Influence of resin chemistry on water uptake and environmental ageing in glass fibre reinforced composites-polyester and vinyl ester laminates

Hand laid up glass fibre reinforced isophthalic polyester resin and transfer moulded vinyl ester resin composites were subjected to environmental ageing. Gravimetric measurements were performed on laminate samples exposed to air and distilled water at 30°C and 60°C. Complementary porosimetry measurements on the original and fully aged samples indicated significant changes in the structure arising on ageing. Blistering showed that hydrolysis and de-bonding at the fibre-matrix interface occurred in polyester laminate to a greater extent at 60°C than 30°C. Leaching of monomer residues led to an anti-plasticisation effect in vinyl ester laminate. The influence of the fabrication method on the ageing behaviour is observed through porosity measurements on aged samples.     

Performance analysis of fibre metal laminated thin conical frusta under axial compression

The crushing behaviour and energy absorption capacity of frustrated conical shells made-up of bare aluminium (AC) and E-glass fibre/epoxy resin composite overwrapped aluminium (CWAC) was studied under quasi-static axial compression condition. Using spinning process, the hollow frustrated conical specimens were fabricated with the help of wooden conical shaped mandrill with semi apical angles of 16° and 21°. Thin commercial aluminium sheets of average thickness 0.87 mm were obtained for making aluminium conical specimen. CWAC frusta were fabricated by wrapping glass fibre/epoxy resin over aluminium conical shell to form hybrid composite with required thickness by hand layup process. Quasi-static axial compression load was applied over top end of the specimen with cross head speed as 2 mm/min using Universal Testing Machine (UTM). From the experiment results, the load–deformation characteristics of different AC and CWAC frusta were investigated. Energy absorption capacities or crashworthiness and mode of collapse of all models of AC and CWAC are determined from load–deformation curve and the same was validated with finite element analysis package ABAQUS®.     

Effect of resin system on the mechanical properties and water absorption of kenaf fibre reinforced laminates

The objective of this study is to compare the mechanical and water absorption properties of kenaf (Hibiscus cannabinus L.) fibre reinforced laminates made of three different resin systems. The use of different resin systems is considered so that potentially complex and expensive fibre treatments are avoided. The resin systems used include a polyester, a vinyl ester and an epoxy. Laminates of 15%, 22.5% and 30% fibre volume fraction were manufactured by resin transfer moulding. The laminates were tested for strength and modulus under tensile and flexural loading. Additionally, tests were carried out on laminates to determine the impact energy, impact strength and water absorption. The results revealed that properties were affected in markedly different ways by the resin system and the fibre volume fraction. Polyester laminates showed good modulus and impact properties, epoxy laminates displayed good strength values and vinyl ester laminates exhibited good water absorption characteristics. Scanning electron microscope studies show that epoxy laminates fail by fibre fracture, polyester laminates by fibre pull-out and vinyl ester laminates by a combination of the two. A comparison between kenaf and glass laminates revealed that the specific tensile and flexural moduli of both laminates are comparable at the volume fraction of 15%. However, glass laminates have much better specific properties than the kenaf laminates at high fibre volume fractions for all three resins used.     

Morphology, dynamic mechanical and thermal studies on poly(styrene-co-acrylonitrile) modified epoxy resin/glass fibre composites

Poly(styrene-co-acrylonitrile) (SAN) was used to modify diglycidyl ether of bisphenol-A (DGEBA) type epoxy resin cured with diamino diphenyl sulfone (DDS) and the modified epoxy resin was used as the matrix for fibre reinforced composites (FRPs) in order to get improved mechanical and thermal properties. E-glass fibre was used as the fibre reinforcement. The morphology, dynamic mechanical and thermal characteristics of the systems were analyzed. Morphological analysis revealed heterogeneous dispersed morphology. There was good adhesion between the matrix polymer and the glass fibre. The dynamic moduli, mechanical loss and damping behaviour as a function of temperature of the systems were studied using dynamic mechanical analysis (DMA). DMA studies showed that DDS cured epoxy resin/SAN/glass fibre composite systems have two T_gs corresponding to epoxy rich and SAN rich phases. The effect of thermoplastic modification and fibre loading on the dynamic mechanical properties of the composites were also analyzed. Thermogravimetric analysis (TGA) revealed the superior thermal stability of composite system.     

