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.     

A new method to normalize the effect of matrix properties on the value of interfacial shear strength obtained from the fragmentation test

A new method, based on tensile yield strength and strain, has been developed to normalize the effect of matrix properties on the critical fibre length and the interfacial shear strength obtained from the fragmentation test. It is argued that the conventional data normalization technique which employs elastic properties of the matrix, is fundamentally flawed because the model employed to calculate interfacial shear strength assumes perfect plasticity. Single embedded fibre fragmentation in a range of epoxy resins with differing mechanical properties has been used to validate the new method. Stoichiometric quantities of the current agent were used to keep the same interfacial chemistry. The proposed method provides more consistent interfacial shear strength data than existing theories. Furthermore, this normalization technique can also be used to predict the interfacial shear strength of glass fibres embedded in a range of support resins, such as vinyl ester or epoxy resins. For these cases, a thin layer of the phenolic resin was used on the glass fibre to keep the interface chemistry the same. This revised version was published online in November 2006 with corrections to the Cover Date.     

拉伸速率对炭纤维/尼龙6微复合材料微观力学行为的影响

采用微球脱粘实验法研究了炭纤维/尼龙6微复合材料的界面微观力学性能,考察了不同的拉伸速率对界面微观力学行为的影响.结果显示:在拉伸速率为3.5,4,4.5mm/min,纤维在树脂基体中的包埋长度le＜140μm时,树脂微球从纤维上脱粘时的力值Fmax与包埋长度le保持着良好的线性关系;当拉伸速率为5mm/min,包埋长度le＞70μm时,纤维就断裂了,试样失效.     

A novel microbond bundle pullout technique to evaluate the interfacial properties of fibre-reinforced plastic composites

The interfacial properties of the fibre composite systems decide the overall usability of a composite in simple and complex shapes, as they are the deciding factors in determination of the mechanical properties, structural properties and above all a complete understanding of the reliability of composite systems. In the present investigation, the interfacial properties of carbon fibre/epoxy composites viz., matrix shrinkage pressure, interfacial frictional stress, interfacial shear stress and coefficient of friction were evaluated through a novel microbond bundle pullout test. This test is different from the single fibre pull out, fibre fragmentation or the fibre push in test. Based on some of the physical principles involving the single fibre microbond pullout test, like the contact angle of the microbond matrix drop with the fibre surface, the surface tension/energy of the two surfaces before and after adhesion and the interfacial fibre/matrix chemistry, this is simple to perform and statistically averaged mesomechanical test is also easy to evaluate and is shown to be a test method that enables a conservative prediction of the laminate level or macromechanical shear properties of fibre composite systems. This test demonstrates the validity of the mesomechanical tests that are more relevant to the macromechanical tests than the micromechanical tests. Fractography carried out to corroborate the observed mechanical properties with the fracture features is also reported. The general advantages of the mesomechanical interfacial tests over those based on micromechanical assumptions is also discussed along with some common limitations.     

The role of sizing resins in carbon fibre-reinforced polyethersulfone (PES)

The role of sizing resin in carbon fibre-reinforced polyethersulfone has been studied using surface-treated Type A carbon fibre sized with different polymeric coatings. To investigate their influence on the adhesion of the carbon fibre to the matrix, the single embedded filament fragmentation test was used. Sized carbon fibres showed a higher interfacial shear strength than the unsized ones. Analysis by time-of-flight secondary ion mass spectrometry suggests that this arises from a strong interaction between sizing resin, the fibre and the matrix.     

Fibre fragment distribution in a single-fibre composite tension test

Single fibre fragmentation tests are performed for brittle fibres with Weibull strength distribution and different surface treatments. The fragmentation process is modelled and closed-form expressions for break spacing distribution are obtained. The model accounts for the effect of finite fibre length on the initial fragmentation as well as for break interaction on the advanced fragmentation stage. It is assumed that the exclusion zone due to fibre–matrix interface failure and stress recovery in the fibre is linearly dependent on the applied load. This assumption is validated experimentally. The derived theoretical average fragment length dependence on applied load is used to determine the fibre strength distribution parameters and the effective interfacial shear stress for carbon/epoxy single fibre composites with different fibre surface treatment and for glass/vinylester single fibre composite. Fragment length distribution is predicted for several load levels. Predictions are in good agreement with experimental data.     

