Page 311 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of flax fibers 285

A plant fiber can be assimilated to a stack of plies reinforced by cellulose fibrils
arranged in a helix and oriented at an angle of about 10 degrees (Bourmaud et al.,
2013b). During a tensile test, the fiber behavior depends on the constituents but is
also influenced by two mechanisms (Baley, 2002; K€ ohler and Spatz, 2002; Spatz
et al., 1999): The partial reorientation is toward the axis of stress of the fibrils and
the sliding of the latter with respect to one another. The tensile behavior is therefore
not perfectly linearly elastic and, beyond a threshold, the reorientation of the cellulose
fibrils results in an increase in the stiffness during tests with the increase in the strain
(Baley, 2002; Lefeuvre et al., 2014a).
Moreover, this specific nonlinear behavior is also seen with single hemp fibers
(Duval et al., 2011; Placet et al., 2011), abaca (Cai et al., 2015), jute (Lee et al.,
2009), kenaf (Lee et al., 2009) or sisal fiber bundles (Silva et al., 2008). It is a strong
specificity of plant fibers.

8.4.4 Standard tests
For reinforcement purposes, it is necessary to take into account of the fiber’s specific
tensile behavior. This has led to the introduction of appropriate standards dedicated to
the specific vocabulary (AFNOR NT T 25-501-1, 2015), the tests on single fibers
(AFNOR NF T25-501-2, 2015) and on fiber bundles called technical fibers (AFNOR
NF T25-501-3, 2015). The procedure described by a standard is always open to
criticism, but it has the advantage of defining a common language and allowing
comparisons. The tensile behavior of an elementary fiber is generally not linear
(Fig. 8.4) and, to allow stiffness comparisons, it is important to clearly specify the
strain corresponding to the determined tangent modulus. The standard AFNOR NF
T25-501-2 (2015) proposes the measurement of the tangent modulus on the second
part of the tensile curve, this part being quasi linear (Baley, 2002).

8.4.5 Influence of sampling area according to the stem height
Fiber morphology is strongly dependent on the weather conditions during plant
growth. Indeed, Chemikosova et al. (2006) observed that lack of water and sunshine
slows down plant development, thus impacting on the fiber elongation and cellulose
filling. Temperature has been also reported to have a negative effect on plant growth
(Casa et al., 1999). Flax seeds are sown in March, when the weather can be dry and
cold, so fibers from the bottom of the stem, which are the first to develop, are likely
to undergo perturbations due to stresses during cell wall formation. Conversely, a rise
in temperature in April and May leads to a rapid vegetative growth. Thus, fibers
from the middle of the stem are reported to be longer and with a higher percentage
of cellulose filling (Charlet et al., 2007). Then, floweringisfollowedbyseed
maturation in June, which, depending on the variety, leads to the cessation and slow-
ing down of the stem and cell development. Therefore, fibers from the top of the
stem, which are initiated at the end of the plant growth do not have enough time
to fully develop. Bourmaud et al. (2015) found that fibers from the top of the stem
are thinner and also less numerous compared to those from the middle and bottom.