Page 86 - Plastics Engineering

Mechanical Behaviour of Plastics 69

Solid skin

Solid skin

Original solid/foam/ Equivalent solid section
Solid section
Fig. 2.19

Using the information given

D = 200/935 = 0.214, p, = 935 x g/mm3, W = 0.14 g/m
so
d = 1.61 mm
Using the expression for h from above, the thickness of the foam is given by

h = 31.6 IWII
The cost of the solid beam is given by

(Cost), = 935 x 10 x 15 x x C, = 0.14 C,

The cost of the sandwich beam is given by
(C0st)F = pcbhCc + pS2bdCs = 0.159 C,

Hence
0.159 - 0.14
the additional cost = = 13.5%
0.14
It is interesting to generalise this solution for any core: skin density ratio.
Fig. 2.20 shows how the optimum skin thickness varies with D. This is
independent of the solid material density or modulus but is based on a weight