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

a: Raw hemp bast
c: P. rad. treated hemp
b: Water retted hemp
d: Hemp yarn
Figure 4. SEM microscopy photos of transverse composite sections recorded at low and
high magnification with the defibrated hemp fibres. The small inset pictures have a scale
bar length of 200 μm and the background pictures 20 μm.
Composite mechanical properties
The tensile strength and stiffness of the fabricated composites, corrected for porosity are
presented in Figure 6 and 7, respectively. The terms σcup and Ecp are calculated from the
measured strength and stiffness (σcu and Ec) using nσ=2.1 and nE=1 in Equation 9.
Finally, the fibre strength and stiffness determined from σcup and Ecp are calculated from
Equation 9 and presented in Table 4.
Filament winding and hand lay-up were used for preparation of fibre sheets for
impregnation in epoxy leading to the procedures for composite fabrication. These two
fabrication techniques were compared using hemp yarn. The curve for composite tensile
strength versus fibre volume fraction showed a slightly higher slope with the wound
fibres than with the lay-up of fibres (Figure 6a). The calculated fibre tensile strength with
the wound hemp yarn was therefore higher (728 MPa) and also with lower standard
deviation (44 MPa) than with the hand lay-up of hemp yarn (677 ± 92 MPa, Table 4).
The failure strain of these composites were 1.6 – 1.8% and based on this the failure stress
of the epoxy matrix was 39 – 43 MPa (Table 4). For composite stiffness versus the fibre
content, the curves for the wound fibres and the lay-up of fibres showed similar slopes
(Figure a). The determined fibre stiffness did not therefore appear to be much affected by
the preparation method, and the determined fibre stiffness was 60 – 61 GPa (Table 4).
For both fibre tensile strength and stiffness, the standard deviation obtained with hand
lay-up was twice the standard deviation obtained with filament winding.
132 Risø-PhD-11