×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Modal Analysis: Why high stress/displacement?

Modal Analysis: Why high stress/displacement?

Modal Analysis: Why high stress/displacement?

(OP)
Good evening, working in Workbench.  Been analyzing a small plastic (Valox) gear, and was interested in modes.  Where reasonable frequencies are shown in results, 6 modes within 1200 Hz to 1500 Hz, stresses and displacements are unreasonably high, 1.5 e8 PSI with lowest stress 50 KSI.  With a 7500 PSI yield, it seems this gear would've exploded.  This doesn't seem right... any help would be greatly appreciated.  Thanks.  

RE: Modal Analysis: Why high stress/displacement?

Modal analysis as such doesn't produce meaningful stresses. That is, the analysis is for an undamped mode, and the displacement scaling is not meaningful, directly.


You can get menaingful stresses if you add damping to your model and then drive it with representative forces. That may well be an add-in module to your post processor.


 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Modal Analysis: Why high stress/displacement?

Normally, computed modes are normalized to the mass matrix.  Hence, the computed stresses and displacements are meaningful in a relative sense but meaningless in an absolute sense.

As greg suggested if you want to know something other than natural frequencies or mode shapes you'll need to do a forced response analysis or something of the sort based on the information you have at hand.

RE: Modal Analysis: Why high stress/displacement?

(OP)
Greg, Stringmaker,
   Thanks for the insight, will be sure to do more research on forced responses.  Knew there's an obvious explanation for this.

Chad

 

RE: Modal Analysis: Why high stress/displacement?

I think you are probably looking for "Frequency Response Analysis".  Check www.femci.gov for some basic understanding.

RE: Modal Analysis: Why high stress/displacement?

You need to perform a harmonic response analysis.  If you have an ANSYS/mechanical or multiphysics license you can do this.  If you have the watered down version of ANSYS than a modal analysis is the best you can do.

RE: Modal Analysis: Why high stress/displacement?

Ah, not quite true.

Producing FRFs from a normal modes analysis is fairly easy in a spreadsheet.

Fairly easy...

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources