Page 145 - Plastics Engineering

128 Mechanical Behaviour of Plastics

Normal stnsro
f t t

hriation of uy
along x axis (O=O)

Fig. 2.66 Stress distribution in vicinity of a crack

defined as
K = o(Ru)'/~
The stress intensity factor is a means of charactensing the elastic stress
distribution near the crack tip but in itself has no physical reality. It has units
of MN m-3/2 and should not be confused with the elastic stress concentration
factor (K,) referred to earlier.
In order to extend the applicability of LEFM beyond the case of a central
crack in an infinite plate, K is usually expressed in the more general form

K = Yo(Ru)'/~ (2. loo)

where Y is a geometry factor and 'u' is the half length of a central crack or
the full length of an edge crack.
Fig. 2.67 shows some crack configurations of practical interest and expres-
sions for K are as follows.
(a) Central crack of length 2u in a sheet of finite width

(2.101)

(b) Edge cracks in a plate of finite width

(2.102)