Aligned short glass fibre/epoxy composites

An orientation system based on the principles of the ERDE glycerine process was built and used to prepare short glass fibre mats of known orientation distributions. The mats were subsequently impregnated with epoxy, B staged to yield partially cured prepregs and finally compression moulded. Mechanical characterization and comparison with theoretical formulations were performed on longitudinal, transverse and off-axis specimens. The mechanical characteristics of specimens with different orientation patterns were compared with analytical predicitions based on ‘Laminate analogy’ methods. Fracture surfaces were studied by optical and scanning electron microscopy techniques.     

Monitoring the micromechanics of reinforcement in carbon fibre/epoxy resin systems

The technique of laser Raman spectroscopy (LRS) was employed to obtain the interfacial shear stress (ISS) distribution along a short high-modulus carbon fibre embedded in epoxy resin at different levels of applied stress. Up to 0.6% applied strain, the ISS reached a maximum at the bonded fibre ends and decayed to zero at the middle of the fibre. At higher applied strains, the maximum value of the ISS distribution shifted away from the fibre ends towards the middle of the fibre. At the point of fibre fracture, fibre/matrix debonding was found to initiate at the fibre breaks. Further increase of applied strain resulted also in debonding initiation at the fibre ends. Current analytical stress-transfer models are reviewed in the light of the experimental data.     

Interface chemistry in carbon fibre/epoxy matrix composites

Microcomposite test methods were used to measure the properties of the interphase of HTA/922 carbon/epoxy composites. The shear strength of the interphase resin is lower than that of the bulk resin. It is suggested that the discrepancy arises from changes in resin chemistry at the fibre/matrix interface. Bulk resin samples where the proportions of the constituents had been altered were tested. Resin with a reduced level of hardener matched the mechanical behaviour of the interphase resin. It is concluded that, for the system examined, the interphase resin had a lower hardener concentration than the bulk resin.     

Fatigue life of E-glass fibre/zirconia/epoxy hybrid composites

An ultrasonic technique has been used to evaluate the degradated fatigue life of glass fibre/zirconia hybrid composites in different concentrations of sodium chloride and sulphuric acid. Results indicate that reduction in fatigue life of hybrid composites increases with increase in ultrasonic attenuation. Ultrasonic attenuation can be used to predict the fatigue life of hybrid composites before and after service.     

Pull-through performance of carbon fibre epoxy composites

An investigation of the through-thickness properties of carbon fibre prepreg laminates, Non-Crimp Fabric laminates and non-crimp 3D orthogonal woven composites by pull-through testing was performed. Influence of matrix system and curing temperature on the performance of the 3D woven composites was investigated.     

Thermomechanical behaviour of interfacial region in carbon fibre/epoxy composites

A study of the thermomechanical stability of the fibre-matrix interphase in carbon/epoxy composites has been carried out. The thermodynamic work of adhesion has been evaluated at room temperature by wetting measurements. The interfacial shear stress transfer level τ for sized and desized carbon fibre has been measured as a function of temperature by means of a single-fibre fragmentation test. As the test temperature increased τ values were found to decrease, with values being higher for the desized carbon fibre. The dependence of interfacial shear stress transfer on bulk matrix mechanical properties (modulus and shear strength) has also been discussed. Dynamic mechanical measurements performed on single-bundle composites confirmed the better thermomechanical stability of the desized fibre interphase.     

Interfacial separation of fibres in fibre reinforced composites

An analytical model is proposed to predict the ultimate tensile strength of fibre-reinforced composites when the failure is governed by fibre debonding.

The analytical analysis is based on the principle of the compliance method in fracture mechanics with the presence of an interfacial crack at the fibre/matrix interface. The model is developed on the basis of the assumption that both the matrix and the fibre behave elastically and the matrix strain at a zone far from the matrix-fibre interface is equal to the composite strain. Furthermore, it is assumed that a complete bond exists between the fibre and the matrix and that the crack faces are traction free.

