Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 173
they preferentially absorb the harmful radiation and convert it to thermal
energy.
The ESC originates from microscopic surface defects where the active
medium interacts with the high stress region at the crack tip. The material
becomes locally weakened and the crack spreads. The commonest examples
are those involving amorphous polymers in contact with organic solvents
(e.g. polycarbonate in contact with low molecular weight hydrocarbons such
as acetone), although detergents can cause ESC failure when in contact with
semi-crystalline materials such as polythene. In polymers which craze under
stress, the liquid environment enters the crazes and penetrates between the
molecular chains, giving rise to a swelling of the structure.
This local swelling effect of the environment reduces the amount of external
work necessary for fracture to about 0.1 J m –2 , which is of the order of the
chemical bond energy and several orders of magnitude lower than that required
to fracture most tough plastics in the absence of ESC. The rate-controlling
mechanism for the cracking process appears to be the diffusion of the deleterious
medium in the polymer structure, the process being assisted by the applied
stress. Resistance to ESC can usually be enhanced by using material of
higher molecular mass, so that longer molecular chains bridge the craze and
inhibit the growth of a crack within it.
The propensity for ESC is often assessed by empirical standard tests. One
simple test involves immersing bent strips of plastic in a particular medium
and, subsequently, to examine them for signs of defects. Among the parameters
that can be measured is the time to initiate visible damage such as crazing.
Fatigue
When subjected to fluctuating strains, polymers may fracture by fatigue,
although, in contrast to the behaviour of metals, there are two processes by
which the failure may occur. One process is analogous to that encountered in
metallic materials, in that fatigue cracks may initiate and propagate to final
failure. The other process arises from the hysteretic energy generated during
each loading cycle. Since this energy is dissipated in the form of heat, a
temperature rise will take place when isothermal conditions are not met.
This temperature rise can lead to melting of the polymer and failure of the
component occurs essentially by viscous flow.
Thermal fatigue
Consider a polymer subjected to a sinusoidal variation of cyclic stress, σ =
σ 0 sin ωt, where σ is the stress at time t, σ 0 is the peak stress and ω = 2πf,
f being the test frequency. The viscoelastic response of the polymer implies
that the corresponding variation in strain will be out of phase with the stress