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.     

Suppression of crack growth in hybrid fibre composites

A theoretical analysis based on the assumed form of the strain field surrounding a crack bridged by reinforcing elements has been used to examine the growth of a crack propagating transversely to the fibres in hybrid fibre composites. An intermingled carbon fibre/glass fibre polymer matrix system has been considered. Two situations have been investigated. In the first of these the effect of the addition of carbon fibres on the development of cracks resulting from the failure of the glass fibres by stress corrosion has been studied. The analysis indicates that crack growth can be severely inhibited by a 5% volume fraction of type III carbon fibres. The analysis has been used also to investigate the process by which strong high failing strain glass fibres inhibit the growth of cracks caused by the fracture of localized clusters of low failing strain carbon fibres. The predictions of this analysis agree with existing experimental data on glass fibre/carbon fibre hybrids.     

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.     

Extraction and preparation of bamboo fibre-reinforced composites

Natural plant fibre composites have been developed for the production of a variety of industrial products, with benefits including biodegradability and environmental protection. Bamboo fibre materials have attracted broad attention as reinforcement polymer composites due to their environmental sustainability, mechanical properties, and recyclability, and they can be compared with glass fibres. This review classifies and describes the various procedures that have been developed to extract fibres from raw bamboo culm. There are three main types of procedures: mechanical, chemical and combined mechanical and chemical extraction. Composite preparation from extracted bamboo fibres and various thermal analysis methods are also classified and analysed. Many parameters affect the mechanical properties and composite characteristics of bamboo fibres and bamboo composites, including fibre extraction methods, fibre length, fibre size, resin application, temperature, moisture content and composite preparation techniques. Mechanical extraction methods are more eco-friendly than chemical methods, and steam explosion and chemical methods significantly affect the microstructure of bamboo fibres. The development of bamboo fibre-reinforced composites and interfacial adhesion fabrication techniques must consider the type of matrix, the microstructure of bamboo and fibre extraction methods.     

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.     

Mechanical properties and failure of etched UHMW-PE fibres

The structural hierarchy of fibrillar ultra-high molecular weight polyethylene (UHMW-PE) fibres is investigated and related to fibre mechanical properties. Chemical etching has been used to change the surface properties of these UHMW-PE fibres through the removal of a skin layer and UHMW-PE oxidation. The physical and chemical changes to the fibre surface introduced by etching affect single-fibre mechanical properties. The effects of etchant and etching time on failure properties and mechanisms is discussed. The decrease in failure strain and strength with etching is associated with the change from an energy-absorbing fibril delamination failure to brittle fracture.     

Influence of temperature on fracture initiation in PET and PA66 fibres under cyclic loading

The fatigue of single thermoplastic fibres has been well documented to occur in a reproducible manner when they are subjected to certain cyclic loading conditions. The fatigue fracture morphologies of these fibres are very distinctive and differ markedly from other types of failure. This type of behaviour, which is clearly seen with the unambiguous tests on single fibres, must reflect behaviour of fibres in more complex structures which are subjected to cyclic loading. Only limited numbers of reports have, however, shown similar fracture morphologies with fibres extracted from fibre bundles embedded in a matrix material such as rubber. Usually the fractured ends of fibres taken from structures are seen to be shorter than those obtained in single fibre tests and also they show more complex and confused crack growth. The present study reveals that the low thermal conductivity of the fibres, exacerbated when they are embedded in a rubber matrix, leads to very high temperature rises, which is not the case in single fibre tests and under these conditions, crack initiation occurs across the fibre section instead of being restricted to the near surface region. Tests on single fibres at temperatures up to and beyond the glass transition temperature have shown how the fracture morphologies become modified. The fatigue process has been seen to become generalised throughout the fibre and failure occurs due to the coalescence of several cracks, some of which are initiated in the core of the fibre. In all cases, the cracks can be seen to have been initiated by solid inclusions in the fibres.     

