A study of mechanisms of stress transfer in continuous- and discontinuous-fibre model composites by laser Raman spectroscopy

The micromechanics of stress transfer in single-fibre/epoxy-resin model composites has been investigated. Two specimen geometries are examined incorporating both continuous and discontinuous fibres in epoxy resin blocks. In both cases, the point-by-point strain in the fibre is measured from the fibre Raman spectrum and its strain dependence. The continuous-fibre model composites (CFMC) are subjected to incremental tensile loading and the fibre fragmentation process is continuously monitored. The short-fibre model composites (SFMC) are loaded incrementally to levels of stress of sufficient magnitude to cause interfacial failure and the fibre strain profiles are obtained at each level of applied stress.

In addition to fibre strain measurements, the interfacial shear stress distribution is derived at each increment of applied stress by means of a balance-of-forces argument. The effects of fibre surface treatment and fibre modulus on the strain transfer profile and the distribution of the interfacial shear stress along the fibre are examined. The importance of parameters such as fibre/matrix debonding and interphase yielding in the vicinity of fibre breaks or fibre ends is discussed.     

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.     

Effect of water on the mechanical adhesion of the glass/epoxy interface

In recent years, the quality of the fibre/matrix bonding in polymer composites has been quantified by means of a single mechanical parameter, the interfacial shear strength, based on measurements made using micromechanical techniques. It has gradually appeared, however, that this parameter is both ambiguous in terms of its physical meaning and, at the same time, difficult to measure reliably in many cases. Moreover, different micromechanical techniques yield differing values of the interfacial shear strength. Finally, it has been suggested in a few studies that it may not be the critical factor governing fibre/matrix debonding. In this paper an energy balance approach is proposed, by which the degree of fibre/matrix bonding is now quantified by means of the interfacial energy, as a function of the fibre geometrical and mechanical characteristics, the stress transfer length and the debonding length. The validity of the approach is discussed in the case of the single-fibre composite test, in which progressive fragmentation of a single brittle fibre in a more ductile polymeric matrix takes place, using data for E-glass fibres embedded in epoxy, both in the dry state and in the presence of hot distilled water.     

Evaluation of interface fracture energy for single-fibre composites

Raman and luminescence spectroscopy have been used for the first time to determine the interface fracture energy for single-fibre composites. By using the measured fibre stress distributions in single-fibre fragmentation composite specimens and a simple energy-balance scheme, the energy for the initiation of interfacial debonding has been estimated for carbon (T50) and α-alumina (PRD-166 and Nextel 610) fibres embedded in epoxy resins. It has been found that the interface fracture energy shows good sensitivity to changes in the level of fibre/matrix adhesion due to surface treatment and sizing of the fibres. It is also found that the values of interface fracture energy correlate well with measured values of interfacial shear strength determined for the same fibre/matrix systems.     

Influence of fibre surface oxidation treatment on mechanical interfacial properties of carbon fibre/ polyarylacetylene composites

Abstract

In the present work, the mechanical interfacial properties of carbon fibre (CF) reinforced polyarylacetylene (PAA) resin composites were modified through the surface oxidation treatment of carbon fibres by ozone. Both X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy showed that oxidation treatment could increase the amount of elemental oxygen on the fibre surface markedly by introducing more oxygen groups. Atomic force microscopy (AFM) images indicated that weak surface regions of fibres had been etched and removed, and the degree of fibre surface roughness was increased. The interlaminar shear strength (ILSS) and the interfacial shear strength (IFSS) of CF/PAA composites were both improved notably (no less than 50%). It could be concluded that an improvement of fibre surface chemical activity, better wettability of resin on the carbon fibre surface, and stronger mechanical joining between fibres and resin all resulted in the modification of interfacial properties of carbon fibre reinforced PAA composites. The influences of temperature, ozone concentration, and treatment time on the oxidation results were studied, and optimal treatment parameters determined.     

