CivilEngineerUK:
You would do well to do a little studying and reading on fatigue and corrosion, on your own. There are some good texts and papers on the subjects, dig them up. These two topics are complex enough that they are hard to codify or put down in some clean/clear flow chart or some such. Fatigue involves cyclic loading in a destructive stress range, and its study usually starts with testing a pristine sample which tries to emulate an actual, real world condition. But, the real world conditions which exist, so quickly deviate from the test assumptions, that a good deal of engineering experience and judgement is involved/needed in making real world interpretations of test results and design decisions. It tries to account for various known stress raisers and other anomalies which disrupt normal stress flow patterns, but again, quickly starts to fall short of real world conditions. Corrosion, of course, starts by reducing the net section of a member, which increases the stresses in the member under a cyclic situation. It causes pitting, which is a stress raiser itself, and enlarges cracks and other stress raisers and can start to alter the steel properties in detrimental ways. These are just not things which are easily codified or tabulated, thus the kinda indefinite treatment of the subjects by the codes. Everything we do as engineers is not covered by a code paragraph, a design guide and some worked examples. Sometimes, with some problems, we have to use our engineering experience and best judgement to proceed, with caution.
