纤维对短切碳纤维/AZ91D复合材料热变形行为和加工性能的影响纤维对短切碳纤维/AZ91D复合材料热变形行为和加工性能的影响

采用等温压缩试验研究了不同碳纤维体积分数的镁基复合材料（CFs/AZ91D）和镁合金（AZ91D）在变形温度310~430℃、应变速率10-3~10-1 s-1范围内的塑性变形行为。根据实验结果建立了CFs/AZ91D和AZ91D的热加工图,分析了纤维对CFs/AZ91D塑性加工性能与变形机制的影响。结果表明:相比ZA91D,纤维在提高复合材料流动应力的同时促进了基体动态再结晶和应变软化,但纤维体积分数对流动应力与应变软化程度影响较小,CFs/AZ91D热变形时表现出比ZA91D更高的应变速率敏感指数和变形激活能;ZA91D热加工图不存在变形失稳区且其高温低速率区变形时的能量耗散效率大于30%,CFs/AZ91D高温低应变速率区变形时的能量耗散效率大于50%,此时纤维激励了基体合金动态再结晶而使复合材料表现出极高的能量耗散效率,但在低温高应变速率变形时,基体合金与纤维之间的界面开裂极易导致CFs/AZ91D出现塑性流变失稳行为。      

纤维类别对三维浅交弯联复合材料弯曲性能影响的数值模拟

借助绘图软件PRO/E建立一种三层三维浅交弯联机织复合材料结构细观模型,并借助大型有限元软件ANSYS对该复合材料的弯曲力学性能进行模拟分析。分别将纤维材料定义为玻璃纤维和碳纤维,树脂基体定义为环氧树脂E51。对比在1kN的弯曲载荷作用下,两种不同类别纤维作为增强体时的复合材料、纤维增强体和树脂基体的应力、应变分布情况,预测复合材料的破坏形式,并与实验结果定性对比。结果表明,玻纤作为增强体时比碳纤维表现出更大的弯曲应力和弯曲应变,更容易发生破坏;1kN弯曲载荷作用下复合材料的破坏形式主要为纤维增强体的变形,树脂基体的碎裂以及纤维增强体和树脂基体间的脱粘。     

NiTi纤维增强粘弹性基体与弹性基体复合材料的力学特性

借助Voigt混合律,针对马氏体逆相变过程,建立了NiTi纤维增强Kelvin粘弹性基体与弹性基体两种复合材料的应力-应变关系.在同样的升温过程及应力作用下,对两种复合材料的变形(应变)、约束态NiTi纤维逆相变开始温度进行了分析对比;升温、加载经历一段时间后,限制两种复合材料的应变为固定值,分析其受力特征.分析有助于了解NiTi纤维增强粘弹性基体与弹性基体复合材料的力学性能.     

2D-C/SiC复合材料低温动态Z向压缩性能

采用Hopkinson压杆试验装置,对2D-C/SiC复合材料进行了低温条件下的Z向动态压缩性能试验研究,低温条件通过控制酒精和液氮的配比系数得到,通过改变波形整形器几何尺寸的方法来实现恒应变率加载,以得到准确、可信的试验结果。试验结果表明: 由于复合材料内部含有大量初始微缺陷,2D-C/SiC复合材料在低温动态加载条件下呈现伪塑性行为,其破坏时并未表现出典型的脆性破坏,而是在应力达到压缩强度时出现了显著的应变软化现象,在经历了较大的变形后才最终破坏。随着温度的降低,复合材料的动态Z向压缩强度增加,但失效应变减小。2D-C/SiC复合材料在低温环境下,其内部纤维和基体之间界面结合力增强,同时强的界面结合力可以导致高的压缩强度。     

金龟子外甲壳异型纤维结构分析

运用扫描电镜实验观察了金龟子外甲壳内部的微结构,发现其类似于人造纤维增强复合材料,具有几丁质纤维增强角质化蛋白质基体的结构特征.对其仔细观察发现其中存在非均匀的纤维铺层以及奇特的异型刺状和球形纤维.针对发现的异型刺状纤维结构,建立纤维端头模型,用有限元分析方法对其进行了应力和变形分析,结果表明刺状和球形端头纤维能显著地降低其端头界面剪应力集中现象,提高纤维增强复合材料抵抗变形破坏的能力,增加纤维增强复合材料的强度.分析结果对仿生复合材料的设计具有指导意义.     

