Page 148 - Plastics Engineering

Mechanical Behaviour of Plastics 131

Solution The stress intensity factor for this configuration is

The information given in the question may be substituted directly into this
equation to give the bursting pressure, PB, as

1.0(8 x 10-3)(32 x
PB = = 5.6 MN/m2
3.0qi2 x 10-3)2(Ir x 3.5 x 10-3)1/2

2.19 General Fracture Behaviour of Plastics
If the defect or crack in the plastic is very blunt then the stress intensification
effect will be small and although failure will originate from the crack, the
failure stress based on the net section will correspond to the failure stress in
the uncracked material. If the stress on the material is based on the gross area
then what will be observed is a reduction in the failure stress which is directly
proportional to the size of the crack. This is shown as line A in Fig. 2.68.

"
oo Crack size ( 20 I
Fig. 2.68 Brittle and ductile failure characteristics for plastics

If, however, the defect or crack is sharp then the picture can change
significantly. Although ABS and MDPE are special cases, where the materials