7 Replies Latest reply on Nov 25, 2014 2:17 PM by Jared Conway

Stress Values Not Corresponding to Published Values

Cory Nation Nov 17, 2014 6:49 PM

I am conducting an analysis on a simple part to determine the stress concentration factor of a shaft with shoulder fillet in torsion. I have referenced Pilkey's Stress Concentration factors and other texts and the values I am achieving from solidworks are much higher than what is published. I have referenced a youtube video and the same issue occurs.

Torsional Static Analysis and Design Study Example Using Solidworks - YouTube

If you go to 5:01 of the video, the author shows a Kts table to relate his model to. When he finishes his analysis, instead of having Kt values of 1.48, 1.25, 1.17, and 1.13 he gets 2.303, 2.235, 2.132, 2.06. Why is the stress level so high? My stress values are charted below for D/d=1.111.

Re: Stress Values Not Corresponding to Published Values

James Riddell Nov 18, 2014 7:10 AM (in response to Cory Nation)

Cory,
Even your data looks high compared to accepted values in sources such as Machinery's Handbook. I've found that SW is very bad at analysis at 'sharp' contacts and don't trust refined meshes all that much.
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- Re: Stress Values Not Corresponding to Published Values
  
  Shaun Densberger Nov 18, 2014 3:28 PM (in response to James Riddell)
  
  I've found that SW is very bad at analysis at 'sharp' contacts and don't trust refined meshes all that much.
  
  What do you mean by, "sharp contacts?" Are you referring to re-entrant corners? If so, then this isn't something specific to SW, but rather the finite element method in general. That being said, the fillet in this model resolve the singularity issue, so this shouldn't be an issue.
  
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Re: Stress Values Not Corresponding to Published Values

Shaun Densberger Nov 18, 2014 3:26 PM (in response to Cory Nation)
Couple things I noticed with his model:

At ~2:58 you see the deformed shape and it looks like the rod is bending down. This isn't right and it's hard to say if this is a bug or something else, but we should see the shaft artificially expand due to the linear assumption.
His stress sensor for the fillet is at a point, which is a poor choice. He should have looked at the maximum across the entire fillet.
His stress sensor for the shaft is at a point, which is (again) a poor choice. He should have looked at the maximum around the circular edge of the split line.
He never does a convergence study, so it's possible that he had a poorly constructed mesh.

I went ahead and repeated his work and got the following Kt factors:

Fillet Kt
0.05 in 1.36
0.10 in 1.23
0.15 in 1.16
0.20 in 1.12
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Re: Stress Values Not Corresponding to Published Values

Cory Nation Nov 18, 2014 3:43 PM (in response to Cory Nation)

UPDATE: Solidworks outputted is estimated stress using the Von Mises Crtierion. When generating a stress concentration factor, where the stress is in pure torsion, the von mises stress must be divided by sqrt(3). (since sigma(1)=-sigma(2)=tau(y)) tau(yield)=sigma(yield, VMS)/sqrt(3).

When I did this conversion, the stress concentration factor lined up with the predicted values form Peterson.

I did a convergence graph and applied sensors in various regions around the fillet to ensure the stress is accurate and precise. The ERR plot shows error in the energy norm to be less than .0098%. After this aforementioned change, the stress values deviated by .0038 on average from the published values.
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- Re: Stress Values Not Corresponding to Published Values
  
  Shaun Densberger Nov 18, 2014 5:12 PM (in response to Cory Nation)
  
  ...the von mises stress must be divided by sqrt(3)...
  
  How were you calculating your stress concentration factor?
  
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  - Re: Stress Values Not Corresponding to Published Values
    
    Cory Nation Nov 24, 2014 2:11 PM (in response to Shaun Densberger)
    
    Kt=max stress/nom stress.
    
    Max stress=VMSmax/sqrt(3)
    VMSmax is the maximum von mises stress calculated by solidworks in the region of interest. To convert the VMS to principle stress for a part subjected to pure torsion, you must divide by the square root of 3.
    
    nom stress= 16*Torque/(pi*small diameter^3)
    Torque applied to this part was 2500 in-lbf
    
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    - Re: Stress Values Not Corresponding to Published Values
      
      Jared Conway Nov 25, 2014 2:17 PM (in response to Cory Nation)
      
      so in this case the issue was intepretation of the data, not a software/calculation issue
      
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Fillet	Kt
0.05 in	1.36
0.10 in	1.23
0.15 in	1.16
0.20 in	1.12

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