Resistance to stress corrosion cracking of unidirectional ECR-glass/polymer composites for high voltage composite insulator applications

Stress corrosion experiments were performed on unidirectional ECR-glass/polymer composites with the modified polyester, epoxy and vinyl ester resins for the use in high voltage composite insulator applications. Two types of ECR-glass fibers were investigated with low and high counts of gaseous inclusions (seeds) within the glass. The stress corrosion tests were performed in nitric acid under four point bending conditions and the resistance to stress corrosion cracking (SCC) of the composites was determined for as-supplied and sandblasted surface conditions. In addition, the materials were analyzed for their micro-hardness and surface fiber exposure. The resistance to SCC of the ECR-glass/polymer composites was compared with that of the E-glass/polymer systems. It has been found that the ECR-glass fiber composites vastly out-perform their E-glass counterparts regarding their resistance to SCC in nitric acid. The stress corrosion data presented in this work should help composite insulator manufacturers in the selection of composite rod materials with substantially increased resistance to SCC to lower the number of brittle fracture failures among their products.     

Effect of fibre angle on fracture properties of unidirectional flyash filled FRP composites

The fracture properties of unidirectional flyash filled and unfilled glass fibre and carbon fibre reinforced epoxy resin composites are studied in relation to the variation of width ratio (a/W) and fibre angle. The results indicate that the fracture toughness, fracture surface energy and change in elastic strain energy are dependent on the width ratio but the effect of fibre angle between 30 and 60° is not very dependent on fracture properties due to the arrest of the crack path in fibre composites by flyash particles.     

Thermal degradation of the mode I interlaminar fracture properties of stitched glass fibre/vinyl ester composites

Changes to the Mode I interlaminar fracture toughness, GIc, and fracture mechanisms of stitched and unstitched fibreglass/vinyl ester composites were investigated after exposure to elevated temperatures. The fibreglass was stitched through the thickness with Kevlar®-49 thread in two orientations with two stitch densities, and then resin transfer moulded with a cold-curing vinyl ester resin. After curing at room temperature (20°C) for several weeks, the composites were heated to between 100 and 300°C for 1 h or at 175°C for times ranging from 0.25–100 h. The GIc values, which were measured using the double cantilever beam method, of stitched composites in the cold-cured condition were between 1.5 and 2.3 times higher than the unstitched composite. It was observed with scanning electron microscopy that this toughening occurred by deflection of the crack tip at the stitches, by the ability of the stitches to remain intact for a short distance (7–15 mm) behind the crack front, and by partial pull-out of broken stitches. The interlaminar fracture toughness of the unstitched composite increased slightly following heating, despite a possible breakdown of the chemical structure of the vinyl ester between 150 and 300°C. In contrast, the interlaminar toughness of the stitched composites was degraded significantly by heating, and this was probably caused by thermal deterioration of the Kevlar® stitches. This study reveals that the elevated-temperature post-curing of stitched composites will reduce the effectiveness of Kevlar® stitching in raising the Mode I interlaminar fracture toughness. © 1998 Chapman & Hall     

Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of γ-aminopropyltriethoxy silane (γ-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with γ-APS/PU sizing is much softer than the PPm matrix, while the interphase with the γ-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and ≈300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of ‘boundary effect’, nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites.     

An experimental evaluation of glass–polymer interfacial toughness

The evaluation of interfacial fracture toughness in glass fibre reinforced polymer composites is examined using macroscopic polymer–glass joints. Different bi-material systems providing various degrees of mixity were tested using various tests configurations. The test configurations were analysed in the frame of interfacial linear elastic fracture mechanics with the finite element method in order to investigate their properties and obtain relations between fracture mechanics parameters and joints’ geometry useful for the experimental data reduction. The analysis showed that the stress intensity factor phase angle characterizing mode mixity is almost constant with crack length for each test configuration examined: hence these tests lend themselves to be used to study the crack propagation behaviour when stable crack growth is observed.Joints made of combinations of E-glass, either treated or untreated with a surface finishing agent, and two polymers, poly (methylmethacrylate) and an epoxy resin, were employed.An attempt to model the fracture process of these systems with cohesive elements yielded not completely satisfactory results.     

