Materials for engineering, 3rd Edition - (Malestrom)

172
Materials for engineering
10
Temperature (°C)
–200 –100 0 100 200 300
Normalized strength (strength/E 0 )
10 –1
10 –2
10 –3
10 –4
10 –5
Brittle fracture
1 s –1
10 –6 s –1 Crazing and
Cold
shear yielding
drawing
PMMA
E 0 = 8.57 GPa
Tg = 378 K
Contours of
strain rate
10 –6 s –1
Viscous
flow
1 s –1
Decomposition
10 3
10 2
10 1
10
Strength (MPa)
10 –1
0 0.4 0.8 1.2 1.6
Normalized temperature (T/T g )
5.8 Deformation map for PMMA (After M. F. Ashby and D. R. H.
Jones, Engineering Materials 2, Oxford, Pergamon Press, 1986).
by drawing and eventually by viscous flow. We have already discussed how
the properties of polymers are strain-rate dependent and Fig. 5.8 shows
contours of constant strain-rate, so the deformation map shows how the
strength varies with temperature and strain-rate.
Environment-assisted cracking in polymers
Environmental stress cracking (ESC) is an important cause of embrittlement
in plastics. Many polymers are strongly affected by environments such as
water, vapours or organic liquids in an analogous way to the stress-corrosion
cracking of metals described in Chapter 2. A stressed plastic will fail under
these conditions when it would not be expected to do so in the absence of the
sensitizing environment. Again, in the absence of applied stress, the effect of
the environment alone would not be deleterious, and a critical strain can be
determined below which ESC does not occur. The phenomenon is thus
synergistic in nature. Internal stresses resulting from moulding or welding
processes may be sufficient to cause problems.
The most serious example of ESC is the oxidative cracking of rubber. The
‘perishing’ of both natural and synthetic rubbers arises from interaction
with the ordinary atmosphere, often accelerated by ultraviolet radiation. The
presence of carbon black or other special additives alleviate the effect, since