剪切载荷下含椭圆形大开口层合板的试验研究

在面内纯剪切载荷作用下, 采用试验与有限元模拟方法研究了结构中心设置椭圆形大开口的正方形复合材料层合板的应力/应变集中现象及屈曲、 后屈曲行为, 通过测试结果对试验件失效模式进行了评估。研究结果表明: 层合板开口附近应力/应变集中程度很高; 大尺寸开口使结构稳定性显著降低, 且开口层合板具有较好的后屈曲承载能力; 由于弯曲产生高水平的层间应力, 导致局部分层损伤并伴有基纤剪切破坏; 随着横向挠度的增加, 各应力集中区域的纤维发生拉伸断裂, 导致整体结构瞬间发生脆性失效。有限元模拟结果与试验结果符合较好。     

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。     

晶须增强铝基复合材料的热压缩变形行为研究

采用试验和数值模拟相结合的方法研究了纵向、横向和倾斜分布的晶须增强铝基复合材料热压缩变形行为.研究表明:SiCw/6061Al复合材料300℃压缩的应力-应变行为与晶须取向角密切相关;在热变形过程中,纵向分布晶须折断严重,导致复合材料表现为应变软化行为;而倾斜于压缩方向30°的晶须折断和转动明显,引起相应复合材料应变软化;横向分布晶须没有转动而折断程度很小,使复合材料呈现出加工硬化行为.     

填隙补偿对碳纤维/环氧树脂复合材料-铝合金装配结构力学性能的影响

碳纤维/环氧树脂复合材料和铝合金作为主要的航空材料，在飞机结构中存在着大量装配关系，但受成型工艺方法的限制，两种材料在制造和装配偏差的情况下，构件配合面间会产生装配间隙，当间隙超过一定大小时，需要采取填隙补偿措施。本研究基于实际结构抽象出碳纤维/环氧树脂复合材料-铝合金装配模型，使用装配试验台模拟施加螺栓预紧力，通过应变片实验比较强迫装配及垫片补偿情况下试件局部表面的应变分布，结合三维数字图像相关(3D-DIC)实验测得的试件表面应变场分析变形规律；通过有限元进行层间应力分析，提取内聚力单元各应力分量和损伤情况，研究填隙补偿对碳纤维/环氧树脂复合材料层间应力和局部损伤的影响。结合实验和仿真分析结果表明:强迫装配时，碳纤维/环氧树脂复合材料-铝合金试件主要受弯曲变形和螺栓头挤压的影响，且随着装配间隙的增大，各应变值均增大；垫片补偿在改善弯曲变形引起的应变状态的同时，也使中间贴合部位的螺栓头挤压区应变增大，但总体而言，垫片的引入使碳纤维/环氧树脂复合材料-铝合金试件表面应变分布趋于均匀，降低了碳纤维/环氧树脂复合材料损伤情况，且液体垫片补偿效果略好于可剥垫片。      

碳纤维复合材料的压缩响应的微观分析

设计了实验平台,对纤维压缩应变与电阻间灵敏系数进行了标定,将标定后的纤维制作成单丝和多丝复合材料体系压缩试验件.在压缩载荷条件下,通过电阻测量,揭示了多丝复合材料体系纤维断裂对周围纤维的影响规律,并准确测量出纤维发生压缩破坏时的最大变形量;利用数值方法模拟了断口处基体应力分布导致附近纤维微屈曲的损伤过程.     

碳纤维/环氧树脂预浸带铺放应力场及制品微观结构的模拟与实验

碳纤维/环氧树脂预浸带复合材料在铺放成型时,由于树脂基体与碳纤维之间的热膨胀系数存在差异以及成型时热-力参数作用下由于纤维的变形而导致纤维与基体接触处产生应力集中等原因,在制品材料中会产生热残余应力。针对碳纤维/环氧树脂预浸带复合材料的实际结构特点,利用ABAQUS有限元软件建立含有界面的碳纤维/环氧树脂预浸带复合材料的细观代表性体积单元(Representative volume element, RVE)有限元模型,采用实验研究和有限元仿真分析的方法,研究在温度-压力参数作用下预浸带铺放制品残余应力的分布规律及影响机理。首先,建立预浸带铺放时的温度和压力模型,研究不同温度和压力参数条件下碳纤维/环氧树脂预浸带铺放制品残余应力的分布情况。其次,采用耦合降温法模拟碳纤维/环氧树脂预浸带残余应力随纤维体积含量、铺放压力以及铺放温度的变化规律,并采用扫描电镜对不同工艺参数条件下预浸带铺放制品的微观结构进行分析。通过对模拟结果进行分析比较得到各因素对制品残余应力的基本影响规律;最后进行不同温度和压力等铺放参数对预浸带铺放成型时残余应力影响的实验测试研究。      

