基于RTM技术的碳纤维/聚酰亚胺复合材料舵面一体化制备与验证

设计了一种碳纤维/聚酰亚胺复合材料舵面结构,采用PAM-RTM软件模拟了舵面在注胶过程中的树脂流动,根据模拟结果设计了成型模具,并通过树脂传递模塑(RTM)工艺制备了耐高温碳纤维/聚酰亚胺复合材料舵面,对其进行了力学试验,并将三维有限元分析结果与试验结果对比。试验结果表明,碳纤维/聚酰亚胺复合材料舵面在150%的使用载荷下保持了结构的完整性,骨架的最大应变为2 408×10–6,复合材料蒙皮的最大应变为2 371×10–6。有限元分析结果表明,金属骨架的最大应力出现在舵轴根部圆弧过渡区,而碳纤维/聚酰亚胺复合材料蒙皮的最大应力出现在与垫片外圆弧接触处;碳纤维/聚酰亚胺复合材料舵面的初始破坏为蒙皮单向带横向拉伸失效。      

典型损伤对复合材料机翼振动特性的影响

利用大型商用软件MSC.Patran建立了某型无人机复合材料机翼的有限元模型。在机翼中分别模拟了蒙皮分层、蒙皮与翼梁翼肋脱粘以及翼梁裂纹这3种典型损伤,通过与无损伤复合材料机翼固有振动频率进行比较,分析了损伤位置和大小对机翼振动特性的影响。结果表明:3种典型损伤一般不会引起机翼各阶振动模态的改变,但会使各阶振动频率发生变化;对各阶振动频率的影响既与损伤位置有关,也与主承力结构的损伤程度有关;蒙皮分层和翼梁裂纹出现在机翼根部时对机翼振动频率的影响最明显。     

RTM整体成型复合材料翼盒的固有模态及稳定性

采用基于水溶性型芯的RTM成型技术制备了整体化的复合材料翼盒, 并对翼盒进行了自由振动模态试验; 采用三维壳单元, 建立了翼盒固有模态及稳定性有限元分析模型, 该模型分析的翼盒固有模态与试验结果吻合很好, 验证了有限元模型的有效性; 为研究翼盒固有模态及稳定性的铺层效应, 采用该模型分析计算了4种铺层方案的翼盒的固有模态及稳定性。研究结果表明: 对称铺层蒙皮有利于提高翼盒轴向压缩与轴向扭转屈曲载荷及固有频率, 而不利于面外弯曲和弯扭组合情况; 腹板减薄和增加腹板45°铺层均不利于提高, 甚至会大幅度降低屈曲载荷及固有频率, 弯扭组合加载最容易导致失稳。     

Natural modal and stability of integrated composite wing box fabricated by RTM process

采用基于水溶性型芯的RTM成型技术制备了整体化的复合材料翼盒,并对翼盒进行了自由振动模态试验;采用三维壳单元,建立了翼盒固有模态及稳定性有限元分析模型,该模型分析的翼盒固有模态与试验结果吻合很好,验证了有限元模型的有效性;为研究翼盒固有模态及稳定性的铺层效应,采用该模型分析计算了4种铺层方案的翼盒的固有模态及稳定性。研究结果表明：对称铺层蒙皮有利于提高翼盒轴向压缩与轴向扭转屈曲载荷及固有频率,而不利于面外弯曲和弯扭组合情况;腹板减薄和增加腹板45°铺层均不利于提高,甚至会大幅度降低屈曲载荷及固有频率,弯扭组合加载最容易导致失稳。     

SiC/Ti-6Al-4V复合材料在横向拉伸载荷下界面失效行为的数值模拟

采用有限元模拟了SiC/Ti-6Al-4V复合材料冷却过程和横向拉伸试验过程, 横向拉伸试样采用十字形试样。分别建立了平面应力和轴对称有限元模型, 采用平面应力有限元模型计算环绕纤维圆周的界面微区应力分布, 预测界面失效机制。采用轴对称有限元模型分析复合材料界面脱粘过程以及残余应力对界面径向应力分布的影响。结果表明: 对于SiC/Ti-6Al-4V复合材料十字形试样,在横向拉伸载荷下的界面失效由径向应力导致,界面失效模式为法向失效, 剪切失效模式未发生; 十字形试样在横向拉伸载荷下界面初始脱粘位置处于界面中间; 随横向拉伸应力增加, 十字形试样的界面脱粘对称向两边扩展; 界面径向应力随残余应力降低而升高。     

