Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Influence of fibre-surface treatment on structural, thermal and mechanical properties of jute fibre and its composite

Jute fibres (Corchorus olitorious), an environmentally and ecologically friendly product, were chemically modified and treated with 5% NaOH solution at room temperature for 2 h, 4 h and 8 h. The above samples were characterized and morphologically analysed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and Instron 1185. Alkali treatment affects the supramolecular structure of the fibre as shown by XRD by improving the degree of crystallinity of the fibre. Surface chemistry of the fibre also altered as depicted by FT-IR studies. This chemical treatment was also found to alter the characteristic of the fibre surface topography as seen by the SEM. From the mechanical single fibre test it was found that the tenacity and modulus of the fibre improved after alkali treatment. This might be due to the improvement in the crystallinity. DSC data demonstrated that the thermal degradation temperature for the cellulose get lowered from 365·26°C to 360·62°C after alkali treatment led to the reduction in fibre thermal stability. Jute fibre reinforced composite were prepared with treated and untreated jute fibre (15 wt%) reinforced unsaturated polyester (UPE). Effectiveness of these composites was experimentally investigated through the study of the composites by DSC, Instron 1195 for mechanical property of composites, volume fraction of the porosity and hydrophobic finishing of the composite. From the DSC analysis it was found that thermal stability enhanced for treated fibre reinforced composite. This could be due to the resistance offered by the closely packed cellulose chain in combination with the resin. Flexural strength of the composite prepared with 2 h and 4 h alkali treated fibre were found to increase by 3·16% and 9·5%, respectively. Although 8 h treated fibre exhibited maximum strength properties, but the composite prepared with them showed lower strength value. Alkali treatment helped in the development of hydrophobicity and reduction in volume fraction of the porosity. This may be due to the better fibre matrix interface adhesion caused due to the fibre surface treatment by alkali.     

Impact of bleaching pine fibre on the fibre/cement interface

The goal of this article was to evaluate the surface characteristics of the pine fibres and its impact on the performance of fibre–cement composites. Lower polar contribution of the surface energy indicates that unbleached fibres have less hydrophilic nature than the bleached fibres. Bleaching the pulp makes the fibres less stronger, more fibrillated and permeable to liquids due to removal the amorphous lignin and its extraction from the fibre surface. Atomic force microscopy reveals these changes occurring on the fibre surface and contributes to understanding the mechanism of adhesion of the resulting fibre to cement interface. Scanning electron microscopy shows that pulp bleaching increased fibre/cement interfacial bonding, whilst unbleached fibres were less susceptible to cement precipitation into the fibre cavities (lumens) in the prepared composites. Consequently, bleached fibre-reinforced composites had lower ductility due to the high interfacial adhesion between the fibre and the cement and elevated rates of fibre mineralization.     

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.     

Surface oxidation of carbon fibre on tribological properties of PEEK composites

Abstract

In this work, ozone modification method and air oxidation were used for the surface treatment of polyacrylonitrile (PAN) based carbon fibre. The surface characteristics of carbon fibres were characterised by X-ray photoelectron spectroscopy. The interfacial properties of carbon fibre reinforced PEEK (CF/PEEK) composites were investigated by means of the single fibre pull-out tests. As a result, it was found that IFSS values of the composites with ozone treated carbon fibre are increased by 60% compared with that without treatment. X-ray photoelectron spectroscopy results show that ozone treatment increases the amount of carboxyl groups on carbon fibre surface, thus the interfacial adhesion between carbon fibre and PEEK matrix is effectively promoted. The effect of surface treatment of carbon fibres on the tribological properties of CF/PEEK composites was comparatively investigated. Experimental results revealed that surface treatment can effectively improve the interfacial adhesion between carbon fibre and PEEK matrix. Thus the wear resistance was significantly improved.     

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.     

