Page 95 - Plastics Engineering

78 Mechanical Behaviour of Plastics

Therefore,
2 x 5.5 x 2
E= = 0.34%
802
From the creep curves for acetal, at 1 month (2.6 x lo6 s) and 0.34% strain,
the modulus (0,'~) is approximately 1.3 GN/m2.

Therefore, using (2.23) and ignoring (W/12) on each side
70 x 23 = 1.3 x D3

D = 7.6 mm
Hence an acetal beam 7.6 mm deep and of the same width and length as the
aluminium beam will perform in exactly the same way when the load of 200 N
is applied.
However, as will be shown in Chapters 4 and 5, it would not be good design
practice to use a plastic beam Section 7.6 mm thick. Such a section would take
a long time to mould and would probably have sink marks on its surface. It
would be preferable to use thinner wall sections and introduce ribs to provide
the desired stiffness. This is illustrated below.

(b) Consider the flat beam to be replaced by a ribbed section as shown in
Fig. 2.27. It is necessary for the second moment of area, I, to be the same for
both sections. Clearly there are many permutations for the dimensions of the

W
4

Fig. 2.27 Flat beam and ribbed section, Example 2.11