Stochastic analysis of fibre volume fraction and permeability in fibre bundles with random filament arrangement

For fibre bundles with non-uniformly distributed filaments, probabilities for the occurrence of defined local filament arrangements, determining geometrical constants for permeability calculation, were described as functions of the local fibre volume fraction. Because of changing inter-filament gap geometries, the local transverse permeability decreases discontinuously with increasing local fibre volume fraction, while the local axial permeability decreases continuously. Axial bundle permeabilities, derived from log-normally distributed local permeabilities, tend to be higher than predictions for uniform filament arrangements, while transverse bundle permeabilities tend to be lower. This implies that assuming uniformity of filament arrangements is inappropriate for realistic modelling of bundle properties. For fibre bundles with rectangular cross-section, the transverse permeability decreases in width-direction and increases in thickness-direction with increasing aspect ratio of the bundle cross-section, while the axial permeability is not affected. This is of high practical relevance, in particular for spread-tow reinforcements with high aspect ratios.     

Transverse permeability of chopped fibre bundle beds

The out-of-plane permeability of 25 mm-long chopped glass fibre bundle beds was measured. These bundles, used in sheet moulding compound (SMC) composites, have an elliptical cross-section with the major axis perpendicular to the flow direction. An adapted experimental set-up was developed, which prevented flow along the sides of the packed bundle bed. The set-up was tested with needled glass fibre mats, and satisfactory agreement was found with previous results. The transverse permeability of the loose SMC bundle beds was then measured and linked to the bundle sizing type, geometry and volume fraction. Finally, the results could be well described by models taking into account the elliptical nature of the bundles.     

Hyperelastic modelling for mesoscopic analyses of composite reinforcements

A hyperelastic constitutive law is proposed to describe the mechanical behaviour of fibre bundles of woven composite reinforcements. The objective of this model is to compute the 3D geometry of the deformed woven unit cell. This geometry is important for permeability calculations and for the mechanical behaviour of the composite into service. The finite element models of a woven unit cell can also be used as virtual mechanical tests. The highlight of four deformation modes of the fibre bundle leads to definition of a strain energy potential from four specific invariants. The parameters of the hyperelastic constitutive law are identified in the case of a glass plain weave reinforcement thanks to uniaxial and equibiaxial tensile tests on the fibre bundle and on the whole reinforcement. This constitutive law is then validated in comparison to biaxial tension and in-plane shear tests.     

Continuous monitoring of three-dimensional resin flow through a fibre preform

A basic requirement for an accurate numerical simulation of the resin transfer moulding process is a set of exact permeability coefficients of the applied textile reinforcement. The permeabilities are usually obtained from the measurement of flow front propagation through a stack of planar fibre preforms. While measuring the in-plane flow is considered as rather simple, the detection of a flow front that moves perpendicular to the laminate plane emerges to be difficult. Therefore, the knowledge of the transverse impregnation behaviour is still sparse and transverse permeability values were determined for a very few fabrics only. The knowledge of an exact transverse permeability is important for the three-dimensional simulation of flow through thick sectioned parts and for specific RTM-related processes like resin film infusion or SCRIMP^®. This paper addresses the monitoring of flow front propagation utilizing ultrasound transmission. A testing rig was developed for monitoring three-dimensional ellipsoidal impregnation, which is induced by point injection. Two multidirectional non-crimped fabrics were characterised by the three-dimensional measurement. For the determination of the transverse permeability the three-dimensional filling was emulated by a numerical flow simulation software.     

Strength variability and size effect of Nicalon fibre bundles

Statistical strength and size effect of Nicalon fibre bundles are studied. The Weibull type of statistical theory underlying predictions of bounding Nicalon fibre bundle strength is presented and discussed. The relationship of bundle strength to single Nicalon filament strength and a model explaining the correlation are also discussed. The predicted values for Nicalon fibre bundles were in close agreement with the experimental data. Characterization of Nicalon fibres or bundles provides an insight into the ultimate mechanical performance of ceramic-matrix composites.     

Process monitoring of partially reinforced metal-matrix composite components by acoustic emission

Partial reinforcement using a fibre bundle embedded in a part of a component has been investigated in the development of a composite piston for use in an internal combustion engine. A trial piston was fabricated by a casting operation in which molten aluminium was poured into a die containing an annular continuous fibre bundle. The most probable defects introduced during the manufacturing operation are (i) microcracks generated in the fibre bundle due to residual thermal stresses and (ii) imperfect impregnation of the molten aluminium into the fibre bundle. Acoustic emission measurements have been used as a technique to detect the presence of defects in the trial pistons. The acoustic emission was measured during cooling of the trial piston after casting. Microcracking in the fibre bundle during cooling could be detected. Imperfect impregnation of the aluminium into the fibre bundle could also be detected. The acoustic emission due to microcracking was found to be strongly dependent on the mechanical properties of the fibres, while the acoustic emission from incomplete impregnation was found to depend on process conditions. It is believed that acoustic emission measurements can not only be used for the detection of microcracks but can also be of value in the selection of fibre materials and in the adjustment of the process conditions.     

