M.G. Bader

On the re-inforcement of thermoplastics by imperfectly aligned discontinuous fibres

Studies of deformation behaviour of short fibre reinforced thermoplastics are complicated by the facts that usually a wide range of fibre lengths are present in moulded test pieces and that the fibres are not systematically oriented with respect to any test direction. An equation has been derived for the stress/strain curve of such a material. This has been used to determine fibre/matrix bond strengths in two glass/nylon 6.6 and two glass/polypropylene composites from measured stress/strain curves and fibre length distributions.It is concluded that major improvements in the properties of these materials will only be achieved by modifying processing to retain longer fibres.

The stiffness and strength of a polyamide thermoplastic reinforced with glass and carbon fibres

It has been established that the optimum degree of mechanical property enhancement by fibre reinforcement of a typical thermoplastic material (polyamide 6.6) is achieved if comparatively long fibres are used, the fibre length required being determined by the properties of the interface between the fibre and the thermoplastic matrix. The extent of stiffness improvement at low strains is described by simple modifications to the law of mixtures to allow for fibre orientation and length. The strength enhancement is limited by an embrittlement effect which reduces the strain to fracture as the stiffness of the composite is improved. The cause of this effect has been identified as matrix crack formation at the ends of the reinforcing fibres. At strains of between 0.5% and 1.0%, according to fibre type, length andVf, cracks form at the tips of the longest fibres aligned in the straining direction. Subsequently as the strain is increased more cracks form progressively at the ends of shorter and/or more misaligned fibres. It has been shown that initially this cracking can be accommodated by load transfer to adjacent fibres which “bridge” the cracked region. Final failure occurs when the extent of cracking across the weakest section reaches a critical level when the surrounding fibres and matrix can no longer support the increasing load.

The strength of hybrid glass/carbon fibre composites

The tensile mechanical properties of hybrid composites fabricated from glass and carbon fibres in an epoxy matrix have been evaluated over a range of glass: carbon ratios and states of dispersion of the two phases. The failure strain of the carbon phase increased as the relative proportion of carbon fibre was decreased, and as the carbon fibre was more finely dispersed. This behaviour is commonly termed the “hybrid effect”, and failure strain enhancement of up to 50% has been measured. Only part of the effect may be attributed to internal compressive strains induced in the carbon phase by differential thermal contraction as the composite is cooled from its cure temperature. The laminae or ligaments of carbon fibre dispersed in the glass fibre phase show a multiple failure mode, and when the constitution is favourable catastrophic failure does not occur until a considerable number of ligament fractures have accumulated. Failure is thus progressive, and the material is effectively “tougher” than equivalent all-carbon fibre composites.

The use of a δ-alumina fibre for metal-matrix composites

Composites formed by infiltration of an array of fine alumina fibres with aluminium alloy melts have been investigated in terms of fabrication characteristics, microstructural features and mechanical properties. A production method has been developed in which the application of pressure ensures very low porosity levels and strong fibre-matrix bonding. Details of the transport phenomena occurring during fabrication have been explored with a view to optimizing selection of applied pressure, thermal fields, alloy composition and the structure of the fibrous preform. Microstructural examinations revealed an intimate fibre-matrix bond, but the virtual absence of any chemical reaction at the interface. Comparison of property measurements with data from unreinforced alloys revealed increased elastic moduli and marked improvement in tensile strength at elevated temperature, accompanied by reductions in elongation.

Selection of composite materials and manufacturing routes for cost-effective performance

Abstract A study has been conducted to estimate the costs of manufacture of a simple component in a number of different composite materials and by different manufacturing routes. The materials and routes selected span the range of composites from those appropriate for general engineering applications to aerospace. A simple methodology is introduced for a comparison on the basis of cost-performance efficiency. It is demonstrated that more economic solutions may often be realised by choice of ‘expensive’ carbon rather than ‘cheaper’ E-glass as the reinforcing fibre.

