F.J. Guild

The effect of delamination geometry on the compressive failure of composite laminates

One of the causes of a reduction in the compressive strength of a composite material containing delaminations is the buckling of the delaminated plies. To understand the effect of delamination geometry on the compressive behaviour of laminated composite materials, compression tests were carried out on glass-fibre-reinforced plastic (GRP) test specimens containing artificial delaminations of various geometry, created by inserting PTFE film into the laminate during lay-up. Finite-element modelling was also carried out to gain further understanding of the mechanisms of compressive failure. Good agreement between finite-element predictions and experimental measurements were found for the whole range of delamination geometries that were tested. Finite-element and simple closed-form models were also developed for delaminated panels with isotropic properties. This enabled a study of the effect of delamination geometry on compressive failure without the complicating effects of orthotropic material properties. The results of this study can be used to derive a graph of non-dimensional failure load versus non-dimensional failure stress, where the results for any one delamination geometry superimpose on those for all others.

Understanding and control of adhesive crack propagation in bonded joints between carbon fibre composite adherends I. Experimental

Carbon fibre composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. However, such bonded joints must be shown to behave in a predictable and reliable way. A major aspect of this is to demonstrate that the progress of cracks through the bonds is well understood. The work presented here illustrates one possibility for establishing a measure of control over that crack propagation.

An investigation into the effects of fabric architecture on the processing and properties of fibre reinforced composites produced by resin transfer moulding

Abstract The use of resin transfer moulding (RTM) as an economic and efficient means of producing high performance fibre-reinforced composites is critically limited by the permeability of the fabrics employed. Commercial fabrics are available where the architecture of their reinforcement is designed to cluster the fibres, giving higher permeabilities than conventional fabrics. This has been shown to improve processing times, but there is evidence that such clustering is detrimental to the mechanical performance of the resulting composite materials. The objective of this work was to relate variations in permeability and mechanical performance to differences in composite microstructure. This was achieved by producing carbon/epoxy plates of different weave styles by RTM in a transparent mould. The progress of the resin was recorded by a video camera during injection, and the images were processed by a frame-grabbing computer, permitting the permeabilities of the fabrics to be calculated. Further plates were manufactured using the same fabrics, and sectioned for microstructural image analysis and interlaminar shear strength (ILSS) testing to CRAG standards. Relationships were sought between measured permeabilities and finished microstructures using a Quantimet 570 automatic image analyser. It has been shown that variations in permeabilities and mechanical properties can be related to observed differences in the microstructure.

Understanding and control of adhesive crack propagation in bonded joints between carbon fibre composite adherends II. Finite element analysis

Carbon fibre composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. However, such bonded joints must be shown to behave in a predictable and reliable way. A major aspect of this is to demonstrate that the progress of cracks through the bonds is well understood. The simulation work presented here complements the experimental work presented in Part I. The observed failure processes and their sequence are successfully described and modelled.

Delaminations in flat and curved composite laminates subjected to compressive load

Abstract Delaminations reduce the compressive strength of composite laminates because they allow out of plane displacement of plies to occur more easily. This work is a study of the effect of delaminations in laminates with initial curvature. Initial curvature typically results in an increased buckling load and the hypothesis was that the reduction in compressive strength due to delamination would be less in a curved specimen than a flat one. Glass fibre reinforced plastic test laminates were manufactured containing artificial delaminations of different sizes and through thickness positions. The laminates were either flat or curved, where the plane of curvature was normal to the loading direction. Compressive load was applied to the laminates and measurements of out of plane displacements and failure loads were made. Contrary to expectations, the curved laminates gave lower failure loads than flat laminates, for the same delamination size and through thickness position. Finite element simulation was carried out to predict out of plane displacement and failure load, with failure being assessed using a maximum stress criterion. Good agreement between finite element predictions and experimental measurements of load versus deflection behaviour was observed for both flat and curved laminates. Ultimate failure loads were also well predicted for the flat laminates but relatively poorly for the curved laminates. The finite element predictions depend on modelling accurately the out of plane displacement behaviour, which was found to be very sensitive to the boundary conditions for the case of curved laminates.