The interfacial properties of aramid/epoxy model composites

Many attempts have been made to measure, evaluate and improve the level of interfacial adhesion in aramid/epoxy composites. Different surface treatments have been developed in order to promote chemical bonding between the fibre and the matrix but it is found that most of the surface treatments developed have shown little or no improvement in the level of interfacial adhesion. The interfacial properties of a model composite are often determined by measuring the interfacial shear strength using micromechanical test methods that employ different loading configurations. However, the values of interfacial shear strength determined using different test methods are found to be dependent upon the variation of localized stress in the samples due to the different loading configurations and often give different results. Using Raman spectroscopy it is shown that the strain-dependent shift of the 1610 cm^–1 aramid Raman band can be used to determine the point-to-point variation of axial fibre strain along aramid fibres embedded in epoxy resin matrices from which the interfacial properties can be derived. The interfacial properties of aramid/epoxy model composites have been determined using Raman spectroscopy where the properties of the fibre, including different surface treatments, and the matrix have been changed systematically. The results are reviewed here and compared to those obtained using conventional micromechanical test methods. It is also demonstrated that the Raman technique can be used to characterize the interfacial properties of aramid/epoxy model composites deformed using different micromechanical test methods. In this way the interfacial properties can be determined at different loading levels enabling the progressive failure of the fibre/matrix interface to be monitored and defined accurately.     

Investigation on adhesion, interphases, and failure behaviour of cyclic butylene terephthalate (CBT)/glass fiber composites

Interphases exist in hybrid materials and significantly influence their mechanical performance. To find a bridge between the microscopic and macroscopic mechanical properties, this work investigates the microscopic nature of glass fiber surfaces and glass/CBT interphases in terms of topography, fractography, and adhesion properties. The variations in glass fiber surface properties result from the different sizings. Using the single fiber pull-out test, AFM, and ζ potential tests, it is shown that the interfacial bond strengths in CBT resin composites can vary depending on the kind of sizing formulation and properties. The greatest adhesion strength is achieved by aminosilane sizings with epoxy resin film former. The surface roughness of the fibers can be varied by sizings with different content and ζ potential values, which has no significant contribution to interphase adhesion strength from ‘mechanical interlocking’. For the systems with film formers, cohesive failure occurs and similar values of both interfacial adhesion strength, τ_d, and fracture energy release rate, G_ic, are obtained, in which τ_d approaches the shear yield strength of CBT matrix. A further enhancement of interfacial adhesion is limited by the mechanical properties and the non-homogeneous microstructure of CBT resin due to the less-than-perfect CBT polymerization.     

Investigation of the long term effects of moisture on carbon fibre and epoxy matrix composites

The aim of this work is to investigate the long term effects of moisture on the interface between a carbon fibre and an epoxy matrix. High modulus carbon fibres were used to prepare single fibre model composites based on an epoxy resin. The samples were immersed in the seawater and demineralised water and their moisture uptake behaviour was monitored. The equilibrium moisture content and diffusion coefficients for the samples were determined. DSC has been used to analyse the moisture effects on glass transition temperature and thermal stability of the pure epoxy specimens. These results showed a reduction in the glass transition temperature (T_g) after moisture absorption. Tensile tests were also carried out for the epoxy specimens and a general decrease in the mechanical properties of the epoxy matrix was observed. Raman spectroscopy was used to observe the effects of moisture on the axial strain of the carbon fibre within the composite and stress transfer at the interface as a function of exposure time. The results show that the decrease in the mechanical and interfacial properties of the model composites under the seawater immersion is more significant than under demineralised water immersion.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Comparative assessment of stress transfer efficiency in tension and compression

Raman spectroscopy is used to get an insight into the microstructural aspects of the compressional behaviour of carbon fibre composites. This is done by a comparative assessment of the stress transfer efficiency in tension and compression in single-fibre discontinuous model geometries. It was found that the axial stress is transferred in the fibre through the generation of shear stresses at the interface. The mechanism of stress transfer is independent of the loading mode. Furthermore, the values of maximum interfacial shear stress are a function of the applied strain for both tension and compression loading. Significant differences were found, however, in the mode of failure of the two systems. In tension, interfacial failure initiates from the fibre ends at relatively high applied strains and the stress transfer efficiency is affected by the onset of matrix plasticity. On the other hand, in compression, deterioration of the stress transfer efficiency occurs prior to any noticeable interfacial failure at the fibre ends due to fibre collapse at low strains. Finally, it is worth noting that in compression, the fibre fragments remain in contact, and thus can still bear load.     