Acoustic emission analysis for characterisation of damage mechanisms in fibre reinforced thermosetting polyurethane and epoxy

Acoustic emission analysis is used to investigate microscopic damage mechanisms and damage progress in unidirectional glass and carbon fibre reinforced composites. Under static loading the influence of fibre orientation on damage initiation and propagation is determined. A novel polyurethane matrix system significantly enhances material performance in terms of crack initiation load levels, crack growth, damage tolerance and off-axis tensile strength. Hysteresis measurements during stepwise increasing dynamic load tests highlight the effect of fibre–matrix-adhesion and resin fracture toughness in unidirectional 0° fibre reinforced composites. Acoustic detection of beginning fibre breakage correlates with a significant increase of loss work per cycle.     

Some aspects of the fatigue behaviour of fibre reinforced thermosetting resins

This paper reports the continuing work to establish the fatigue response of commercial fibre reinforced plastics now being specified by designers, particularly those materials using thermosetting resins as the matrix.

Results are reported of the flexural fatigue of balanced needled fabrics and of the tensile fatigue of filament wound E-glass and R-glass rings. Only a small dependence on the ratio of the minimum to the maximum stress (R-ratio) has been observed for flexural fatigue between R = 0·05 and R = 0·6.

It is shown that the fatigue behaviour of a wide variety of glass fibre reinforced composites (unidirectional/bidirectional, long and short fibres, epoxy and polyester resins), subject to either tensile or flexural loading, can be rationalised, like others have done, by normalising the maximum fatigue stress with respect to the corresponding ultimate tensile or flexural strength, obtained at an equivalent strain rate.     

Mode I interfacial toughening through discontinuous interleaves for damage suppression and control

An investigation is described concerning the interaction of propagating interlaminar cracks with embedded strips of interleaved materials in E-glass fibre reinforced epoxy composites. The approach deploys interlayer strips of a thermoplastic film, thermoplastic particles, chopped fibres, glass/epoxy prepreg, thermoset adhesive film and thermoset adhesive particles ahead of the crack path on mid-plane of Double Cantilever Beam (DCB) specimens. During these mode I tests, the interlayers were observed to confer an apparent increase in the toughness of the host material. The crack arrest performance of individual inclusion types are discussed and the underlying mechanisms for energy absorption and the behaviour of the crack at the interaction point of the interleave edge were analysed using scanning electron microscopy.     

Fracture behaviour of fly-ash filled FRP composites

The major objective of this study was to determine the fracture toughness and fracture surface energy of epoxy, epoxy/fly-ash, epoxy/carbon fibre, epoxy/carbon fibre/fly-ash, epoxy/glass fibre and epoxy/glass fibre/fly-ash composites. The quality of composite specimens was evaluated by the ultrasonic method. The results show that a fly-ash particle can arrest the crack path and thus improve the fracture properties of fibre reinforced plastic (FRP) composites. The results of this study have further significance in view of the fact that fly-ash powder is far cheaper than carbon fibre, glass fibre and epoxy resin.     

Investigation on effect of fibre hybridization and orientation on mechanical behaviour of natural fibre epoxy composite

Nowadays bio fibre composites play a vital role by replacing conventional materials used in automotive and aerospace industries owing to their high strength to weight ratio, biodegradability and ease of production. This paper aims to find the effect of fibre hybridization and orientation on mechanical behaviour of composite fabricated with neem, abaca fibres and epoxy resin. Here, three varieties of composites are fabricated namely, composite 1 which consists of abaca fibre and glass fibre, composite 2, which consists of neem fibre and glass fibre, whereas composite 3 consists of abaca, neem fibres and glass fibres. In all the above three varieties, fibres are arranged in three types of orientations namely, horizontal (type I), vertical (type II) and 45\(^{\circ }\) inclination (type III). The result shows that composites made up of abaca and neem fibres with inclined orientation (45\(^{\circ }\)) have better mechanical properties when compared with other types of composites. In addition, morphological analysis is carried out using scanning electron microscope to know the fibre distribution, fibre pull out, fibre breakage and crack propagation on tested composites.     

The environmental stress corrosion cracking of glass fibre-reinforced laminates and single E-glass filaments

The environmental stress corrosion cracking of epoxy/glass fibre crossply, unidirectional coupons and single E-glass filaments have been compared. At initial applied strains > 0.15% the resin does not protect the fibres as shown by their equivalent failure times. The failure occurs in the environment and planar fractures occur because of the localized stress in the load bearing plies adjacent to a transverse crack in the 90° ply of the 0°/ 90°/0° coupons. These transverse cracks result from stress corrosion of the glass/resin interface, which leads to a reduction of the transverse cracking strain. At applied strains < 0.15% fracture occurs within the unexposed half of the coupons and is thought to be caused by rapid transport of glass corrosion products where they crystallize within the coupon. This phenomenon is also responsible for the progressive transverse cracking that occurs in both the 0° and 90° plies of the unimmersed half of the crossply coupon under zero load.     

