Unidirectional compression of fibre reinforcements. Part 2: A continuous permeability tensor measurement

Modelling LCM processes, where resin flow and compression of the fibre reinforcements are involved, requires a proper modelling of the fibre reinforcements behaviour in compression and to define how the permeability tensor evolves. The permeability measurement techniques based on fluid injection lead to data scattering. Using a material testing machine, unidirectional compression tests are performed on impregnated fibre reinforcements to induce in-plane and through-thickness fluid flows. With proper viscosities and compression speeds, the expelled-fluid pressure becomes sufficiently high to be measured. Then, using that fluid pressure and following a methodology based on numerical optimizations, the in-plane equivalent permeability and through-thickness permeabilities can be extracted. An additional measurement is needed to know the anisotropy ratio and calculate the two in-plane principal permeabilities. Due to the nature of the compression test, a major advantage of the present method is that it is continuous with respect to fibre volume fraction and that it limits errors involved with injection techniques. Samples can previously be sheared before compression and permeability measurements.     

Hydro-mechanical loading and compressibility of fibrous media for resin infusion processes

The modelling of composite manufacturing processes where hydro-mechanical coupling takes place depends on the validity of compressibility and permeability models. In this work, the computer code initially used to simulate the effect of coupled hydro-mechanical load on composite preform (Ouahbi et al. Composites Part A, 38:1646–1654, 2007) is integrated into an inverse method to predict the compaction behaviour of the reinforcements. An experimental device developed at Le Havre is used to apply hydro-mechanical loads to the preforms. Two ramps of stress are imposed to the preform and the thickness evolution is measured as a function of time. The speed of thickness reduction is not constant and varies in the range of 0.1 to 12 mm/min. The effect of compression speed upon the saturated fabrics is investigated. For a fixed fibre volume fraction, an increase in stress is observed in increasing compression speed. The experimental results are compared to the compressibility curves determined by an inverse method. The calculated curves correspond to the compressibility curves experimentally obtained with low compression speed (～0.25 mm/min). As a consequence, this suggests that a low compression speed should be applied when investigating the compressibility behaviour of composite preform with a view of modelling resin infusion processes.     

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.     

Rate constitutive equations for computational analyses of textile composite reinforcement mechanical behaviour during forming

Textile composite reinforcements are made up of fibres. Consequently, their mechanical behaviour is a result of the possible sliding and the interactions between the fibres. When they are formed on double curved shapes, these fabrics are submitted to large strains, in particular large in-plane shear. Among the mechanical behaviour models for these textile reinforcements, continuous models are most commonly used for forming simulations because they can be used with standard finite elements. The objective of the present paper is to propose a continuous approach for textile reinforcement deformation analysis based on a rate constitutive equation specific to materials made of fibres. The objective derivative of this constitutive model is defined by the fibre rotation. This constitutive model is implemented in ABAQUS and can be used in most commercial F.E. software. The approach is extended to materials with two-fibre directions in order to perform simulations of woven fabric forming processes. A set of simulations of large deformations of textile composite reinforcements at the mesoscopic scale (deformation of a woven unit cell) and at the macroscopic scale (deep drawing) is presented to show the efficiency of the proposed approach.     

Modelling of composite sheet forming: a review

The use of composite materials in sheet forming applications is gaining popularity with the rise of consumer demands and specific mechanical properties. In addition to unidirectional (UD) fibres, the use of textile reinforcements such as woven fabric and knitted fabric has been shown to be feasible in recent years. This paper gives a survey on the modelling of composite sheet forming for both UD fibre and textile composites. Two broad approaches are reviewed here—the mapping approach and the mechanics approach. Mapping approaches for UD fibre composites, woven fabric composites and knitted fabric composites are elucidated on the basis of their fibre geometry. For the mechanics approach both the viscous fluid models and elastic solid models, as a means of describing the constitutive properties, are reviewed. Various updating methods for modelling large deformation found in sheet forming are then described. Finally, a guideline for the choice of modelling techniques for various types of fibre/fabric reinforcements and suggestions for future work are given.     

