Page 154 - Plastics Engineering

Mechanical Behaviour of Plastics 137

related to the stress intensity factor K by the following equation
da
-= CIK" (2.1 14)
dt
where C1 and rn are material constants.
Now using equation (2.100) we may write

If the material contains defects of size (hi) fai-dre occurs w...en these
and
reach a size (h,) then the time to failure, tf, may be obtained by integrating
the above equation.

Although equations (2.1 12), (2.1 13) and (2.1 15) can be useful they must not
be used indiscriminately. For example, they are seldom accurate at short times
but this is not a major wony since such short-time failures are usually not of
practical interest. At long times there can also be inaccurate due to the embrit-
tlement problem referred to earlier. In practice therefore it is generally advisable
to use the equations in combination with safety factors as recommended by the
appropriate National Standard.

2.20.2 Crazing in Plastics
When a tensile stress is applied to an amorphous (glassy) plastic, such as
polystyrene, crazes may be observed to occur before fracture. Crazes are like
cracks in the sense that they are wedge shaped and form perpendicular to the
applied stress. However, they may be differentiated from cracks by the fact
that they contain polymeric material which is stretched in a highly oriented
manner perpendicular to the plane of the craze, i.e. parallel to the applied stress
direction. Another major distinguishing feature is that unlike cracks, they are
able to bear stress. Under static loading, the strain at which crazes start to
form, decreases as the applied stress decreases. In constant strain rate testing
the crazes always start to form at a well defined stress level. Of course, as with
all aspects of the behaviour of plastics other factors such as temperature will
influence the levels of stress and strain involved. Even a relatively low stress
may induce crazing after a period of time, although in some glassy plastics there
is a lower stress limit below which crazes will never occur. This is clearly an
important stress for design considerations. However, the presence of certain
liquids (organic solvents) can initiate crazing at stresses far below this lower
stress limit. This phenomenon of solvent crazing has been the cause of many
service failures because it is usually impossible to foresee every environment
in which a plastic article will be used.