PIERRE OUAGNE and DAMIEN SOULAT

ABSTRACT

The possibility of manufacturing complex shape composite parts with a good pro­duction rate is crucial for the automotive industry. The sheet forming of woven reinforcements is particularly interesting as complex shapes with double or triple curvatures with low curvature radiuses can be obtained. To limit the impact of the part on the environment, the use of flax fiber based reinforcements may be consid­ered for structural or semistructural parts. This study examines the possibility to develop composite parts with complex geometries such as a tetrahedron without defect by using flax based fabrics. An experimental approach is used to identify and quantify the defects that may take place during the sheet forming process of woven natural fiber reinforcements. Wrinkling, tow sliding, tow homogeneity defects and tow buckling are discussed. The origins of the defects are discussed, and solutions to prevent their appearance are proposed. Particularly, solutions to avoid tow buckling caused by the bending of tows during forming are developed. Specially designed flax based reinforcement architecture has been developed. However, if this fabric design has been successful for the tetrahedron shape, it may not be sufficient for other types of shapes and that is why the optimization of the process parameters to prevent occurrence of buckles from a wide range of commercial fabrics was also investigated with success.

7.1 INTRODUCTION

With the view to answer the weight reduction question, the replacement of metallic materials by composite materials exhibiting lower densities and higher stiffness, has been a great success in the aeronautic industry. Composite materials are an assem­bly of reinforcement materials, which confer the stiffness and a binder, (generally a polymeric resin) for the cohesion of the composite. For aeronautical parts, carbon

or glass reinforcements are generally used. These carbon and glass fiber composites cannot be easily recycled even though processes such as mechanical grinding, py­rolysis, fluidized bed or solvolysis are studied at the laboratory scale 4 In order to reduce the environmental impact of the composite part on the environment, the idea to replace the synthetic carbon and glass fibers by natural fibers such as flax or hemp has motivated numerous studies 37. However, very few studies deal with the scale of the reinforcement and particularly of structural or semistructural reinforcements constituted of aligned tows woven for example according to a specifying fabric style. Few publications can be used to constitute a database reporting the mechani­cal properties of the natural fiber based reinforcements. At this scale, the choice of reinforcement structure (size of the tows, weaving style, etc.) is essential as it in­fluences its mechanical characteristics89. Indeed, to manufacture high performance composite parts, it is necessary to organize and to align the fibers. As a consequence, aligned fibers architectures such as unidirectional sheets, noncrimped fabrics and woven fabrics (bidirectional) are usually used as reinforcement.

When dealing with weight reduction, it appears that the best gain can be ob­tained on complex shape parts. However, the possibility to realize these shapes in composite materials is still a problem to be solved. For example, only 25% of the Airbus A380 is constituted of composite materials. Several low scale manual manufacturing processes exists to realize these complex shape composite parts par­ticularly for the military or the luxury car industries. The sheet forming of dry or comingled (reinforcement and matrix fibers mixed in a same tow) can be considered as a solution to manufacture at the industrial scale complex shape composite parts as this process shows a good production rate/cost ratio. Numerical approaches have been used to determine the process parameters to be used 1012. However, few of these studies dealt with complex shape parts for which specific defects such as tow buck­les may appear 13. The appearance of such defects may prevent the qualification of the part and indicate the limit of the reinforcement material behavior under a single or a combination of deformation modes. It is therefore important to quantify and un­derstand the mechanisms controlling the appearance of defects so that the numerical tools developed in the literature for complex shape forming 14 can simulate them.

