In order for a material to absorb sound, it must be porous, so that the sound waves can move through the material.17 Porosity is the ratio of open space volume to the total volume of the porous material. Only connected pores, which are accessible to air flow, should be included in porosity calculation.14

The gravimetric measurement of the porosity requires the knowledge of the vol­ume of the porous material and the density of the fibers. The porous material is weighed and fabric density is calculated. The porosity is calculated by using the following equation:

where h is porosity, rw is density of fibrous material, rf is the density of the fiber. Reciprocal of porosity, massivity, ¥, is the ratio of fiber volume to the total volume of the porous material:18
(27)

Another method of measuring porosity involves saturating the porous materials with a kind of fluid such as water or mercury and determining the porosity from the relative weights of saturated and unsaturated samples. The disadvantage with this method is the deformation of the pore structure due to the introduction of the fluid.17

A dry method of porosity determination is based on the measurement of the change in pressure within a container, which contains the porous material, with ad­dition of a small volume of air. It is advantageous as it only measures the connected pores, through which sound propagates.17

The range of porosity is higher than 0.95 for mineral and glass wools and porous plastic foams. The porosity values of fibrous mats have been reported to range be­tween 0.83 and 0.95 by Cox and D’antonio.14 Porosity determines the ratio between the average particle velocity in the channels and on the cross-sectional average par­ticle velocity of the absorber material.13 A number of models use the assumption that the porosity is close to unity including that of Delany and Bazley.28 The effect of porosity should be included in air flow resistivity terms that takes a significant role in predictive models as the quotient between the density of the biocomposite to its constituent fibers, , given in Eq. (25) corresponds to massivity, ¥, which is porosity subtracted from unity.

In the construction of auditorium, classroom, office buildings, residential and com­mercial buildings architects pay attention to the acoustic quality of such spaces. The acoustic qualities of such spaces are evaluated by few important measured / design parameters like noise Criterion (NC) rating, reverberation time and speech interfer­ence level (SIL). While designing such spaces the architects and engineers aim to obtain desirable value of the above parameters. The acoustical property of the build­ing materials plays a significant role to obtain the target parameters. In this section the use of jute and its derivatives for improvement of the acoustical quality of the architectural space is presented.

The reverberation time of an architectural space is defined as the time taken for a sound level in a room to decrease by 60 dB1. The reverberation time is given by Eq. (15),

where RT is the reverberation time defined as the time taken for a sound to decay by 60 dB after the sound source is suddenly switched off.

V is the volume of the auditorium in m3.

A is the total absorption of the auditorium in m2-sabins.

The absorption unit of 1 m2-sabins represents a surface capable of absorbing sound at the same rate as 1 m2 of a perfectly absorbing surface, for example an open window.

However for practical purposes people use generally the Eyrings’s formula for reverberation time is used which is given in Eq. (16) below

al, a2,•••,an is the respective absorption coefficient

Figure 6Л2 gives the typical variation of overall reverberation time as a func­tion of room volume^ By an international standard the reverberation time of an architectural space can be measured and improvement of the reverberation can be

made by having the appropriate amount of absorption on the wall surface57. The speech interference level (SIL) is the arithmetic average of the sound pressure level in octave band centered at 500, 1000, 2000 and 4000 Hz and the preferred Speech Interference Level (PSIL) is the arithmetic average of the SPL in octave bands of 500 Hz, 1 kHz and 2 kHz. The PSIL as a function of distance from the receiver in presence of background noise is shown in Fig. 6.132. Apart from using reverbera­tion time and speech interference level to characterize the acoustics of a room the noise criterion (NC) curve is widely used in engineering. The NC curves are a set of sound pressure level in the various octave bands. To have a particular NC rating of the room the measures SPL in octave band should be less than or equal to the SPL given for the particular NC rating. A typical NC Curve is shown in Fig. 6.14 and the recommended NC values for various environments are given in Table 6.12.

R VL SH

50 60 70 80 90 100 110 125

13 Communication limits in the presence of background noise (with permission

The NFCs have been studied by several researchers, using injection molding as a fabrication process. Several issues and challenges regarding the fabrication of NFCs by injection molding have been observed.

