Influence of adsorption behaviour of a silane coupling agent on interlaminar fracture in glass fibre fabric-reinforced unsaturated polyester laminates

The relationship between the interphase consisting of physisorbed and chemisorbed silane on glass fibres and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate, was studied. The Mode I interlaminar fracture toughness of the laminate specimen was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of the laminate specimen finished with two silane concentrations and washed in methanol solvent, is discussed on the basis of the interlaminar fracture toughness. In order to determine the amount of physisorbed and chemisorbed silane on the glass fibre, the amount of total carbon was determined using an analysis instrument. The physisorbed silane migrated into the resin matrix and influenced the mechanical properties and interlaminar fracture of the laminate specimen. The amount of unsaturated polyester resin blended with a silane coupling agent was measured using dynamic mechanical spectroscopy, and a DCB specimen for mechanical properties and fracture toughness.     

Impact response of thick glass fibre reinforced polyester laminates

Thick glass/polyester woven roving laminated plates subject to low-velocity impact have been investigated using a guided drop-weight test rig in ascending energy order up to 1500 J. The impact response and energy-absorbing characteristics have been determined by impact-forces and absorbed-energy histories, and by force-displacement relationships. Impact damage is examined by visual inspection, ultrasonic C-scan and an optical microscope so that a three-staged sequential damage model is proposed to characterize damage growth. Static tests are also conducted to examine strain rate effect. It is demonstrated that the maximum impact forces are increased by 36% for the thin plates and by 22% for the thick plates, although the initial threshold forces are less strain-rate sensitive. Two thicknesses of laminated plates are used to study the thickness effect, and the scaling rules are developed for the delaminated plates. It is shown that both the maximum static and impact forces, and the incident kinetic energy can be scaled by the thickness ratio if these laminates have the same diameter and their behaviour is dominated by shear. The finite element modelling is carried out for relatively low energy cases, the impact structural response is well captured by linear elastic solution.     

Influence of in-plane fibre orientation on mode I interlaminar fracture toughness of stitched glass/polyester composites

The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, G_Ic of unstitched and stitched glass/polyester composites is investigated in this paper. The G_Ic of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/−30, 45/−45, 60/−60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/−30, 45/−45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch². The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on G_Ic is studied. Double cantilever beam (DCB) tests are carried out and the G_Ic is determined using modified beam theory. The G_Ic of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The G_Ic values of unstitched 45/−45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on G_Ic is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the G_Ic and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle.     

Influence of weathering on unreinforced and short glass fibre reinforced thermoplastic polyester

The influence of weathering on fracture toughness, Jc, yield strength, y, local ultrasonic velocity, VR, and microhardness, Hu, in unreinforced and a short glass-fibre-reinforced polyester-based thermoplastic Xenoy has been investigated. Unreinforced material weathered for 11 months outdoors in Perth, West Australia, exhibited a significant decrease in VR and Jc, whereas little change was shown in y, Hu and the fracture surface morphology. Irradiation for 1000 h by artificial ultraviolet rays upon the unreinforced material caused a considerable increase in Hu and only a slight deterioration in VR of the surface layer. Filling with the short fibres induced an improvement in y and a large reduction in Jc for the unreinforced material. The natural weathering of the reinforced material caused a small reduction in Jc but a large degradation in the slope of the R-curve. It was concluded that the measurement of Jc and the slope of R-curve in combination with VR and Hu was an effective way to study the effects of weathering on engineering plastics. © 1998 Chapman & Hall     

Effect of silane coupling agent on mechanical performance of glass fibre

Mechanical performance of commercially manufactured unsized and γ-aminopropylsilane sized boron-free E-glass fibres has been characterised using single-fibre tensile test. Both apparent fibre modulus and fibre strength were found to strongly depend on fibre gauge length. The average strength of sized fibres was found 40–80 % higher than unsized fibres at different gauge lengths. Weibull analysis suggested that the failure mode of unsized fibres could be described by unimodal Weibull distribution, whereas the strength distribution of sized fibres appeared to be controlled by two exclusive types of flaw population, types A and B. Comparison of the Weibull plots between unsized and sized fibres revealed that the strength of unsized fibres was likely to be dominated by type A flaws existing on the bare glass surface and type B flaws may be related to the defects on the glass surface coated with silane. This was partially supported by the observation of fractured cross-sectional area using SEM. It was, therefore, proposed that the strength difference between unsized and sized glass fibres may be more reasonably interpreted from the surface protection standpoint as opposed to the flaw healing effect. The results obtained from this study showed that silane coupling agent plays a critical role in the strength retention of commercially manufactured E-glass fibres and the silane effect on the fibre strength is also affected by the change in gauge length of the sample.     

