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

9 Composites reinforced with hemp fibres
9.1 Effect of fibre orientation on mechanical properties
Plant fibre reinforced composites are generally made with randomly orientated fibres or
aligned fibres. Composites with aligned fibres are generally stronger and stiffer in the
fibre direction than composites with randomly orientated fibres.
Both the strain and stress at which a composite fractures decrease when the fibre axis is
inclined to increasing angles to the test direction. The composite tensile strength
decreases by four times when the inclination angle to the fibre orientation is increased to
26° (Figure 32; Madsen, 2004) while the composite stiffness only decreases by two
times.
Ultimate stress (MPa)
200
150
100
50
0
0 10 20 30 40 50 60 70 80 90
Yarn angle (degrees)
Figure 32. Ultimate stress of hemp yarn-PET composites with the yarn axis inclined at
different angles to the test direction. The full lines are ultimate stress predicted from the
maximum stress theory with a shear stress of 22 MPa. The dotted line is ultimate stress
predicted from the Tsai-Hill criterion with a shear stress of 26 MPa (Madsen, 2004).
The prediction of ultimate stress for composites at a given fibre angle to the test
orientation (sq) is made with two models: the maximum stress theory and the Tsai-Hill
criterion. The maximum stress theory characterises three modes of failure, which are
defined by the three following equations developed by Stowell and Liu (1961):
First mode: Failure is caused by tensile fracture of the fibres, when the fibre angle is
inclined with 0°-15°.
σx
σθ 2
cos θ
=
50 Risø-PhD-11