Shear deformation and micromechanics of woven fabrics

Abstract This paper includes an experimental study and a mathematical analysis of the shear deformation of woven fabrics by using picture-frame type shear testing. Four types of weaves were tested and compared: a loose plain weave, a tight plain weave, a twill and a satin weave. The locking shear angle was determined both in picture-frame tests and manual shear tests. The experimental data presented for each fabric include curves of shear load–shear stress as a function of either the shear angle or the shear rate, and measured locking shear angles. The shear deformation data were analysed by following elasticity principles and taking into account the effects of fibre inextensibility. A microstructural analysis was carried out in all four fabrics to investigate the shear locking on the basis of a geometrical approach and the maximum packing fibre fraction.

Compression in the processing of polymer composites 1. A mechanical and microstructural study for different glass fabrics and resins

This paper focuses on the compression of fibre reinforcements during the processing of polymer composites, covering a range of fabrics, namely a plain weave, a twill, a satin and a non-crimped, stitch-bonded fabric. The compression of assemblies of fabrics has been studied in both dry and wet states where, in the latter case, the fabrics were impregnated with three alternative resins: a non-Newtonian polyester of relatively high viscosity, a non-Newtonian polyester of relatively low viscosity, and a Newtonian epoxy of equally low viscosity. Investigations of mechanical behaviour included the influence of compression speed, type of fabric and viscosity and type of resin. Microstructural studies of laminates produced under different degrees of maximum compression elucidated further the results of mechanical compression and provided data of average area porosity (resin-rich areas), area pore structure, average area voidage and voids for the different types of fabrics.

Measurement techniques and effects on in-plane permeability of woven cloths in resin transfer moulding

The resin transfer moulding process studied in this paper involves the injection of a liquid polymer resin into a mould containing a pre-placed assembly of woven cloths. Two techniques of measuring the in-plane permeability of the reinforcement are considered. The first technique involves rectilinear flow of a model liquid, whereas the second involves radial flow of the liquid injected from a central gate. A comparison of the two techniques is given, including problems and differences in the obtained permeability values. The effects of a number of parameters are studied in each case, including the flow rate, porosity, pre-wetting the cloths and the number of layers of cloth.

Transverse Ply Cracking in Cross-Ply CFRP Laminates—Initiation or Propagation Controlled?

The initiation and propagation stages of transverse ply cracking in crossply CFRP laminates under quasi-static loading have been studied using laminates with a range of transverse ply thicknesses. Specimens of two types have been tested-coupons with polished edges and coupons with defects (notches) introduced into the transverse ply prior to testing. Detailed observations of crack propagation across the transverse ply thickness and width have been made, along with measurements of crack density and residual properties. In laminates with thin transverse plies (less than 0.25 mm for the materials tested here), fully formed transverse cracks are observed at about the same strain in undamaged and notched laminates. In laminates with thicker transverse plies (greater than 0.25 mm), the notched laminates show fully formed transverse cracks at lower strains than the undamaged laminates. The experimental results have illustrated the situations (laminate properties, presence of flaws) in which fracture mechanics models for transverse cracking available in the literature can be applied. In particular, it is shown that the fracture mechanics models which assume implicitly the presence of initial defects spanning the thickness of the ply may underestimate the failure strain of laminates with thick transverse plies. The effect of composite intralaminar toughness is also discussed.

Mathematical modelling of macro- and micro-infiltration in resin transfer moulding (RTM)

Abstract A mathematical model is proposed to describe the macro- and micro-infiltration through reinforcements of bimodal porosity distribution in resin transfer moulding. The model is based on Dareys law incorporating mechanical, capillary and vacuum pressures and covers three modes of infiltration. Permeabilities and capillary pressures are calculated at macro- and micro-levels. A numerical analysis is presented where at each numerical location the flowrate is split between three possible types of flow on the basis of mass balance and a combination of permeability magnitude and local flow potential. Parametric computational studies are carried out to study the flow of a model Newtonian fluid through woven cloths where the following parameters are varied: fibre volume fraction, fibre tow diameter and injection pressure. Predicted variables include micro- and macro-infiltration times and apparent global permeability. The latter was found to depend on the injection pressure, in the regime of relatively low injection pressures, where the variations also depended on the fabric architecture and fibre volume fraction.