Carbon nanotube grafted silica fibres: Characterising the interface at the single fibre level

Model polymer composites containing carbon nanotube (CNT) grafted fibres provide a means to investigate the influence of nanostructures on interfacial properties. Well-aligned nanotubes, with controllable length, were grown on silica fibres by using the injection chemical vapour deposition method, leading to a significant increase of the fibre surface area. In single fibre tensile tests, this CNT growth reaction reduced the fibre strength, apparently due to catalyst etching; however, the fibre modulus increased significantly. Contact angle measurements, using the drop-on-fibre method, indicated an excellent wettability of the CNT-grafted fibres by poly(methyl methacrylate) (PMMA). PMMA model composites were fabricated and studied using the single fibre fragmentation tests. A dramatic improvement (up to 150%) of the apparent interfacial shear strength (IFSS) was obtained for the composites containing CNT-grafted fibres. The improvement of IFSS was also influenced by the length and morphology of the grafted CNTs.     

SWNT composite coatings as a strain sensor on glass fibres in model epoxy composites

The preparation of a model glass-fibre/epoxy composite with single-walled carbon nanotubes (SWNTs) incorporated as a strain sensor on the fibre surface is described. A micromechanical study of stress transfer at the fibre–matrix interface followed using Raman spectroscopy properties is reported. The SWNTs were distributed along the fibre surface either by dispersing them in an amino-silane coupling agent or coating with an epoxy resin solution containing the SWNTs. The point-by-point mapping of the fibre strain in single fibre fragmentation tests has been undertaken for the first time using SWNTs on the fibres and the interfacial shear stress distribution along the fibre length was determined using the embedded SWNTs. The behaviour was found to be consistent with the classical shear-lag model. The effects of SWNT type and preparation procedure on the sensitivity of the technique were evaluated and optimized from single fibre deformation tests.     

Interfacial shear stress distribution in model composites: the effect of fibre modulus

The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis.A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.     

Analysis of the fragmentation test for carbon-fibre/epoxy model composites by means of Raman spectroscopy

The interfaces between high-modulus PAN-(T50) and mesophase pitch-based (P55) carbon fibres and an epoxy matrix have been studied by using the conventional fragmentation test in conjunction with polarised-light optical microscopy. Raman spectroscopy has also been used to follow stress transfer from the matrix to the fibres for the same fragmentation geometries. The level of fibre/matrix adhesion and mechanisms by which the stress is transfered from the matrix to the fibres has been determined from both the stress birefringence patterns and strain-induced Raman band shifts in the fibres. The values of interfacial shear strength have been determined by means of both the conventional analysis and the Raman technique. It is found that the Raman method gives a much more detailed picture of stress transfer in the test specimens and that the two methods give somewhat different values of the interfacial shear strength. The values of interfacial shear stress have been discussed with respect to fibre surface energy, surface chemistry and surface morphology. It was found that the surface chemical functional groups appear to have no direct correlation with interfacial shear strength. Furthermore, it appears that mechanical interlocking due to surface roughness could contribute to the higher values of interfacial shear strength determined for the PAN-based fibre.     

Determining the Interfacial Shear Strength in the Presence of Transcrystallinity in Composites by the ‘Single-Fibre Microcomposite Compressive Fragmentation Test‘

Based on the compressive thermal fragmentation test and model proposed recently, a transcrystalline phase grown adjacent to the surface of a high modulus carbon fibre embedded in polycarbonate was found to lower the thermal stresses in the fibre. The interfacial shear stress, calculated from the compressive stress profile generated by the model, was lowered in the presence of a transcrystalline layer. It is proposed that the transcrystalline layer reduces the radial stresses acting on the fibre, thereby reducing the friction component of the interfacial bond strength.     

Stress transfer in the fibre fragmentation test

An improved micromechanics model has been developed of the stress transfer for a single fibre embedded in a matrix subjected to uniaxial loading. Debond crack growth is analysed based on the shear strength criterion such that when the interfacial shear stress reaches the shear bond strength, debonding occurs; and the average strength concept based on Weibull statistics is considered for fibre fragmentation. The influences of the interfacial shear bond strength and the fibre strength on the stress distributions in the composite constituents are evaluated. Depending on the relative magnitudes of these two strength parameters and given the elastic constants and geometric factors, three distinct conditions of the fibre-matrix interface are properly identified which include full bonding, partial debonding and full frictional bonding. Also quantified are the necessary criteria which must be satisfied in order for each interface condition to be valid. Finally, the mean fibre fragment length is predicted as a function of applied strain using a model composite of carbon fibre-epoxy matrix. The parametric study suggests that the critical transfer length predicted when the applied strain (or stress) required for further fibre fragmentation approaches infinity, can be regarded as a material constant, which is the sum of the bonded and the debonded lengths for the model composite.     

