基于宏微观分析的碳纤维增强高分子复合材料强度性能表征

微观力学强度理论(MMF)是一种新型的基于物理失效模式的复合材料强度理论。通过对碳纤维/树脂(UTS50/E51)复合材料单向层合板进行纵向、横向静载拉伸、压缩和弯曲试验, 得到层合板的基本力学性能和宏观强度指标。建立了碳纤维增强树脂基复合材料微观力学模型, 获取树脂基体和纤维不同位置的机械载荷应力放大系数和热载荷应力放大系数。结合获取的应力放大系数及试验测得的单向层合板宏观强度, 计算出层合板组分的MMF强度特征值。绘制了基于MMF强度理论的层合板破坏包络线, 并与Tsai-Wu失效准则预测结果进行对比。实现了对UTS50/E51层合板MMF强度特征值的表征。     

基于微观力学失效理论的CFRP多向层合板低速冲击行为预测

基于微观力学失效(MMF)理论对碳纤维增强复合材料(CFRP)多向层合板在低速冲击载荷下失效机制及损伤过程进行分析和预测。建立基于MMF理论的层合板结构冲击损伤行为分析方法。首先, 使用MMF理论对冲击过程中组分的失效类别进行判别; 然后, 根据组分失效的类别制定出相应的材料性能退化方案来实现对复合材料在低速冲击下的逐步失效分析;在ABAQUS平台上开发了基于显示分析的用户材料子程序(VUMAT), 即基于MMF理论的层合板冲击损伤分析程序;最后, 利用MMF理论冲击损伤行为分析方法, 对UTS50/E51碳纤维增强复合材料多向层合板在小能量低速冲击情况下的失效机制和损伤形貌进行预测, 并将预测结果与试验结果进行对比, 分析了利用MMF理论预测冲击损伤这一方法的准确性。结果表明理论预测的凹坑直径与试验测试的凹坑直径误差为4.8%, 预测的失效机制和损伤形貌与实际观察的一致。      

复合材料层合板多尺度交互渐进损伤分析

纤维增强复合材料强度的准确表征是复合材料力学性能研究的核心问题之一。该文以碳纤维增强树脂基复合材料层合板为研究对象,基于宏观-细观多尺度分析方法,根据复合材料的物理失效模式分别给出了基体和纤维的细观失效准则,同时考虑基体失效对复合材料层合板纤维轴向力学性能的影响。提出了新的刚度退化方式,可准确表征复合材料层合板的损伤演化过程,开展了复合材料层合板四点弯模型的多尺度交互渐进损伤分析和试验验证。结果表明:基于多尺度方法的复合材料层合板宏-细观交互渐进损伤分析结果与试验结果吻合较好,新的刚度退化方式可以准确模拟层合板的失效过程。     

接头尺寸对玻璃纤维/热塑性树脂复合材料机械连接性能的影响接头尺寸对玻璃纤维/热塑性树脂复合材料机械连接性能的影响

对玻璃纤维/聚酰胺（GF/PA）、玻璃纤维/聚甲醛（GF/POM）、玻璃纤维/聚丙烯（GF/PP）这三种玻璃纤维增强热塑性树脂基复合材料进行机械连接试样的常规拉伸试验,以及低周疲劳拉伸试验,并对疲劳前后的试样断裂面进行SEM观察,研究了接头尺寸（宽径比w/d （试样宽度/开孔直径）和端径比e/d （试样端距/开孔直径））对机械连接件破坏载荷和破坏模式的影响。实验结果表明:玻璃纤维增强纤热塑性树脂复合材料机械连接件的承载能力在一定的宽径比时会随着e/d的增加而增加,当w/d≥3、e/d≥2时趋于稳定;破坏模式以拉伸破坏为主;低周疲劳拉伸对GF/POM和GF/PA机械连接试样拉伸强度产生一定的影响,而对GF/PP的拉伸强度无明显影响,低周疲劳拉伸对玻璃纤维增强热塑性树脂复合材料机械连接试样的破坏模式没有影响。SEM观察显示,随着疲劳载荷水平的增加,GF/POM和GF/PA的断裂面上被抽拔纤维数量增加,而GF/PP断裂面纤维与基体的存在状态无明显变化。      