Continuous transverse permeability of fibrous media

The compaction of composite preforms and the flow of resin through the fibrous network take place simultaneously during the Resin Film Infusion process. Therefore there is a coupled loading of the porous reinforcements. A new experimental device to impose combinations of hydraulic and mechanical loadings (Hydro-Mechanical loadings) to fibrous preforms is used to evaluate the transverse permeability in a continuous manner (under Hydro-Mechanical conditions) for a flax mat, a flax non-crimped fabric, a carbon plain weave and a glass satin weave. For a 0.5 mm/min compression speed the continuous technique gives values similar to the “classical” technique for the four composite reinforcements of very different nature. This suggests that it is possible to use the continuous technique to evaluate the transverse permeability behaviour of fibrous reinforcements (permeability vs. fibre volume) with a time reduction of about 8–10. Increasing the compression speed gives slowly decreasing continuous transverse permeability values.     

Characterising wood fibre mats as reinforcements for liquid composite moulding processes

Liquid composite moulding (LCM) processes are commonly used techniques for the manufacture of advanced composite structures. This study explores the potential of wood fibres as reinforcement for LCM preforms, considering discontinuous fibre mats produced using four different methods. Modified paper manufacturing techniques were employed to produce two types of wet formed mats, the other two being manufactured using dry methods. The dry compaction response of these mats has been investigated, required compression loads being measured up to a fibre volume fraction of 0.4. A complex non-elastic compression response was observed which has significant influence on forces generated within moulds. Saturated compaction tests were also carried out, the samples infiltrated with two different test fluids. A significant reduction in compaction load was observed due to wood softening when using a water based fluid. On the other hand, a non-water-based solution had little less influence on the compaction of the wood fibre mats. In addition, permeability of all four types of mats was measured as a function of fibre volume fraction. Reinforcement permeability and compaction response data are required to simulate LCM processes.     

The role of reinforcement architecture on impact damage mechanisms and post-impact compression behaviour

This review considers the link between the damage tolerance of composite laminates and the nature and organization of the fibre reinforcement. This embraces composites made from unidirectional prepregs through composites based on a variety of textile forms such as woven fabrics, multiaxial fabrics, braids and knits. The objective has been firstly to detail how the differing varieties of composite exhibit different properties under impact conditions and under subsequent loading after impact. This includes both fracture mechanisms and data such as energy absorption, and peak failure loads. The second objective is to describe the links that have been found between these properties and the specific fibre architectures and damage development processes in the various composite forms. The post impact compression properties are highlighted as this is the area of greatest interest by end-users. The review describes the different forms of textiles that are used for composite reinforcement, considers different impact conditions (e.g. low velocity and ballistics), general materials variables such as fibre and resin type, and ultimately looks at specific textile systems. Some consideration is also given to the value and role of numerical modelling in the field of damage formation and damage tolerance. Clear differences have been found in the literature between composites based on different textile forms in terms of damage states after impact and the consequences of this damage on subsequent properties. While the literature is clearly incomplete at this time there is sufficient information available to indicate that control of fibre organization by the use of textiles may be an effective method of optimizing composite properties for specific end use properties.     

Aspects of the stitch formation process on the quality of sewn multi-textile-preforms

Stitching technologies are considered to be one of the key technologies for automated manufacturing of complex textile preforms which are used for liquid composite moulding of fibre reinforced plastic parts [Proceedings of Sixth International Conference on Automated Composites (1999)]. The sewing or stitching process is applied for different purposes during the production of dry fibrous reinforcements as well as for structural aims in the composite component (through-the-thickness reinforcement), thus, the requirements on the stitch itself are wide spread [Complex Multi-textile Preforms – The Potential of Sewing. 90(4) (2000) 43; Stress Conc Compos Sci Technol, 59 (1999) 2125; Tech Textiles 43 (2000) 120; Compos Part A 31 (2000) 571; D 82 Dissertation RWTH Aaachen (1999)]. A detailed prediction of properties of the stitched reinforcements requires an understanding of the stitch formation process and the interaction between textile reinforcements, the stacking sequence thereof, the stitching process parameters and sewing thread properties.     

Mesoscopic scale analyses of textile composite reinforcement compaction

Transverse compaction of textile composite reinforcements is an important deformation mode arising during composite forming and manufacture. The mesoscopic simulations of the transverse compression of textile preforms presented in this paper are based on 3D FE models of each yarn in contact with friction with its neighbours. A hypoelastic model based on the fibre rotation depicts the mechanical behaviour of the yarn. The compression responses of several layer stacks with parallel or different orientations are computed. The numerical simulations show good agreement when compared to compaction experiments. The mesoscopic simulations can be used as virtual compression tests. In addition they determine the internal geometry of the reinforcement after compaction. The internal geometry can be used to compute the permeability of the deformed reinforcement and to calculate the homogenised mechanical properties of the final composite part.     