8.4.1 DISTRIBUTION AND ORIENTATION OF NATURAL FIBERS

The distribution and orientation of the fibers in the composite plays an important role in determining the mechanical properties of the composites. In injection molded composites, the orientation of the reinforced fibers is a critical factor to control. The fiber orientation of short fibers vary with respect to the thickness as well as the in plane direction. During the injection molding of composites, the fibers are oriented according to the complex molten polymer flow generated during the process. Con­vergent flow of the fiber matrix mixture results in fiber alignment along the flow axis while the divergent flow causes the fibers to align perpendicular to the flow direction.25The fibers near the surface are generally aligned parallel to the direction of flow and are aligned perpendicular to the direction of flow at the center. A similar orientation of fibers was also observed, in a study where distribution of flax fibers reinforced PP was studied. The fibers close to the surface and sides of the molded part were well aligned with the direction of flow, whereas, near the mid plane the fibers were randomly distributed.26This shows the dependence of fibers to align in the direction of shearing and stretching. During injection of the matrix-fiber mixture into the mold, the shear flow near the mold walls aligns the fibers in the direction of flow. This outer layer in contact with the mold surface is called as skin. Below this layer, the mixture in the molten state continues to experience shear and fibers are aligned along the flow direction. Then a core layer is formed in the center where bulk deformation of the flow occurs causing the material to stretch in and out of the paper direction aligning the fibers.25 The fiber distribution in PLA/Jute composites was studied using long fiber pellets (LFT) (fabricated via pultrusion process) and recompounded pellets (RP) (extruded LFT) and it was found that the fiber dispersion and separation in RP was better than that of LFT, resulting in a better interfacial interaction and improved mechanical properties, although severe fiber attrition was observed in case of recompounding process.27 Hence, it shows that the orientation and distribution of the fibers not only depends on the processing and flow conditions but also on the precompounding of the fibers and the matrix. A balance between the amount of compounding and the distribution of the fibers is required to keep the fiber attrition to minimum. Also, along with the processing parameters, the rheologi­cal properties of the matrix should be taken into account while designing the mold for injection molding to ensure a better flow and less fiber attrition.

Different aqueous solutions, which can be single, or bicomponents were used in this study. In a single component solution, only one chemical is dispersed in demineral­ized water. In a bi-component solution or a bi-component suspension, separate solu­tions or suspensions of each of the two chemicals were prepared in an equal amount of demineralized water and then they were mixed together.

Hemicelluloses (or heteropolysaccharides) can also be used as a raw feedstock for numerous polymeric biomaterial applications, but thus far they have not received as much attention as cellulose or startch polysaccharides despite their abundance and structural diversity.32In many lignocellulosic refining processes to obtain cellulose, hemicelluloses are partly degraded. Hemicelluloses are hydrophilic components of the cell wall and they can be extracted from plant-material either by water and/or alkaline media. Hemicelluloses are classified according to their structure, compris­ing monomers (D-xylose, D-mannose, D-galactose, L-arabinose and D-glucose) forming xylans, mannans, galactans, arabinans and p-glucans, respectively, in the polymer main chain. In addition, most hemicelluloses possess side groups of 1-2 monosaccharide units and also acetyl groups. The most abundant hemicelluloses are xylans and mannans. While xylans are the most common hemicelluloses and considered to be the major noncellulosic cell wall polysaccharide component of an — giosperms (e. g., hardwood, grasses, and cereals), mannans (galactoglucomannans) are the predominant hemicelluloses in softwoods.

Recently, the incorporation of xylans and mannans for biodegradable films has been explored. They have the potential to be blended with other polymers or mixed with nanoparticles to achieve desirable properties.33

Xylan consists of р-D-xylopyranose units, linked by (1-4)-bonds, and different side groups depending on the plant source. The degree of polymerization (DP) of native cellulose is ten to one hundred times higher than that of hemicellulose. For instance, the DP of wood cellulose was reported to be 10,000 while the DP of wood arabino glucuronoxylan is about 100 and that of glucuronoxylan is about 200. In hardwood xylans, seven out of ten xylose units in the backbone contain an O-acetyl group at C-3 or C-2 (Fig. 14.7). The acetyl groups cannot be found in softwood xylans.

Alternating D-glucopyranosyl and D-mannopyranosyl units attached by P-(1—4) bonds characterize galactoglucomannans and glucomannans. The ratio of glucose to mannose in hardwood glucomannans can vary between 1:2 and 1:1 based on the wood species. Softwood galactoglucomannans can be basically categorized into two fractions with different galactose contents that contain D-galactopyranosyl units at­tached by a-(1-6) linkages to the backbone mannose chain. Figure 14.8 illustrates the structure of O-acetylated galactoglucomannan.