单向碳纤维增强树脂基复合材料的超低温力学性能

采用真空袋-热压罐工艺制备单向碳纤维增强树脂基复合材料(CFs/EP)层合板,并将高低温试验箱与万能试验机相结合,通过合理使用低温胶和低温引伸计,并在降温过程中实施应力-应变实时调零等关键技术,在室温和液氧超低温度(-183℃)下对单向CFs/EP层合板进行拉伸和弯曲试验,研究了其超低温力学性能,并根据室温和超低温试验后试样的微观和宏观特征,揭示了超低温环境下复合材料力学性能变化机制。结果表明,与室温力学性能相比,单向CFs/EP层合板超低温拉伸强度下降约为9.5%,而拉伸模量上升约为6.2%,主要是由于超低温环境下,树脂的收缩使绝大部分碳纤维与树脂间形成了强界面,拉伸后试样呈"劈裂式"破坏形式,无法使每根纤维都充分发挥其强度,拉伸强度下降,同时超低温也限制了树脂大分子链的运动,所以导致单向CFs/EP层合板拉伸模量上升;单向CFs/EP层合板超低温弯曲强度和弯曲模量分别提高约54.75%和11.64%,这是由于单向CFs/EP层合板的常温和超低温的弯曲破坏形式均为分层剪切破坏,超低温下复合材料的界面增强,提高了单向CFs/EP层合板抵抗剪切分层的能力,进而使CFs/EP的弯曲性能得到提高。     

国产碳纤维增强树脂基复合材料应变不变量性能

应变不变量失效理论是一种新型的基于物理失效模式的复合材料强度理论, 广泛应用于复合材料结构失效分析。根据该理论, 建立了碳纤维增强树脂基复合材料微观力学模型, 获取树脂基体和纤维不同位置的机械应变放大系数和热应变放大系数。对国产复合材料CCF300/5228、CCF300/5428和T700/5428不同铺层角单向层合板进行拉伸试验, 并根据试验结果得到对应复合材料的应变不变量性能。分析了增强体纤维和树脂基体对复合材料应变不变量性能的影响; 并将应变不变量失效理论应用于国产复合材料失效分析。结果表明碳纤维增强树脂基复合材料应变不变量性能中的第一应变不变量和基体Von-Mises应变临界值取决于树脂, 而纤维Von-Mises应变临界值取决于增强体纤维; 应变不变量失效理论能够用于国产复合材料失效分析。      

基于纤维束/环氧树脂复合材料试验的单向层合板横向拉伸强度预测方法

实验研究表明,纤维束/环氧树脂复合材料试件的横向拉伸强度与工程上常用的单向层合板横向拉伸强度在趋势上具有很好的相关性,但是数值上存在一定差距。本文使用两种碳纤维和两种环氧树脂制备了三种纤维束/环氧树脂复合材料和单向层合板,并分别测量了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度,以及环氧基体的拉伸强度。在实验基础上,应用Griffith断裂强度理论建立了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度的关系模型,通过两种复合材料实验的结果拟合了该模型中的参数。利用第三种复合材料实验进行校验,发现该模型预测的单向层合板横向拉伸强度与实测强度之间达到很好的一致性,相对偏差为9%。采用本文提出的方法,可以用较为简单的纤维束/环氧树脂复合材料和环氧基体拉伸试验预测单向层合板的横向拉伸强度。     

接头局部增强的复合材料层合板螺栓连接试验与数值模拟

采用真空辅助成型工艺（VARI）制备了四种局部增强的复合材料层合板螺栓连接试件，通过试验及数值模拟对其力学性能进行了研究。数值研究中将复合材料层合板连接件的拉伸作为一个准静态问题，运用ABAQUS的显示分析算法及所编写用户材料子程序VUMAT对连接件进行了三维渐进失效模拟，同时在有限元模型中采用内聚力单元模拟了层合板与所设增强层的界面分层失效。数值计算结果与试验结果取得了较好的一致，验证了本文中数值方法的有效性。研究结果表明，不同的局部增强方案对复合材料螺栓连接性能的影响较大，设置[0/90/0/90]_S铺层的内置纤维增强层能显著提高层合板的螺栓连接性能。     