Fractographic study of transverse cracks in a fibre composite

Transverse fracture of unidirectional fibre composites was studied in a model glass/epoxy composite in which 1 mm-diameter rods had been used in place of fibres. The fracture surface resulting from transverse cracking in this model system was studied by scanning electron microscopy (SEM). The interaction of the crack with the epoxy matrix resin and the glass rods was the following: Cracks in the resin appeared to have effected a debonding at the glassmatrix interface before reaching the glass. The debonding then propagated along the interface and induced secondary cracks ahead of the primary debonding crack. The confluence of the secondary and primary cracks resulted in sharp ridges being formed on the matrix resin surface, produced by plastic deformation of the rigid epoxy resin. These appeared as a field of parabolic marks. Considering the brittleness of the resin, the amount of plastic deformation indicated by the ridges was astonishing. As the debonding continued around the glass rod, a transverse corrugated texture developed on the resin surface, again produced by plastic deformation. Finally, the cracks reentered the matrix from small patches of polymer adhering especially strongly to the glass surface. The overall fracture energy of transverse cracking of unidirectional fibre composites is suggested to consist, therefore, of the following elements in addition to crack propagation in the matrix resin: (a) the glass-resin debonding before the incoming cracks reach the glass, (b) the initiation of secondary cracks or debonds at the interface, (c) the plastic deformation in generating the ridges on the rigid resin surface, appearing both as the paraboloids and the transverse corrugation, and (d) cracking of the matrix reinitiated at the opposite side of the glass. The use of an enlarged glass reinforcement in this study provided a more direct observation of the properties of transverse crack propagation in composite materials than would have been possible with the small, roughly 10m fibres.     

Influence of matrix material on flexural fatigue performance of unidirectional composites

A deflection-controlled flexural fatigue study of unidirectional glass fiber reinforced epoxy and vinyl ester composites was undertaken. Damage initiation and growth for various deflection levels were evaluated. Also, quantitative assessment of damage was made by monitoring stiffness loss in the composites as a function of fatigue cycles. Results show that the glass/epoxy composite has better performance compared with the glass/vinyl ester composite, especially at low deflection amplitudes. Fatigue behavior of the composites at low deflection amplitudes is found to be primarily influenced by matric and fiber-matrix interfacial damage in the form of longitudinal splitting.     

TWINTEX纤维增强塑料的应力腐蚀开裂

研究了TWINTEX纤维增强塑料在酸中的应力腐蚀,并与玻璃纤维增强塑料的应力腐蚀进行了比较.结果表明,TWINTEX纤维增强塑料具有比玻璃纤维增强塑料好得多的耐应力腐蚀性能,而且这种好的耐应力腐蚀性主要表现在长的裂纹孕育期.聚丙烯纤维良好的耐蚀性是造成长的裂纹孕育期的主要原因.外加载荷的变化对TWINTEX纤维增强塑料的应力腐蚀裂纹扩展速率的影响不如其对玻璃纤维增强塑料的应力腐蚀裂纹扩展速率的影响显著.     

Effects of fibre length on tensile strength of carbon/glass fibre hybrid composites

The tensile strength of epoxy resin reinforced with random-planar orientation of short carbon and glass fibres increased as the length of the reinforcing fibres increased, and the increase in tensile strength remained almost unchanged after the fibre length reached a certain level. The tensile strength of composites at any fibre length could be estimated by taking the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the mean critical fibre length into consideration. The tensile strength of the hybrid composite could be estimated by the additive rule of hybrid mixtures, using the tensile strength of both composites.     

Propagation of stress corrosion cracks in aligned glass fibre composite materials

A modified fracture toughness test has been used to measure the growth of cracks in unidirectional glass fibre/polyester resin composite materials in the presence of a dilute hydrochloric acid. Crack growth rates perpendicular to the fibre axis have been measured over a range of stress intensities. Scanning electron microscope studies of the fracture surfaces have shown that the micromechanisms of nucleation and propagation are dependent on stress intensity. The use of crack growth data to predict component lifetimes and the existence of inherent flaws in the material are discussed.