34212The Effect of Corrosion of Fatigue Resistance

The full-scale fatigue experiments on which the fatigue rules are based were carried out in moist air and therefore reflect some degree of environmental effect or corrosion fatigue. Therefore, the lower-bound S-N curves in Figure 34.1 can be used for the design of details with a mildly corrosive environment (such as the environment for bridges, even if salt or other corrosive chemicals are used for deicing) or provided with suitable corrosion protection (galvanizing, other coating, or cathodic protection). Some design codes for offshore structures have reduced fatigue life (by approximately a factor of 2) when details are exposed to seawater [21,23].

At relatively high stress ranges at which most accelerated tests are conducted, the effect of seawater is clearly detrimental. However, there is evidence that the effect of corrosion in seawater is not so severe for long-life variable-amplitude fatigue of welded details. Full-scale fatigue experiments in seawater at realistic service stress ranges do not show significantly lower fatigue lives [24], provided that corrosion is not so severe that it causes pitting or significant section loss. At relatively low stress ranges near the CAFL, typical of service loading, it appears that build-up of corrosion product in the crack may increase crack closure and retard crack growth, at least enough to offset the increase that would otherwise occur due to the environmental effect. The fatigue lives seem to be more significantly affected by the stress concentration at the toe of welds than by the corrosive environment.

Severely corroded members may be evaluated as Category E details [25], regardless of their original category (unless of course they were Category E' or worse to start with). However, pitting or significant section loss from severe corrosion can lower that fatigue strength [25,26].

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