复合材料机身曲板环向弯曲加载试验及失效机制

对复合材料机身曲板进行了环向弯曲加载试验,采用四点弯加载方式对考核段进行纯弯加载,设计一种加强连接方式避免加载段提前破坏,通过试验对机身曲板的环向稳定性和破坏模式进行了分析。同时,建立了基于内聚力单元的考虑长桁与蒙皮粘接界面损伤的有限元模型,分别使用Quads准则和Hashin准则作为界面和层合板的失效判据分析曲板结构的失效机制,计算结果与试验结果吻合较好。试验及有限元分析结果表明,长桁帽底蒙皮的局部屈曲引起长桁与蒙皮粘接的R区出现初始开裂,并最终扩展为长桁脱粘。随着蒙皮屈曲及长桁脱粘的扩大,蒙皮由局部屈曲变为整体失稳而失去承载能力,最终导致隔框承载过大而发生断裂。根据初始损伤模式,采取了长桁帽内全包工艺改进设计,改进后的曲板结构稳定性和承载能力分别提高了21.9%和16.8%。      

翼肋支撑对复合材料加筋板轴压性能的影响

为确定翼肋支撑对复合材料加筋板轴压性能的影响,对施加翼肋支撑前后的复合材料工型加筋板和帽型加筋板进行压缩试验和数值模拟研究。轴压试验中,通过应变计和影像云纹法实时监测试验件的失稳载荷及失稳模态,通过断面观测分析结构损伤破坏机制。基于ABAQUS软件建立有限元模型模拟加筋板屈曲及后屈曲过程,通过失稳节线及反节线上的应力分布变化分析加筋板破坏机制。计算结果与试验结果相吻合,表明翼肋支撑对不同筋条加筋板失稳模态有影响但均不改变结构失稳载荷,位于节线上的翼肋支撑对工型加筋板破坏载荷影响较小,但位于反节线上的翼肋支撑使帽型加筋板的承载能力提高了26.2%。试验件失稳后应力向反节线上筋条蒙皮界面集中,过高的应力导致界面脱粘,使得结构集中在反节线上破坏。      

三维机织复合材料预制体经密与弯曲性能的有限元分析

借助绘图软件PRO/E构建出不同经密的三维浅交弯联机织复合材料结构模型。使用有限元分析软件ANSYS对构建出的复合材料三维模型在位移为1mm的弯曲载荷作用下复合材料的弯曲力学性能进行模拟,并分别对纤维、树脂的应力、应变分布进行模拟,分析其弯曲破坏机理。并通过定性实验,得到了与模拟结果相同的结论,验证了模拟结果的准确性。结果表明:在1mm的弯曲载荷作用下经密为4根/cm的复合材料弯曲性能优于经密为3根/cm的复合材料;复合材料与弯曲试样接触的位置更容易发生破坏;该复合材料模型在1mm的弯曲载荷作用下破坏模式主要为树脂的破碎、纤维与树脂间的脱粘。     

复合材料翼面壁板剪切稳定性

基于长桁铺层数不同的两块复合材料翼面T型加筋壁板试件SS-1和SS-2开展剪切稳定性试验。试件SS-1和SS-2的L型层合板铺层分别为11层和14层,腹板铺层分别为22层和28层,缘条铺层分别为15层和18层。采用提出的工程方法进行壁板的剪切屈曲应变分析,方法中考虑了长桁尺寸和铺层数的影响,并应用有限元弧长法进行试件屈曲载荷、后屈曲承载能力及剪切屈曲模态分析。试验结果表明,屈曲发生之前试件蒙皮处于均匀纯剪切应变状态,后屈曲阶段试件发生了长桁-蒙皮脱粘破坏失效,长桁铺层数较多的试件SS-2具有更高的屈曲载荷和蒙皮局部屈曲应变。工程方法计算得到试件SS-1和SS-2的剪切屈曲应变相对于试验结果的误差分别为–14.9%和–9.2%。有限元弧长法分析得到试件SS-1的屈曲载荷和屈曲应变误差分别为1.9%和2.7%,且剪切屈曲模态与试验结果一致。弧长法对不同长桁铺层数的研究结果表明,长桁铺层较少时,壁板发生整体失稳的材料破坏,而长桁铺层数较多时,更容易发生长桁与蒙皮的脱粘失效。      