Thermoplastic filament winding with online-impregnation. Part B. Experimental study of processing parameters

In part A of this paper, a novel process technology was presented which allows to combine thermoplastic filament winding with online melt impregnation of fibre bundles. In this part, a comprehensive study of process parameters was conducted. Circular glass fibre (GF) reinforced polypropylene (PP) or polyamide 12 (PA12) tubes were produced as sample component. Processing speeds of up to 15 m/min could be achieved in case of GF/PP before a drop in quality of the parts had to be accepted. The present limitation in winding speed is not attributed to impregnation problems but to an excessive rise in force required to pull the fibre tow off the impregnation device. Measures for process improvement and increase in productivity were proposed.     

Particle deposition mechanisms during processing of advanced composite materials

Liquid composite moulding of advanced composite materials often comprises infiltration of a particle-filled resin into a multi-scale porous fabric. These injections/infusions are subject to severe particle depositions inside the reinforcement, leading to undesired inhomogeneous mechanical and functional properties. Hence, the mechanisms for particle depositions are investigated by detailed meso-scale experiments, analysed by microscopic imaging and micro-particle image velocimetry, and macroscopic infusions of a biaxial non-crimp fabric. It is shown that two main particle deposition mechanisms are filtration during fibre bundle impregnation and filtration induced by stationary flow through fibre bundles. It is also clarified where in the reinforcement the particles will deposit. Finally, a number of suggestions on how to process advanced composite materials with a more homogeneous particle distribution are launched.     

Bubble formation and motion in non-crimp fabrics with perturbed bundle geometry

The motion of the liquid front during impregnation of non-crimp fabrics has been considered by using Sethian’s level set method. Particular attention is put on the creation of bubbles at the liquid front and a virtual 3D model mimicking biaxial fabrics has been built for this purpose. The saturated fluid flow is governed by the Navier–Stokes Equations and Darcy law, while capillary pressure has been accounted for at the liquid flow front and continuity maintained. The influence of perturbation in the bundle geometry has been investigated. Local correlations of the dimensions of neighbouring gaps formed between the bundles are of paramount importance. Focus is on inter-bundle bubbles. An existing model for bubble dynamics is used based on a probabilistic approach for bubbles moving, splitting, merging, and dissolving. The same approach was used for intra-bundle bubbles, the difference being that their motion appears to be much slower. The obtained void fractions of inter-bundle bubbles at different vacuum levels applied at the liquid flow front are compared to those from real mouldings with a high degree of conformity.     

Statistical fracture of E-glass fibres using a bundle tensile test and acoustic emission monitoring

Mechanical strength studies have been carried out on fibre bundles used in composite manufacturing. The variability in mechanical properties of glass fibres has been studied using bundles of about 2000 filaments. The fibre strength distributions were analysed using the survival probability-applied strain (S–ε) curve, in relation with various experimental conditions. We also examine the effect of lubricant’s viscosity on the fracture behaviour of E-glass fibre bundles. Acoustic emission (AE) was monitored during the bundle tensile tests in order to verify that individual filament failures are statistically independent. On tensile tests with lubricated bundles of E-glass fibres, it is shown that each individual fibre break can be detected using AE. Hence, AE monitoring of a lubricated bundle of E-glass fibres provides a convenient and relatively quick method to obtain the Weibull parameters of strength distribution.     

Preparation of C-fibre borosilicate glass composites: Influence of the fibre distribution on mechanical properties

Optimum conditions with respect to preparation-determined fibre distribution in borosilicate glass composites were investigated. Continuous C-fibre bundles were impregnated with glass powder in silicon alkoxide solution and wound in parallel to prepregs which were hot pressed into unidirectional composites. The influence of the glass particle size during the impregnation of fibre bundles and during hot pressing on the homogeneity of the fibre distribution was of special interest, as well as the influence of pressure and temperature on the densification of the composites. Optimum conditions were related to optimum values in the bending strength of the resulting composites. Under the optimum hot-pressing conditions the fibre volume content was varied. It has been shown that the fibre distribution was much more homogeneous when fine-grained glass powder was used for impregnation. At high fibre volume concentrations of the composites the distribution was better than at low concentrations. High fibre concentrations were connected with fracture toughness of the sample, as shown by the three-point bending experiments, whereas samples with low C-fibre concentration showed brittle behaviour.     

Quantification of micro-scale variability in fibre bundles

Local variations in the random filament arrangement in carbon fibre bundles were determined by optical microscopy and automated image analysis. Successive steps of abrading, polishing and acquiring micrographs of the sample surface made it feasible to analyse the micro-structure over a series of cross-sections along the fibre bundle path. Random and systematic changes in local filament arrangements were determined. Systematic changes were related to the interaction of a fibre bundle with an intersecting binder thread leading to a local increase of the fibre volume fraction at the interface. Random clustering of filaments in areas of high or low fibre volume fractions within the fibre bundles were found to be unaffected by the relative position of the bundle.     

