Fracture Toughness of Hybrid Carbon Fibre/Epoxy Enhanced by Graphene and Carbon Nanotubes

Carbon-based nanoparticles have attracted considerable attention in materials science and engineering fields as they can significantly improve the electro-thermo-mechanical properties of polymer-based materials. With the need of enhancing the mechanical property through the thickness direction of a carbon fibre reinforced polymer (CFRP) system, this study investigates the effect of graphene nanoplatelets (GNP), multi-walled carbon nanotubes (MWCNT) and their hybridisations on its Mode I interlaminar fracture toughness. Various nanoplatelet sizes and weight percentages are compared to prohibit their agglomeration in epoxy which can drastically reduce the mechanical properties of CFRP. The smallest GNP size, 1 μm, dispersed in the n-methyl-2-pyrrolidone solvent leads to an advanced 146% enhancement of Mode I interlaminar fracture toughness on the CFRP system. The acetone solvent is found less surface compatible with the nanoplatelets, but provides a simple and environmentally friendly manufacturing process. The hybrid GNP/MWCNT with 1wt% content dispersed in acetone solvent shows the synergistic effect and reaches a 120% enhancement of Mode I interlaminar fracture toughness of CFRP. Additionally, the application of the thin film hot press technique on nanoplatelets enhanced CFRP demonstrates an effective and promising solution to manufacture homogeneous multi-phase composites.

     

Effect of Fibre Volume Fraction on Mixed-Mode Fracture of a Fabric Carbon/Epoxy Composite

Variation in fibre volume fraction is a common characteristic of composites made by an injection moulding process. The effect of this variation on fracture toughness is not yet fully investigated. This paper examines the fracture in fabric carbon/epoxy composite laminates under a wide range of combined mode-I and mode-II delamination. A total of 60 double cantilever beam and edge-notched flexure specimens are manufactured by resin transfer moulding with two different fibre volume fractions. It was observed that increasing the fibre volume fraction decreased the initiation fracture toughness in all mixed-mode ratios. This behaviour is believed to relate to the fact that the initiation fracture energy is dominantly absorbed by the resin-rich regions at the delamination tip. In contrast, an increase in fibre volume fraction was found to increase the propagation fracture toughness at high mode-I contribution where the fibre bridging is believed to be the major energy dissipating mechanism. Fractographic analysis also demonstrated that an increase in contribution of mode-II delamination is accompanied by a decrease in fibre bridging and an increase in shear hackles.     

Loading Rate Dependence of Mode II Fracture Behavior in Interleaved Carbon Fibre/Epoxy Composite Laminates

The mode II delamination fracture toughness (G _IIC ) for interleaved and non-interleaved CFRP were measured at loading rates ranging from 0.2 to 125 mm/min. It was observed that interleaving could enhance G _IIC for CFRP significantly. Furthermore, G _IIC was observed to be independent of the loading rate for the non-interleaved CFRP. For the interleaved CFRP, the loading rate has a strong influence on G _IIC , and the loading rate dependence can be explained in terms of the fracture surface morphologies.     

复合材料厚壁圆筒的损伤问题

基于连续介质损伤力学理论,引入表征材料内部微细缺陷的损伤变量,导出了三维复合材料厚壁圆筒的损伤模型,预测该结构内各处的损伤过程;针对不同损伤模式,推导出包含不同结合力和损伤变量的损伤扩展准则;利用三维有限元分析软件模拟计算出结构损伤破坏的全过程,分析了复合材料圆筒的损伤模式与破坏机理,以及能量变化关系。     

炭纤维特性与炭纤维/环氧树脂界面断裂能关联分析

采用基于WND(Wagner-Nairn-Detassis)能量模型的单丝断裂法,测试了5种国产炭纤维、2种国外炭纤维与航空结构用环氧树脂复合体系的界面断裂能,通过SEM,AFM,IR以及XPS等手段分析了7种炭纤维的表面物理化学特性,并研究了炭纤维特性与界面断裂能的关联。结果表明:对于所研究的炭纤维/环氧树脂体系,去除炭纤维表面上浆剂后界面断裂能下降,说明上浆剂可以在一定程度上提高界面的韧性。此外,实验范围内,纤维拉伸强度较高时,测得的界面断裂能较高,炭纤维表面粗糙度较高时,测得界面断裂能较高,说明纤维拉伸性能和表面粗糙度对界面韧性有重要影响,而与这两种因素相比,上浆剂的种类影响相对较小。研究结果为高性能国产炭纤维的研发和炭纤维/树脂匹配性的评价提供了重要的实验数据。     

