The flax fabric (Fig. 7.4.a) used in this study, is a plain weave fabric with an areal weight of 280±19 g/m2, manufactured by GroupeDepestele (France)68. The fabric is not balanced, as the space between the weft tows (1.59± 0.09 mm) is different to the one between the warp tows (0.26± 0.03 mm). The width of the warp and the weft tows are, respectively 2.53±0.12 mm and 3.25±0.04 mm. As a consequence, there are 360 warp tows and 206 weft tows per meter of fabric. The linear mass of the warp and the weft tows is the same and is equal to 494±17 g/km. The tows are constituted by globally aligned groups of fibers. The length of these groups of fibers varies between 40 to 600 mm with a maximum occurrence-taking place at 80 mm. This fabric is constituted of continuous tows (Fig. 4.b). Generally, when natural fibers are considered, twisted yarns are elaborated to increase its tensile properties. Indeed, as discussed by Goutianos et al.69 sufficient tensile properties of the yarns are necessary for these ones to be considered for textile manufacturing or for pro­cesses such as pultrusion or filament winding. In this study, the flax tows used to elaborate the plain weave fabric are un-twisted and exhibit a rectangular shape. The fibers or groups of fibers are slightly entangled to provide a minimum rigidity to the tows. This geometry has been chosen as it generates low bending stiffness tows, therefore limiting the crimp effect in the fabric and therefore limiting empty zones between tows. It has also been chosen because fabric manufactured from highly twisted yarns exhibit low yarn permeability preventing or partially preventing the use of processes from the LCM (Liquid Composite Moulding) family. Un-twisted tows have also been chosen because manufactured composites display better me­chanical properties than composites made with twisted yarns70. However, this rein­forcement was originally developed to manufacture large panels with low curvature and was therefore not optimized for complex shape forming.


FIGURE 7.4 Reinforcement 1: (a) Flax fabric; (b) flax tow.

A second flax woven reinforcement has also been used. This reinforcement 2 is a 4/4 flax woven hopsack construction with an areal weight of 508 ± 11 g/m2 manu­factured by the Composites Evolution Company, UK71. The hopsack is presented in Fig. 7.5. The cylindrical yarns are manufactured from aligned fibers held together by a polyester yarn going in a spiral manner along the flax yarn. The linear density of the flax yarns is 250 ± 9 tex (g/km). The lineic mass of the yarn holding the flax fibers is 20 ± 3 tex. The reinforcement used in this work is not balanced. A differ­ence of 20% in the number of flax yarns has been measured between warp and weft directions.


FIGURE 7.5 Reinforcement 2: (a) flax fabric; (b) 4 aligned yarns; (c) individual yarn.