Counting carbon fibres by electrical resistance measurement

Electrical Impedance Measurement has been used to measure the diameter of single carbon fibres to within 3% of the actual value measured by Scanning Electron Microscopy (SEM). The precision of the technique developed also allows for the accurate determination of the number of fibres present in a carbon fibre bundle, such data are important for the calculation of fibre tensile strength from the tensile force applied to carbon fibre bundles. The impedance of a single carbon fibre and carbon fibre bundles of up to 20 fibres have been measured, with results showing good agreement with theoretical values. The impedance of multiple lengths of carbon fibres ranging from 80 to 300 mm has also been studied, with the impedance being directly proportional to the fibre length, as per electrical theory. This technique will be suitable for determining the number of fibres in a virgin or recycled carbon fibre bundle.     

Failure of hierarchical distributions of fibre bundles. I

We consider by computational and analytic means the failure properties of hierarchically organized bundles of fibres with equal load sharing, a problem that may be treated exactly by renormalization methods. We show, independent of the specific failure properties of an individual fibre, that the threshold for failure of a fibre bundle obeys a universal scaling law with respect to the size of the bundle. Moreover, this scaling law is preserved for any hierarchical organization of fibre bundles.     

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

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.     

Testing E-glass fibre bundles using acoustic emission

In tensile tests on lubricated bundles of a few hundred parallel E-glass fibres it is shown that individual fibre breaks, to the last fibre in the bundle, can be detected using acoustic emission (AE). By this means the single-fibre strength distribution is deduced. Relationships are obtained between some AE signal parameters and the fibre fracture stress which are consistent with theoretical expectations. Studies are made of the distribution of fibre break locations, the occurrences of multiple (stimulated) fibre breaks and the attenuation of the AE signals.     

Influence of Tow Architecture on Compaction and Nesting in Textile Preforms

Transverse compression response of tows during processes such as vacuum infusion or autoclave curing has significant influence on resin permeability in fabrics as well as the laminate thickness, fibre volume fraction and tow orientations in the finished composite. This paper reports macro –scale deformations in dry fibre assemblies due to transverse compaction. In this study, influence of weave geometry and the presence of interlacements or stitches on the ply-level compaction as well as nesting have been investigated. 2D woven fabrics with a variety of interlacement patterns - plain, twill and sateen- as well as stitched Non-crimp (NCF) fabrics have been investigated for macro-level deformations. Compression response of single layer and multilayer stacks has been studied as a function of external pressure in order to establish nesting behaviour. It appears that the degree of individual ply compaction and degree of nesting between the plies are influenced by tow architectures. Inter-tow spacing and stitching thread thickness appears to influence the degree of nesting in non-crimp fabrics.     

Mechanical properties of flax fibre/polypropylene composites. Influence of fibre/matrix modification and glass fibre hybridization

The effect of fibre treatments and matrix modification on mechanical properties of flax fibre bundle/polypropylene composites was investigated. Treatments using chemicals such as maleic anhydride, vinyltrimethoxy silane, maleic anhydride-polypropylene copolymer and also fibre alkalization were carried out in order to modify the interfacial bonding between fibre bundles and polymeric matrix. Composites were produced by employing two compounding ways: internal mixing and extrusion. Mechanical behaviour of both flax fibre bundle and hybrid glass/flax fibre bundle composites was studied. Fracture surfaces were investigated by scanning electron microscopy. Results suggest that matrix modification led to better mechanical performance than fibre surface modification. A relevant fact is that silanes or MA grafted onto PP matrix lead to mechanical properties of composites even better than those for MAPP modification, and close to those for glass fibre/PP.     

Finite Element Assisted Modelling of the Microscopic Impregnation Process in Thermoplastic Preforms

Fibre reinforced composite materials incorporating thermoplastic matrices are gaining increasing popularity in many industrial applications. One of the potential preforms for the manufacture of technical components is commingled yarn composed of reinforcement and matrix in fibre form. These are often employed in the pultrusion process. Another innovative preform consists of polymer powder preimpregnated sheath surrounding fibre bundles. To achieve adequate mechanical properties of the final product it is essential, when producing laminates by a process such as pultrusion with both types of preform, that sufficient matrix impregnation is achieved. The prevention of voids and dry-spots in the laminate requires a theoretical understanding of the mechanisms involved. On a microscopic scale, several finite element (FE) models can be used to simulate the progress of the matrix flow into the interstitial spaces between the single reinforcement fibres. In the present simulations, a hexagonal and a square arrangement account for two of the various fibre packings occurring in a laminate. It permits an estimation of the impregnation performance of commingled and powder impregnated yarns. For each preform the shear rate, to which the polymer matrix is subjected during the impregnation and consolidation process, can be predicted.