Lifting lug FEA results vs hand calculation

[JR_Inn]

Hello,

I'm designing a lift lug for 25000lbs. Minimum required factor of safety is 3. My FEA results show maximum stress 19.89ksi - way to high for A36 material.

I used a bearing force in Inventor and tried the same in Solidworks with identical results.

I did some simple hand calcs and stress was significantly lower.

Then I found this calculator:

https://mechanicalc.com/calculators/lifting-lug-analysis/#divResults

This calculator is using an Air Force Method and my lug would pass with safety factor grater than 3.

I'm fairly new to FEA so I did some googling and found this video talking about St. Venant's Principle:

https://www.youtube.com/watch?v=TgpM6xOTfuE

Could anybody comment on this? 

Thanks

[JDMather]

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johnes.ruml wrote:

Could anybody comment on this?  

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No Inventor file attached?

No hand calcs attached?

[MSD_takaseh]

Hello johnes.ruml,

I am not familiar with Air Force Method but I think Air Force Method and Finite Element Analysis are different solver, so the result could be different.
If the result in Inventor is suspicious for you then please check the "Convergence Plot". And then if the analysis is not convergence then increase value of "Maximum Number of h-Refinements" in the "Convergence Settings" and run analysis again.

Thanks.

[JR_Inn]

Sorry for not including the files. 

For clarification, the lug is part of a safety device and the force will be applied only in normal direction. 

As of my had calc, I did only tension stress in the weakest section of the lug. I don't have it in a formal format so I just type it here:

Lug height: 6.5"

Lug thickness: 1.5"

Pin diameter: 3"

Force: 25000lbf

Cross sectional area: (6.5 - 3) x 1.5 = 5.25 in^2

Stress in this area: 25000 / 5.25 = 4761.9 psi

My question was more about the youtube video and St. Venants principle, I didn't find anywhere else similar approach to FEA results. 

I also tried the same in Nastran in Cad, and I'm getting a different results. Around 11 ksi in same area as Inventor reported 19.9ksi. 

The model that I uploaded includes both FEAs.

[MechMachineMan]

Nvm. Thought FEA files weren't attached because a file was missing, but the important FEA files appear to be embedded.

[JDMather]

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@JR_Inn wrote:

Cross sectional area: (6.5 - 3) x 1.5 = 5.25 in^2 

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It appears that you are calculating the average stress on a cross-sectional area, while the software is returning results for localized stress rather than across a cross-section of the part.

Calculate Stress

The analysis stress is highly dependent on the mesh.  Examine though that the highest stress is localized to a small volume of the part.  A few Probes away from this localized stress indicates much higher safety factor (and keep in mind that safety factor is not a measure of "safety" - it only means the part would permanently deform if less than one - not an indication of fracture.

Now, let's take a look at that actual deformation.  Max .002"  about half the thickness of a sheet of paper.  You should find that the Deformation reported across different software programs is consistent  (not so sensitive to the mesh). 

Finally, if we look at the Stress XX (+x and -x directions) we can see where the material is in compression and tension along that direction.

[JR_Inn]

JDMather thank you for your comment. I really appreciate your help.

I am aware that I calculated the average stress, and I am not trying to compare it with the local stress that FEA reports. 

I saw a lot of FEA results on the internet and most of them would say my part doesn't pass the safety factor of 3 so it needs some design changes. 

Could you please explain why Nastran In Cad reports significantly different result in comparison to Inventor or Solidworks. You mentioned that results are mesh sensitive. I understand that, I tried to create the mesh same across all 3 simulations. 

I tried some other simple simulations in the past to see differences among software and the results were almost identical.  

[JDMather]

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@JR_Inn wrote:

1. I saw a lot of FEA results on the internet and most of them would say my part doesn't pass the safety factor of 3 so it needs some design changes. 

2. Could you please explain why Nastran In Cad reports significantly different result in comparison to Inventor or Solidworks. You mentioned that results are mesh sensitive. I understand that, I tried to create the mesh same across all 3 simulations. .  

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1. Can you cite sources?  (And please - provide the complete information here so that I, or others, don't have to reproduce the analysis.  For example - I do not have an account on the Lug Calculator link that you referenced earlier and have no desire to register on that site.)

2. I haven't take the time (yet) to run Convergence Analysis in Inventor and SolidWorks and Nastran, but just casual observation I can see that the mesh in Nastran is quite different.

I expect that once I find time to run Convergence Analysis that I will get roughly equivalent results.

Have you run any physical tests to validate your digital analysis model?

Do you have access to the physical testing equipment?

[swalton · Accepted solution]

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@JR_Inn wrote:

JDMather thank you for your comment. I really appreciate your help.

I am aware that I calculated the average stress, and I am not trying to compare it with the local stress that FEA reports. 

I saw a lot of FEA results on the internet and most of them would say my part doesn't pass the safety factor of 3 so it needs some design changes. 

Could you please explain why Nastran In Cad reports significantly different result in comparison to Inventor or Solidworks. You mentioned that results are mesh sensitive. I understand that, I tried to create the mesh same across all 3 simulations. 

I tried some other simple simulations in the past to see differences among software and the results were almost identical.  

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There seems to be an issue with the Nastran In-Cad 2018 bearing load.  See https://forums.autodesk.com/t5/nastran-in-cad-forum/bearing-vs-force/td-p/7260827

I am leery of any Nastran In-Cad 2018 bearing load results until after Autodesk releases a patch.

[JR_Inn]

Here is one example: 

https://www.youtube.com/watch?v=zuAQ-4kHzng

The presenter judges the part based on a surface stress very similar to my lug. 

Most tutorials are talking about singularity and stress concentrations, but I didn't find any source stating that stress should be also judged by how deeply is penetrating the surface.

I don't have an account on the Lug calculator either, but they have two materials for free. I should have saved the results and uploaded them here. 

Unfortunately, I don't have access to a physical testing equipment. 

[Anonymous]

Hi, 

Have you published your work? I would like to read and use it if you have funding for your modeling. 

Regards

[Anonymous]

@johnes.ruml 

Any published work for this model?

[JDMather]

https://www.youtube.com/playlist?list=PLp5izJt_zvN0KUccGh5OrgbNTZ_c4vYxs

Finite Element Analysis for the Casual User in Inventor | Autodesk University

Be sure to let me know if you find any errors so that I can resolve them for future work.

[cadman777]

I'm no engineer.

But when I designed things like this, I used Roark's Formulas for Stress and Strain.

Then we'd send out the design to an engineer to sign-off on it or mark it up.

I think only one time did one of my designs come back with minor markups over the course of 30 years.