Fatigue verification of steel section exposed to corrosion 1

[CivilEngineerUK]

Do anyone have experience with fatigue verification of a steel (S355) section exposed to corrosion? Will the detail category decrease due to the corrosion.
I can't find anything in Eurocode describing this subject?

 

[r13]

I think you need to take a thickness reading/mapping to determine the useful thickness, the perform the fatigue analysis as usual.

 

[CivilEngineerUK]

Eurocode for fatigue states that it's only valid (stated detailed category) if the steel is protected by coating, stainless steel or weathering steel. Corrosion may result in notches. In DNV it is stated that rust putting reduces the detail category with 2 classes. Therefore, proper corrosion (let's say 5mm) must be even worse? Or maybe I'm wrong.

 

[HTURKAK]

I have copied and pasted relevant section of Eurocode 3: Part 1-9: Fatigue

IMO, the code is clear ..the structure subject to fatigue loading should have sufficient corrosion protection and regular maintenance.


I think it should be rust pitting. ( A.1 Non-welded details For members that can acquire stress concentrations due to rust pitting etc. curve C is required.).

The RECOMMENDED PRACTICE DNV-RP-C203 proposes a different S-N curve For high strength steel with yield strength above 500 MPa and also you can see the proposed S-N curves for corrosion .

and a reference book (Reducing Brittle and Fatigue Failures in Steel Structures by Peter Maranian sponsored by ASCE )
(...High strength steels, which may be considered to be 790 MPa to 1000 MPa , can be particularly susceptible and may be subject to brittle behavior..)

Hence, seems nothing special for steel (S355) .

 

[CivilEngineerUK]

I have also looked in DNV. Is rust pitting and 120 years corrosion the same? The DNV describe a SN curve for free corrosion in saltwater, maybe that should be used even through the steel section is subjected to corrosion in soil and air?

 

[dhengr]

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.

 

[HTURKAK]

Rust pitting or pitting corrosion , sometimes called "crevice corrosion" localized corrosion in the shape of a pit . These local pits are important factor for stress concentration.

DNV-RP-C203 is for fatigue design of offshore steel structures and proposed design life limited with 50 years and for the design charts

120 years of design life is very long period. I will suggest you provide corrosion allowance , corrosion protection and follow the standard BS EN 1993-1-9:2005.

 

[CivilEngineerUK]

@HTURKAK: It is a steel post for a noise barrier, subjected aerodynamic loads. Do you know if "normal" corrosion is worse compared to rust pitting?

 

[HTURKAK]

IMHO, for carbon steel , the surface corrosion is more probable and pitting corrosion does not occur at atmospheric conditions .In general pitting corrosion is a problem for SS material. I will suggest you to search the threads for pitting corrosion to get an idea.


Fatigue is a problem for tall stacks, slender masts due to to Vortex Shedding Loads. If a tall slender mast is subjected to a
steady wind, vortices will cause swaying oscillations in a direction normal to the wind and if the vortex shedding frequency is resonant with the natural frequency of the stack , it may result in large vibrations and fatigue will be a serious issue.. For your case, fatigue could be a concern for the anchor bolts and pretensionning will eliminate this problem.

For pitting corrosion, pls look thread338-443782
and for fatigue ,thread330-128281