FRP复合材料剩余刚度退化复合模型

为建立剩余刚度与材料损伤量及剩余寿命的关系, 将纤维增强树脂复合材料(FRP)层合板在拉-拉疲劳载荷作用下的失效模式划分为纤维间破坏、纤维随机断裂与分层3种类型, 分析不同失效模式与剩余刚度退化量的定量关系, 提出一个集成各失效模式影响的剩余刚度退化复合模型。该模型适用于占寿命绝大多数比例的Ⅰ、Ⅱ阶段, 避免了Ⅲ阶段刚度降不确定性的影响。剩余刚度退化曲线按时间尺度归一化, 消除了试件个体分散性影响, 分散性显著降低。对4种E-glass/Epoxy玻璃纤维复合材料层压板与3种AS-4/聚醚醚酮(PEEK)碳纤维复合材料层压板的疲劳试验结果进行了统计分析, 表明本文模型适于精确描述复合材料的剩余刚度下降规律。      

VARI工艺成型纤维增强树脂复合材料层合板厚度和纤维体积分数的影响因素

分析了影响真空辅助成型技术（VARI）工艺成型复合材料的纤维体积分数和厚度均匀性的关键因素,即VARI成型工艺的树脂流动控制形式、纤维预制体状态、织物状态、树脂黏度,通过试验分析了各因素对VARI成型复合材料厚度和纤维体积分数的影响。试验结果表明,采用HFVI（high fiber-volume vacuum infusion）工艺、BA9914树脂及真空处理后的U3160单向机织物成型的纤维增强树脂复合材料层合板,其纤维体积分数和厚度均匀性能够接近预浸料/热压罐成型的复合材料制件的水平。     

Evaluation of fatigue behavior on repaired carbon fiber/epoxy composites

The continuous use of structural polymer composites in aeronautical industry has required the development of repairing techniques of damages found in different types of laminates. The most usually adopted procedure to investigate the repair of composite laminates has been by repairing damages simulated in laminated composite specimens. This work shows the influence of structural repair technique on mechanical properties of a typical carbon fiber/epoxy laminate used in aerospace industry. When analyzed by tensile test, the laminates with and without repair present tensile strength values of 670 and 892 MPa, respectively, and tensile modulus of 53.0 and 67.2 GPa, respectively. By this result, it is possible to observe a decrease of the measured mechanical properties of the repaired composites. When submitted to fatigue test, it is observed that in loads higher than 250 MPa, this laminate presents a low life cycle (lower than 400,000 cycles). The fatigue performance of both laminates is comparable, but the non-repaired laminate presented higher tensile and fatigue resistance when compared with the repaired laminate.     

基于纤维束增强树脂基复合材料测试的单向层合板层间剪切性能的预估方法

纤维束增强树脂基复合材料（FBC）及其单向层合板在标准Iosipescu剪切实验中表现出非常相似的破坏特征,然而测量得到的剪切强度却有明显差异。本文使用两种碳纤维和两种环氧树脂制备了3种FBC和单向层合板,对FBC剪切强度和单向层合板层间剪切强度进行了测量与分析。应用界面单元方法分析了纤维束与基体之间的界面应力场,发现FBC剪切试件中纤维束/基体界面附近的应力状态为拉剪耦合,而单向层合板中界面处于纯剪切应力状态,这一差异导致FBC剪切实验测量的强度低于单向层合板的剪切强度。本文基于Yamada-Sun强度理论建立了FBC剪切强度与单向层合板剪切强度之间的关系模型,应用该模型预测的单向层合板剪切强度与实测强度之间达到良好的一致性,相对偏差为10%左右。根据本文提出的方法,通过制样较简单的FBC试验能够预测和评估相应单向层合板的层间剪切性能。     

Investigation of fatigue crack growth rate in CARALL,ARALL and GLARE

Fibre metal laminates (FMLs) are being used to manufacture many structural components in aerospace industry because of their very high strength to weight ratios, yet the exact model for estimating fatigue crack propagation in FMLs cannot be developed because of many variable parameters affecting it. In this research, tensile strength, fatigue life and fracture toughness values of 2/1 configuration carbon reinforced aluminium laminate (CARALL), aramid reinforced aluminium laminate and glass laminate aluminium reinforced epoxy specimens have been investigated. Mechanical, chemical and electrochemical surface treatments were applied to AA 1050 face sheets to improve the adhesive properties of the laminates. The specimens were prepared using vacuum assisted resin transfer moulding technique and were cut to desired shapes. Fatigue tests were conducted on centre notched specimens according to ASTM Standard E399. Real time material data and properties of adhesive were used in definition of numerical simulation model to obtain the values of stress intensity factor at different crack lengths. It was observed that CARALL shows very superior tensile and fatigue strength because of stress distribution during failure. Numerical simulation model developed in this research accurately predicts fracture toughness of aramid reinforced aluminium laminate, CARALL and glass laminate aluminium reinforced epoxy with less than 2% error. An empirical analytical model using experimental data obtained during research was developed which accurately predicts the trend of FMLs fatigue life.     

