Page 445 - Mechanical Behavior of Materials

Section 9.6 Trends in S-N Curves 445

250
Air 30
S , Stress Amplitude, MPa 150 Al-7.5Zn-2.5Mg 10 S ksi
3% NaCl solution
200
a
20

100

50
a
m
0 S = 0 0
4 5 6 7 8
10 10 10 10 10
N , Cycles to Failure
f
Figure 9.28 Effect of a salt solution similar to seawater on the bending fatigue behavior of
an aluminum alloy. (Data from [Stubbington 61].)

9.6.2 Effects of Environment and Frequency of Cycling
Hostile chemical environments can accelerate the initiation and growth of fatigue cracks. One
mechanism is the development of corrosion pits, which then act as stress raisers. In other cases,
the environment causes cracks to grow faster by chemical reactions and dissolution of material at
the crack tip. For example, testing in a salt solution similar to seawater lowers the S-N curve of one
aluminum alloy as shown in Fig. 9.28.
Even the moisture and gases in air can act as a hostile environment, especially at high
temperature. Time-dependent deformation (creep) is also more likely at high temperature, and when
combined with cyclic loading, creep may have a synergistic effect that unexpectedly shortens the
life. In general, chemical or thermal effects are greater if more time is available for them to occur.
This leads to the fatigue life varying with frequency of cycling in such situations, the life in cycles
being shorter for slower frequencies. Such effects are evident in Fig. 9.29.
Polymers may increase in temperature during cyclic loading, as these materials often produce
considerable internal energy due to their viscoelastic deformation, which must be dissipated as heat.
The effect is compounded because such materials have a poor ability to conduct heat away to their
surroundings. A consequence of this is that the S-N curve is affected not only by frequency, but also
by specimen thickness, since thinner test specimens are more efficient at conducting their heat away.

9.6.3 Effects of Microstructure

Any change in the microstructure or surface condition has the potential of altering the S-N curve,
especially at long fatigue lives. In metals, resistance to fatigue is generally enhanced by reducing the
size of inclusions and voids, by small grain size, and by a dense network of dislocations. However,
special processing aimed at improvements due to microstructure may not be successful unless it
can be accomplished without substantially decreasing the ductility. Some S-N curves for brass
illustrating effects due to microstructure are shown in Fig. 9.30. In this material, a higher degree