Page 164 - Plastics Engineering

Mechanical Behaviour of Plastics 147

Assuming n # 2

(2.119)

The way in which this sort of approach may be used to design articles
subjected to fatigue loading is illustrated in the following example.
Example 2.22 A certain grade of acrylic has a K, value of 1.6 MN m-3/2
and the fatigue crack growth data as shown in Fig. 2.77. If a moulding in
this material is subjected to a stress cycle which varies from 0 to 15 MN/m2,
estimate the maximum internal flaw size which can be tolerated if the fatigue
endurance is to be at least 16 cycles.
Solution The first step is to calculate the critical flaw size which will cause
brittle failure to occur in one cycle. This may be obtained from equation (2.100)
assuming Y = 1.
K = u(xu)'/*
or
21 21
-
n
=
a, = ($) ; (s) = 3.62 x m
During cyclic loading, any cracks in the material will propagate until they
reach this critical size. If the article is to have an endurance of at least lo5
cycles then equation (2.1 19) may be used to determine the size of the smallest
flaw which can be present in the material before cycling commences.

Using C2 = 1.8 x and n = 3.315 from Fig. 2.77 then ai = 1.67 pm.
Therefore the inspection procedures must ensure that there are no defects larger
than (2 x 1.67) = 3.34 pm in the material before the cyclic stress is applied.

2.22 Impact Behaviour of Plastics
The resistance to impact is one of the key properties of materials. The ability
of a material to withstand accidental knocks can decide its success or failure
in a particular application. It is ironical therefore that for plastics this is one of
the least well defined properties. Although impact test data is widely quoted
in the literature, most of it is of little value because impact strength is not
an inherent material property and hence it is not possible to specify a unique
universal value for the impact strength of any plastic. Impact strength depends
on a range of variables including temperature, straining rate, stress system,
anisotropy, geometry of the article, fabrication conditions, environment and so
on. As a result of this there is often a poor correlation (a) between laboratory