Permeability measurement of textile reinforcements with several test fluids

In liquid composite moulding (LCM) techniques, the liquid resin has to flow a long distance to impregnate the dry fibres. The measure for the resistance of the fibre preform to the resin flow is the permeability of the fibre preform. Because of the dual-scale porous structure of the textile preforms, test fluid can influence the unsaturated permeability values through the interaction of the fluid and fibres. In this study, the influence of test fluid on the permeability measurement of several types of textile reinforcements is investigated. First the contact angle of various fluids and fibres was measured. Then the permeability measurement of the textile reinforcements was carried out. The results showed that the influence of test fluid is small under the test conditions.     

Modelling fabric reinforced composites under impact loads

The paper describes recent progress on the materials modelling and numerical simulation of the in-plane response of fibre reinforced composite structures. A continuum damage mechanics model for fabric reinforced composites under in-plane loads is presented. It is based on methods developed for UD ply materials (Compos. Sci. Technol., 43 (1992) 257), which are generalised here to fabric reinforcements. The model contains elastic damage in the fibre directions, with an elastic–plastic model for inelastic shear effects. Test data on a glass fabric/epoxy laminate show the importance of inelastic effects in shear. A strategy is described for determining model parameters from the test data. The fabric model is being implemented in an explicit FE code for use in crash and impact studies and preliminary results are presented on a plate impact simulation.     

Development of Flax Fibre based Textile Reinforcements for Composite Applications

Most developments in the area of natural fibre reinforced composites have focused on random discontinuous fibre composite systems. The development of continuous fibre reinforced composites is, however, essential for manufacturing materials, which can be used in load-bearing/structural applications. The current work aims to develop high-performance natural fibre composite systems for structural applications using continuous textile reinforcements like UD-tapes or woven fabrics. One of the main problems in this case is the optimisation of the yarn to be used to manufacture the textile reinforcement. Low twisted yarns display a very low strength when tested dry in air and therefore they cannot be used in processes such as pultrusion or textile manufacturing routes. On the other hand, by increasing the level of twist, a degradation of the mechanical properties is observed in impregnated yarns (e.g., unidirectional composites) similar to off-axis composites. Therefore, an optimum twist should be used to balance processability and mechanical properties. Subsequently, different types of fabrics (i.e., biaxial plain weaves, unidirectional fabrics and non-crimp fabrics) were produced and evaluated as reinforcement in composites manufactured by well established manufacturing techniques such as hand lay-up, vacuum infusion, pultrusion and resin transfer moulding (RTM). Clearly, as expected, the developed materials cannot directly compete in terms of strength with glass fibre composites. However, they are clearly able to compete with these materials in terms of stiffness, especially if the low density of flax is taken into account. Their properties are however very favourable when compared with non-woven glass composites.     

Mechanical repair of timber beams fractured in flexure using bonded-in reinforcements

The flexural properties of strength class C16 spruce beams have been compared to the flexural properties of the same beams repaired with bonded-in reinforcements in the form of steel or composite pultruded rods. Reinforcing materials included rectangular sections of mild steel, pultruded carbon fibre reinforced plastic (CFRP), glass fibre reinforced plastic (GFRP) and a thermoplastic matrix glass fibre reinforced polyurethane (FULCRUM). Grooves were routed into the faces of the fractured beams following straightening and the reinforcements adhesively bonded into the top, bottom or both faces of the beams. The steel and CFRP reinforcements are most effective in restoring the flexural strength which often exceeds its original value. These reinforcements are also effective in enhancing flexural strength but the CFRP reinforcement endows the greatest transformed flexural strength. The fracture mechanisms in the repaired beams depend on the placement of reinforcement and the quality of the adhesive to reinforcement bond. All properties are optimised by bonding reinforcement into both faces of the fractured beams.     

Postbuckling of functionally graded fiber reinforced composite laminated cylindrical shells, Part I: Theory and solutions

Buckling and postbuckling behavior are presented for fiber reinforced composite (FRC) laminated cylindrical shells subjected to axial compression or a uniform external pressure in thermal environments. Two kinds of fiber reinforced composite laminated shells, namely, uniformly distributed (UD) and functionally graded (FG) reinforcements, are considered. The governing equations are based on a higher order shear deformation shell theory with von Kármán-type of kinematic non-linearity and including the extension-twist, extension-flexural and flexural-twist couplings. The thermal effects are also included, and the material properties of FRC laminated cylindrical shells are estimated through a micromechanical model and are assumed to be temperature dependent. The non-linear prebuckling deformations and the initial geometric imperfections of the shell are both taken into account. A singular perturbation technique is employed to determine the buckling loads and postbuckling equilibrium paths of FRC laminated cylindrical shells.     