Inherent sensing and interfacial evaluation of carbon nanofiber and nanotube/epoxy composites using electrical resistance measurement and micromechanical technique

Inherent sensing of load, micro-damage and stress transferring effects were evaluated for carbon nanotube (CNT) and carbon nanofiber (CNF)/epoxy composites (with various added contents) by an electro-micromechanical technique, using the four-point probe method. Carbon black (CB)/epoxy composites, with conventional nanosize material added, were used for the comparison with CNT and CNF composites. Subsequent fracture of the carbon fiber in the dual matrix composites (DMC) was detected by acoustic emission (AE) and by the change in electrical resistance, ΔR due to electrical contacts of neighboring CNMs. Stress/strain sensing of the nanocomposites was detected by an electro-pullout test under uniform cyclic loading/subsequent unloading. CNT/epoxy composites showed the best sensitivity to fiber fracture, matrix deformation and stress/strain sensing, whereas CB/epoxy composite exhibited poorer sensitivity. From the apparent modulus (as a result of matrix modulus and interfacial adhesion), the stress transferring effects reinforced by CNT was highest among three CNMs. The thermodynamic work of adhesion, W_a as found by dynamic contact angle measurements of the CNT/epoxy composite as a function of added CNT content was correlated and found to be consistent with the apparent mechanical modulus. Uniform dispersion and interfacial adhesion appear to be key factors for improving both sensing and mechanical performance of nanocomposite. Thermally treated-CNF composites exhibited a slightly higher apparent modulus, whereas higher testing temperatures appeared to lower the apparent modulus.     

三维编织碳纤维/环氧树脂复合材料横向压缩性质的温度效应

采用实验和有限元方法,研究了三维编织碳纤维/环氧树脂复合材料在低温场（20、0、-50、-100℃）中横向压缩性质温度效应。研究结果表明：温度对碳纤维/环氧树脂横向压缩模量、屈服应力及切向模量均有不同程度影响。三维编织碳纤维/环氧树脂复合材料横向压缩后,试样表面形貌受温度影响显著。低温场中,表面鳞纹现象减弱,且纱线-树脂间界面出现开裂。温度降低导致碳纤维/环氧树脂内部产生热应力。热应力对碳纤维/环氧树脂力学性能影响有限,不是温度效应的主导因素。基体性质随温度变化是三维编织碳纤维/环氧树脂复合材料横向压缩性质温度效应的主要机制。     

Evaluation of time-dependent change in fiber stress profiles during long-term pull-out tests at constant loads using Raman spectroscopy

Constant-load pull-out tests were carried out on single-fiber model composite specimens for 500 to 1,000 hours in order to investigate the time-dependent change in fiber axial stress profiles resulting from matrix creep in unidirectional continuous fiber-reinforced composites. Three resins used as the matrix materials, in which single carbon fibers were embedded, were normal epoxy, a blend with a more flexible epoxy, and UV-curable acrylic. The time-dependent change in fiber stress profiles in the constant-load pull-out tests was measured using Raman spectroscopy, and creep and relaxation tests for the matrix resins themselves were performed. It was observed that the normal epoxy matrix composite exhibited only a negligible change in the fiber stress profile with time whereas the flexible epoxy and UV-curable acrylic matrices allowed, respectively, considerable and significant changes. These observations were shown to be consistent with the creep and stress relaxation test results of the matrix resins. It was also found that the time-dependent change in fiber stress was much slower in the experiment than in the prediction based on perfect bonding at the fiber/matrix interface. The interfacial slip that occurred in the composites tested could be responsible for the gradual variation in fiber stress profiles.     

The effect of thermoplastic coating on the mechanical properties of woven fabric carbon/epoxy composites

The effect of a polyetherimide (PEI) coating on the mechanical properties of woven fabric carbon/epoxy composites was investigated by thermal mechanical analysis, fractographical analysis and mechanical properties measurements. PEI coating enhanced the mechanical properties of carbon/epoxy composites mainly through the improvement of matrix properties. This was because most of the PEI coated on the carbon fiber diffused into the bulk of epoxy matrix due to its good miscibility with epoxy resin. As for mechanical properties of woven fabric carbon/epoxy composites, the extent of improvement by PEI coating highly depended on the applied stress state. Among the mechanical properties, mode II delamination resistance of carbon/epoxy composites showed the highest increment because matrix shear property played an important role in delamination resistance of woven fabric carbon/epoxy composite. Because of the woven geometry of carbon fiber, the improvement in impact property of carbon/epoxy composite was trivial except the large amount of PEI coated case.     