Impact fatigue behaviour of vinylester resin matrix composites reinforced with alkali treated jute fibres

An impact fatigue study has been made for the first time on 35% jute/vinylester composites containing both untreated and alkali treated fibres. Longer alkali treatment removed the hemicellulose and improved the crystallinity and gave better fibre dispersion. The flexural strength properties of the composites made from treated fibre were superior. 4 h alkali treated jute fibres gave the optimum combination of improved interfacial bonding and fibre strength properties. However this was not reflected in their impact fatigue behaviour. On the contrary, the composites reinforced with 8 h alkali treated fibres displayed superior impact fatigue properties. Here, the fibres suffered catastrophic fracture with microfibrillar pull-out at some places and improved the fatigue resistance property of the composites as evident from SEM micrographs.     

Extrusion and mechanical properties of highly filled cellulose fibre–polypropylene composites

This study focused on manufacturing of highly filled cellulose fibre–polypropylene composites and evaluation of the mechanical properties of the composites. Cellulose fibre reinforced polypropylene composites with up to 60 wt% of fibres with and without coupling agent were manufactured by extrusion. In order to achieve consistent feeding of the fibres into the extruder a pelletization technique was used where the fibres were pressed into pellets. Two commercial grades of cellulose fibres were used in the study, bleached sulfite and bleached kraft fibres. Fibre dimension measurements showed that the pelletization process and extrusion at high fibre loading caused the most severe fibre breakage. Flexural testing showed that increased fibre loading made the composites stiffer but reduced the toughness. Addition of maleic anhydride grafted coupling agent (MAPP) increased the stiffness and strength of the composites significantly. In general, there was no significant difference in the mechanical properties between the composites with kraft and sulfite fibres. An interesting finding was that the flexural modulus and strength of the MAPP modified cellulose fibre–polypropylene composites were not higher than what has previously been reported for wood flour–polyolefin composites. Scanning electron microscopy showed that addition of coupling agent improved the interfacial adhesion between the fibres and polypropylene matrix.     

甘蔗渣的碱处理对其纤维增强全降解复合材料的影响

利用压制成型的方法制备了甘蔗渣纤维增强全降解复合材料, 探讨了碱处理对材料性能的影响。结果表明, 1 %碱液处理后材料的力学性能得到了提高。碱处理后纤维的分解细化和表面优化改善了纤维/ 基材的粘结性能, 从而使材料力学性能得到提高。而且, 处理后纤维拉伸强度和长径比的增大以及纤维缺陷的降低也会增强材料性能。      

The influence of fibre microstructure on fibre breakage and mechanical properties of natural fibre reinforced polypropylene

The aim of the study was to investigate the influence of fibre morphology of different natural fibres on the composites mechanical properties and on the fibre breakage due to extrusion process. The composite materials were manufactured using LTF (long fibre thermoplastic) extrusion and compression moulding and the used fibres were sisal, banana, jute and flax, and the matrix was a polypropylene. The results showed that sisal composites had the best impact properties and the longest fibres after the extrusion. Generally, the composites flexural stiffness was increased with increased fibre content for all fibres, being highest for flax composites. The flexural strength was not affected by the addition of fibres because of the low compatibility. The addition of 2 wt.% maleated polypropylene significantly improved the composites properties. Unlike the other three fibres, flax fibres were separated into individual elementary fibres during the process due to enzymatic retting and low lignin content.     

Characterisation of carbon fibres recycled from carbon fibre/epoxy resin composites using supercritical n-propanol

Three different PAN based carbon fibres (Toray T600S, T700S and Tenax STS5631) were recycled from epoxy resin/carbon fibre composites using supercritical n-propanol. The recycled carbon fibres were characterised using single fibre tensile tests, SEM, XPS and micro-droplet test. The tensile strength and modulus of the recycled carbon fibre was very similar to the corresponding as-received carbon fibres. However, the surface oxygen concentration decreased significantly, which caused a reduction of the interfacial shear strength with epoxy resin.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

Effect of fibre content and alkali treatment on mechanical properties of Roystonea regia-reinforced epoxy partially biodegradable composites

