Page 171 - Plastics Engineering

154 Mechanical Behaviour of Plastics
Occasionally the less satisfactory term of energy to break per unit width may
be quoted in units of J/m.
In some applications impact performance may not be critical and only a
general knowledge of materials behaviour is needed. In these circumstances
it would be unrealistic to expect the designer to sift through all the combina-
tion of multi-point data. Therefore diagrams such as Fig. 2.82 can be useful
for providing an overall indication of the general impact performance to be
expected from different plastics. However, this type of general guide should be
used with caution because it oversimplifies in at least two important respects.
It ignores the plane stresdplane strain toughness transition which causes the
order of merit to depend on the material thickness. Also it ignores the effect of
molecular orientation except insofar as this particular diagram refers to speci-
mens cut from one sort of moulding.

Fig. 2.82 Comparison of impact strengths as measured by Chqy test

2.225 Fracture Mechanics Approach to Impact
In recent years impact testing of plastics has been rationalised to a certain
extent by the use of fracture mechanics. The most successful results have been
achieved by assuming that LEFM assumptions (bulk linear elastic behaviour
and presence of sharp notch) apply during the Izod and Charpy testing of a
plastic.