Page 77 - Plastics Engineering

60 Mechanical Behaviour of Plastics

Thickness = 1.2 mm

Fig. 2.14 Acetal bottle cap

given by

3pR2(1 + u) - 3 x 0.375 x (13)2(1.33)
o= -
8h2 8( 1 .2)2
o = 22 m/m2

After 1 month (2.6 x lo6 seconds) at this stress, the strain is obtained from
Fig. 2.15 as 2.2%. Hence E = 22 x 106/0.022 = 1 GN/m2.

pp 0.375(13)412(1 - 0.332)
6=--
640 - 64 x io00 x (1.213
S=lmm

Example 2.5 In a small polypropylene pump the flange on the cover plate
is 2 mm thick. When the rigid clamping screws are tightened, the flange is
reduced in thickness by 0.03 mm. Estimate the initial stress in the plastic and
the stress after 1 week
Solution The strain in the material is given by
0.03
E=- x 100 = 1.5%
2
This is a stress relaxation problem and strictly speaking stress relaxation data
should be used. However, for most purposes isometric curves obtained from
the creep curves are sufficiently accurate. By considering the 1.5% isometric
curve shown in Fig. 2.8 it may be seen that the initial stress is 16 MN/m2 and
the stress after 1 week is 7 MN/m2.