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douglas.cm
Posts: 10
Registered: ‎10-06-2012
Accepted Solution

What is wrong with fatigue wizard? Results are absurdly wrong

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10-06-2012 07:01 PM

I'm trying to do a simulation of fatigue on a 737 airplane landing gear. I have it modeled, calculated its force in each axle, and ran the fatigue for the force applied within 0.2s The landing gear is not perfectly equal to the real one, but its close enough. After doing the simulation, i get 1 cycle endurance with 322000N on each axle, even with the normal aircraft weight (125000N) on each axle, the design fails. A plane most certainly doesn't break when its supported by its landing gear. I have basically a T modeled, with loads on the small ends. Ill add any files if requested

 

Also, just to be safe i tested with doubled dimensions, still break, with 263 MPa max von misses stress

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Active Member
douglas.cm
Posts: 10
Registered: ‎10-06-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-06-2012 07:51 PM in reply to: douglas.cm

Just tested file of Module 9

 

http://students.autodesk.com/?nd=protected_content&resource_id=cae_section_2

 

Should have gotten like 71000 cycles just like the video, but i'm getting 0.5

 

My file is attached, rename it to rar and extract the FEM file, im getting 0.5 cycles with 32200N ಠ_ಠ, with 322N around 3500 cycles, so a toddler jumping on a 60mm steel reinforced bar could brake it jumping a little on it ಠ_ಠ

 

Something is wrong in the software configuration, since i get bad results with the example file.

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Active Member
douglas.cm
Posts: 10
Registered: ‎10-06-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-09-2012 04:32 PM in reply to: douglas.cm

Anyone?!?!

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Valued Mentor
AstroJohnPE
Posts: 498
Registered: ‎08-30-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-12-2012 09:27 AM in reply to: douglas.cm

Hi Douglas,

 

I wonder if there is there an issue with the units?

 

Otherwise, can you provide a link to the video (or printed information) that goes along with the "module 9" model? Hopefully, the problem can be resolved by reproducing the example problem.

 

Note that the .FEM file does not contain the complete model. The best way to transmit simulation models is to create an archive. See the thread "Create, Post, or Provide an Archive of your model".

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Active Member
douglas.cm
Posts: 10
Registered: ‎10-06-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-12-2012 11:20 AM in reply to: douglas.cm

http://engineeringexploration.autodesk.com/content/module-9-fatigue-analysis

 

This is the guide i was followong and trying to replicate results, i used the files linked on the page and i didn't get the results expected from the wizard, about the units, the stress results were the same, so i believe the issue is with the wizard itself

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Valued Mentor
AstroJohnPE
Posts: 498
Registered: ‎08-30-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-12-2012 12:27 PM in reply to: douglas.cm

Thanks for the links. I went through the module 9 exercise and got the correct result with version 2013. :smileysad: Well, that is good for me but not for helping you. :smileyhappy:

 

Something must be misread by the Fatigue Wizard, such as a missing decimal point.

  • What version of Simulation Mechanical are you using?
  • What regional settings, or Region and Language, is setup on your computer? (It's somewhere under the Control Panel but at different locations depending on which version of Windows is being used.) If you normally use a , (comma) as the decimal symbol, trying switching that to . (period).
  • If you have access to a screen capture utility, it might help to make a screen capture of each input screen and the results.

 

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Active Member
douglas.cm
Posts: 10
Registered: ‎10-06-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-15-2012 11:56 AM in reply to: AstroJohnPE

First thank you for the help. I was out of town so i couldnt reply.

 

- i am using the 2013 student version win x64

- regional is set for portuguese, so my commas and dots are inverted

- i will take pictures of the proccess and post them asap

 

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Active Member
douglas.cm
Posts: 10
Registered: ‎10-06-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-15-2012 05:36 PM in reply to: douglas.cm

Here is my steps.

 

http://imgur.com/a/5eXYM

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Active Member
douglas.cm
Posts: 10
Registered: ‎10-06-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-15-2012 06:50 PM in reply to: douglas.cm

Ok, i can now run the example file correctly after the separators change. (thanks a lot for it)

But my design still fails miserably. Only 52 cycles

 

I have the dimensions mostly right i believe, i've selected the reinforced steel from the materials.

The landing force is calculated to be 320000N on each wheel following many sources.

I have the force applied on each of the outmost "cilinder".

I need to come close to 70 thousand cycles, with is believed to be the norm on aircrafts.

I don't know what i can do to get these results. I can't reduce the force, i think its applied on the right location, and the material while not a perfect fit, comes pretty **** close

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Valued Mentor
AstroJohnPE
Posts: 498
Registered: ‎08-30-2012

Re: What is wrong with fatigue wizard? Results are absurdly wrong

10-17-2012 07:53 AM in reply to: douglas.cm

Hi Douglas,

 

The stress result of 2617 MPa is very high, isn't it? That equals 380000 psi for those familiar with the English unit system, and I doubt there are steels that have a yield strength that high.

 

  • Do the displacement results make sense? If not, there is a problem with your model setup.
  • Either the load is too large,
  • Or applied incorrectly. (Does the sum of the reaction forces in the (Y?) direction equal the applied load?)
  • If all of the above look reasonble, then the stress may be too high due to a mathematical "stress concentration". For example, if there is a sharp corner at the location of the high stress, the maximum stress results are inaccurate and should be ignored. Likewise, the calculated life (Results Contours > Other Results > Fatigue Life Results (Strain based) > No mean stress correction > life (cycles)) can be ignored in those area. Likewise if there is a fillet in the model but the mesh is too coarse to capture an accurate representation of the fillet.

 

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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