In terms of properties, hemicelluloses have the ability to absorb large amounts of water. They are highly hydrophilic due to the numerous free hydroxyl groups in their structure. Depending on the hemicellulose type, the solubility in water varies ac­cording to the degree of substitution: the higher the degree of substitution, the more water-soluble is the hemicellulose. The degree of polymerization also influences the solubility of hemicelluloses. In general, long chains are less water-soluble.34

Differently from cellulose, hemicelluloses do not have crystalline domains and they present low degrees of polymerization compared to cellulose, which probably lower their chemical and thermal stability compared to cellulose.

Cellulose which is a linear homopolymer of D-anhydro glucopyranose unit (AGU) polymerized via P-1, 4-linkage (Fig. 16.1), can be obtained from plant biomass, bacteria and marine animals called tunicates. During its biosynthesis, cellulose is formed as linear and highly crystalline fibrils at nanoscale with outstanding me­chanical properties via intra and intermolecular hydrogen bonding and organized into microscale elementary fibril-bundles separated and cemented by noncrystalline regions containing lignin, pectin, hemicelluloses. The highly crystalline nanofibrils which exhibit higher axial mechanical properties (strength and modulus), can be obtained by eliminating the other amorphous components or defibrillating them via acid hydrolysis, enzymatic hydrolysis, oxidation, mechanical treatment, ultrasonic treatment or combinations thereof.2728

Depending on the source of cellulose and the isolation protocol used and the sources, they can be obtained as shorter cellulose nanocrystals (CNC) or whiskers (with 5-20 x 500 nm), longer cellulose nanofibers (CNF) or nanofibrillated cellu­lose (NFC) (3-20 nm x few microns) and microfibrillated cellulose (20-100 nm x few microns). In addition, nano-scale cellulose can also be directly by microorgan­isms.27 Bacterial cellulose (BC) is an emerging and unique biopolymer character­ized by very long polymer chains with a degree of polymerization in the range of 4000-10,000, high crystallinity, and high purity with an extremely large amount of water containing. The individual nanofibrils of BC exhibit a high tensile strength of equivalent to that of steel or Kevlar.2931

A critical step that remains for the commercial production of synthetic silk fibers is successful spinning of recombinant proteins into materials that resemble natu­ral silks, specifically in their mechanical and thermal properties. One technique in­volves spinning fibers into a liquid coagulation bath, which is referred to as wet spinning. In 1993, it was first described by the DuPont® group with fibroins spun from silkworms62 and then later modified by Nexia Biotechnologies in 2002.63 Suc­cessful reports that integrate arthropod biomimicry to produce synthetic spider silks also have been reported in the scientific literature.64 These procedures have relied on purifying recombinant spidroins, followed by spidroin concentration via lyophi — lization, and then solubilization of spidroins with chaotropic solvents to produce a highly concentrated spinning dope (Fig. 1.5).64b This solution is then pushed through a syringe equipped with a needle into an alcohol bath, allowing a slower solidifica­tion (Fig. 1.5). The flow of the liquid is best controlled by a syringe infusion pump that can move the liquid through the syringe at a constant rate. Isopropanol is of­ten used as coagulation medium during extrusion and provides a dehydration step that removes water from the fiber (Fig. 1.5). The resulting products are referred to “as-spun threads,” which can be subsequently subject to postspin draw, a process that dramatically improves the breaking stress, toughness, and Young’s modulus.4063 This methodology has led to fibers (some biocomposites) with mechanical proper­ties that are lower quality relative to natural spider silks. For example, truncated recombinant fibroins spun into fibers have reported breaking stress values that range from 35-350 MPa (Table 1.2), which are lower than the typical 1000 MPa values reported for natural dragline silk. The strongest fibers produced via this technique have been reported from a fiber spun from a recombinant 96-mer MaSp1 protein construct expressed in E. coli, which lead to fibers that exceeded a breaking stress of 500 MPa, a value about 50% lower relative to natural dragline silk fibers (Table 1.2).41 This recombinant protein was approximately 285 kDa and represents the larg­est molecular weight recombinant protein used for synthetic fiber production.