Characterisation of the cross-linking process in an E-glass fibre/epoxy composite using evanescent wave spectroscopy

The term “self-sensing composites” is sometimes used to describe the case where the reinforcing glass fibres in advanced fibre-reinforced composites are used as the sensors for chemical process-monitoring (cure monitoring). This paper presents conclusive evidence to demonstrate that reinforcing E-glass fibres can be used for in situ cure monitoring. The cure behaviour of an epoxy/amine resin system was compared using evanescent wave spectroscopy via the reinforcing E-glass fibres and conventional transmission Fourier transform infrared spectroscopy. This paper also reports for the first time that evanescent wave spectroscopy via E-glass fibres can be used to detect the presence of silane coupling agents. Preliminary results indicated that the cure kinetics on the E-glass fibre surface, as observed using evanescent wave spectroscopy, were influenced by the silane coupling agent.     

Evaluation of fibre bridging stress of short fatigue cracks in SCS-6/Ti-15-3 composite

Single‐edge notched specimens of a unidirectional SiC long fibre reinforced titanium alloy, were fatigued under four point bending. The propagation behaviour of short fatigue cracks from a notch was observed on the basis of the effects of fibre bridging. The branched fatigue cracks were initiated from the notch root. The fatigue cracks propagated only in the matrix and without fibre breakage. The crack propagation rate decreased with crack extension due to the crack bridging by reinforced fibres. After fatigue testing the loading and residual stresses in the reinforced fibres were measured for the arrested cracks by the X‐ray diffraction method. The longitudinal stresses in the reinforced fibres were measured using high spatial resolution synchrotron radiation. A stress map around the fatigue cracks was then successfully constructed. The longitudinal stress decreased linearly with increasing distance from a location adjacent to the wake of the matrix crack. This region of decreasing stress corresponded to the debonding area between the fibre and the matrix. The interfacial frictional stress between the matrix and the fibre could be determined from the fibre stresses. The bridging stress on the crack wake was also measured as a function of a distance from a notch root. The threshold stress intensity factor range, corrected on the basis of the shielding stress, was similar to the propagation behaviour of the monolithic matrix. Hence the main factor influencing the shielding effect in composites is fibre bridging.     

The effects of matrix microcracking on the oxidation behaviour of carbon-fibre/glass-matrix composites

Carbon-fibre/glass-matrix composites were fabricated using Fortafil fibres and two different glass matrices: a sodium-borosilicate glass (CGW 7740), and a calcium-aluminosilicate glass (CGW 1723). Upon cooling from the hot-pressing temperature used to fabricate the composites (approximately 1250°C), the glass matrices cracked due to differences in the coefficients of thermal expansion between the fibres and the matrix. At elevated temperatures these cracks serve as short-circuit diffusion paths for oxygen transport, and the majority of the weight loss from the cracked samples was caused by oxygen diffusing along these microcracks and reacting with the fibres. Because of the relatively large diameter of these cracks compared to the mean free path for diffusing oxygen, traditional gas kinetics can be applied to the various transport processes occurring in the oxidation reactions, and there is no need to allow for capillary size or to apply Knüdsen diffusion. The composites made of 1723 glass exhibited linear relationships between specific-mass loss ( mass/initial exposed surface area of carbon fibres) and time at all oxidation temperatures (450, 500, 550 and 600 °C). With the 7740 composites, a parabolic relationship between specific-mass loss and time was obtained. As the oxidation temperature approached or exceeded the glass-transition temperature, T _g, for the 7740 composites (560 °C), this parabolic relationship became more pronounced. Microstructural evidence revealed that at temperatures near or exceeding the T _g for the 7740 glass the microcracks in the matrix heal, thereby decreasing the amount of fibre surface area available for chemical reaction. Because the rate of oxidation is directly proportional to the amount of available fibre-surface area, the weight-loss data appear parabolic with time. Additionally, the mechanism for the oxidation of the carbon fibres does not appear to change once the fibres are placed in a glass matrix. The apparent activation energy for oxidation remained constant at approximately 174 kJ mol^–1.     

