Environmental Engineering Reference
In-Depth Information
Figure 3.57
Fatigue striations in aluminum alloy 2024-T851 (2000
).
open for the entire cycle and the attack is aggravated in contrast to the situation
when the mean stress is zero, as in the case of a sinusoidally varying stress of
equal amplitude. The situation is further aggravated if a static tensile stress is
superimposed on the cyclic stress. The mean stress in such a case is raised to a
higher tensile value.
Cyclic stresses of low frequency and high amplitude lead to greater crack
propagation per cycle. This arises from the fact that such conditions allow longer
interaction between material and environment. However, if the frequency is high,
the crack tip may not get the chance to be exposed to the environment, and the
corrosion fatigue behavior becomes one like the mechanical fatigue behavior in
an inert environment.
The presence of stress raisers, such as notches or surface roughness, increases
susceptibility to corrosion fatigue. The fatigue limit decreases sharply (Fig. 3.58).
Corrosion pits also act as stress raisers and cracks have often been observed to
initiate from corrosion pits.
Environmental Factors
Unlike SCC, corrosion fatigue is not restricted to specific environments. Any
environment causing general attack in a metal or alloy beyond a certain minimum
corrosion rate is capable of causing corrosion fatigue. For steels, the minimum
corrosion rate required is about 1 mpy [53]. Thus corrosion fatigue of steel is
encountered in varied environments like freshwater, seawater, combustion prod-
uct condensates, and many chemicals.
The fatigue strength or fatigue life of a material decreases sharply when ex-