Water absorption study on pultruded jute fibre reinforced unsaturated polyester composites

Water absorption of natural fibre plastic composites is a serious concern especially for their potential outdoor applications. In this research, jute fibre reinforced with unsaturated polyester composites are subjected to water immersion tests in order to study the effects of water absorption in its mechanical properties. Water absorption tests were conducted by immersing composite specimens into three different environmental conditions included distilled water, sea water and acidic solutions at room temperature for a period up to 3 weeks. Water absorption curves obtained and characteristic parameter D (diffusion coefficient) and M_m (maximum moisture content) were determined. The water absorption of jute fibre reinforced unsaturated polyester composites were found to follow a so-called pseudo-Fickian behaviour. The effects of the immersion treatment on the flexural and compression characteristics were investigated. The flexural and compression properties were found to decrease with the increase in percentage water uptake. These flexural and compression behaviours were explained by the plasticization of the matrix–fibre interface and swelling of the jute fibres.     

Influence of coupling agents on material behaviour of glass flake reinforced polypropylene

This paper presents a study evaluating the effects of aminosilane and titanate coupling agents on the material properties of glass flake reinforced polypropylene and compares the results with pure polypropylene and untreated glass flake reinforced polypropylene. Tensile and flexural properties, impact resistance and residual stresses are strongly dependent on the type and level of coupling agent employed, whereas changes in melt-crystallisation and crystalline content of the polypropylene in the composite materials was minimal. Glass flakes modified with aminosilane gave higher stiffness and strength than the titanate treatment. Thermal expansion hysteresis provided an indication of the relative levels of residual stresses in the polymeric materials, which was verified using the layer removal method for determining residual stress distributions. Ultrasonic velocity measurements conducted on the composite materials before and after exposure to sub-zero temperatures proved useful in assessing the effectiveness of the various surface treatments in promoting interfacial adhesion.     

Characterization of interlaminar fracture behaviour of woven fabric reinforced polymeric composites

An experimental study was conducted to characterize the interlaminar fracture behaviour of 2-D woven fabric reinforced epoxy composites under mode I loading using double cantilever beam tests. A large displacement, small strain non-linear beam model was used to calculate the interlaminar fracture toughness. The fabrics used included fibreglass and Kevlar woven structures with different weave patterns. An attempt was made to enhance the composite interlaminar toughness by adding different types of microfibres into the matrix. Toughening mechanisms of the composites were analysed using scanning electron microscopy. It was found that the weave patterns of fabrics exhibited a strong influence on the interlaminar fracture behaviour, and that the addition of the microfibres to the epoxy matrix could improve the interlaminar fracture toughness significantly.     

Flexural properties of hemp fibre reinforced polylactide and unsaturated polyester composites

In this work, flexural strength and flexural modulus of chemically treated random short and aligned long hemp fibre reinforced polylactide and unsaturated polyester composites were investigated over a range of fibre content (0-50 wt%). Flexural strength of the composites was found to decrease with increased fibre content; however, flexural modulus increased with increased fibre content. The reason for this decrease in flexural strength was found to be due to fibre defects (i.e. kinks) which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fibre treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fibre/matrix adhesion.     

Composites of glass/modified jute fabric and unsaturated polyester resin

Hybrid composites of glass and jute fabric modified by treatment with γ-aminopropyl trimethoxy silane (silane), isopropyl triisostearoyl titanate (titanate) and tolylene diisocyanate (tdi) were fabricated using unsaturated polyester resin (usp). An improvement in mechanical properties of laminates was observed when jute fabric was modified by titanate treatment. Better retention of mechanical properties in humid environments was observed in these laminates.     

Water absorption characteristics of aluminised E-glass fibre reinforced unsaturated polyester composites

Novel aluminised E-glass fibre reinforced unsaturated polyester composites, originally formulated for enhanced thermal and electrical shielding properties were evaluated in terms of their water absorption. One of the major obstacles delaying the acceptance of novel composites in engineering applications is the degradation of the polymer matrix material by moisture, which effects the physical and mechanical performance over time. The objective of this study was to characterise and quantify the degree of water absorption of novel aluminised E-glass reinforced unsaturated polyester composites. Aluminised E-glass composites were compared alongside their unmetallised E-glass counterparts. Two sets of temperature were used for this study. Results show that aluminised E-glass significantly reduces the saturation point compared to unmetallised E-glass. The differences between aluminised and unmetallised are correlated to fibre coatings. At elevated temperatures the aluminised E-glass sample is unstable and exhibits significantly higher water absorption indicating that a new failure mechanism is occurring.     

Nonlinear response in glass fibre non-crimp fabric reinforced vinylester composites

This paper addresses the nonlinear stress-strain response in glass fibre non-crimp fabric reinforced vinylester composite laminates subjected to in-plane tensile loading. The nonlinearity is shown to be a combination of brittle and plastic failure. It is argued that the shift from plastic to brittle behaviour in the vinylester is caused by the state of stress triaxiality caused by the interaction between fibre and vinylester. A model combining damage and plasticity is calibrated and evaluated using data from extensive experimental testing. The onset of damage is predicted using the Puck failure criterion, and the evolution of damage is calibrated from the observed softening in plies loaded in transverse tension. Shear loading beyond linear elastic response is observed to result in irreversible strains. A yield criterion is implemented for shear deformation. A strain hardening law is fitted to the stress-strain response observed in shear loaded plies. Experimental results from a selection of laminates with different layups are used to verify the numerical models. A complete set of model parameters for predicting elastic behaviour, strength and post failure softening is presented for glass fibre non-crimped fabric reinforced vinylester. The predicted behaviour from using these model parameters are shown to be in good agreement with experimental results.     