Natural fibers and particularly plant fibers have been explored as an alternative to synthetic fiber reinforcement for composite as they are characterized by lower density than glass fibers (1.5 for flax fibers; 2.6 for glass fibers), and because they potentially can be recycled or even degraded at the end of the composite life14. As these fibers are extracted from vegetal resources, a lot of studies deal with the prop­erties of the natural fibers and particularly with their variability according to the place they are extracted in the plant, the climatic conditions during the growth of the plant, the treatments used to extract the fibers from the plant (retting, combing, etc.)15-20

If natural fibers show a lot of advantages such as biodegradability, nontoxicity, good insulation properties, low machine wear, etc.2122, the level of production of these fibers needs to be considered in such a way that food production is not af­fected. This also needs to be placed in parallel to socioecological impacts that may be encountered around the sand mining necessary for the production of glass fibers 2325. As a consequence, a large amount of studies has been devoted to investigate the behavior of individual fibers or group of few fibers of different types 2629. The studies globally showed that the tensile properties of the natural fibers can advanta­geously be compared to the ones of the glass fibers especially if one considers the specific tensile modulus and strength of flax fibers. As a consequence, the automo­tive industry is a candidate for the use of such fibers as this could lead for the same part performance to weight reduction of the composite 30.

All these studies, at the fiber scale, are justified by the fact that the natural fi­bers may show important variability in their mechanical properties and particularly when tensile strength and modulus are considered, because an apparent diameter is generally considered instead of a true cross-sectional area in the calculation of mechanical properties.31 Review articles synthesize and compare at the fiber scale the performances of the natural fibers considered for technical application such as flax, hemp, jute, sisal, kenaf, etc.3234 The property variability is also discussed in these reviews as well as the disadvantages that may appear when considering the use of natural fiber in composites for large-scale production (the variability of the mechanical properties, the compatibility between matrix and natural fiber and the moisture absorption).

In order to avoid long considerations about the variability of the fiber properties, it may be interesting to consider for some manufacturing processes such as filament winding35 or pultrusion36 the scale of the tow or the scale of the yarn. The scale of the composite (natural fiber reinforcement combined to a polymeric resin) is also in­teresting if one wants to avoid considering the variability of the fiber properties3741. The homogenized behavior at the composite scale depends on the reinforcement type (mat, woven fabric, noncrimped fabric), the resin used and the process chosen to manufacture the composite. The study of composite samples is also used to ana­lyze the impact of the composite part all along its life cycle4243. The energetic record to produce flax fibers for composite materials has been analyzed by Dissanayake et al.4445 They showed, in the case of traditional production of flax mats, with the use of synthetic fertilizers and pesticides associated to traditional fiber extraction such as dew retting and hackling, that the energy consumption linked to the production of a flax mat is comparable to the energy consumed during the production of a glass mat. They also showed that the spinning to produce yarns is an energy intensive operation and in that case, the glass woven fabric may show lower impact on the environment than an equivalent flax woven fabric if one considers an environmental energy viewpoint. As a consequence, it is recommended to use aligned fibers instead of spun yarns tows to produce natural fiber based woven fabrics.

Between the fiber and the composite scales, it may be interesting to study the behavior of natural fibers assemblies such as strands, tows, fabric with the view to optimize the composite manufacturing processes using these entry materials. As an example, studies for glass and carbon reinforcements showed that during the sheet forming process of dry (first step of the Resin Transfer Moulding (RTM)46) that the fabrics may be the subject to tension, shear and bending loads. Some of them may even be combined4750. These loads may induce specific deformation states at the origin of forming defects. These defects may impact the quality of the com­posite part5153 and also modify the reinforcement permeability5458 that is a crucial parameter to control the impregnation of the porous reinforcements by liquid resin if Liquid Composite Molding (LCM) processes and particularly the RTM process are considered. The presence of defects also indicates the behavior limits of the fabric in a specific deformation mode. As an example the presence of wrinkles is generally associated to a limit in-plane shear behavior. These limits may be tested for each fabric on the different modes of deformation independently of the form­ing process47’52’5962. For shear and tension, the strains taking place during the sheet forming process can be evaluated in-situ 62,63. This has been performed for carbon and glass reinforcements, but which will be presented in this Chapter proposes to analyze the forming potentialities of a flax based fabric. After presenting the sheet­forming device used to shape the flax reinforcements, the work will introduce the different types of defects that may be encountered during sheet forming of textile fabrics, before concentrating on the feasibility of forming complex shapes without defects. Particularly, the solutions developed to reach this goal will be presented and discussed.