Page 150 - Plastics Engineering

Mechanical Behaviour of Plastics 133

- *Yl
$
(3 ii

t
3 1%
0
1

cb a1 02
Crack depth
Fig. 2.69 Effect of varying stress field on flaw size for ductilehittle transition (K = constant)

,Plastic zone

Fig. 2.70 Extent of plastic zone at crack tip

The size of the plastic zone can be a useful parameter in assessing toughness
and so the ratio (K~,/O,.)~ has been defined as a ducrilityfactor. Table 2.2 gives
typical values of this for a range of plastics. Note that although the ratio used
in the ductility factor is conceptually related to plastic zone size, it utilises KI,.
This is to simplify the definition and to remove any ambiguity in relation to
the stress field conditions when related to the plastic zone size. It is important
that consistent strain rates are used to determine K1, and aY, particularly when
materials are being compared. For this reason the values in Table 2.2. should
not be regarded as definitive. They are given simply to illustrate typical orders
of magnitude.