Page 151 - Plastics Engineering

134 Mechanical Behaviour of Plastics

2.20 Creep Failure of Plastics
When a constant stress is applied to a plastic it will gradually change in
size due to the creep effect which was described earlier. Clearly the material
cannot continue indefinitely to get larger and eventually fracture will occur.
This behaviour is referred to as Creep Rupture although occasionally the less
acceptable (to engineers) term of Static Fatigue is used. The time taken for the
material to fracture will depend on the stress level, the ambient temperature,
the type of environment, the component geometry, the molecular structure, the
fabrication method, etc. At some stresses the creep rate may be sufficiently low
that for most practical purposes the endurance of the material may be regarded
as infinite. On the other hand, at high stresses the material is likely to fail
shortly after the stress is applied.
The mechanism of time-dependent failure in polymeric materials is not
completely understood and is the subject of much current research. In the
simplest terms it may be considered that as the material creeps, the stress
at some point in the material becomes sufficiently high to cause a micro-
crack to develop but not propagate catastrophically. The stress in the remaining
unbroken section of the material will than be increment4 by a small amount.
This causes a further stable growth of the microcrack so that over a period of
time the combined effects of creep and stable crack growth cause a build up
of true stress in the material. Eventually a stage is reached when the localised
stress at the crack reaches a value which the remaining cross-section of the
material is unable to sustain. At this point the crack propagates rapidly across
the whole cross-section of the material.
Creep rupture data is usually presented as applied static stress, 0, against the
logarithm of time to fracture, t, as shown in Fig. 2.71. If fracture is preceded
by phenomena such as crazing (see Section 2.20.2), whitening and/or necking,
then it is usual to indicate on the creep rupture characteristics the stage at which
these were first observed. It may be seen from Fig. 2.71 that the appearance
of crazing or whitening is not necessarily a sign the fracture is imminent. In
many cases the material can continue to sustain the applied load for weeks,
months or even years after these phenomena are observed. However, there is
no doubt that when a load bearing component starts to craze or whiten, it can
be disconcerting and so it is very likely that it would be taken out of service
at this point. For this reason it is sometimes preferable to use the term Creep
Failure rather than creep rupture because the material may have been deemed
to have failed before it fractures.
Isometric data from the creep curves may also be superimposed on the
creep rupture data in order to give an indication of the magnitudes of the
strains involved. Most plastics behave in a ductile manner under the action of
a steady load. The most notable exceptions are polystyrene, injection moulding
grade acrylic and glass-filled nylon. However, even those materials which are
ductile at short times tend to become embrittled at long times. This can cause