Page 166 - Plastics Engineering

Mechanical Behaviour of Plastics 149

shows the triaxial stresses which exist at the tip of a notch. It is this triaxiality
which promotes brittleness in the material. Therefore, in practice one should
avoid abrupt changes in section, holes, notches, keyways etc at critical, highly
stressed areas in a moulding.
In the laboratory the impact behaviour of a material could be examined by
testing plain samples, but since brittle failures are of particular interest it is
more useful to ensure that the stress system is triaxial. This may be achieved
most conveniently by means of a notch in the sample. The choice of notch
depth and tip radius will affect the impact strengths observed. A sharp notch
is usually taken as 0.25 mm radius and a blunt notch as 2 mm radius.
Fig. 2.79 shows the typical variation of impact strength with notch tip radius
for several thermoplastics. The first important fact to be noted from this graph is
that the use of a sharp notch will rank the plastics materials in a different order
to that obtained using a blunt notch. This may be explained by considering the
total impact strength as consisting of both crack initiation and crack propagation

I
40

c polyethylene

30

N- E
2
c
P 20
g!
e
u)
'G
mp
-
E
10

-

-
0 ( 5 1 1.75
Notch tip radius (mm)

Fig. 2.79 Variation of impact strength with notch radius for several thermoplastics