Improving the resin transfer moulding process for fabric-reinforced composites by modification of the fabric architecture

Abstract The use of resin transfer moulding (RTM) as an economic and efficient means of producing high-performance fibre-reinforced composites is critically limited by the permeability of the fabrics employed. Commercial fabrics are available where the architecture of the reinforcement is designed to cluster the fibres giving higher permeabilities than conventional fabrics. This has been shown to improve processing times, but there is evidence that such clustering is detrimental to the mechanical performance of the resulting composite material. The objective of this work was to relate variations in permeability, and in the laminate mechanical properties, to differences in microstructure. A series of experimental carbon fibre fabrics woven to incorporate a novel flow enhancement concept (use of 3K tows in a 6K fabric) were used to manufacture plates by RTM in a transparent mould. The progress of the resin front was recorded to computer disc during injection, thus allowing the permeabilities of the fabrics to be calculated. The manufactured plates were subsequently sectioned for mechanical testing (moduli and strengths in tension and compression) and automated image analysis. Relationships were sought between measured permeabilities, mechanical properties and microstructures using a Quantimet 570 automatic image analyser to determine fractal dimensions from polished sections. It has been shown that variations in the microstructures can be related to the permeability and mechanical property values obtained. Further the deterioration of mechanical properties for the novel fabrics with reduced fibre volume fractions is less than has been reported for fabrics with clustered flow-enhancing tows at constant fibre volume fraction.

The effect of reinforcement architecture on the long-range flow in fibrous reinforcements

Abstract The resin transfer moulding process involves the long-range flow of resin into a closed mould which is filled with dry fibre reinforcement. The rate of resin flow can be calculated using the Darcy and Kozeny-Carman equations. The flow rate is thus a function of the pressure drop across the fibre bed, the resin viscosity and the permeability of the fibre bed. The permeability constant is dependent on the fibre radius and the porosity of the bed. A number of reinforcement fabrics are now available commercially which promote faster resin flow than that in equivalent fabrics of the same areal weight at the same fibre volume fraction. The KozenyCarman equation includes a parameter known as the mean hydraulic radius. If this parameter is varied by calculating specific hydraulic radii, then the flow enhancement may be modelled. Calculations for model materials have been published and demonstrate that this approach predicts that significant changes in flow rate are possible. The commercial fabrics do not have model structures, but feature variations in the mesoscale architecture of the reinforcement: fibres clustered into tows and uneven distribution of pore space. The paper will report on the correlation of quantitative image analysis of optical micrographs with the flow rates in a range of reinforcement fabrics.

A study of the effects of convergent flow fronts on the properties of fibre reinforced composites produced by RTM

The processing speed of resin transfer moulding (RTM) can be improved by using multiple injection ports. Out of necessity this results in the convergence of resin flow fronts. Such convergence can result in the entrapment of voids within the composite leading to a degradation of mechanical properties. A series of carbon/epoxy plates of differing weave styles were manufactured by RTM in a transparent mould with porting arrangements designed to cause resin flows to converge. The plates were analysed to determine the effect of injection strategy, injection temperature and differences in weave style. Analysis was performed by qualitative ultrasound scanning, quantitative image analysis and interlaminar shear strength testing. It has been shown that there is a marked increase in voidage in the areas where flows meet and this is correlated to a deterioration of mechanical properties.

Use of the thick adherend shear test for shear stress-strain measurements of stiff and flexible adhesives

Five commercial structural adhesives were tested using the thick adherend shear test (TAST). These adhesives have mechanical properties ranging from those of high-strength, heat-cured epoxies to ductile, acrylic-based materials. Consideration was given to the adherend selection and dimensions to approach a uniform shear stress-strain in the bonded area, so that the test could be used with both stiff and flexible adhesives. Comparison of the TAST results was also made with those obtained using the butt torsion test. The TAST extensometry has been shown to be suitable for measuring the shear strain properties of the adhesives tested without modification. From the shear behavior of the five adhesives measured using the TAST method, and from the results presented in this paper, it can be seen that the TAST method is repeatable and reproducible for a wide range of adhesive types and adhesive properties. From these results, it is possible to generate comprehensive adhesive shear data. Also, the curves from the butt torsion test and the TAST were found to be consistent and give the same behavior of the adhesives tested.

Voronoi cells, fractal dimensions and fibre composites

The use of fibre‐reinforced polymer matrix composite materials is growing at a faster rate than the gross domestic product (GDP) in many countries. An improved understanding of their processing and mechanical behaviour would extend the potential applications of these materials. For unidirectional composites, it is predicted that localized absence of fibres is related to longitudinal compression failure. The use of woven reinforcements permits more effective manufacture than for unidirectional fibres. It has been demonstrated experimentally that compression strengths of woven composites are reduced when fibres are clustered. Summerscales predicted that clustering of fibres would increase the permeability of the reinforcement and hence expedite the processing of these materials. Commercial fabrics are available which employ this concept using flow‐enhancing bound tows. The net effect of clustering fibres is to enhance processability whilst reducing the mechanical properties.