Page 152 - Plastics Engineering
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Mechanical Behaviour of Plastics 135
Log time to failure (s)
Fracture
--- Whitening or crazing
----- Isometric curves
Fig. 2.71 Qpical creep rupture behaviour of plastics
difficulties in the extrapolation of short-term tests, as shown in Fig. 2.71. This
problem has come to the fore in recent years with the unexpected brittle fracture
of polyethylene pipes after many years of being subjected to moderate pres-
sures. On this basis the British Standards Institution (Code of Practice 312)
has given the following stresses as the design values for long term usage of
plastics.
Plastic Safe working stresses
LDPE 2.1 m/m2
HDPE 5.0 MN/m2
PP 5.0 MN/m2
ABS 6.3 MN/m2
UPVC 10.0- 12.0 MN/m2
Other factors which promote brittleness are geometrical discontinuities (stress
concentrations) and aggressive environments which are likely to cause ESC (see
Section 1.4.2). The absorption of fluids into plastics (e.g. water into nylon) can
also affect their creep rupture characteristics, so advice should be sought where
it is envisaged that this may occur.
It may be seen from Fig. 2.71 that in most cases where the failure is ductile
the isometric curves are approximately parallel to the fracture curve, suggesting
that this type of failure is primarily strain dominated. However, the brittle
