Adhesive properties and failure of etched UHMW-PE fibres

The effect of chemical etching on the surface of ultra-high molecular weight polyethylene (UHMW-PE) fibres with emphasis on the adhesion of epoxy to the fibres was studied. The presence of an oxygen-rich weak boundary layer on the non-polar UHMW-PE fibre yields poor adhesion for the as-received fibre and for fibres etched with the weaker etchants. A significant improvement in adhesion resulted when the weak boundary layer was removed and the UHMW-PE oxidized through etching with chromic acid, a stronger etchant. This significant improvement in adhesion was reflected not only in a higher interfacial shear strength but also in the presence of epoxy cohesive failure. The debonding of droplet microbonds was found to be a suitable technique for the characterization of adhesion in the UHMW-PE/epoxy system.     

Effect of low-temperature-plasma surface treatment on the adhesion of ultra-high-molecular-weight-polyethylene fibres

The poor adhesion of ultra-high-molecular-weight polyethylene (UHMW-PE) fibres to epoxy resin (due to characteristics of their surface structure) makes it difficult to develop light composite materials with good mechanical properties. In our experiments a low-temperature plasma was applied to treat the surfaces of the fibres. The results showed that the surface energy of the treated fibres was increased greatly so that the fibres could be wetted very well by resin, which was an essential prerequisite in improving the adhesion of the fibre/epoxy resin systems; and the results also showed that the pull-out adhesion strength of the treated fibres to resin was increased significantly by about 10 times. This paper discusses the mechanism of the improvement of the adhesion. Two reasons for improvement are that: varieties of polaroxygen-containing groups are produced in the non-polar surface layer, forming chemical bonds; and plasma-etched pits spread all over the surface of the fibres into which the resin penetrates to form a mechanical interlock between fibre and resin. A synthetic analysis and discussion of these two factors and of non-polar interactions influencing the adhesion is given, and a preliminary relationship between them is presented.     

超高分子量聚乙烯纤维的等离子体表面处理

本文采用低温等离子体对超高分子量聚乙烯纤维进行表面处理,以改善其与环氧树脂的粘接性能,为进一步研制高性能轻型复合材料提供科学依据。实验结果表明处理后的纤维表面能大大提高,使环氧树脂能良好地浸润纤维;纤维与环氧树脂间粘接强度可提高近5—10倍。本文进一步分析了粘接性能改善的原因,并对粘接强度做出贡献的各种作用进行了综合的定量分析。     

PLASMA SURFACE TREATMENT OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE FIBRES YUAN Chaoting GAO Shangling MU Qiwu(Chongqing Institute of Architecture and Engineering)——DING Yiping(Textiel Academy,Ministry of Textile Industry)

本文采用低温等离子体对超高分子量聚乙烯纤维进行表面处理,以改善其与环氧树脂的粘接性能,为进一步研制高性能轻型复合材料提供科学依据。实验结果表明处理后的纤维表面能大大提高,使环氧树脂能良好地浸润纤维;纤维与环氧树脂间粘接强度可提高近5—10倍。本文进一步分析了粘接性能改善的原因,并对粘接强度做出贡献的各种作用进行了综合的定量分析。     

A study of the corrosion resistance of glass fibre reinforced polymers

Specific interactions between chemical environments (hydrochloric acid, sulphuric acid and distilled water) and glass fibre cause stress corrosion cracking in the glass fibre surface. The etching of the glass fibre gives rise to an extraction process. Axial or spiral cracks can then be observed. These effects depend on the fibre diameter, the etching time and the chemical environment and cause a drop in tensile stresses. The glass fibre crumbles with increasing etching time.Strict etching procedures lead to definite extraction processes and crack structures in the glass fibres and will be discussed in connection with strength tests.In addition to investigations of individual elements, e.g. glass fibres, it is also possible that whole glass fibre reinforced composites are damaged during service under the influence of aggressive surrounding media. In such cases, circular or spiral-shaped cracks can also be observed preferentially in the glass fibre. The fibres can then no longer contribute to an increase in strength and the result is the untimely failure of the composite material.     

The mechanical properties of oxyfluorinated hybrids of glass fibre and polypropylene fibre

The mechanical properties of a low-cost system comprising orthophthalic polyester resin reinforced with hybrids of glass and polypropylene fibres were investigated. The fibres were oxyfluorinated to overcome the poor surface adhesion properties of polypropylene. Interlaminar shear tests, Izod-type impact tests and tensile tests were considered. It would be expected that increasing polypropylene fibre content corresponds with a decrease in mechanical properties due to the poor properties of polypropylene. Oxyfluorinated laminates containing approximately 25% and 50% polypropylene in the warp direction were, however, found to exhibit significant improvements in interlaminar shear strength, in peak shear stress under impact loading as well as in impact resistance over untreated glass fibre laminates. Scanning electron microscope images show that the reason for this improvement is that the interfacial bond between the polypropylene fibres and the resin is strengthened to such an extent that failure occurs within the polypropylene fibres rather than at the interface.     