Impact damage and residual strengths of woven fabric glass/polyester laminates

Thick glass/polyester laminates of four different dimensions subject to low-velocity impact have been investigated using a guided drop-weight test rig with a flat-ended impactor in ascending energy order up to 3100 J. The characteristics of impact response and energy absorption have been determined by impact force and absorbed energy histories, and impact damage incurred was examined by cross-sectioning and ultrasonic C-scanning. Residual compressive strengths were measured, and the damage tolerance of the laminates was assessed by the retaining ability of these strengths. It is found that the salient features in force-time history curves can be related to fracture processes occurring in the laminates, and that the established relationships between impact force and incident kinetic energy (IKE) can be used to identify damage initiation without examining impacted specimens, which is later confirmed by the damage force maps. The constructed damage force and energy maps have shown not only damage initiation in an unstable fashion but also increase of damage size with IKE and force until reaching their load-bearing capabilities. Residual compressive strengths are reduced very rapidly with the increase of impact damage due to extensive delamination.     

Improving interlaminar fracture toughness of carbon fibre/epoxy laminates by incorporation of nano-particles

Double-cantilever-beam tests were applied to investigate the mode I interlaminar fracture toughness of carbon fibre/epoxy laminates, in which the epoxy matrices were incorporated with rubber and silica nano-particles, either singly or jointly. It is shown that the toughness is improved owing to the presence of these nano-particles although nano-rubber is more effective than nano-silica. Further, by keeping the total particle weight percentage constant in epoxies (e.g., at 8 and 12 wt.%) filled with equal amount of nano-silica and nano-rubber, the interlaminar toughness values of the hybrid laminates are always higher than those with nano-silica filled epoxies but lower than those with nano-rubber filled matrices. Scanning electron microscopy examination of the delaminated surfaces of composite laminates filled with nano-particles revealed that cavitation of nano-rubber particles/void growth and debonding of nano-silica from epoxy matrix are responsible for the improved interlaminar toughness observed. It is also shown that the bulk toughness of nano-particle filled epoxies cannot be fully transferred to the interlaminar toughness of composite laminates, being limited by the constraint effect imposed by the carbon fibres. Finally, the role of fibre-bridging on the delaminated crack and hence delamination toughness is discussed.     

Interlaminar shear fracture of chopped strand mat glass fibre-reinforced polyester laminates

The interlaminar shear fracture of chopped strand mat glass fibre-reinforced polyester laminates has been studied both experimentally and analytically. Lap shear (double-grooved) specimens were used to measure the interlaminar shear strength and the cracking mechanism was studied using photomicrography. The finite element method was used to calculate the stress distribution along the shear surface and the mixed-mode stress intensity factors K_I and K_II. The length of the shear surface was found to have a significant effect on the results. Based on the experimental and analytical results, the validity of the British Standard for GRP pressure vessels (BS4994, 1973) was evaluated and the critical stress intensity factors K_Ic and K_IIc for this material were estimated.     

Influence of fibre treatment and glass fibre hybridisation on thermal degradation and surface energy characteristics of hemp/unsaturated polyester composites

The effect of fibre treatment on the thermal degradation and surface energy characteristics of hemp fibre reinforced unsaturated polyester (HFRUP) composites was investigated by means of a thermogravimetric analyser (TGA) in a nitrogen atmosphere and contact angle measurement. In order to modify the fibre/matrix interface, NaOH treatment and glass fibre hybridisation were employed. HFRUP composites were compared to the unreinforced UP, NaOH treated hemp and glass fibre hybridised hemp/UP composites. TGA test results show that the weight loss for all samples occurred between 200 and 415 °C. The unreinforced UP had a maximum weight loss of 1.011%/°C. For the HFRUP composites, the maximum rate of weight loss was 0.81%/°C. For the NaOH treated and glass fibre hybridised hemp/UP composites, the maximum rate of weight loss was 0.78%/°C and 0.79%/°C, respectively. The effect of fibre treatment on the surface energy of studied samples and their dispersive and polar components were also investigated. Surface energy characteristics obtained from contact angle measurement revealed that for unreinforced UP, the contact angle measured with glycerol is 49.37°. For hemp/UP composites, the contact angle is 76.05°. For NaOH treated hemp/UP composites sample, the contact angle was recorded 78.89°, higher than untreated one. For hemp/CSM/UP specimen, the contact angle was recorded 69.80°. Both TGA and contact angle results indicated that surface treatment and glass fibre hybridisation led to better thermal stability and the wetting behaviour of hemp/UP composites.