Local Vs global stress and fatigue analysis in fea analysis

[krzychu1904]

Hello
Hello,

I have three questions about FEA analysis according to EN 13445 (though I believe these issues are similar in ASME).

The first one: during stress categorization, local and global stresses should be distinguished. Do you know of any simple method for doing this? I read that there is a formula to set the radius where local stress occurs. But how can I interpret this in ANSYS? I’ve seen some people create new edges around discontinuities, but if you have dozens of junctions, it becomes very time-consuming.

AND I would also like to ask about singularities at the place of geometric discontinuities. in these places there is a greater yield point, (F*1,5 or f*3) however when we make a mesh refirment, the stresses tend to infinity. When doing calculations according to the norm, can we ignore the peak stress as a singularity? Or should we rigidly stick to the established yield point, even if one node exceeds it. I mean ignoring it due to Saint-Venant rule.

The last question is simpler: when should I perform fatigue analysis? In the table with assessment criteria, it states "only if required."

 

[TGS4]

I can speak in the context of the ASME rules. Regarding global vs local - I would assume that you are talking P_m vs P_L. P_m is a hand calc. Never use FEA to calculate a P_m. And as for answering the question about how local is local - there are limits on the extent - read the rules.

Singularities are handled by the stress liberalization process.

Please read and then reread paragraph 5.2.1.2 of ASME Section VIII, Division 2, Part 5 when it comes to stress linearization and categorization.

Fatigue analysis is performed when a fatigue screening assessment does not pass.

 

[RadiateurFou]

TGS4 could you elaborate on why Pm is a hand calc only?

 

[TGS4]

The definition of P_m is (from 5.2.2.2(a)(1)) "is the equivalent stress, derived from the average value across the thickness of a section, of the general primary stresses produced by the design internal pressure and other specified mechanical loads but excluding all secondary and peak stresses". What is that but P*r/t, M*c/I, and F/A? It's a hand calc.

 

[RadiateurFou]

I agree but for specific geometries like the nozzle/header assembly of my previous post subjected to external loads, i think it will be difficult with just an hand calc