiber-reinforced thermoset polymer composite skins: Fabrication and evaluation

The application of biofiber based paper-reinforced polymer (PRP) composites as skin materials for light-weight sandwich panel constructions was explored. Various sandwich panels with PRP composite skins and a commercial resin-impregnated aramid paper honeycomb core of different cell sizes and core heights were fabricated in the laboratory. The effects of honeycomb core height and cell size on the flexural properties of the lab-made sandwich panels were evaluated. The flexural moduli and strengths of the lab-made panels were compared to the reported values for three existing commercial products used for automotive load floor applications. The lab-made PRP composite/honeycomb core sandwich panels had comparable bending rigidity and flexural load bearing capability but lower areal weights when compared to the commercial products suggesting that PRP composites have the potential to be used as an alternative to glass fiber-reinforced polymer composites as skin materials in sandwich panel fabrication.     

长桁-翼肋连接对复合材料单加筋板压缩性能的影响

采用试验和有限元方法研究了复合材料含翼肋单加筋板试验件的压缩性能,试验件包括带有长桁-翼肋连接和不含长桁-翼肋连接2种类型。试验和数值计算研究结果表明:与不含长桁-翼肋连接的试验件相比,带有长桁-翼肋连接的试验件具有较高的刚度和较高的临界屈曲失稳载荷,在后屈曲承载过程中具有较小的形变和较小的最终破坏载荷。试验件的最终破坏模式总是长桁与蒙皮间的界面脱粘,这表明长桁-翼肋连接对加筋板试验件的最终破坏模式无影响。在复合材料翼面结构设计中,需要综合考虑长桁-翼肋连接对加筋板初始临界失稳载荷、后屈曲变形和结构承载能力等方面的影响。     

复合材料蜂窝夹层结构T型接头拉伸性能研究

通过试验测量了复合材料蜂窝夹层结构T型整体接头的拉伸性能,得到其拉伸强度与破坏模式。建立了接头结构有限元模型,利用分类损伤判据、失效准则与刚度退化准则对结构的损伤情况进行模拟,研究了接头的拉伸破坏行为。有限元分析结果与试验结果吻合良好。研究结果表明,结构的薄弱点位于腹板内靠近蒙皮的位置。蜂窝在此处发生面外拉伸破坏,从而导致结构的最终破坏。腹板上的拉伸载荷主要通过过渡区填料传递给蒙皮,腹板与蒙皮间的搭接段对载荷传递的贡献较小。参数研究表明,对于复合材料蜂窝夹层结构T型接头,搭接段长度对结构的强度几乎没有影响,而增大蒙皮蜂窝的高度或采用低模量蜂窝可以提高结构强度。