Primary/Secondary stress or Fatigue analysis

[Irwin]

Dear Everybody,

I would like to know in which case should I use primary/secondary analysis method and in which case the fatigue SN curve analysis during a pressure vessel analysis?

Irwin

 

[rb1957]

?

"primary/secondary analysis method" ... never heard of this term

"in which case the fatigue SN curve analysis" ... if you're asking what load case to apply for fatigue anaysis ? that'd be the duty cycle

 

[Irwin]

Maybe I was not accurate in the question.
In the DIN EN13445 standard there is a description about primary stress and secondary stress analysis on pressure vessel structure in the Annex C. In the Chapter 17 and 18 you can find a description about a fatigue analysis method also for the pressure vessel structures. What is the preconditions of these analysis? If we have bad results from the comparison of the limits with primary and secondary stresses in this case we should use one of a fatigue analysis methods?

E

 

[rb1957]

i don't know your reference, but it sounds like it's a standard defining required loads; and it sounds like your pressure vessel doesn't stand up to these loads ??

if that's the case, then i don't think you need a fatigue analysis, i think you need more material ...

but maybe the standard is for infinite life, in which case a fatigue analysis would define a safe (limited) life.

maybe someone more knowledgable will chime in ...

 

[TGS4]

Do you actually understand the failure modes that the primary/secondary stress analyses are protecting against? If something fails the primary stress limits, you no longer have the required margin against it suffering from an unrestrained plastic deformation (aka blowing up). If something fails the secondary stress limits, upon repeated application of the load, it will ratchet. If something fails the fatigue analysis, after exceeding the prescribed number of load cycles, you no longer have the required margin against a through-thickness crack forming (may be either initiation or complete through-thickness, depending on the version of the fatigue analysis).

Each of these is a separate failure mode. Per EN13445, you have to satisfy ALL of the limits. It really doesn't do you much good to have a vessel or part that won't suffer from a fatigue failure, because it has already blown up.

 

[corus]

EN13445 is the european standard for pressure vessel design life, that took over the old BS 5500 British Standard. There's an ASME equivalent somewhere too. You really need to read the standard properly, or books on the design philosophy behind it.

Primary/Secondary/Fatigue are just classifications of the stress components. If you have no cyclic loading then clearly you don't need to consider fatigue damage. You always need to separate primary from secondary stresses however as each have their own stress limits.

Tata

 

[Irwin]

Do I understand well, if I have static load in this case I should use primary, secondary analysis method, if I have dynamic load I have to post-process everything according to the fatigue method?

Ervin

 

[TGS4]

Almost. If you have a static load, then you have to satisfy the primary and secondary stress limits. If you have a cyclic load, then you have to satisfy the primary and secondary stress limits, as well as the fatigue limits.

 

[Irwin]

An example:
I have the following loads:
- gravity (1g)
- temperature
- extrem wind load
- extrem horizontal acceleration (+-0.16g)
- extrem vertical acceleration (+-0,23g)

How should I define load combinations and limits?

Do you have any addvice?

E

 

[EngAddict]

Have you read through the pressure vessel standards? You don't apply wind and earthquake loads at the same time. Some client specifications also state required load combinations. These can't be a reduction on what is in the code but a supplement to it.

 

[Irwin]

So what is your suggestion?
Gravity and Temperature load for simple primary and secondary load approach and independently fatigue analysis for wind and independently fatigue for the acceleration loads?
Or something else?
Anyway: In which chapter I can find this info what you mentioned?

I

 

[cbrn]

Hi,
combinations are suggested by logic...
Can you switch off gravity? I suppose not. So it is a permanent, continuous load. It will be present with full intensity and load multiplier 1.
You are dealing with a pressure vessel: where is your pressure load? Is it constant? Can it vary? It will be your main service load, I presume, so there are appropriate load factors for operating loadcases as well as for exceptional loadcases.
And so on...
I'd suggest you better read the EN 13445 Design By Analysis section, all the relevant information and procedures are herein. As EN 13445 is "thought" for a Limit-States approach, you could complement your study with some book describing the approach itself, or with the Eurocodes themselves where all the information / procedures are described.
Also pay attention to the "new" non-linear approaches known as the "Direct Route", as they can offer you savings in material and most of all a better comprehension of the "real" behaviour of your component under extreme loads (see GPD check, PD check, etc...). This will require you understand the Melan theorem and the Deviatoric Map. Not easy, but extremely useful (as long as the Customer doesn't "force" you to use an ASME-like approach)..

Regards!

 

[rb1957]

irwin, i'm surprised you didn't list pressure as one of your loads ?

my 2c ... what loads can happen at the same time ? gravity as you've said, is always acting. wind and earthquake appear to be extreme loads, and probably should be considered as special cases (with gravity). in-service loads seem to be your lateral and vertical accel ... which would also combine with gravity. fatigue would require your typical in-service duty cycle. and pressure needs to be there somewhere.

once you've decided how to combine your loads you need to be aware that different loads have different safety factors (well they do in aero.).