三维浅交弯联机织复合材料拉伸性能的有限元分析

本文通过绘图软件PRO/E5.0构建了用于拉伸性能研究的三层浅交弯联机织复合材料的数字化结构模型。借助大型有限元分析软件ANSYS Workbench对复合材料在1mm的拉伸变形下复合材料、纤维增强体及树脂基体的拉伸应力、应变分布情况进行模拟、计算,并对1mm拉伸位移下复合材料的破坏行为和破坏机理进行分析。结果表明:复合材料承受拉伸载荷时,纤维增强体起主要承载作用,树脂基体起次要承载作用;平行于拉伸方向的纬纱比垂直于拉伸方向的经纱承受更大的载荷作用;1mm拉伸位移下,复合材料破坏行为主要为纤维拉伸变形、纤维与树脂间脱粘及树脂的破碎。     

RTM成型非对称复合材料T型接头的拉伸失效机制

采用树脂传递模塑(RTM)工艺制备了结构对称和非对称两种复合材料T型接头试样,并对其进行了静态拉伸力学试验,对比分析了两种结构的拉伸破坏模式、结构刚度及破坏载荷。同时基于T接头内聚力模型(CZM),研究了两种不同结构T型接头的拉伸破坏过程及失效机制,并对比分析了不同偏转角下T接头的层间应力。结果表明：不同结构T型接头的拉伸破坏模式不同,偏转角的存在使结构非对称T型接头夹角大侧圆弧受力明显高于小侧圆弧,导致接头首先在大侧夹角圆弧与三角区界面定向萌生初始裂纹,随后裂纹主要沿大侧腹板翻边与蒙皮的界面扩展,进而导致接头最终破坏,最终失效载荷较对称T型接头提高了15.3%,且结构刚度更大。有限元结果表明T型接头三角区的初始失效主要由层间正应力及剪应力引起,有限元分析的失效模式与试验一致,结构对称及非对称T型接头最终失效载荷与试验值均吻合较好;且随着偏转角的增加,腹板圆弧处层间应力逐渐减小,初始失效载荷将随之增大;初始破坏位置将转移至大侧夹角圆弧末端。     

Investigation into Z-Pin Reinforced Composite Skin/Stiffener Debond under Monotonic and Cyclic Bending

Skin/stiffener debonding has been a longstanding concern for the users of stiffened composite panels in long-term service. Z-pinning technology is an emerging solution to reinforce the composite assembly joints. This work experimentally characterizes the progressive debonding of Z-pinned skin/stiffener interface with the skin under static bend loading. The three-stage failure process is identified as: flange edge debonding, pin/laminate debonding, and ultimate structural failure. Three different distribution patterns were compared in terms of the static debonding properties revealed the affirmative fact that locating pins in high normal stress regions, that is close to the flange edges in skin/stiffener structures, is more beneficial to utilize the full potential of Z-pinning reinforcement. The unit strip FE model was developed and demonstrated effective to analysis the effect of Z-pin distribution on the ultimate debond load. On the other hand, the evolution of fatigue cracks at Z-pinned skin/flange interface was investigated with a series of displacement-controlled fatigue bending tests and microscopic observations. Results show that Z-pinning postpones crack initiations at low displacement levels, and the remarkable crack-arresting function of pins enables the structure a prolonged fatigue life. However, pins become less effective when the maximum displacement exceeds the crack initiation level due to gradually pullout of pins.     

Experimental,Theoretical and Numerical Investigation of the Flexural Behaviour of the Composite Sandwich Panels with PVC Foam Core

This study presents the main results of an experimental, theoretical and numerical investigation on the flexural behaviour and failure mode of composite sandwich panels primarily developed for marine applications. The face sheets of the sandwich panels are made up of glass fibre reinforced polymer (GFRP), while polyvinylchloride (PVC) foam was used as core material. Four-point bending test was carried out to investigate the flexural behaviour of the sandwich panel under quasi static load. The finite element (FE) analysis taking into account the cohesive nature of the skin-core interaction as well as the geometry and materials nonlinearity was performed, while a classical beam theory was used to estimate the flexural response. Although the FE results accurately represented the initial and post yield flexural response, the theoretical one restricted to the initial response of the sandwich panel due to the linearity assumptions. Core shear failure associate with skin-core debonding close to the loading points was the dominant failure mode observed experimentally and validated numerically and theoretically.     

