IMECH-IR  > 非线性力学国家重点实验室
先进复合材料体系的界面力学特性研究
姚寅
Thesis Advisor陈少华
2013-02
Degree Grantor中国科学院力学研究所
Place of Conferral北京
Subtype博士后
Degree Discipline固体力学
Keyword碳纤维增强树脂基复合材料 界面改性 表面粗糙度 剪滞模型 应力传输 纤维拔出 热残余应力 桥联机制 梯度界面相
Abstract以碳纤维复合材料为典型代表的先进复合材料在国防及民用工业领域中有着极为广泛的应用。然而碳、芳纶等先进纤维普遍具有惰性表面,与基体粘合性能差,使得复合材料中界面结合性能较差,极易出现界面脱粘、纤维拔出等失效机制,降低复合材料内部的应力传输效率。为克服这一缺陷,研究者们提出了多种界面改性方法用以改善界面粘合状况,包括纤维表面刻蚀、界面涂层、改变界面处化学键等。界面改性处理对界面力学特性和复合材料整体性能均有显著影响,但目前关于这方面的研究仍大多限于实验观测,相应的理论工作较少,缺乏系统的研究和规律性的认识。
为了揭示界面性质对复合材料性能的影响机理,为界面优化设计提供理论指导,本文以碳纤维增强聚合物基复合材料为主要研究对象,对若干典型界面特性,如纤维表面粗糙度、梯度界面相等进行解析描述,并建立理论模型分析其对界面结合和复合材料整体力学性能的影响。主要研究内容及成果如下:
首先,研究了纤维表面横向粗糙度对具有交错结构聚合物基复合材料等效刚度的增强效应。推导得到的改进剪滞方程能够反映粗糙度增加使界面接触面积增大的改性作用。针对聚合物基体具有的两种界面剪应力传递方式,即摩擦和弹性传递分别进行研究,发现对于前者,粗糙度增加使纤维应力增大,而在后一种情况下则有利于降低界面剪应力集中;通过上述对应力传输机制的改善,复合材料刚度得以增强。
根据实验观察到的现象,对碳纤维表面真实形貌进行解析描述,将其与轴对称单纤维拔出模型相结合,分析碳纤维表面轴向粗糙度对碳/环氧树脂基复合材料界面结合性能的影响。所得到的理论预测结果与相关实验符合较好,同时发现当碳纤维长细比在30~45的范围变化时,粗糙度对界面强度的增强效果最为明显。
建立改进的桥联模型研究界面热残余应力和碳纤维表面轴向粗糙度对碳/环氧树脂基复合材料断裂行为的影响。经分析发现界面残余应力和粗糙度的增加能够改善界面粘合性能,有利于提高基体断裂强度,从而达到增强复合材料承载能力的效果;但同时界面强度的提高使纤维桥联机制的形成受到抑制,降低复合材料整体断裂韧性。与传统桥联模型相比,使用改进桥联模型得到的理论结果更加符合实际。
最后,研究了界面相梯度特性对碳纤维树脂基复合材料内部应力传输机制的影响。将纤维和基体间界面相层的弹性模量设为沿厚度方向幂次或线性分布的函数。发现当界面相模量为幂次分布时,纤维应力随界面层厚度增加而增加;而线性分布时则呈现相反的趋势,且对应幂次型界面相的纤维应力和界面剪应力均大于线性情况下的值,这些现象均可通过界面相平均刚度随厚度的变化关系来解释。对理论结果进行了有限元验证,发现数值模拟结果与理论结果符合较好,同时泊松比和热膨胀系数的梯度分布特性对结果的影响基本可以忽略不计。
Other AbstractThe advanced composites, such as carbon fiber-reinforced composites, have a wide range of applications in national defense and civilian industries. However, the advanced fibers (carbon、armaid) usually have invert surfaces, leading to a poor adhesion to the matrix. As a result, the interfacial bonding between fibers and matrix in advanced composites is always very weak, and interfacial failure mechanisms, such as debonding and fiber pull-out, are likely to occur, which result in a reduction of load transfer efficiency. To overcome this shortcoming, researchers developed many interface modification techniques for improving the interfacial adhesions, including surface etching, interface coating and changing the chemical bonds at the interfaces. These treatments have pronounced effects on the mechanical properties of interfaces and the overall performces of composites. As yet, researches on the interface modification are almost confined to experimental studies, while theoretical works are very few. Systematic studies and regular understandings on this issue still lack.  
In order to reveal the influences of interface properties on the performance of composites and provide some guidance for the optimal design of interfacial zone, we choose a carbon fiber-reinforced polymer composite as the main studied object in this paper. Some typical interface properties, such as fiber’s surface roughness and the graded interphase, are characterized analytically, and theoretical models are established to analyze their effects on the interface adhesions and overall performances of composites. The main contents and results are as follows:
Firstly, the enhancing effects of fiber’s transverse surface roughness on the effective stiffness of a staggered polymer composite are studied. The enlargement of interfacial contact area due to an increasing roughness, which is one of the main results of interface modification, can be characterized by the improved shear-lag equation. Two stress transfer modes of the polymer matrix, i.e., the frictional and elastic shear transfer, are analyzed, respectively. We find that in the former case, the increase of surface roughness leads to an increase of fiber stress, while in the latter, an increasing roughness can help to reduce the shear stress concentration at the interface. Consequently, the effective stiffness of composites can be enhanced due to the improvement of stress transfer mechanicsms.
Based on the experimental observations, an analytical characterization of the carbon fiber’s real surface morphlogy is presented, which is combined with the single fiber pull-out model to investigate the effects of carbon fiber’s longitudinal surface roughness on the interfacial adhesions in carbon/epoxy resin composites. The theoretical results agree well with the experimental ones. An interesting phenomenon is also found: when the fiber’s aspect ration lies within a range of 30~45, the increasing surface roughness has the most significant enhancing effects on the interfacial shear strength.
An improved fiber bridging model is established to study the effects of interfacial thermal residual stress and carbon fiber’s longitudinal surface roughness on the matrix cracking behavior of the carbon/epoxy composites. It can be found that the increases of thermal residual stress and surface roughness help to improve the interfacial adhesions. On the one hand, a stronger interfacial bonding leads to an increase of the matrix cracking stress and the load bearing capacity of composites can be enhanced; on the other, the overall toughness reduces since the formation of fiber bridging behavior is suppressed by the enhanced interfacial strength. The theoretical results predicted by our model are more consistent with the reality, as compared to those obtained by former bridging models.
Finally, the effects of graded interphase on the stress transfer mechanisms in carbon fiber-reinforced epoxy composites are studied. The elastic modulus of interphase layer between fiber and matrix is considered to vary according to a power law or a linear one in the thickness direction. For the power variation case, the fiber stress increases with an increasing interphase thickness, while an opposite trend can be found for the linear variation case. Moreover, the fiber stress and interfacial shear stress for the power variation case are apparently larger than those for the linear variation one. All these phenomena can be explained by the change of interphase average modulus versus the thickness. Meanwhile, finite element (FE) simulations are carried out and the numerical results agree with the theoretical ones, which show that the graded features of Poisson’s ratio and thermal expansion coefficient can hardly affect the results.
Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/49879
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
姚寅. 先进复合材料体系的界面力学特性研究[D]. 北京. 中国科学院力学研究所,2013.
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