Although these results are good indicators of progress, several caveats are worth noting for synthetic fiber production. Firstly, the reproducibility of the quality of the synthetic fibers has been a major issue. There is still much variability between the mechanical properties of the fibers generated from different regions of the same spun fiber, which highlights several technical issues that remain to be resolved dur­ing the manufacturing process. Much variability is due to the introduction of hu­man errors during the processing steps and can be circumvented by automation of the process. Secondly, the solvent choice for the majority of the laboratories has been a challenge. Hexafluoro-2-propanol (HFIP) and formic acid are two solvents that are excellent at dissolving the silk proteins to achieve the necessary spinning dope concentrations, but are volatile and toxic compounds that are less than ideal for manufacturing conditions. Additionally, maintaining protein solubility during the spinning process can be difficult. Too high of protein concentration can lead to precipitation or gelation, making the spinning process unmanageable. During fiber curation, HFIP has been reported to evaporate, leaving voids within the interior of the material that impact the mechanical properties of the fibers (Fig. 1.6).40 One laboratory has produced recombinant silk protein constructs that have been demon­strated to form fibers from an aqueous solution, potentially offering an advantage to fiber production without the use of HFIP and formic acid.65

1.5.1 SPE/QC/WF BIOCOMPOSITES

1.5.1.1 PROPERTIES OF CURED EPOXY RESINS

In order to optimize the curing condition of SPE and QC, the curing tem­perature and epoxy/hydroxy ratio were changed.24 Table 4.8 summarizes the tan 5 peak temperature measured by DMA and 5% weight loss temperature of SPE/QC cured at various conditions. When the curing temperature was changed from 150 °C to 190 °C for SPE/QC at a typical epoxy/hydroxy ratio of 1/1, the SPE/QC cured at 170 °C had the highest tan 5 peak temperature (78.4 °C) and 5% weigh loss temperature (335.4 °C). Because SPE is an aliphatic epoxy resin, it is presumed that some thermal degradation starts to occur at around 190 °C. Therefore, the curing temperature was fixed to 170 °C. When the epoxy/hydroxy ratio was changed from 1/0.8 to 1/1.2 at the curing temperature of 170 °C, the SPE/QC 1/1.2 had the highest tan 5 peak temperature (85.5 °C) and 5% weigh loss temperature (342.5 °C). This result suggests that four hydroxy groups of QC with five hydroxy groups per molecule actually reacted with SPE. Although it is not clear why SPE/ QC 1/0.8 had a higher tan 5 peak temperature than SPE/QC 1/0.9-1/1.1, it is supposed that cationic homopolymerization of SPE occurs by the action of acidic hydroxy proton at 5-position of carbonyl group of QC. Consequently, the epoxy/hydroxy ratio of 1/1.2 and curing temperature of 170 °C were selected as the optimized curing condition for SPE/QC. Table 4.8 also sum­marizes the thermal properties of the cured resins of SPE/PN, diglycidyl — ether of bisphenol A (DGEBA)/QC and DGEBA/PN. As a result of optimi­zation of the epoxy/hydroxy ratio for SPE/PN, SPE/PN(1/0.8) showed the highest tan 5 peak temperature (81.0 °C), which was still lower than that of SPE/QC(1/1.2) (85.5 °C). Although we did not fully optimize the epoxy/hy­droxy ratio for DGEBA/QC and DGEBA/PN, DGEBA/QC(1/1) and DGE — BA/QC(1/1.2) showed higher tan 5 peak temperature than DGEBA/PN(1/1). These results indicate that QC is a superior epoxy hardener to produce the cured resin with a high T

Natural fibers have recently been used for making composite materials and they offer several advantages over synthetic materials. While these natural fibers can be extracted from many sources such as sisal, jute, coir, flax, hemp, pineapple and banana3; jute has been promoted as the most readily available, environment friendly, abundant, economic and bio-renewable source. It is specifically cultivated in large quantities in the eastern part of India and in Bangladesh4. It is a lignin-cellulose fiber

which is composed primarily of the plant materials; cellulose (major component of plant fiber) and lignin (major components wood fiber). It falls into one of the bast fiber category (fiber collected from bast or skin of the plant) along with kenaf, in­dustrial hemp, flax (linen), ramie and so forth.

Jute is used in various forms for noise control applications. The raw jute fiber after cleaning are used to produce jute yarn by a spinning process. The jute yarn is then weaved to make jute textile or cloth. Stacks of jute yarn laid in a random or a definite sequence are pressed under temperature to produce jute felt. The jute felt/ fiber in turn can be chemically treated with a bonding agent usually natural rubber latex as a resin and pressed under certain temperature to form jute-based biocom­posite panels. In few instances the raw fibers after appropriate processing can be chopped and used as fills in noise control blankets and pads. In jute mills where jute-based textiles are manufactured, during the trimming operations of these tex­tiles many waste trims are produced. These trims can be used as acoustical fills as well, for noise control. All the above forms ofjute derivatives are shown in Fig. 6.3 in some form or the other can be used for noise control purposes.

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.