The present paper investigates the effect of fibre content and alkali treatment on tensile, flexural and impact properties of unidirectional Roystonea regia natural-fibre-reinforced epoxy composites which are partially biodegradable. The reinforcement Roystonea regia (royal palm) fibre was collected from the foliage of locally available royal palm tree through the process of water retting and mechanical extraction. The poor adhesion between fibre and matrix is commonly encountered problem in natural-fibre-reinforced composites. To overcome this problem, specific physical and chemical treatments were suggested for surface modification of fibres by investigators. Alkali treatment is one of the simple and effective surface modification techniques which is widely used in natural fibre composites. In the present study both untreated and alkali-treated fibres were used as reinforcement in Roystonea regia epoxy composites and the tensile, flexural and impact properties were determined at different fibre contents. The alkali treatment found to be effective in improving the tensile and flexural properties while the impact strength decreased.     

The crystallization and interfacial bond strength of nylon 6 at carbon and glass fibre surfaces

The nucleation and crystallization of nylon at the interface in glass-fibre and carbon-fibre reinforced nylon 6 composites has been investigated by electron microscope studies of sectioned and etched bulk specimens and solution cast and melt crystallized thin films. The fracture energies of the composites were obtained from tensile strength tests and the interfacial bond strengths were calculated from fibre pullout measurements. The fibres are shown to nucleate a columnar structure at the interface with marked differences between the structures nucleated by glass fibres and by carbon fibres and also between that nucleated by type I and type II carbon fibres. The structure around glass fibres was non-uniform and influenced to some extent by the presence of the size coating on the fibre surface. In the carbon-fibre composites the columnar structure was due primarily to physical matching of the graphite crystallites. Surface treatment of the carbon fibres to improve chemical bonding is shown to have a significant effect on bond strength which cannot be explained in terms of the columnar structure at the fibre surface. The treated fibres gave rise to only small amounts of fibre pull-out and low fracture energies whereas the untreated fibres showed extensive pull-out which was reflected in high fracture energies.     

Crack bridging and fibre pull-out in polyethylene fibre reinforced epoxy resins

An investigation has been undertaken of the stress distributions in high-performance polyethylene fibres bridging cracks in model epoxy composites. The axial fibre stress has been determined from stress-induced Raman band shifts and the effect of fibre surface treatment has been followed using untreated and plasma-treated polyethylene fibres. It is found that when the specimen is cracked, the fibres do not break and stress is transmitted from the matrix to the fibre across the fibre/matrix interface. A debond propagates along the fibre/matrix interface accompanied by friction along the debonded interface. The axial stress distributions in the fibres can be analysed using a partial-debonding model based upon shear-lag theory and it is found that the maximum interfacial shear stress at the bond/debond transition is a function of the debond length. The debonding process has been modelled successfully in terms of the interfacial fracture energy-based criterion developed by Hsueh for the propagation of a debond along a fibre/matrix interface accompanied by constant friction along the interface.     

The Effect of Alkali Treatment on the Adhesion Characteristics of Sisal Fibres

The effect of alkali treatment on the wetting ability and coherence of sisal-epoxy composites has been examined. Treatment of sisal fibre in a 0.5N solution of sodium hydroxide, resulted into more rigid composites with lower porosity and hence higher density. The treatment has been shown to improve the adhesion characteristics, due to improved work of adhesion because it increases the surface tension and surface roughness. The resulting composites showed improvements in the compressive strength and water resistance. It has been suggested that the removal of intracrystalline and intercrystalline lignin and other surface waxy substances by the alkali substantially increases the possibility for mechanical interlocking and chemical bonding. The alkali treatment is simple and is recommended to precede other sophisticated surface modification treatments on plant fibres similar to sisal fibre.     