It is shown that the separation strain energy release rate for fibre-reinforced composites can be obtained for cases with and without the existence of an interfacial crack. Numerical examples are presented and compared with results obtained in the literature by finite element analyses and from experimental tests. The comparison demonstrates the accuracy and the convergence of the model.     

碳纳米管纤维/环氧树脂复合材料的界面剪切强度及微观结构

研究了碳纳米管纤维的微观结构和拉伸性能,并进一步分析了其与环氧树脂形成界面剪切强度及微观结构。采用单丝断裂试验测试了碳纳米管纤维/环氧树脂复合材料体系的界面剪切强度,结合单丝断裂过程中的偏光显微镜照片、复合材料的拉曼谱图和断口扫描电镜照片,研究了碳纳米管纤维/环氧树脂复合材料界面的微观结构。结果表明: 碳纳米管纤维/环氧树脂复合材料的界面剪切强度约为14 MPa;在碳纳米管纤维和环氧树脂形成界面的过程中,环氧树脂可以浸渍纤维,形成具有一定厚度的复合相,这种浸渍过程和界面相的形成都有利于碳纳米管纤维与基体之间的连接。     

激光对碳纤维及碳纤维/环氧树脂复合材料性能影响

采用高能激光束对聚丙烯腈（PAN）基碳纤维进行表面改性。利用SEM、EDS、FTIR、XRD、万能试验机等表征手段,对改性前后碳纤维微观形态、成分变化、物相结构、力学性能进行表征,系统地研究了激光束对碳纤维微观组织变化、性能变化等的影响规律,探索激光束对碳纤维的作用机制。结果表明,碳纤维经激光表面改性后,其表面的粗糙度和比表面积增加,碳纤维的浸润性得到提升,且激光束的功率越高、扫描速度越低,碳纤维浸润性越好。改性后的碳纤维化学成分、微观结构及官能团种类没有改变;改性后的碳纤维官能团种类没有改变,说明激光改性过程主要以物理过程为主;激光改性没有改变碳纤维的微观结构,改性后微晶尺寸略有减小,有利于改善碳纤维与环氧树脂的界面黏结性能。激光表面改性碳纤维/环氧树脂复合材料的拉伸强度和冲击强度均有不同程度的提高,当碳纤维质量分数为0.2wt%、激光改性功率为150 W时,碳纤维/环氧树脂复合材料的拉伸强度提高了59%,冲击强度提高了52%。     

Jute fibre/epoxy composites: Surface properties and interfacial adhesion

Jute fibres were surface treated in order to enhance the interfacial interaction between jute natural fibres and an epoxy matrix. The fibres are exposed to alkali treatment in combination with organosilane coupling agents and aqueous epoxy dispersions. The surface topography and surface energy influenced by the treatments were characterized. Single fibre pull-out tests combined with SEM and AFM characterization of the fracture surfaces were used to identify the interfacial strengths and to reveal the mechanisms of failure.     

Interfacial slip and damping in fibre reinforced composites

The effect of interfacial slip on the stress—strain properties of a rubbery polymer reinforced with short fibres is discussed with particular reference to hysteresis. The amount of energy dissipated by interface sliding and by the viscoelastic response of the matrix is calculated in the light of a simple model. This is compared with the dynamic properties of the composites, both with a view to designing composites with a useful combination of stiffness and damping, and also with the possibility of evaluating the integrity of the fibre-matrix interface from dynamic measurements.     

Investigation of flax fibre reinforced epoxy friction composites

The use of eco‐materials on the basis of natural fibre reinforced polymer composites has found its way to many applications. In the automobile sector, the use of such composites has long been established for applications in the car interior. The use of natural fibre reinforced composites for braking applications has however not been confirmed yet. In this study the use of flax fibres as a candidate reinforcement substitution for glass or even the carcinogenic asbestos fibres has been investigated. Typical frictional materials such as alumina, iron and brass particulates have been used together with graphite as a lubricant. Epoxy resin was used as the binding matrix. Results show frictional behaviours comparable with commercially available brake linings at acceptable wear rates.     

Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement

Low-velocity impact tests were performed to investigate the impact behaviour of carbon fibre/epoxy composite laminates reinforced by short fibres and other interleaving materials. Characterisation techniques, such as cross-sectional fractography and scanning acoustic microscopy, were employed quantitatively to assess the internal damage of some composite laminates at the sub-surface under impact. Scanning electron microscopy was used to observe impact fractures and damage modes at the fracture surfaces of the laminate specimens. The results show that composite laminates experience various types of fracture; delamination, intra-ply cracking, matrix cracking, fibre breakage and damage depending on the interlayer materials. The trade-off between impact resistance and residual strength is minimised for composites reinforced by Zylon fibres, while that for composites interleaved by poly(ethylene-co-acrylic acid) (PEEA) film is substantial because of deteriorating residual strength, even though the damaged area is significantly reduced. Damages produced on the front and back surfaces of impact were also observed and compared for some laminates.     

Tensile behaviour of stainless steel fibre/epoxy composites with modified adhesion

In this study we investigate the tensile behaviour of unidirectional and cross-ply composites reinforced with ductile stainless steel fibres and modified adhesion to the epoxy matrix. Results show that annealed stainless steel fibres have a potential in designing tough polymer composites for structural applications. The stiffness of the UD composites made from these fibres is 77GPa combined with the strain-to-failure between 15% and 18% depending on the level of adhesion. Silane treatments were used to modify the adhesion. By treating the stainless steel fibres with different silane coupling agents, an increase of 50% in the transverse 3-point-bending strength was realised. Increasing the adhesion by 50% leads to a higher tensile strength and strain-to-failure in both UD and cross-ply laminates and a higher in-situ strength of the 90° plies. It also delays formation of matrix cracks and hinders growth of debonding.     

Stacking sequence effects on fracture of notched carbon fibre/epoxy composites

The purpose of this study was to investigate the ability of the so-called damage zone model (DZM) to predict the influence of stacking sequence on the strength of notched carbon fibre/epoxy composites. The DZM is in essence based on the unnotched tensile strength, σ₀, and the apparent fracture energy, G_c^*, and the damage zone is modelled as a crack with cohesive forces acting on the crack surfaces. The DZM predicts fracture loads for three-point bend (TPB) specimens and specimens with circular holes quite accurately. As an attempt to explain the difference in strengths, the damage zone extension in the TPB specimens with different stacking sequence was examined.     

Mode I delamination behaviour of short fibre reinforced carbon fibre/epoxy composites following environmental conditioning

Carbon fibre laminates based on Newport Adhesives NCT-301 uniaxial pre preg were prepared for Mode I DCB testing. Polyamide and Polyolefin web materials supplied by Spunfab Ltd and Toyobos’ Zylon HM fibres were applied to the interlaminar region to enhance fracture toughness. Three of each specimen type were “treated”, subjected to immersion in 95 °C water for a period of 600 h. Two “untreated” control specimens of each type were aged at 21 °C and 40-95% relative humidity. Specimen weight gains in the order of 2% were found to occur in all immersed samples. Fracture toughness increases were evidenced in all treated samples with the exception of the Polyamide Web sample. Average G_Ic increases in treated samples were 250–300%. Untreated samples were found to have exclusively interlaminar crack propagation. Untreated samples containing Polyamide or Polyolefin Web reinforcement displayed increased fracture toughnesses. These toughness increases were attributed to improved interlaminar bonding.     

运用神经网络设计碳纤维织物/环氧复合材料界面性能

提出一种设计碳纤维织物／环氧复合材料界面性能的新方法。利用BP(back propagation)神经网络，建立起工艺参数与复合材料界面性能的关系模型，同时给出了实例用来验征此方法的有效性。