Page 317 - Handbook of Properties of Textile and Technical Fibres
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Tensile properties of flax fibers 291
Figure 8.8 Flax fabric consisting of an assembly of unidirectional tapes.
Inside a composite material, the load transfer between fibers is ensured by the inter-
facial bond and the matrix. In view of the anisotropy of flax fibers, polymer strength-
ening with twisted yarns leads to a decrease of the tensile properties compared to that
of unidirectional laminates (Baets et al., 2014; Shah et al., 2013) because fibers are not
perfectly aligned. In the same way, it is questionable to use flax fabric (with twisted
yarns) produced for textile applications. Moreover, the use of twisted strands leads
to a less homogeneous distribution of the fibers within the plies (Shah et al., 2013;
Berges et al., 2016) and, possibly, impregnation difficulties by the matrix inside
strands. For different processing methods for composite materials (weaving, filament
winding, automated fiber placement (AFP)), it is useful to employ unidirectional tapes
(Baley et al., 2016a). In this case, fibers (or bundles) are held, for example, by a binder,
a polymer, or a weft yarn. Fig. 8.8 shows, for example, a flax fabric consisting of an
assembly of unidirectional tapes.
Evidently it is necessary to ensure compatibility between the matrix and the binder
if they do not have the same composition. Similarly, glass rovings are coated, and one
of the functions of the sizing is to ensure fiber cohesion. When the layer is woven, the
fiber yarns are identified as warp and weft tows; warp tows can be considered as
lengthwise yarns (0 degrees), whereas the weft tows (or fill tows) (90 degrees) are
inserted over and under warp yarns to produce the weave pattern. The weave pattern
is also a reason for disorientation, and this is a parameter studied for the damping
capacity of biocomposites (Duc et al., 2014).
8.5.3 Biocomposite process and thermal cycle
The choice of matrix (thermosetting or thermoplastic) depends on the specifications,
the process used, and the know-how of the factory. A thermoplastic matrix allows a
recyclable composite to be obtained (Bourmaud and Baley, 2007) and in some cases
a biodegradable composite (Le Duigou et al., 2008; Bodros et al., 2007). With this