Strength and adhesion characteristics of elementary flax fibres with different surface treatments

It is known that the best flax fibres can compete in terms of mechanical properties with glass fibres. However, during the manufacturing process flax fibres are often damaged, and hence, the properties can be lowered. Furthermore, these properties change from batch to batch (depending on the time and place of harvest), which means that they are somewhat unpredictable. The most affected fibre property is strength, which can vary in very wide interval due to defects introduced by the manufacturing process. Therefore, there is a need for a simple but reliable testing procedure that allows the estimation of the strength of flax fibres, so called quality control. Regarding the final goal, that is the development of natural fibre composites, another crucial property is the fibre/matrix adhesion. The objective of this study is to investigate the possibility to use the single fibre fragmentation test to characterize strength distribution of flax fibres and to evaluate the adhesion. Untreated flax fibres and fibres coated by a special surface treatment are used. Fragmentation tests are performed on flax fibres embedded in thermoset, vinylester and polyester, resins. Results show that there is a definite improvement in interfacial strength when a fibre surface treatment is applied. Fibre strength distribution is obtained from SFFT and compared with limited results available from single flax fibre tests.     

Identification of interfacial and interphasal failure in composites of plasma polymer coated fibres

It has been demonstrated in previous work by this group that the mechanical properties of a glass fibre reinforced polymer composite can be improved by the deposition of a plasma polymer coating onto the fibres prior to their incorporation into the matrix. It was further demonstrated that the mechanical properties of the resulting composite could be tailored by adjusting the composition of the coating. More complete fractography of these composites has now been performed. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to identify the chemical species present on both the surface of the fibres and the composite fracture surfaces and so deduce their role in achieving adhesion between the fibre and matrix. By imaging the fracture surfaces with matrix, interphase and fibre specific secondary ions the locus of failure could be identified. It was found that changing the composition of the plasma polymer coating applied to the fibre moved the locus of failure from within the interphase to the interface. This demonstrates how the chemistry of the interphase can produce the desired fracture mechanics to optimise composite mechanical properties.     

Strength variability of single flax fibres

Due to the typical large variability in the measured mechanical properties of flax fibres, they are often employed only in low grade composite applications. The present study aims to investigate the reasons for the variability in tensile properties of flax fibres. It is found that an inaccuracy in the determination of the cross-sectional area of the fibres is one major reason for the variability in properties. By applying a typical circular fibre area assumption, a considerable error is introduced into the calculated mechanical properties. Experimental data, together with a simple analytical model, are presented to show that the error is increased when the aspect ratio of the fibre cross-sectional shape is increased. A variability in properties due to the flax fibres themselves is found to originate from the distribution of defects along the fibres. Two distinctive types of stress–strain behaviours (linear and nonlinear) of the fibres are found to be correlated with the amount of defects. The linear stress–strain curves tend to show a higher tensile strength, a higher Young’s modulus, and a lower strain to failure than the nonlinear curves. Finally, the fibres are found to fracture by a complex microscale failure mechanism. Large fracture zones are governed by both surface and internal defects; and these cause cracks to propagate in the transverse and longitudinal directions.     

Mechanical properties of poly(butylene succinate) (PBS) biocomposites reinforced with surface modified jute fibre

A biocomposite was originally fabricated with biodegradable polymer PBS and jute fibre, and the effects of fibre surface modification on characteristics of jute fibre and mechanical properties of the biocomposite were evaluated in this paper. The experimental results show that surface modification can remove surface impurities and reduce diameter of jute fibres. Regarding the mechanical properties of biocomposites, it is observed that the biocomposites with jute fibres treated by 2% NaOH, 2 + 5% NaOH or coupling agent, respectively, an optimum in mechanical properties can obtain at fibre content of 20 wt.%, which exhibit an obvious enhancement in mechanical strength and modulus compared to the ones with untreated jute fibre. Furthermore, surface modification also exhibits less effects on flexural properties compared to tensile properties and more on flexural or tensile modulus than on the strength.     

Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials. I. hemp fibres

In this study a thorough analysis of physical and fine structure of hemp fibre bundles, namely surface topography, diameter, cellulose content and crystallinity index, have been presented. The fibre bundles have been alkalised and physical and mechanical properties analysed. Alkalisation was found to change the surface topography of fibre bundles and the diameter decreased with increased concentration of caustic soda. Cellulose content increase slightly at lower NaOH concentrations and decrease at higher NaOH concentrations. The crystallinity index decrease with increase in caustic soda concentration up to 0.24% NaOH beyond which, it decreases with increase in NaOH concentration. It was also found that the tensile strength and stiffness increases with increase in the concentration of NaOH up to a limit. Tensile strength and Young’s modulus increase with decrease in cellulose content, while crystalline cellulose decreases slightly but with improved crystalline packing order resulting in increased mechanical properties. Similar observations are elucidated by the crystallinity index. Alkalised hemp fibre bundles were found to exhibit a similar specific stiffness to steel, E-glass and Kevlar 29 fibres. The results also show that crystallinity index obtained following alkalisation has a reverse correlation to the mechanical properties. Stiffer alkalised hemp fibre bundles are suitable candidates as reinforcements to replace synthetic fibres. The improvement in mechanical properties of alkali treated hemp fibre bundles confirms their use as reinforcement materials.     

Atmospheric plasma fluorination as a means to improve the mechanical properties of short-carbon fibre reinforced poly (vinylidene fluoride)

The impact of fluorination of carbon fibres on the properties of short fibre reinforced polyvinylidene fluoride (PVDF) composites was studied. As received and continuously atmospheric plasma fluorinated (APF) carbon fibres were cut to an average fibre length of 2 mm. Short fibre composites (SFC) containing 5, 10 and 15 wt.% carbon fibres were manufactured using a twin-screw mixer. Test specimens were produced by injection moulding. The mechanical properties of the SFC were studied using tensile and compression testing. As expected, the incorporation of short-carbon fibres into PVDF led to an increase in strength and stiffness. The tensile strength and Young’s modulus of the SFC containing APF-treated carbon fibres increased by up to 17% and 190%, respectively. Furthermore, the compressive strength and modulus of the SFC containing APF-treated carbon fibres also increased by 19% and 35%, respectively. APF of carbon fibres results only in a marginal increase in the bulk matrix crystallinity of PVDF as determined by DSC. Scanning electron micrographs of fracture surfaces from tensile tested specimens exhibited a typical brittle failure mode with low fibre loading fraction. Despite the presence of up to 5% of voids and visible resin rich regions at fracture surface, SFC containing APF-treated fibres suggest better bonding at the fibre/matrix interface which led to the much enhanced mechanical properties.     

The mechanical behaviour of porous austenitic stainless steel fibre structures

The mechanical properties of metal fibre porous structures were studied in the light of their potential application as surface coatings of implants. Stainless steel AISI 316 L fibres with diameters of 50 and 100μm were compacted and sintered. The variation of the modulus of elasticity with density, as obtained in tension, corresponds closely with theoretical models. The ultimate failure of the tensile specimens proceeds through the fibres, and not through the sinter bonds, except at lower densities. Differences in yield strength between 50 and 100 μm fibre tensile specimens are explained on the basis of the onset of plastic deformation of the individual fibres. Upon compression the modulus of elasticity is nearly 10 times smaller than in tension. This result is due to the different deformation patterns of the fibres in compression and tension.     

Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement

“Hairy” bacterial cellulose coated sisal fibres were created using a simple slurry dipping process. Neat sisal fibres were coated with BC to create (i) a dense BC coating around the fibres or (ii) “hairy” fibres with BC oriented perpendicular to the fibre surface. These fibres were used to produce hierarchical sisal fibre reinforced BC polylactide (PLLA) nanocomposites. The specific surface area of the BC coated fibres increased when compared to neat sisal. Single fibre tensile tests revealed no significant difference in the tensile modulus and tensile strength of “hairy fibres”. However, when sisal fibres were coated with a dense BC layer, the mechanical fibre properties decreased. The tensile, flexural and visco-elastic properties of the hierarchical PLLA nanocomposites reinforced by both types of BC coated sisal fibres showed significant improvements over neat PLLA.     

Oberflächenbehandlung von Kohlenstoffasern und mechanische Eigenschaften von Laminaten

Surface Treatment of Carbon Fibres and Resulting Composite Properties In composites carbon fibres are used as reinforcing fibres with thermosetting and thermoplastic resins as martices. These carbon fibres differ strong in their micro-structure and therefrom in fibre properties. To achieve sufficiant composite properties special carbon fibre surface treatment methods are necessary. This paper describes a systematic study on oxidative surface treatment of carbon fibres by wet-, dry- and anodic oxidation. Further investigations by matrix variation show us the influence of matrix strength on the mechanical composite properties. Finally it is shown that in case of impact load composite fracture behaviour is controlled only by the fibre itself.     

