Page 155 - Plastics Engineering

138 Mechanical Behaviour of Plastics
There is considerable evidence to show that there is a close connection
between crazing and crack formation in amorphous plastics. At certain stress
levels, crazes will form and studies have shown that cracks can nucleate in the
crazes and then propagate through the preformed craze matter. In polystyrene,
crazes are known to form at relatively low stresses and this has a significant
effect on crack growth mechanisms in the material. In particular, during fracture
toughness testing, unless great care is taken the material can appear to have a
greater toughness than acrylic to which it is known to be inferior in practice.
The reason is that the polystyrene can very easily form bundles of crazes at
the crack tip and these tend to blunt the crack.
If a plastic article has been machined then it is likely that crazes will form
at the surface. In moulded components, internal nucleation is common due to
the presence of localised residual stresses.

2.21 Fatigue of Plastics

The failure of a material under the action of a fluctuating load, namely fatigue,
has been recognised as one of the major causes of fracture in metals. Although
plastics are susceptible to a wider range of failure mechanisms it is likely that
fatigue still has an important part to play. For metals the fatigue process is
generally well understood, being attributed to stable crack propagation from
existing crack-like defects or crack initiation and propagation from structural
microflaws known as dislocations. The cyclic action of the load causes the
crack to grow until it is so large that the remainder of the cross-section cannot
support the load. At this stage there is a catastrophic propagation of the crack
across the material in a single cycle. Fatigue failures in metals are always brittle
and are particularly serious because there is no visual warning that failure is
imminent. The knowledge of dislocations in metals stems from a thorough
understanding of crystal structure, and dislocation theory for metals is at an
advanced stage. Unfortunately the same cannot be said for polymer fatigue.
In this case the completely different molecular structure means that there is
unlikely to be a similar type of crack initiation process although it is possible
that once a crack has been initiated, the subsequent propagation phase may be
similar.
If a plastic article has been machined then it is likely that this will introduce
surface flaws capable of propagation, and the initiation phase of failure will be
negligible. If the article has been moulded this tends to produce a protective
skin layer which inhibits fatigue crack initiatiodpropagation. In such cases it
is more probable that fatigue cracks will develop from within the bulk of the
material. In this case the initiation of cracks capable of propagation may occur
through slip of molecules if the polymer is crystalline. There is also evidence
to suggest that the boundaries of spherulites are areas of weakness which may
develop cracks during straining as well as acting as a crack propagation path.