单丝断裂能量法研究温度对碳纤维/环氧界面性能的影响

针对两种不同上浆剂碳纤维/高温固化环氧树脂体系, 采用基于WND(Wagner-Nairn-Detassis)能量模型的单丝断裂法, 测试分析了从室温到130 ℃范围内单丝复合体系界面断裂能的变化规律, 研究了碳纤维上浆剂对界面耐热性能的影响, 并结合复合材料层板的短梁剪切性能, 分析了微观和宏观界面性能的关联性。结果表明: 在测试温度范围内, 碳纤维/环氧体系的界面断裂能随温度升高呈先下降而后基本不变的趋势, 去除上浆剂后界面断裂能及其随温度的变化程度与未去除上浆剂的情况存在差异, 说明上浆剂对界面耐热性有重要作用。碳纤维/环氧树脂层板层间剪切强度随温度升高线性下降, 与界面断裂能的变化规律不一致, 这与两种测试方法的原理及界面破坏位置的不同有关。      

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 interphase properties on fracture behavior of TCP/PLA composites

In the present study, the effects of interphase on the mechanical properties of β-tricalcium phosphate (β-TCP) particle-dispersed bioabsorbable poly(lactic acid) (PLA) composites were investigated. In order to improve interfacial strength between PLA and β-TCP, the surface of β-TCP was modified with L-lactic acid (LLA) monomer. The weight ratios of LLA to β-TCP are selected as 1.5, 3, 4.5, 6, 7.5%. Tensile strength decreased with the incorporation of β-TCP to PLA, whereas the tensile strength of composites was improved in the case of modification with the weight ratio of 3%. From acoustic emission measurements, initial interfacial debonding was also suppressed up to 3%. From finite elemental analysis, elastic modulus of interphase is about 30–70% of that of PLA and elastic modulus of the composites do not depend on interphase thickness. From the stress distributions at initial debonding onset, it is clarified that interfacial debonding is caused by shear stress between particle and interphase and interfacial shear strength seems to be about 20 MPa.     

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.     

Development of sizing-free multi-functional carbon fibre nanocomposites

High quality multi-walled carbon nanotubes (CNTs) grown at high density using a low temperature growth method are used as an alternative material to polymer sizing and is utilised in a series of epoxy composites reinforced with carbon fibres to provide improved physical and electrical properties. We report improvements for sizing-sensitive mechanical and physical properties, such as the interfacial adhesion, shear properties and handling of the fibres, whilst retaining resin-infusion capability. Following fibre volume fraction normalisation, the carbon nanotube-modified carbon fibre composite offers improvements of 146% increase in Young’s modulus; 20% increase in ultimate shear stress; 74% increase in shear chord modulus and an 83% improvement in the initial fracture toughness. The addition of CNTs imparts electrical functionalisation to the composite, enhancements in the surface direction are 400%, demonstrating a suitable route to sizing-free composites with enhanced mechanical and electrical functionality.     

Photoelastic study of the durability of interfacial bonding of carbon fibre-epoxy resin composites

The physical techniques of polarizing microscopy, including the quantitative measurements of small optical retardations, have been used to investigate elastic fields adjacent to short carbon fibres in epoxy resin composites. The elastic fields associated with shear stress distribution along the fibre-matrix interface have been employed to monitor the initiation of interface debonding during hot (100 °C) water uptake. By examining the development of stress birefringence during resin swelling in the resin adjacent to individual fibres, the differences in the durability of interfacial bonding and the fibre failure modes for differently coated fibres have been obtained. The results show that the state of self-stress in model composites, comprising a single carbon fibre in a film of epoxy resin, can, by immersion in hot distilled water, be enhanced to such an extent that the axial tension in the fibre can be sufficient to initiate fibre fracture. The results also show that, for fibres that have been given certain proprietary surface treatments, the fibre fractures by different failure modes.     

