|Place of Conferral||北京|
|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|
|Other Abstract||The 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.
|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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