玻璃纤维增强灌注型聚氨酯泡沫塑料的拉伸、压缩性能和破坏机理

研究了用短切玻璃纤维对硬质聚氨酯泡沫体的增强效果及拉伸、压缩的破坏行为。结果表明当纤维长为12 mm 时, 6 w t% 纤维含量的增强效果为最好, 可以使泡沫体的拉伸强度提高75% , 压缩强度提高25% , 压缩模量增加约30%。纤维增强的泡沫体拉伸产生的裂纹扩展时, 遇到纤维可能终止扩展(应力不大时) , 也可能发生偏转(应力较大时) ; 泡沫破坏时, 可能出现纤维拉出、拉断等不同的破坏形式。增强泡沫体在压缩破坏时, 主要是泡沫结构的支柱弯曲、扭转变形引起泡壁破裂和支柱失稳, 并导致材料的破坏。      

A quantitative study on the plastic shrinkage cracking in high strength hybrid fibre reinforced concrete

High strength concrete using silica fume is prone to plastic shrinkage cracking in dry and windy conditions. Addition of fibres is known to restrict the growth of shrinkage cracks. The present study was aimed at controlling plastic shrinkage cracks in high strength silica fume concrete by means of adding fibre reinforcement up to 0.5% by volume of concrete. Individual steel fibres as well as hybrid combinations of steel and non-metallic (polyester, polypropylene and glass) fibres were evaluated for their influence on plastic shrinkage cracking. Results showed that hybrid fibres were most effective in reducing shrinkage cracks. Among the hybrid fibre combinations, the steel and polyester combination was found to reduce plastic shrinkage cracks by more than 99% compared to the plain concrete. Increased fibre availability and low stress levels at early ages were the main factors contributing to the good performance of hybrid fibre mixtures.     

Analysis of flax fibres viscoelastic behaviour at micro and nano scales

In glass or carbon fibres reinforced plastics, creep or stress relaxation, arise from the polymeric nature of the matrix. Plant fibres, used in bio-composites, are also polymers. Therefore, the issue of their service life requires studying the viscoelastic behaviour of both the matrix and the fibres. In this study, we investigate, at different length scales, the response of elementary flax fibres to tensile tests, as well as to nano-indentation tests on their secondary cell walls. The results of these experiments are then analysed via linear viscoelastic rheological models and identification procedures. The values of the identified parameters (relaxation time, viscosity and elastic stiffness) are discussed in relation to the microstructure of the flax fibre (cellulose microfibrils, hemicelluloses and pectins). The nano-indentation technique provides much more deterministic results than tension tests on an entire fibre. The scale of the secondary wall cell is then relevant to assess the viscoelastic behaviour of the fibres.     

Statistical changes during the corrosion of glass fibre bundles

The mechanical properties of E-glass fibre bundles have been measured after corrosive attack by hydrochloric acid of various concentrations for various times. The effective stiffness of the fibre bundles is seen to be proportional to the effective cross-sectional area of the fibres as identified with the characteristic core-sheath geometry found in fibres exposed to long-term acid attack. However, the strength of the fibre bundles is not simply related to the effective area of the fibres and the statistics of fibre strength vary considerably with time. In particular the Weibull shape parameter is seen to increase rapidly at short times, before core-sheath formation is observed, and then fall slowly with core-sheath formation. Hence we have a shortterm narrowing of the strength distribution followed by a long-term broadening.     

Direct observation of compressive deformation of PBO fibre in the SEM

Polybenzobisoxazole fibres were compressed uniaxially in a tension-compression stage for direct observation of deformation in a scanning electron microscope. Kink bands were observed to develop on the fibre surface at early stages of deformation. They seemed to initiate heterogeneously and propagate across a large part of the fibre. Both the length and thickness of the band increased dramatically with slight compression. Upon continued deformation, cracks or openings were seen to develop from surface irregularities near the kink bands. The cracks or openings extended along the fibre length while new bands continued to develop adjacent to them. Cracks and kink bands seemed to be mutually inducible to result in a localized bulge at later stages of deformation. In Kevlar 49 fibres, cracks or openings seemed to develop at higher compressive strains notably at intersections of kink bands.     