国内外碳/碳复合材料连接研究进展

介绍了近年来国内外碳/碳复合材料连接领域的研究进展,系统总结了碳/碳复合材料与自身及其与铜合金、钛合金、铌合金之间连接新技术与新工艺,并对有限元分析方法在碳/碳复合材料连接中的应用进行了简要的概括.针对碳/碳复合材料连接中存在的问题,提出了一些可行性解决方案,最后展望了碳/碳复合材料连接的研究前景.     

An Assessment of the Double-Notch Shear Test for Interlaminar Shear Characterization of a Unidirectional Graphite/Epoxy under Static and Fatigue Loading

This research explores a modified testing technique for measuring interlaminar shear properties of orthotropic composite materials. An existing test method (double-notched test method) is examined here to characterize the interlaminar shear properties (strength and fatigue life) of a unidirectional ply under both static and fatigue loading conditions. No complicated fixture is required for the testing method which is beneficial for fatigue testing of the materials. The testing method is verified by a finite element technique where an optimized geometry for the specimen is found. AS4/3501-6 graphite/epoxy material is used in this study. The experimental results show that final failure occurs in the gage area for both static and fatigue loading conditions. Moreover, a reasonable amount of scatter for both the static strength and fatigue life is achieved.     

An Assessment of the Double-Notch Shear Test for Interlaminar Shear Characterization of a Unidirectional Graphite/Epoxy under Static and Fatigue Loading

This research explores a modified testing technique for measuring interlaminar shear properties of orthotropic composite materials. An existing test method (double-notched test method) is examined here to characterize the interlaminar shear properties (strength and fatigue life) of a unidirectional ply under both static and fatigue loading conditions. No complicated fixture is required for the testing method which is beneficial for fatigue testing of the materials. The testing method is verified by a finite element technique where an optimized geometry for the specimen is found. AS4/3501-6 graphite/epoxy material is used in this study. The experimental results show that final failure occurs in the gage area for both static and fatigue loading conditions. Moreover, a reasonable amount of scatter for both the static strength and fatigue life is achieved.     

炭纤维增强树脂基复合材料模拟加速腐蚀方法的研究

对自然海洋大气环境下炭纤维/环氧树脂基复合材料的模拟加速腐蚀方法进行探索.根据已有的环境谱,初步设计实验室仿自然海洋大气环境的加速腐蚀方法,并对腐蚀前后的试样进行一系列力学性能测试及微观观察.结果表明:模拟日光、温度和湿度的共同作用会引起炭纤维/环氧树脂基复合材料表面及内部的腐蚀,并且破坏树脂基体与炭纤维之间的界面.与...     

Stress analysis of a Fe3Al/Q235 diffusion bonded joint using finite element analysis

Fe₃Al intermetallics and Q235 steel are bonded using vacuum diffusion bonding technology. The interfacial shear strength was measured and the stress of Fe₃Al/Q235 diffusion bonded joint was analyzed using finite element analysis (FEA). The results indicate that interfacial shear strength increases from 39.9 to 112.3 MPa with the enhancement of heating temperature from 1000 to 1060°C. Also, shear fracture has more characteristics of cleavage fracture. The farther from the central axis of the interface, the larger the stress is, and the maximum stress appears on the surface of the joint. The maximum stress increases with the heating enhancement and increase of material thickness. When the thickness reaches a critical value (6 mm) and then increases, the stress increases slightly and even to a stable value.     