碳纤维织物布层压复合材料湿热环境疲劳后剩余压缩强度

为了研究湿热环境对碳纤维织物布层压复合材料疲劳性能的影响,开展了碳纤维织物布含中心孔层压板疲劳试验和剩余压缩强度试验,并为湿热环境拉-压疲劳试验自行设计了湿热环境试验箱。参照ASTM标准设计加工了疲劳试验夹具和压缩试验夹具,分别在标准环境条件(温度(23±2)℃、相对湿度(50±5)%RH),湿热环境条件(温度(70±2)℃、相对湿度(85±5)% RH)下对试件进行拉-压疲劳试验,之后分别进行了剩余压缩强度试验。以标准环境疲劳试验件的剩余压缩强度为基准值,对比湿热环境疲劳试验件的破坏载荷对应的极限压缩强度,计算其与基准值的差值。通过试验结果对比,得到湿热环境下试验件疲劳后剩余压缩强度下降了14%,该湿热环境对载荷谱下的碳纤维织物布层压复合材料试验件疲劳后剩余压缩强度影响较小。      

含孔CFRP正交层合板的双轴拉伸力学行为

采用加载臂开槽的中心开孔等厚度十字形试样,实验研究了正交对称铺层碳纤维增强聚合物基复合材料（CFRP）层合板在双轴拉伸载荷作用下的力学行为,分析了3种双轴加载比对其拉伸强度和破坏行为的影响。研究表明:纤维被切断的铺层部分在拉伸作用下容易与其相邻铺层脱粘,导致层合板承载力下降;等双轴加载时,在孔边的被切断纤维与连续纤维间基体在横向拉伸和纵向剪切组合作用下首先开裂;非等双轴加载时,在垂直于快速拉伸方向的铺层中沿孔边应力集中处先出现基体裂纹;随着加载比的增大,快速拉伸方向的细观结构损伤随载荷的增大发展更快,刚度下降更快,破坏时主裂纹的扩展方向更趋于垂直于快速拉伸方向;强度包络线的分析表明快速拉伸方向的拉伸强度随加载比的增大呈缓慢增大的趋势。      

Investigation on the effects of fiber composites at concrete joints

This investigation involves parametric studies of the application of fiber-reinforced polymer (FRP) composite laminates to exterior beam–column joints to increase their moment capacity. Three beam–column joint models were examined using various fiber composite laminates and wraps, and various thickness. Composite laminates and wraps considered were made out of epoxy and fibers such as E-glass, carbon, and kevlar. One beam–column joint model without FRP reinforcement was used as a control specimen for comparison. The other two beam–column joint models studied included laminates bonded to the tensile faces with and without wraps. The wraps were provided to prevent the peeling of the laminates. The finite element analysis results indicated that the choice of the fiber composite materials, the laminate and wraps arrangement and thickness affected the enhancement of the structural joint performance significantly. Furthermore, an increase in the moment capacity of up to 37% was observed when the joints were reinforced with FRP laminates and were compared to the control specimen.     

Compression properties of z-pinned composite laminates

The effect of z-pinning on the in-plane compression properties and failure mechanisms of polymer laminates is experimentally studied in this paper. The reduction to the compression modulus, strength and fatigue performance of carbon/epoxy laminates with increasing volume content and diameter of pins is determined. The elastic modulus decreases at a quasi-linear rate with increasing pin content and pin diameter. Softening is caused by fiber waviness around the pins and reduced fiber volume content due to volumetric swelling of the laminate from the pins. A simple model is presented for calculating the compression modulus of pinned laminates that considers the softening effects of fiber waviness and fiber dilution. The compression strength and fatigue life also decrease with increasing volume content and diameter of the pins. The strength and fatigue properties are reduced by fiber kinking caused by fiber waviness around the pins and the reduced fiber content caused by swelling. The deterioration to the compression properties is also dependent on the fiber lay-up pattern of the laminate, with the magnitude of the loss in properties increasing with the percentage of 0° (load bearing) fibers in the laminate. The paper gives suggestions for minimizing the loss to the compression properties to laminates due to pinning.