Chemical stress cracking of acrylic fibres

The generation of periodic microscopic transverse cracks in oriented acrylic fibres immersed in hot alkaline hypochlorite solution is described in detail and shown to be a variety of chemical stress cracking. It is greatly accelerated by external tensile stress, high fibre permeability, moderate fibre orientation, and water-plasticization. The proposed mechanism for bond cleavage involves cyclization of nitrile groups (similar to the prefatory reaction in pyrolysis of acrylic fibres), followed immediately by N-chlorination and chain scission. Mechanical retractile forces (internal or external) then cause chain retraction and crack growth. Despite the remarkable regularity of the crack pattern, which typically resembles a series of stacked lamellae, the process is independent of any such underlying fibre morphology. The cracking process does, however, appear to be a sensitive indicator of residual latent strain in the fibre, which may persist even after high-temperature annealing.     

Deformation micromechanics of a model cellulose/glass fibre hybrid composite

Interfacial stress transfer in a model hybrid composite has been investigated. An Sm³⁺ doped glass fibre and a high-modulus regenerated cellulose fibre were embedded in close proximity to each other in an epoxy resin matrix dumbbell-shaped model composite. This model composite was then deformed until the glass fibre fragmented. Shifts of the absolute positions of a Raman band from the cellulose fibre, located at 1095 cm⁻¹, and a luminescence band from a doped glass fibre, located at 648 nm, were recorded simultaneously. A calibration of these shifts, for both fibres deformed in air, was used to determine the point-to-point distribution of strain in the fibres around the breaks in the glass fibre. Each break that occurred in the glass fibre during fragmentation was shown to generate a local stress concentration in the cellulose fibre, which was quantified using Raman spectroscopy. Using theoretical model fits to the data it is shown that the interfacial shear stress between both fibres and the resin can be determined. A stress concentration factor (SCF) was also determined for the regenerated cellulose fibre, showing how the presence of debonding reduces this factor. This study offers a new approach for following the micromechanics of the interfaces within hybrid composite materials, in particular where plant fibres are used to replace glass fibres.     

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.     

The fracture energy of hybrid carbon and glass fibre composites

The fracture energy of a model carbon fibre/glass fibre/epoxy resin hybrid composite system has been evaluated as a function of the carbon fibre/glass fibre ratio. Work of fracture measurements were less than a rule of mixtures prediction and a pronounced negative synergistic effect was observed at high carbon fibre and high glass fibre contents. Fibre debonded lengths and fibre pull-out lengths for the carbon and glass fibres were accurately measured using a projection microscope technique. Models of microscopic fracture behaviour, together with these measurements, were successful in quantitatively describing the observed fracture behaviour of the hybrid fibrous composites. It was found that post-debond friction energy provided a major contribution to the fracture energy of the glass fibres. The post debond sliding mechanism was also shown to be primarily responsible for the non-linear behaviour of the work of fracture of the hybrid composite.     

Influence of surface treatment on adhesion of polyethylene fibres

Abstract

A detailed examination has been undertaken of the influence of surface treatment on the adhesion of polyethylene fibres to epoxy resin. The pull-out adhesion has been determined for untreated, chromic acid treated, and plasma etched monofilaments with different draw ratios and thermal annealing treatments. In a few cases, additional chemical treatments were applied to plasma treated fibres before the pull-out test. The polyethylene surface energy also has been determined by measurement of contact angle. The results, taken together, suggest that the adhesion depends on three factors: (i) the wettability (or physicochemical interactions), which is affected by the extent and nature of the surface treatment as well as the fibre draw ratio; (ii) the surface roughness, after plasma etching only, where a honeycomb structure of pits permits mechanical keying between the fibre and the resin (this structure has been examined by scanning electron microscopy); and (iii) the number of chemical bonds per unit area between the fibre and the resin. It is concluded that these three factors can be regarded as additive and that optimum results are obtained when their respective pull-out strengths reach their maximum values, ~2, ~3, and ~1·7 MN m^?2.

MST/640