Page 258 - Plastics Engineering

Mechanical Behaviour of Composites 24 1

Questions

3.1 Compare the energy absorption capabilities of composites produced using carbon fibres,
aramid fibres and glass fibres. Comment on the meaning of the answer, The data in Fig. 3.2 may
be used.
3.2 A hybrid composite material is made up of 20% HS cdn fibres by weight and 30%
E-glass fibres by weight in an epoxy matrix. If the density of the epoxy is 1300 kg/m3 and the
data in Fig. 3.2 may be used for the fibres, calculate the density of the composite.
3.3 What weight of carbon fibres (density = 1800 kg/m3) must be added to 1 kg of epoxy
(density = 1250 kg/m3) to produce a composite with a density of 1600 kg/m3.
3.4 A unidirectional glass fibre/epoxy composite has a fibre volume fraction of 60%. Given
the data below, calculate the density, modulus and thermal conductivity of the composite in the
fibre direction.

Epoxy 1250 6.1 0.25

Glass fibre 2540 80.0 1.05

3.5 In a unidirectional Kevlar/epoxy composite the modular ratio is 20 and the epoxy occupies
60% of the volume. Calculate the modulus of the composite and the stresses in the fibres and
the matrix when a stress of 50 MN/m2 is applied to the composite. The modulus of the epoxy is
6 GN/m2.
3.6 In a unidirectional carbon fibdepoxy composite, the modular ratio is 40 and the fibres
take up 50% of the cross-section. What percentage of the applied force is taken by the fibres?
3.7 A reinforced plastic sheet is to be made from a matrix with a tensile strength of 60 MN/m2
and continuous glass fibres with a modulus of 76 GN/m2. If the resin ratio by volume is 70%
and the modular ratio of the composite is 25, estimate the tensile strength and modulus of the
composite.
3.8 A single ply unidirectional carbon fibre reinforced PEEK material has a volume fraction
of fibres of 0.58. Use the data given below to calculate the Poisson's Ratio for the composite in
the fibre and transverse directions.
Material Modulus (GN/m2) Poisson's Ratio
Carbon fibres (HS) 230 0.23
PEEK 3.8 0.35
3.9 A single ply unidirectional glass fibre/epoxy composite has the fibres aligned at 40" to the
global x-direction. If the ply is 1.5 mm thick and it is subjected to stresses of a, = 30 MN/m2
and uy = 15 MN/m2, calculate the effective moduli for the ply in the x-y directions and the
values of and E~. The properties of the ply in the fibre and transverse directions are
El = 35 GN/m2, E2 = 8 GN/m2, Gl2 = 4 GN/m2 and u12 = 0.26
3.10 A single ply uni-directional carbon fibre/epoxy composite has the fibres aligned at 30"
to the x-direction. If the ply is 2 mm thick and it is subjected to a moment of M, = 180 Nm/m
and to an axial stress of a, = 80 MN/m2, calculate the moduli, strains and curvatures in the x-y
directions. If an additional moment of M, = 250 Nm/m is added, calculate the new curvatures.