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

Long fibres that pass through the entire test specimen length (18 cm long) were in
general used in the fabricated composites except with the hemp fibres defibrated with C.
subvermispora, which were only obtained as short fibres (6 cm long). The effects of fibre
length on composite strength and stiffness were therefore investigated with the fibres
produced by cultivation of P. radiata obtained at both fibre lengths. The curves for
composite strength versus fibre volume fraction were slightly steeper with long fibres
than with short fibres (Figure 6a). The effective fibre strength was therefore higher when
the long fibres were used than when the overlapping short fibres were used (643
MPa/521 MPa = 1.23, Table 4). This is expected due to the high probability of
composite fracture near the fibre-end positions of the short fibres. The curves for
composite stiffness versus fibre volume fraction were also slightly steeper with long
fibres than with short fibres (Figure 7a). The increase in calculated fibre stiffness when
using long fibres as reinforcement instead of short fibres was 94 GPa/82 GPa = 1.14
(Table 4).
The hemp fibres produced in the fungal cultivation experiments were compared based on
results obtained with composites reinforced with fibres of short length (6 cm). The curve
for composite strength was slightly steeper with the fibres produced by defibration with
P. radiata than with C. subvermispora (Figure 6a). The defibration of hemp by
cultivation of P. radiata resulted therefore in a higher calculated fibre strength (521
MPa) than the cultivation of C. subvermispora (434 MPa) (Table 4). This was in
agreement with the measured fibre bundle strength that was also found to be highest for
the P. radiata defibrated hemp fibres. The curves for composite stiffness versus fibre
volume fraction were also slightly steeper with the fibres produced by cultivation of P.
radiata than C. subvermispora (Figure a). The defibration of hemp with P. radiata
resulted therefore in higher fibre stiffness (82 GPa) than the C. subvermispora
cultivation (77 GPa) (Table 4).
In order to compare the fungal defibrated hemp fibres with water-retted hemp and raw
hemp bast, it was necessary to correct for the effect of short fibre length on the fibre
strength and fibre stiffness. This correction was performed with the ratios previously
determined for hemp fibres that were defibrated using P. radiata (1.23 for fibre tensile
strength and 1.14 for fibre stiffness). These corrected values for the hemp fibres
defibrated by C. subvermispora cultivation are 536 MPa for fibre strength and 88 GPa
for fibre stiffness (Table 4).
For raw hemp bast and water-retted hemp (Figure 6b), the curves for composite strength
versus Vf were less steep than for the hemp fibres produced by cultivation of P. radiata
(Figure 6a). The raw hemp bast and the water-retted hemp fibres had therefore lower
tensile strength of 535 MPa and 586 MPa, respectively than the P. radiata defibrated
hemp (643 MPa) (Table 4). The failure strain of hemp fibre composites except the hemp
yarn composites was 0.7 – 0.9% and based on this the failure stress of the epoxy matrix
was 18 – 25 MPa (Table 4). The highest composite strengths obtained using raw hemp
bast, water retted hemp or P. radiata defibrated hemp, as reinforcement were 122 MPa,
153 MPa and 174 MPa, respectively without consideration of porosity (σcu). The
composites with hemp fibres produced by cultivation of P. radiata were strongest due to
the low porosity content, the high fibre strength and the higher obtainable fibre content.
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