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Member
mike
Posts: 4
Registered: ‎04-29-2012

Max/Min Stiffness ratio

615 Views, 4 Replies
05-02-2012 06:42 PM

I'm wondering what is considered a high max/min stiffness ratio 1 x 10^13? , 1 x 10^12? When is a warning triggered in Simulation?

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Employee
INACTIVE_AstroJohn
Posts: 492
Registered: ‎03-25-2010

Re: Max/Min Stiffness ratio

05-03-2012 05:20 AM in reply to: mike

Hi Mike,

 

I believe that 1 x 10^10 is the limit that triggers the high max/min stiffness warning.

 

To put it in a simple example, imagine adding 1 x 10^6 and 1 X 10^-4 (a max/min ratio of 10^10). The result is 1.0000000001 x 10^6. That is getting close to the limit of precision in today's computers (15 or 16 digits normally), so some precision can be lost in the calculation. How the inaccuracy propogates through the solution is difficult to predict. So my suggestion is to be cautious when using results from analyses with such warnings. If you understand why the warning occurs in each model and "know" that it should not influence the answer, then it may be safe to use the results.

 

As with any analysis, you should check all of the results (displacement, stress, reactions, temperature, heat flows, etc) to check for potential errors.

 

Sincerely,
John Holtz, P.E.
Senior User Experience Designer, Simulation
Autodesk, Inc.

Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)
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Employee
S.LI
Posts: 398
Registered: ‎05-06-2010

Re: Max/Min Stiffness ratio

05-03-2012 05:41 AM in reply to: mike

I'm not very clear on your question.

But a ratio like this seems too large for current double precision solver.

Usually, it causes unstable, bad accuracy results.

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Employee
sunj.autodesk
Posts: 32
Registered: ‎11-05-2010

Re: Max/Min Stiffness ratio

05-21-2012 09:01 AM in reply to: mike

Hi Mike,

 

This value is calculated by measuing the max and min diagonal terms in the system stiffness matrix. Usually it will print as a warning as you see in the log file to indicate your matrices might have some problem. However, under some cases such as when MPCs are used with Penalty method, or some initial boundary condition is used, this warnig can also be seen.

 

As long as the solver does not give you error messages, you can pretty much ignore it. But as John suggested, do be cautious you observe the results is against your intuitive. If that happens, this warning might give you some clue to check your modeling details.

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Employee
PipePakPat
Posts: 158
Registered: ‎11-12-2009

Re: Max/Min Stiffness ratio

05-30-2012 10:25 AM in reply to: mike

I have seen only two models that produced problematic results relating to this stiffness warning.  The stress results were obviously incorrect, as they appeared in "spots" on the model... like a 12 year old's acne problem.  In these cases, the stiffness warning exceeded 1e20.  In these cases, the models contained "Gap" or "Boundary Elements" where the stiffness was defined.  Changing these gap and boundary element's stiffness to 1e10 solved the problem both times.

Pat Tessaro, P.E.
Customer Support Engineer

Autodesk, Inc.
285 Kappa Drive
Suite 250
Pittsburgh, PA 15238
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