Page 153 - Plastics Engineering

136 Mechanical Behaviour of Plastics

fracture line cuts across the isometric lines. It may also be seen that whitening
or crazing occur at lower strains when the stress is low.
Many attempts have been made to obtain mathematical expressions which
describe the time dependence of the strength of plastics. Since for many plastics
a plot of stress, 0, against the logarithm of time to failure, tf, is approximately
a straight line, one of the most common expressions used is of the form

tf =Ae-B" (2.112)
where A and B are nominally constants although in reality they depend on such
things as the structure of the material and on the temperature. Some typical
values for A and B at 20°C are given below. It is recommended that the material
manufacturers should be consulted to obtain values for particular grades of their
materials.

Acrylic Polypropylene
Sheet Moulded Homopolymer Copolymer

B(rn2/hfN) 0.404 0.42 0.88 1.19

It is recommended that the material manufacturers should be consulted to
obtain values for particular grades of their materials.
One of the most successful attempts to include the effects of temperature
in a relatively simple expression similar to the one above, has been made by
Zhurkov and Bueche using an equation of the form

(2.1 13)

where '0 is a constant which is approximately s for most plastics
UO is the activation energy of the fracture process
y is a coefficient which depends on the structure of the material
R is the molar gas constant (= 8.314 J/mol" K)
and T is the absolute temperature.
If the values for Uo and y for the material are not known then a series
of creep rupture tests at a fixed temperature would permit these values to be
determined from the above expression. The times to failure at other stresses
and temperatures could then be predicted.

2.20.1 Fracture Mechanics Approach to Creep Fracture
Fracture mechanics has also been used to predict failure under static stresses.
The basis of this is that observed crack growth rates have been found to be