Effects of plasma oxidation on the surface and interfacial properties of ultra-high modulus carbon fibres

An ultra-high modulus (UHM) carbon fibre was submitted to an oxygen plasma treatment. The effects of this treatment on the physical and chemical properties of the carbon surfaces were investigated by using surface characterisation techniques. SEM and STM studies were performed in order to determine the changes in the surface morphology. Observations on the nanometre scale lead to the conclusion that the plasma oxidation “cleaned” the original surfaces of carbonaceous impurities. XPS analysis of the treated fibres revealed a very significant increase of oxygen content. Single-fibre epoxy composites were prepared from as-received and plasma-treated fibres, and fragmentation tests were performed in order to characterise fibre/matrix interfacial adhesion. Raman spectroscopy has been used to map the strain along the fibre during tensile loading of the matrix, and the distribution of interfacial shear stress has been obtained. The quality of the interface improved dramatically after the surface treatment, supporting the ability of cold plasma oxidation to enhance the adhesion of UHM carbon to epoxy matrices. It is concluded that the increase of the oxygen surface content and the removing of the outermost layers may contribute in a co-operative way to the improvement on fibre/matrix adhesion.     

A buckled plate test to measure interfacial toughness in composites

A new, buckled plate (BP) test has been used to measure transverse toughness as the parameter characterizing interfacial adhesion in unidirectional, continuous-fibre composites. The test is simple, with advantages over other interfacial methods. The theory and experimental details are presented. The results of BP tests are discussed for polycarbonate/carbon fibre composites. Evaluations have been made with regard to specimen dimensions, testing speed, crack length, modulus, fibre volume fraction, and processing conditions. Transverse toughness is a sensitive measure of the interfacial adhesion, giving results similar to transverse tensile strength. The test has also been used to measure longitudinal toughness. This test should be widely applicable to many composite systems.     

The use of a single-fibre fragmentation test to study environmental durability of interfaces/interphases between DGEBA/mPDA epoxy and glass fibre: the effect of moisture

The influence of environmental exposure, both thermal and hydrothermal, on the average critical length/diameter (L_c/D) ratio of fibre fragments, as measured by the single-fibre fragmentation test, has been investigated. The increase in _c/D for hydrothermally treated samples indicated that degradation of the interfacial properties was dominant, while L_c/D for the thermally treated samples showed no change. Apparently, degradation of both the interfacial shear strength and the fibres occurred under hydrothermal conditions. Analysis of the distilled water in which the hydrothermally treated samples had been soaked detected the presence of ions from E-glass. Furthermore, fragmentation of the fibre after hydrothermal treatment reached its limit (saturation) at a lower value of applied strain than did fibres after exposure to the dry or thermal environments. Changes in strength at the interface and in the fibre appear to be the major factors influencing the L_c/D values on hydrothermal exposure.     

Stress transfer in the fibre fragmentation test: Part III Effect of matrix cracking and interface debonding

A theoretical stress analysis has been developed for the fibre fragmentation test in the presence of matrix cracks at sites of fibre breaks. The strain energy release rates for both matrix cracking and interface debonding are calculated for a carbon fibre/epoxy matrix composite. By comparing these strain energy release rates with the corresponding specific fracture resistances, the competition between matrix crack growth and interface debonding has been studied. The distributions of fibre axial stress and interfacial shear stress obtained from the present analysis show that the matrix crack substantially reduces the efficiency of stress transfer from the matrix to the fibre. This revised version was published online in November 2006 with corrections to the Cover Date.     

A study on the characteristics of glass fibre reinforced thermoplastics by transfer moulding

Lately, the use of fibre reinforced thermoplastics (FRTP) has increased due to its increased mouldability compared to thermosetting FRP material.

FRTP includes stampable sheet, short and long fibre reinforced thermoplastic pellets, continuous fibre reinforced thermoplastics sheets, etc. The long fibre reinforced thermoplastic (LFRTP) pellet has better mechanical properties than short fibre reinforced pellets and better mouldability than stampable sheet. At present, injection moulding method is mainly used for moulding LFRTP pellets because of its high productivity.

