LOCAL PLASTICITY - Fractal Mechanics of Polymers - page 109

It is well-known [31], that stress concentration at notch tip intensification
results to HDPE samples strength decrease in impact tests. The stress con-
centration coefficient K s for the notch of length a can be estimated according
to the Noiber formula [4], which is similar to the Eq. (5.10):

1/ 2

a

, (5.14)

K =+ 12

s

R

where R is notch tip radius.
Despite the fact, that HDPE testing samples have nominally a sharp
notch, it is well-known [37], that local plasticity zone availability results
to a sharp notch blunting and as the first approximation the value R can be
accepted equal to notch critical opening d c [37], determined according to
Dugdale model [4]:

2
f

ps a

d =

(5.15)

c

loc
Y

E

s

FIGURE 5.9 The relation of local shear deformation zone length r p and plastic constraint
factor k cons at testing temperature: 293 (1), 313 (2), 333 (3) and 353 K (4) for HDPE [32].

In Fig. 5.10 , the dependence of stress concentration coefficient K s on
value

1/2
cons

k

is shown, which is linear and passes through coordinates ori-