Page 137 - Plastics Engineering

120 Mechanical Behaviour of Plastics

if it occurs, can have more catastrophic results. Therefore it is essential that
designers recognise the factors which are likely to initiate fracture in plastics
so that steps can be taken to avoid this.
Fractures are usually classified as brittle or ductile. Although any type of
fracture is serious, brittle fractures are potentially more dangerous because there
is no observable deformation of the material prior to or during breakage. When
a material fails in a ductile fashion, large non-recoverable deformations are
evident and these serve as a warning that all is not well. In polymeric materials,
fracture may be ductile or brittle depending on such variables as the nature of
the additives, the processing conditions, the strain rate, the temperature and the
stress system. The principal external causes of fracture are the application of a
stress in a very short period of time (impact), the prolonged action of a steady
stress (creep rupture) or the continuous application of a cyclically varying stress
(fatigue). In all cases the fracture processes will be accelerated if the plastic is
in an aggressive environment.
Basically there are two approaches to the fracture of a material. These
are usually described as the microscopic and the continuum approaches. The
former approach utilises the fact that the macroscopic fracture of the material
must involve the rupture of atomic or molecular bonds. A study of the forces
necessary to break these bonds should, therefore, lead to an estimate of the
fracture strength of the material. In fact such an estimate is usually many times
greater than the measured strength of the material. This is because any real
solid contains multitudes of very small inherent flaws and microcracks which
give rise to local stresses far in excess of the average stress on the material.
Therefore although the stress calculated on the basis of the cross-sectional area
might appear quite modest, in fact the localised stress at particular defects in the
material could quite possibly have reached the fracture stress level. When this
occurs the failure process will be initiated and cracks will propagate through
the material. As there is no way of knowing the value of the localised stress,
the strength is quoted as the average stress on the section and this is often
surprisingly small in comparison with the theoretical strength.
The second approach to fracture is different in that it treats the material as a
continuum rather than as an assembly of molecules. In this case it is recognised
that failure initiates at microscopic defects and the strength predictions are then
made on the basis of the stress system and the energy release processes around
developing cracks. From the measured strength values it is possible to estimate
the size of the inherent flaws which would have caused failure at this stress
level. In some cases the flaw size prediction is unrealistically large but in many
cases the predicted value agrees well with the size of the defects observed, or
suspected to exist in the material.
In this chapter the various approaches to the fracture of plastics are described
and specific causes such as impact loading, creep and fatigue are described in
detail.