However, the long fibre of LFRTP pellets, whose length is the same as pellet length, is degraded during processing if conventional injection moulding machines are used, and as a result, the mechanical properties are not improved as expected in many cases. Therefore, a new moulding process is required to make good use of LFRTP pellets.

For this study, a transfer moulding apparatus was designed and built to minimize fibre degradation of the moulded parts.

Firstly, the LFRTP pellets with fibre lengths of 3, 6, 9, 12.7 and 17 mm were prepared in order to clarify the difference of mechanical properties due to fibre length. The fibre ratio was 30% in weight for all cases and the same polypropylene was used. They were moulded to the shape of the test specimens. Tensile, bending and Izod impact strengths were measured by using these test specimens. Secondly, LFRTP pellets were moulded to the shape of test specimens by the transfer moulding apparatus and conventional injection moulding machine, and then mechanical properties were measured. At the same time, short fibre pellets were moulded to the smae shape of test specimens by the injection moulding machine, and mechanical properties were compared with those of LFRTP pellets.

With the long glass fibre reinforced polypropylene, good results of fibre preservation and mechanical properties were obtained by the transfer moulding apparatus which was built for this study. The impact strength was increased remarkably as the fibre length increased, and consequently the preservation of fibre length in the moulded parts was especially effective in improving the impact strength.     

Effects of the interface on the mechanical response of CF/EP microcomposites and macrocomposites

The aim of this study was to investigate the effects of the interfacial bond quality on the mechanical response of composite laminates, for example an epoxy matrix reinforced by continuous carbon fibres of varying surface coating. The fibre/matrix adhesion was characterized by determining the interfacial shear strength τ_i in single-fibre fragmentation and microdroplet pull-off tests. The failure mechanisms were deduced from the stress birefringent patterns (fragmentation test) and from fractographic analysis in a scanning electron microscope (microdroplet pull-off test). Selected interface-relevant properties were evaluated in mechanical tests on laminates. The present paper highlights the problems related to the micromechanical characterization and the interface relevance of data resulting from transverse tensile, transverse flexure and interlaminar shear tests. Furthermore, the effects of the interface on the impact performance of unidirectional and cross-ply laminates were studied. Attempts were made to correlate the macroscopic mechanical response with the interface-related characteristics (τ_i and failure mechanisms).     

Comparing single-walled carbon nanotubes and samarium oxide as strain sensors for model glass-fibre/epoxy composites

The study of the interfacial stress transfer for glass fibres in polymer composites through the fragmentation test requires certain assumptions, such as a constant interfacial shear stress. In order to map the local interfacial properties of a composite, both Raman spectroscopy and luminescence spectroscopy have been independently used. Unlike other polymer fibre composites, the local strain state of a glass fibre cannot be obtained using Raman spectroscopy, since only very broad and weak peaks are obtainable. This study shows that when single-walled carbon nanotubes (SWNTs) are added to the silane sizing as a strain sensor, it becomes possible to map the local fibre strain in glass fibres using Raman spectroscopy. Moreover, if this model glass fibre contains a small amount of Sm₂O₃, as one of the components, luminescence spectroscopy can be simultaneously used to confirm this local fibre strain. A combined micromechanical properties study of stress transfer at the fibre–matrix interface using luminescence spectroscopy, together with Raman spectroscopy, is therefore reported. The local strain behaviour of both Sm³⁺ doped glass and SWNTs in the silane coating are shown to be consistent with a shear-lag model. This indicates that Sm³⁺ dopants and SWNTs are excellent sensors for the local deformation of glass fibre composites.     

Effect of anodic oxidation of coal tar pitch-based carbon fibre on adhesion in epoxy matrix: Part 1. Comparison between H2SO4 and NaOH solutions

Anodic oxidation of high modulus coal tar pitch-based carbon fibre in alkaline and sulfuric solutions was investigated. X-ray photoelectron spectroscopy (XPS) analysis was used to evaluate the oxygen concentration (O_1S/C_1S) and surface functional groups (---OH, C=O and COOH). The interfacial shear strength between the epoxy matrix and carbon fibre was measured by a fragmentation test. The results showed that the oxygen concentration on fibre surfaces increased rapidly as electrical charge increased, in both alkaline and acidic solution. The best bonding between epoxy matrix and carbon fibre was achieved in alkaline solution. The Raman spectra of carbon fibre oxidized in alkaline and sulfuric solutions suggested that the weak adhesive strength between the epoxy resin and oxidized carbon fibre in sulfuric solution was due to the existence of an oxidized graphite layer, which might easily come off the surface.