Experimental and numerical investigation of composite bolted π-joint subjected to bending load

This present work investigated the failure mechanism of a novel composite bolted π-joint subjected to bending load by experimental and finite element simulation. A test sample manufactured by resin transfer moulding process (RTM) was tested. A 3D progressive damage model developed in ABAQUS/Standard was used to simulate the failure of the π-joint. Based on good correlation of failure load and damage distribution between experimental results and FE prediction, further investigation was extended to the effect of two primary assembly clearances on mechanical behavior of the π-joint. The study results reveal that delamination of the fillet region in L-preform is the π-joint's failure mode. Moreover, the assembly clearances have little effect on the failure load of the joint.     

侧边边界条件对复合材料加筋板轴压载荷下屈曲和后屈曲性能的影响

为研究侧边边界条件对复合材料加筋板压缩稳定性能的影响,首先采用有限元软件对压缩载荷作用下的复合材料加筋板进行建模数值计算,得到加筋板在侧边简支和自由2种边界条件下的屈曲载荷和形式,然后采用工程计算方法对加筋板轴压承载能力进行了估算,参考计算结果,分别对侧边有支持和侧边自由2组加筋板进行轴向压缩试验,分析侧边边界条件对试验件的屈曲形式、屈曲载荷以及后屈曲破坏过程的影响。试验结果表明：侧边支持条件会影响加筋板的屈曲形式和破坏形式。对于侧边有支持的试验件,屈曲后整体变形较小,筋条的压缩断裂是主要的破坏形式;而侧边自由的试验件屈曲后会逐渐出现整体弯曲变形,变形引起的筋条脱粘和弯曲断裂则是主要的破坏形式,且筋条脱粘会显著降低结构的承载能力。有限元计算结果与试验结果较吻合,验证了有限元模型的合理性。采用工程计算方法对侧边有支持的加筋板承载能力进行估算具有较好的精度。     

An experimental and numerical study on omega stringer debonding

Omega stringers offer interesting structural capabilities and are expected on future aircraft fuselages. In postbuckling mode, the final failure of these structures may occur by stringer debonding between stringer flanges and the skin of the fuselage. In this study, it is demonstrated that the use of fracture mechanics allows to predict skin/omega stringer separation under multiple load cases. Three different load cases and experiments are presented allowing a debonding to start at different locations: at free flange edges or at the inner radius of the omega. Firstly, a skin/stringer configuration subjected to three point bending following the longitudinal axis of the stringer was tested. For this configuration, a numerical study was made and shows the influence of a refined mesh taking into account resin fillets. Secondly, new specimens were obtained by cutting into slices the longitudinal specimen. Those specimens were subjected to four points bending. It has been shown that the upper rolls position of the test jig could modify the debonding location. Numerical models have allowed to determine accurately the debonding location and the associated load level. For some specimens, resin fillets were removed from the flange tips and their effect were assessed numerically and experimentally.     

复合材料双向波纹夹层结构力学性能

为改进传统单向波纹夹层结构横向力学性能较差的缺点,设计了一种新型复合材料双向波纹夹层结构。考虑复合材料双向夹层结构制备困难,研究了整套真空辅助成型工艺(VARI)工艺制备方案,实现双向波纹夹层结构的高效制备,以满足工程应用的需要。对制备出的复合材料双向波纹夹层结构与单向波纹夹层结构分别进行面外压缩、弯曲和剪切实验,分析了双向波纹夹层结构在不同载荷下的破坏模式及其失效机制,计算了该结构在不同荷载条件下的强度和模量,并将其与单向波纹夹层结构进行对比分析。结果表明,在压缩荷载作用下,玻璃纤维/环氧树脂芯子为主要承载部分,结构的失效主要体现在芯子的屈曲、断裂和分层;在弯曲荷载的作用下,由于纤维的抗压强度远小于抗拉强度,所以压头下方的上面板最先达到破坏荷载,结构的弯曲失效形式主要为上面板的断裂和脱粘;结构的剪切失效主要以泡沫与面板的脱粘和压溃为主,芯子和面板未见明显的破坏现象;与单向波纹夹层结构相比,双向波纹夹层结构力学性能显著提升。