Synergy of matrix and fibre modification on adhesion between carbon fibres and poly(vinylidene fluoride)

Interfacial properties between carbon fibres and poly(vinylidene fluoride) (PVDF) were tuned by modifying both constituents. Atmospheric plasma fluorination (APF) was utilised to tailor the surface composition of carbon fibres, which resulted in an incorporation of up to 3.7 at.% of fluorine functionalities in to the fibre surfaces. The PVDF matrix was modified by blending pure PVDF with maleic anhydride (MAH) grafted PVDF. Both fibre and matrix modifications act in synergy with improvements of up to 50% in the apparent interfacial shear strength (IFSS) above the level of pure fibre or matrix modification. Modification of both constituents led to the formation of various interactions at the fibre/matrix interface namely dispersive and polar (H-bonds) between (modified) PVDF and the fluorine as well as oxygen functionalities on the fibre surfaces. The apparent IFSS between the modified fibres and matrix reaches a maximum of 42 MPa, which is almost the tensile strength of the pure PVDF. The improvements in apparent IFSS in single fibre model composites for both fibre and matrix modifications translated to a seven times improvement in the interlaminar shear strength of unidirectional composites.     

Effect of additives on the interfacial strength of poly(l-lactic acid) and poly(3-hydroxy butyric acid)-flax fibre composites

The interfacial shear strength (IFSS), evaluated by single fibre pull-out tests was quantified for various biopolymer-flax fibre composites that were modified with additives. The additives included a plasticiser (glycerol triacetate) (GTA) absorbed onto/into the fibres, 4,4′-thiodiphenol (TDP) that is capable of forming hydrogen bonds between the matrix and cellulose from the fibres, and a hyperbranched polyester (HBP) to impart improved fracture toughness. Fibres were washed with acetone to remove the surface impurities and dried under vacuum before absorption of plasticiser and adsorption of thiodiphenol. It was found that the different additives significantly influenced the IFSS for the biopolymer-flax fibre systems while extraction with acetone had a no effect on the IFSS compared with the untreated fibres. The use of TDP imparted the most significant increase in IFSS whilst the HBP had an opposing effect. The use of ESEM corroborated with the findings of the single fibre pull-out tests.     

Alkali treatment of coir fibres for coir-polyester composites

Coir fibres were subjected to alkali treatment with a view to improving the wettability of coir fibres by a commercially available resin such as polyester. Tensile strength of the fibres increases by 15% when the fibres are soaked in 5% aqueous solution of NaOH at 28±1° C for 72 to 76 h after which it shows a gradual decrease. This decrease is much more pronounced when the alkali is replenished after every 24 h. SEM observations showed the removal of cuticle and tyloses from the surface of coir as a result of alkali treatment, resulting in a rough fibre surface with regularly spaced pits. The debonding stress of alkali-treated fibres from polyester matrix was 90% higher than that of untreated fibres from the same matrix. Untreated fibres tended to float in the polyester whereas alkali-treated fibres were uniformly dispersed in polyester. Incorporation of 0.30 volume fraction of untreated and treated fibres in polyester resulted in composites having 11% and less 5% porosity, respectively. Flexural strength, modulus and impact strength of composites containing alkali-treated fibres were 40% higher than those containing the same volume fractions of untreated fibres. Longitudinal ultrasonic velocity and sound attenuation measurements indicated less fibre segregation and better fibre-matrix bonding in composites containing alkali-treated fibres.     

Matrix morphology and fibre pull-out strength of T700/PPS and T700/PET thermoplastic composites

The main objective of this fundamental study was to investigate effects of processing conditions and resulting matrix morphology on interfacial bond strength of fibre reinforced thermoplastic composites. Using a hot stage microscope, single fibre pull-out samples were produced with T700S high strength carbon fibre and two semicrystalline thermoplastic matrices, polyphenylene sulphide (PPS) and polyethylene terephthalate (PET), respectively. Processing temperatures and cooling histories were the major variables in sample preparation. The T700S fibre had no clear effect on the surrounding PPS and PET matrix morphology, as long as direct cooling at constant rates was selected. A transcrystalline phase around the fibres could be induced in the T700S/PPS system, if isothermal crystallization was carried out at 227°C. Fibre pull-out tests were conducted at room temperature and two basic failure paths were observed, i.e. debonding at the fibre-matrix interface and cohesive failure of the matrix close to the fibre surface. The results indicate that slow cooling rate and a resulting coarse spherulitic morphology around the fibres correlate with high interfacial shear strength. In fact somewhat higher strength values were obtained for samples with transcrystalline layers around the fibres.