Carbon nanotube grafted carbon fibres: A study of wetting and fibre fragmentation

Carbon nanotubes (CNTs) were grafted on IM7 carbon fibres using a chemical vapour deposition method. The overall grafting process resulted in a threefold increase of the BET surface area compared to the original primary carbon fibres (0.57 m²/g). At the same time, there was a degradation of fibre tensile strength by around 15% (depending on gauge length), due to the dissolution of iron catalyst into the carbon; the modulus was not significantly affected. The wetting behaviour between fibres and poly(methyl methacrylate) (PMMA) was directly quantified using contact angle measurements for drop-on-fibre systems and indicated good wettability. Single fibre fragmentation tests were conducted on hierarchical fibre/PMMA model composites, demonstrating a significant (26%) improvement of the apparent interfacial shear strength (IFSS) over the baseline composites. The result is associated with improved stress transfer between the carbon fibres and surrounding matrix, through the grafted CNT layer. The improved IFSS was found to correlate directly with a reduced contact angle between fibre and matrix.     

Interfacial microstructure and mechanical properties of carbon fiber composites by fiber surface modification with poly(amidoamine)/polyhedral oligomeric silsesquioxane

Polyhedral oligomeric silsesquioxane (POSS) was grafted onto carbon fiber surface using poly(amidoamine) (PAMAM) as a novel coupling agent at mild reaction conditions. Firstly, the reinforcement was designed with propagation of PAMAM on the fiber surface by in situ polymerization to improve the surface activities of carbon fiber. Secondly, the POSS further grafted on the fiber could significantly enhance fiber surface energy and wettability, which would greatly increase the interfacial strength of fiber-matrix. The microstructure and mechanical properties of carbon fiber and the resulting composites were investigated. The results indicated that PAMAM and POSS, which could significantly increase the surface roughness and wettability of carbon fiber, were successfully grafted on the fiber surface. Compared with the desized fiber composites, the interlaminar shear strength and the interfacial shear strength of the modified carbon fiber composites increased by 48% and 89%, respectively.     

Enhanced mechanical properties of multiscale carbon fiber/epoxy composites by fiber surface treatment with graphene oxide/polyhedral oligomeric silsesquioxane

Graphene oxide (GO) and polyhedral oligomeric silsesquioxane (POSS) grafted carbon fiber (CF) was demonstrated to reinforce the mechanical properties of fiber composites. Such a fiber composite was prepared by grafting POSS onto the CF surface using GO as the linkage. The presence of GO linkage and POSS could significantly enhance both the area and wettability of fiber surface, leading to an increase in the interfacial strength between fibers and resin. Compared with the desized CF composites, the grafted CF composites fabricated by compression molding method exhibited 53.05% enhancement in the interlaminar shear strength. The changed surface morphology, surface composition and surface energy were supposed to be related with the interfacial performance of unidirectional composites, as revealed by scanning electron microscopy, atomic force microscope, dynamic contact angle test and X-ray photoelectron microscopy charaterizations.     

Interphase behavior of titanium matrix composites at elevated temperature

This paper examines the effect of temperature and thermal exposure on the interphase behavior of continuous fiber reinforced titanium metal matrix composites. The system considered is SCS-6/Timetal-21S. Elevated temperature fiber push-out tests were conducted to determine the effect of test temperature on interphase shear properties. Corresponding variations of debonding shear strength and frictional shear stress with test temperature are presented and discussed. Thermal exposure, both in a vacuum and an air environment, has been conducted on specimens, with temperatures up to 650°C and exposure times of up to 100 h. The resulting size and composition of the interphase have been examined. Fiber push-out tests were carried out at room and elevated temperature on the aged specimens. Results are discussed in terms of the influence of relaxation and oxidation on the debond shear strength. Using the experimentally determined interphase shear properties, the interphase toughness has been calculated and discussed in relation to interface decohesion models.