Page 169 - Plastics Engineering

152 Mechanical Behaviour of Plastics

2.22.3 Miscellaneous Factors Affecting Impact
Other factors which can affect impact behaviour are fabrication defects such as
internal voids, inclusions and additives such as pigments, all of which can cause
stress concentrations within the material. In addition, internal welds caused
by the fusion of partially cooled melt fronts usually turn out to be areas of
weakness. The environment may also affect impact behaviour. Plastics exposed
to sunlight and weathering for prolonged periods tend to become embrittled due
to degradation. Alternatively if the plastic is in the vicinity of a fluid which
attacks it, then the crack initiation energy may be reduced. Some plastics are
affected by very simple fluids e.g. domestic heating oils act as plasticisers for
polyethylene. The effect which water can have on the impact behaviour of
nylon is also spectacular as illustrated in Fig. 2.80.
The surface finish of the specimen may also affect impact behaviour.
Machined surfaces usually have tool marks which act as stress concentrations
whereas moulded surfaces have a characteristic skin which can offer some
protection against crack initiation. If the moulded surface is scratched, then
this protection no longer exists. In addition, mouldings occasionally have an
embossed surface for decorative effect and tests have shown that this can cause
a considerable reduction in impact strength compared to a plain surface.

2.22.4 Impact Test Methods
The main causes of brittleness in materials are known to be
(1) triaxiality of stress
(2) high strain rates, and
(3) low temperatures.
In order to provide information on the impact behaviour of materials, metal-
lurgists developed tests methods which involved striking a notched bar with a
pendulum. This conveniently subjected the material to triaxiality of stress (at
the notch tip) and a high strain rate so as to encourage brittle failures. The
standard test methods are the Izod and Charpy tests which use the test proce-
dures illustrated in Fig. 2.81(a) and (b). The specimens have a standard notch
machined in them and the impact energy absorbed in breaking the specimen is
recorded. With the ever-increasing use of plastics in engineering applications it
seemed appropriate that these well established test methods should be adopted.
However, even the metallurgists recognised that the tests do have certain short-
comings. The main problem is that the test conditions are arbitrary. The speed
of impact, method of stressing and specimen geometry are fixed and experience
has shown that it was too much to expect the results to be representative of
material behaviour under different conditions.
In particular, standard specimens contain a sharp notch so that it is propagation
energy rather than initiation energy which is the dominant factor. In general the
standard tests are useful for quality control and specification purposes but not