Properties of hemp fibre polymer composites -An optimisation of ...

Second mode: Failure is controlled of shear forces between the fibres and not the fibre
strength when the fibre angle is inclined with 15°-40°.
τ
σ θ =
xy
sinθ cosθ
where τ is shear stress in hemp yarn (26 MPa; Madsen, 2004).
Third mode: Tensile failure normal to fibres.
σ
θ =
sin
y
σ 2
θ
The maximum stress theory is in agreement with results for composites with carbon
fibres and epoxy resin. The Tsai-Hill criterion takes these three failure mechanisms into
account in a more complex expression based on the composite strength in the fibre
direction, perpendicular to the fibre direction and the shear stress between fibres and
matrix. This expression is based on the maximum distortion energy and is defined by the
equation below (Tsai, 1968).
2 2 2
4
2 2
1 cos θ ( cos θ − sin θ)
sin θ cos θ sin θ
=
+ +
2
2
2
2
σ
σ
σ τ
θ
x
y
in which the ultimate composite strength has been found to fit well with the three failure
modes described above and with data for hemp yarn reinforced composites (Figure 32;
Madsen, 2004).
9.2 Composition of the formed composites
The hemp fibre reinforced composites were observed using scanning electron
microscopy in order to see the fibres shape, size and impregnation with the epoxy-matrix
(Figure 33). This figure shows the fibres (light grey area), the matrix (dark grey area)
and the air filled voids mentioned as porosity (black area).
There appeared to be porosity inside the fibre bundles and inside the epidermis on the
fibre bundle surfaces of raw hemp bast and water retted hemp fibres when these were
used as reinforcement of composites. Debonding between fibres and matrix had therefore
occurred which generally results in reduced mechanical properties. The hemp fibres that
were defibrated by cultivation of P. radiata Cel 26 appeared well impregnated with
epoxy and no porosity could be observed on the fibre bundle surfaces. The same
appearance was observed on the composites with hemp fibres defibrated by cultivation
of C. subvermispora (Paper IV), showing that the fungal defibration resulted in good
impregnation of the fibres.
The composites with barley straw contained much larger fibre lumens resulting in higher
porosity content than in the hemp fibre composites. Large areas with no contact between
matrix and fibres were observed, which resulted in very low composite strength
perpendicular to the fibre axis (unpublished data).
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xy