Stress concentration factors in two-dimensional composites: effects of material and geometrical parameters

In the study of fracture processes in composite materials, the interactions between broken and intact fibers are of critical importance. Indeed, the redistribution of stress from a failed fiber to its unfailed adjacent neighbors, and the stress concentration induced in these, determine the extent to which a break in one fiber will cause more breaks in neighboring fibers. The overall failure pattern is a direct function of the stress concentration factors (SCFs). In this paper, we propose a new model for the SCFs in two-dimensional unidirectional composites containing broken fibers. A closed-form expression is derived for the SCF profiles as a function of material and geometrical parameters. The model differs significantly from earlier schemes, as the local effect of a fiber break on nearest neighbors is much milder than previously calculated, both as a function of the inter-fiber distance and of the number of adjacent broken fibers. Comparison with experimental results for silicon-carbide/epoxy composites demonstrates the validity of the proposed scheme. Since the overall fracture pattern in fiber composites is a direct function of the SCFs, the model may help shed light on fracture nucleation and growth in composites.     

Micromechanics analysis of Kevlar-29 aramid fiber and epoxy resin microdroplet composite by Micro-Raman spectroscopy

The effect of geometry on interfacial micromechanical behavior of fiber/matrix microdroplets is investigated by means of the combination of microbond test and Micro-Raman spectroscopy in this paper. Microbond test is usually employed to measure the interfacial properties of fiber/matrix composites. Since the existing method was realized through traditional microscopy technique, the test results may not satisfy those accurate analyses such as geometry influence on interfacial micromechanical behavior of composites. To overcome this problem, Micro-Raman spectroscopy is introduced into the microbond test to detect the distributions of micromechanical properties including fiber axial stress, residual stress, interfacial shear stress and stress transfer length along the interfaces between Kevlar-29 aramid fiber and epoxy resin matrix microdroplets. The obtained experimental results show that axial stress transfer will accelerate and then the shear stress concentration will be enhanced along with an increase in the interfacial edge angle. The study indicates that the geometrical characteristics can affect significantly the stress distributions in the fiber/matrix microdroplets.     

多壁碳纳米管-有机蒙脱土协同增韧环氧树脂

采用机械搅拌和离心分散的方法制备了多壁碳纳米管-有机蒙脱土/环氧树脂复合材料。X射线衍射分析表明,当有机蒙脱土含量为2 wt%时, 蒙脱土在树脂体系中能够形成离散性结构。断裂韧性测试结果表明,多壁碳纳米管和有机蒙脱土的混杂对环氧树脂具有协同增韧的作用。当有机蒙脱土含量为2 wt%,多壁碳纳米管含量为0.1 wt%时,所得复合材料的断裂韧性是纯环氧树脂的1.77倍,是2 wt%有机蒙脱土/环氧树脂复合材料的1.45倍,是0.1 wt%多壁碳纳米管/环氧树脂复合材料的1.39倍。扫描电镜分析表明,多壁碳纳米管在环氧树脂体系中分散均匀,并与有机蒙脱土片层形成了一定程度的相互穿插和咬合,多壁碳纳米管与有机蒙脱土协同增韧的主要原因是微裂纹增韧、剪切屈服与纤维拔出。      

基于纳米压痕技术的碳纤维/环氧树脂复合材料各组分原位力学性能测试

基于纳米压痕技术对碳纤维/环氧树脂复合材料各组分的原位硬度、 弹性模量和蠕变性能进行了测试, 实验得到了基体、 纤维和微小厚度界面层的力学性能。结果表明, 从环氧树脂基体到碳纤维过渡过程中, 硬度和弹性模量有明显的梯度变化, 并且纤维和树脂基体的原位弹性模量平均值与其非原位性能有一定的变化, 实验得到纤维的原位弹性模量有所下降, 环氧树脂的弹性模量有所增加。试件制备过程中的机械研磨对其表面产生的残余应力和复合后两种材料的相互影响是组分材料原位性能变化的主要原因。各组分的蠕变性能呈现出明显的差异。