Page 261 - Plastics Engineering

244 Mechanical Behaviour of Composites

3.25 A sheet of chopped strand mat-reinforced polyester is 5 mm thick and 10 mm wide. If its
modulus is 8 GN/mz calculate its flexural stiffness when subjected to a point load of 200 N mid-
way along a simply supported span of 300 mm. Compare this with the stiffness of a composite
beam made up of two 2.5 mm thick layers of this reinforced material separated by a 10 mm thick
core of foamed plastic with a modulus of 40 m/mZ.
3.26 A composite of gfrp skin and foamed core is to have a fixed weight of 200 g/m. If its
width is 15 mm investigate how the stiffness of the composite varies with skin thickness. The
density of the skin material is 1450 kg/m3 and the density of the core material is 450 kg/m3.
State the value of skin thickness which would be best and for this thickness calculate the ratio of
the weight of the skin to the total composite weight.
3.27 In a short carbon fibre reinforced nylon moulding the volume fraction of the fibres is 0.2.
Assuming the fibre length is much greater that the critical fibre length, calculate the modulus
of the moulding. The modulus values for the fibres and nylon are 230 GN/m2 and 2.8 GN/m2
respectively.