Shear mechanism modelling of heavy tow braided composites using a meso-mechanical damage model

Heavy tow braid reinforced composites are a potential substitute for metals in automotive and other transport applications. These composites, if properly designed, can provide lightweight efficient load bearing structural members that can also absorb high specific energy under impact and crash loading. Many of these components are ‘beam like’ members that must resist large transverse deformations at high force levels, thereby absorbing high levels of energy. This class of composite component is particularly considered in this paper.An effective means to achieve high energy absorption is careful design of the fabric architecture so that shearing mechanisms of the fibre/matrix interface, without premature fibre failure, are possible. Characterisation and modelling of progressive shear damage and failure occurring in biaxial carbon and glass braided composites are investigated. Fibre re-orientation and fibre/matrix interface damage is measured using an optical strain measuring method based on digital image correlation (DIC). This is then used to provide input to a meso-mechanical damage model in an explicit finite element code. A modelling approach using coupled layers of equivalent unidirectional plies is used to represent the biaxial braid composite and validation of the approach has been performed against test coupons and beam structures loaded transversally to failure.     

New thermoplastic composite preforms based on split-film warp-knitting

A newly developed type of dry thermoplastic textile preform incorporating non-crimp glass fibre reinforcements and matrix material in the form of split-film is presented. Weft-inserted warp knitting has been chosen as a textile production technique for its low cost. A specialized glass fibre/polypropylene matrix system has been proven to perform favourably in melt impregnation and to provide good composite properties. Some of the processing techniques to be applied to the new textile preform are presented, one of which is the QUIKTEMP concept for fast heating and cooling of tools for thermoplastic moulding. Composite plates produced from preliminary split-warpknit structures reveal a good potential for cost-saving while reasonable mechanical properties can be maintained.     

Simulation of the reinforcement compaction and resin flow during the complete resin infusion process

Resin infusion (a.k.a. VARTM) is one of the LCM processes, for which liquid resin is drawn into dry reinforcements. Significant cavity thickness changes occur during processing, due to the flexibility of the vacuum bag used as one side of the tool, and the complex stress balance within the laminate. While the magnitude of thickness change is often small, the influence is significant on reinforcement properties. Changes in permeability during filling and post-filling have the potential to significantly affect the process. To simulate this behaviour, it is important to accurately model compaction and unloading of reinforcement in dry and wet states. A series of tests were completed to determine compaction behaviour of an isotropic glass fibre mat. From these tests several non-linear elastic compaction models have been determined, and applied within a resin infusion simulation which addresses pre-filling, filling and post-filling. This simulation was then used to assess different post-filling strategies.     

Infiltration processing of fibre reinforced composites: governing phenomena

All three classes of fibre reinforced composite materials (polymer, metal and ceramic matrix) may be produced by flow of liquid matrix into the open spaces left within pores of a fibre preform. Even though several specific issues arise from the nature of each composite matrix class, governing phenomena apply to all infiltration processes, and include in particular: (i) capillary phenomena, (ii) transport phenomena, and (iii) the mechanics of potential fibre preform deformation. These phenomena and their governing laws are reviewed for the case of isothermal infiltration with no phase transformations. Four basic functional quantities, which need to be known to model the processes, are identified, and addressed in turn. The paper concludes with some examples of modelling methodologies and comparison with experimental data.     

Postbuckling of functionally graded fiber reinforced composite laminated cylindrical shells, Part II: Numerical results

In this Part, the extensive parametric studies performed are reported and numerical results are presented for the buckling and postbuckling of fiber reinforced polymer matrix and metal matrix composite laminated shells subjected to axial compression or external pressure under different sets of environmental conditions. Two kinds of fiber reinforced composite laminated shells, namely, uniformly distributed (UD) and functionally graded (FG) reinforcements, are considered. The numerical results show that the buckling loads as well as postbuckling strength of the shell can be increased as a result of functionally graded fiber reinforcements. The results reveal that the effect of functionally graded fiber reinforcements on the buckling loads and postbuckling strength of shell with polymer matrix is more pronounced compared to the shell with metal matrix in the case of axial compression. In contrast, in the case of external pressure, the functionally graded fiber reinforcements may have a significant effect on the buckling pressure and postbuckling strength of the shell with metal matrix.