Rißbildung in Elementarglasfasern durch Einwirkung korrosiver Medien und erhöhter Temperatur

Crack Initiation in Glass Fibres Under the Influence of Chemical Environment and High Temperature Specific interactions between chemical environments (distilled water, hydrochloric acid, and sulfuric acid) and the glass fibre generate stress corrosion cracking in the glass fibre surface. The etching of glass fibre effects either axial or spiral cracks in the glass fibre surface. These effects depend on the fibre diameter, the etching time and the chemical environment. Crack initiation generates a drop of tensile stresses. At least the glass fibre crumbles with increasing etching time. The reasons for this phenomenon are residual stresses in the glass fibre. Therefore it is necessary to know something about residual stresses. Strict etching procedures lead to definite crack structures in glass fibres and should indicate the kind of residual stresses in connection with strength tests.     

A computational model for failure analysis of fibre reinforced concrete with discrete treatment of fibres

Failure patterns and mechanical behaviour of high-performance fibre reinforced cementitious composites depend on the distribution of fibres within a specimen. In this contribution, we propose a novel computational approach to describe failure processes in fibre reinforced concrete. A discrete treatment of fibres enables us to study the influence of various fibre distributions on the mechanical properties of the material. To ensure numerical efficiency, fibres are not explicitly discretized but they are modelled by applying discrete forces to a background mesh. The background mesh represents the matrix while the discrete forces represent the interaction between fibres and matrix. These forces are assumed to be equal to fibre pull-out forces. With this approach experimental data or micro mechanical models, including detailed information about the fibre-matrix interface, can be directly incorporated into the model.     

The application of optical fibres as witness devices for the detection of plastic strain and cracking

Low cost optical fibres have recently become readily available for telecommunications purposes. Silica fibres are characterised by high elastic strains to failure. The feasibility of using these fibres for structural integrity monitoring particularly for offshore structures is investigated. The basis of the technique is that a fibre may be bonded to a critical part of a structure and provides an optical path which will be broken if the fibre fails due to plastic strain or crack opening in the critical area.
Groups of fibres which have been given predetermined fracture strains by surface etching were encapsulated in special packs. These packs were bonded to steel and concrete tensile specimens. Strain transfer occurred successfully between the specimens and individual fibres. The distribution of strain to fibre fracture appeared to be uniform along the fibre. The use of several fibres with a range of fracture strains caused fibres to break progressively with increasing strain. For applications to offshore structures it has been found possible to use water-repellent adhesives which can be applied and cured in sea water and suffer no deterioration.
The advantages of this system include versatility, relatively low cost, adaptability to continuous monitoring and the possibility of being fitted retrospectively and refitted after repair operations.     

Surface Modification of Aramid Fibres with Graphene Oxide for Interface Improvement in Composites

A novel method of biomimetic surface modification was used for aramid fibres aiming to enhance the interface properties between epoxy resin and the modified aramid fibre. Inspired by the composition of adhesive proteins in mussels, a thin layer of poly(dopamine) (PDA) was self-polymerized onto the surface of the aramid fibre. The graphene oxide (GO) was then grafted on the surface of PDA-coated aramid fibres. The microstructure and chemical characteristics of the pristine and modified fibres were characterised using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), indicating successful grafting of GO on the PDA-coated aramid fibres. Single fibre tensile test and microbond test were carried out to evaluate the mechanical properties of the modified fibres. It was found that the fibre surface modification improved the interfacial shear strength by 210% and the fibre tensile strength was protected by GO-PDA coating.     

Can thermally degraded glass fibre be regenerated for closed-loop recycling of thermosetting composites?

Commercially manufactured E-glass fibres were heat-conditioned to mimic the effects of thermal recycling of glass fibre thermosetting composites. Degradation in the strength and surface functionality of heat-treated fibres was identified as a key barrier to reusing the fibres as valuable reinforcement in composite applications. A chemical approach has been developed to address these issues and this included two individual chemical treatments, namely chemical etching and post-silanisation. The effectiveness of the treatments was evaluated for both thermal degraded fibres and corresponding composites. Drastic reduction was observed in the properties of the composites with the heat-conditioned preforms indicating thermally degraded glass fibres have no value for second-life reinforcement without further fibre regeneration. However, significant regeneration to the above properties was successfully obtained through the approach developed in this work and the results strongly demonstrated the feasibility of regeneration of thermally degraded glass fibres for potential closed-loop recycling of thermosetting composites.