Anomalous stress values

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Dear Siemens PLM Community,

 

I am quite confused by the Von-Mises stress obtained with a linear elastic analysis. The stress values developed for a displacement load of 5mm applied on a bent sheet of AISI 304 stainless steel, are more than the elastic limit of the material. Can someone please help me out in confirming the results? I am attaching the model file.

 

Thankfully,

Sabiq

9 REPLIES 9

Re: Anomalous stress values

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Did you check the units system consistency?
Seifeddine Naffoussi
SafeMecha Stress Engineering

Re: Anomalous stress values

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Pioneer

Dear @Seif 

 

Thank you for your reply. I prefer to use SI units. So I set the geometrical dimensions in meters (in File>Preferences). I applied the displacement load in meters. And also I input the material properties in SI units. But, I don't know if there are any other factors to be considered with the units.

 

Thanks,

Sabiq

Re: Anomalous stress values

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Material Young Modulus in Pa ?
Seifeddine Naffoussi
SafeMecha Stress Engineering

Re: Anomalous stress values

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Yes @Seif . I input the young's Modulus in Pa.

Re: Anomalous stress values

Gears Phenom Gears Phenom
Gears Phenom
You can achive any stress you want in linear static, even 10 times biger than ultimate stress. Linear solver "dont know" about plasticity, yeild or ultimate limits and materisl rupture.
You can attach .dat file because femap models created in student version can not be opened in other versions or you can attach some pictures and description of model.

Re: Anomalous stress values

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Dear @Karachun 

 

Thank you for reaching out. Unfortunately, I don't have a .dat format. But I am attaching the snapshots with relavent information. Please check if those details are sufficient to explain the analysis.

 

Thankfully,

Sabiq

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Re: Anomalous stress values

Gears Phenom Gears Phenom
Gears Phenom

Mesh is too coarse. Use plates or refine mesh. If you want to use solids anyway then set element size 1/3 or 1/4 of sheet thickness in all directions, element aspect ratio should be close to 1. Perform mesh convergence - reduce element size until stress level stop changing. In addition, try to use hexa instead of tetra.

Moreover, these measures do not guarantee that material do not yield - this may happen IRL. In this case, perform nonlinear analysis.

Read about mesh convergence/independence study, there are lot of articles on the internet.

Physics (material properties) is ok.

https://enterfea.com/correct-mesh-size-quick-guide/

https://knowledge.autodesk.com/support/simulation-mechanical/learn-explore/caas/sfdcarticles/sfdcart...

https://www.simscale.com/blog/2017/01/convergence-finite-element-analysis/

https://www.xceed-eng.com/finite-element-analysis-convergence-and-mesh-independence/

https://abaqus-docs.mit.edu/2017/English/SIMACAEGSARefMap/simagsa-c-ctmmeshconverg.htm

 

P.S. Give me five! I've thinking that I'm only one at Siemens comunity who use standard, non derived, SI units (kg, m, sec, N, Pa)

Re: Anomalous stress values

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Dear @Karachun 

 

Thank you very much for your detailed answer and valuable advices. I will study the parameters and aspects that you mentioned, to expand my knowledge in FEA, and thank you for the links.

 

Hope I have not done major mistakes with the unit system in the model.

 

I am glad that you are following the international standard units 🙏 But at the same time, I didn't know the standard units are not popular in the community.

 

Thank you very much.

 

Sabiq

Re: Anomalous stress values

Gears Phenom Gears Phenom
Gears Phenom

Depend from type of object you model. If you work with 300 m ship you can easy mess up with geometry using millimeters but in your case, for small plate, you can use more convenient set of units like ton, mm, sec, N, MPa.

Google "FEA consistent units" - there are lot of tables.

Consistent_Units_-_Stress_Analysis