On hybrid stress,hybrid strain and enhanced strain finite element formulations for a geometrically exact shell theory with drilling degrees of freedom

The paper is concerned with the finite element formulation of a recently proposed geometrically exact shell theory with natural inclusion of drilling degrees of freedom. Stress hybrid finite elements are contrasted by strain hybrid elements as well as enhanced strain elements. Numerical investigations and comparison is carried out for a four-node element as well as a nine-node one. As far as the four-node element is concerned it is shown that the stress hybrid element and the enhanced strain one are equivalent. The hybrid strain formulation corresponds to the hybrid stress formulation only in shear dominated problems, that is the case of the plate. © 1998 John Wiley & Sons, Ltd.     

Fracture toughness of the interface between Ni–Cr/ceramic,alumina/ceramic and zirconia/ceramic systems

Few studies have focused on the interface fracture performance of bi‐layered structures, which have an important role in dental restorations, using ceramic materials. The purpose of this study is to evaluate the fracture mechanics performance of the Ni–Cr/ceramic, alumina/ceramic and zirconia/ceramic interfaces by investigating the propagation of an interfacial crack under a wide range of mode mixities. The effect of the mechanical properties of the base materials and the interface, on the crack initiation and crack path, will also be studied. The finite element method (FEM) was used to calibrate the production of the experimental specimens, allowing to obtain the minimum dimensions and amounts of material needed to correctly characterize the fracture event. The specimens were tested until failure using a three‐point bending test machine. The interface fracture parameters were obtained using the FEM. For all specimens, the cracks propagated into the ceramic. The results suggest that, in Ni–Cr/ceramic, alumina/ceramic and zirconia/ceramic bi‐layered structures, the ceramic is weaker than the interface, which can be used to explain the clinical phenomenon that the ceramic chipping rate is larger than interface delamination rate. Consequently, a ceramic material with a larger fracture toughness is needed to decrease the failure rate of ceramic restorations.     

一种碳纤维/环氧预浸料的微波固化特性及其层合板的微波固化工艺研究

采用差示扫描量热法(DSC)和傅里叶变换红外光谱法(FTIR)研究一种碳纤维/环氧预浸料在微波辐射下的固化特性,并对碳纤维/环氧预浸料层合板的微波固化-真空袋成型工艺中存在的真空袋易过热破损的问题进行研究。结果表明:与热固化相比,微波固化能够明显缩短固化时间,而且不会改变最终固化产物的分子结构;碳纤维/环氧预浸料的尺寸大小对其微波固化行为有一定的影响;通过控制微波功率,可以有效地解决在碳纤维/环氧复合材料微波固化过程中存在的真空袋过热破损的问题,且微波固化可获得固化时间控制在60min左右、固化度达95%以上的碳纤维/环氧复合材料层合板,微波固化时间比传统热固化(固化时间大于2h)缩短了一半以上。     

Illustration of stress/strain behavior and yarn fragmentation in a pseudo-hybrid composite system

To study the stress/strain behavior and the fragmentation of fibers in a hybrid composite in tensile loading, a model strand was constructed with cotton yarns as the low-elongation component and polyester yarns as the high-elongation component. The effect of interfacial shear strength on the performance of a hybrid composite was simulated by varying the twist multiplier of the strands. It was observed that a strand with a lower twist level was more likely to show a typical hybrid stress/strain behavior and a multi crack fragmentation pattern. The same was found for strands with the highest twist levels but with cotton contents between 22 and 33%.     

Efficient numerical implementation of pressure,time, and temperature superposition for elasto‐visco‐plastic material model by using a symbolic approach

This article is concerned with the finite element implementation of an elasto‐visco‐plastic constitutive model using a symbolic approach. The model combines the Knauss–Emri (KE) pressure, temperature, and time superposition principle in the implicit finite element scheme. The equation development and code generation was performed using the symbolic tool AceGen. The same symbolic system was applied to derive analytical sensitivities of the numerical model with respect to the material and shape parameters. To enable efficient numerical implementation of the KE model the convolution integrals were transformed into their respective incremental forms, so that radical improvements of code efficiency and computer storage requirements were achieved. The numerical examples derived for polyethylene terephthalate (PET) polymers demonstrate that symbolic systems can be applied to develop complex constitutive models capable of simulating material responses that are in good agreement with experimental measurements over a wide range of strain rates, temperatures, and loading conditions. Copyright © 2010 John Wiley & Sons, Ltd.