Fatigue process using FEM with BS standard 1

[anhken]

Hi everyone,
I am conducting a simulation to calculate stress in order to estimate fatigue of a 3D structure.
In Bs code standard I used, they use nominal stress or hot spot principle stress to calculate stress range and then use it to s-n curve, however, as far as I know, FEM cannot calculate nominal stress (tensor stress, principle stress, .. are ok).
Is that right?
The second question is whether the hot spot principle stress relates to principle stress ?.
I am a very new guy in FEM, thank you so much.

 

[rothers]

What do you mean by nominal stress ?

 

[anhken]

Hi Rothers,
Thanks for your reply.

"Nominal stress" I said is the type of stress Bs 7608 Standard used to get stress range (nominal stress = F/A Link).
However, using the FEM method, I get only stress tensor, principal tensor but nominal stress.
So how can I pass this obstacle?

 

[FEA way]

I don't know this code but if they mean nominal stress as opposed to true stress, in FEA you will get the latter which takes the deformation into account. Nominal stress is based on the initial state of the geometry. Up to some point on the stress-strain curve they won't differ that much.

 

[ThomasH]

Hi

I think that "nominal stress" in this context (fatigue) is the same as principal stress. It is a pure tensile or compressive stress. Is is not the same as von Mises stress where shear also in included.

Edit: Shear can also be a nominal stress. But then it is pure shear.

/Thomas

 

[rb1957]

"nominal" stress seems to be like "gross section" stress.

This can be a problem with FEA, in that it'll give you the stress peak due to the stress concentration (or something like it). So you need to look at elements near the point of interest, say an element away from the concentration.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[ThomasH]

Hi
If I don't remember it wrong the "nominal stress method" and the "hot spot method" both have specific requirements regarding the FEM-modelling. There are requirements regarding meshing and where exactly you should pick the stresses in the model. The models are typically made with volume elements.

I have never used them myself but I had a colleague who used them. And he had done research on fatigue.

Thomas

 

[ThomasH]

Hi ankhen

One additional comment. You refered to BS code Standard. I assumed that you mean British Standard and that means Eurocode. If your reference is something else some parts of my comments may be incorrect. Even if it is Eurocode I won't bet my life on it but I think my comments are correct .

Thomas

 

[SPDL310]

Hi ankhen,

There should be some sort of guidance or annex in you code about using this method, and if not there should be some sort of example problems available. You didn't name the exact code you are using so couldn't search for any example problems, but here are some articles on the Eurocode hotspot method that I did find. I hope they are helpful to you endeavors.

https://www.sciencedirect.com/science/article/abs/pii/S0142112308001072

https://www.sciencedirect.com/scien...ly, the hot spot is,b) as illustrated in Fig.

https://www.twi-global.com/technica...ves-for-fatigue-assessment-of-fpsos-june-2001