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JDMather
Posts: 27,994
Registered: ‎04-20-2006
Message 11 of 30 (442 Views)

Re: FEA interpretation

10-10-2012 10:51 AM in reply to: rmerlob

rmerlob wrote:

 

Like you, I believe any permantent deformation should be avoided, ....

 


How much Actual (computed) displacement are you seeing in the analysis (if we ignore for a moment that results below SF might not be correct)?

Displacement.png

 

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rmerlob
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Message 12 of 30 (428 Views)

Re: FEA interpretation

10-10-2012 12:01 PM in reply to: rmerlob
Its the same as yours, about 0.2 mm, results on autodesk simulation is a little higher around .3 mm but stress seems lower all around.
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karthur1
Posts: 4,296
Registered: ‎04-27-2005
Message 13 of 30 (421 Views)

Re: FEA interpretation

10-10-2012 12:31 PM in reply to: JDMather

Is this with the second one "SoporteB.ipt" that was posted?  I am getting 0.101mm total displacement on the first part that was posted.

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rmerlob
Posts: 135
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Message 14 of 30 (411 Views)

Re: FEA interpretation

10-10-2012 01:08 PM in reply to: karthur1

No karthur, SoporteB is the part that mates with SoporteA, I will attach picture of the actual parts we are going to repoduce when I have the chance.

 

We dont knowand we have a lead on what the material is, thats 

 

Are you sure you have a fine enough mesh? I definitely get more than 0.2 mm on fully converged displacement results, actually I get 0.2749 on 289619 elements

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JDMather
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Message 15 of 30 (391 Views)

Re: FEA interpretation

10-10-2012 02:42 PM in reply to: rmerlob

rmerlob wrote:
Its the same as yours, about 0.2 mm,

My point was, is this really significant (for the usual function of this part), about the thickness of two sheets of paper - can barely see by eye.  A lot of people see the exaggerated representation take interpret it literaly.

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rmerlob
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Registered: ‎10-21-2011
Message 16 of 30 (381 Views)

Re: FEA interpretation

10-10-2012 05:05 PM in reply to: JDMather

I agree, what I meant about no permanent deformation being accepted didn´t quite explain itself, I tried to say that there probably shouldnt be points that are past yield.

 

Of course 0.2 displacement is okay on this part, what worries me is that stresses are past yield, and even though theory tells me that some localized yielding can be acceptable on ducile materials (again, keywords: strain hardening and stress redistribution) theres no guidelines on how much of it is acceptable.

 

This leaves no more options other that testing, even for a case that I consider could be the simplest real life application of finite element analisys.

 

The reason for this thread is that I DO NOT want to believe this is the case and I think there is somebody out there with more experience and training than myself that can say: this local yielding is acceptable or not because x and y reasons.

 

 

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JDMather
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Message 17 of 30 (373 Views)

Re: FEA interpretation

10-10-2012 05:36 PM in reply to: rmerlob

rmerlob wrote:

 

This leaves no more options other that testing, even for a case that I consider could be the simplest real life application of finite element analisys.

 

 


I'm really interested in this too but I suspect the Autodesk Simulation forum might get better response.

I've always been taught that physical testing is a requirement to validate that the virtual test has been set up correctly.

In my class I wait till the last lab of the last semester with my seniors and do a similar problem that casts doubt on all FEA.  I would love to hear more on the topic from the true experts.

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rmerlob
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Message 18 of 30 (368 Views)

Re: FEA interpretation

10-10-2012 06:54 PM in reply to: JDMather

Glad we are on the same page, I did enlist the help of the sim squad hopefully they will come by shortly.

 

On the topic of testing, its clear that it would be helpful, but what if the load were 10 times higher or maybe it wasnt simple tension, like a cantilevered beam or something.

 

As cases get more complex it gets exponentially more dificult to test something out on limited resources, and I always thought finite elements was supposed to fill that void and let a small engineering team test it out virtually on cases that were simple or not critical enough.

 

Theres just 2 possible answers, I´m not experienced/trained enough to make that call, or I have a wrong idea about the capabilities of finite elements. I dont like either lol.

 

Another thing to note is that doing a manual calculation for the area around the holes, I get somethig that was around 100 MPa (very roughly dont remember it exactly) without considering stress concentration, theres a page on Shigleys Mechanical Design textbook that says stress concentration factors are generally not applied to ductile materials in static loading because of the strain hardening effect I mentioned earlier, so if I were to design this manually I would believe there is like a SF of 3.

 

Kind of scary when you think that manual calulations had you thinking you got this and finite elements makes you doubt the entire thing.

 

In all fairness if we were to apply a stress concentration factor SF would drop dramatically but thats not what I should be doing from what I gather from Mr. Shigley himself.

 

Great now im doubting manual calculations too, great, 5 years of education down the drain lol.

 

 

 

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m.granata
Posts: 151
Registered: ‎04-07-2005
Message 19 of 30 (357 Views)

Re: FEA interpretation

10-10-2012 09:15 PM in reply to: JDMather

I didn't read through every entry in this post but I would just like to toss in a few things I was taught years ago in grad school and employ when performing FEA.  When performing FEA it is always good practice to perform some type of hand calculation to validate the model.  The results won't be identical but within 4% or 5% is pretty good.  This will help verify the model, loads, and constraints are correct.  Without that, the accuracy of the FEA model is uncertain.  Often, inaccurate FEA results are related to a poor quality mesh which sometimes stem from a model that is too complex.  One source for various hand calculations with different load and geometry configurations is Roark's Formula for Stress and Strain.  Also, to the best of my knowledge, stress concentration factors are only considered in Fatigue Analysis and not applied to Linear Static Analysis.  Lastly, it is not uncommon to see local yielding at discontinuities, etc. with FEA.  That doesn't always mean the part will fail.

 

Regards,

Mike  

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karthur1
Posts: 4,296
Registered: ‎04-27-2005
Message 20 of 30 (318 Views)

Re: FEA interpretation

10-11-2012 06:21 AM in reply to: rmerlob

The displacement that I am seeing is a little different than yours. Since you have the male part that keeps the ears on the clevis from bending in, I added a frictionless constraint on the inside so they cant move in.  The surfaces I added it to is highlighted in the image.  I get a displacement of around .10mm.  I am using an element size of 0.01.

 

I can get the same displacement as you if I dont have this frictionless constraint.

 

2012-10-11_0816.jpg

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