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Distinguished Contributor
ilyas
Posts: 140
Registered: ‎06-13-2012

Re: surface contact

02-06-2013 07:09 PM in reply to: AstroJohnPE
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Contributor
yasmind
Posts: 23
Registered: ‎02-04-2013

Re: surface contact

02-07-2013 03:41 AM in reply to: ilyas

Hello,

Thank you for your messages. 

The major problem now is that the stiffness values assigned to those weak springs between the parts that only depend on friction do not affect my results to a large extent. However, the coefficients of friction do not have any effect at all. So, I believe my contacts still do not work properly.

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Valued Mentor
AstroJohnPE
Posts: 501
Registered: ‎08-30-2012

Re: surface contact

02-07-2013 07:33 PM in reply to: yasmind

Hi,

 

"the stiffness values assigned to those weak springs ... do not affect my results". I think that is good! The springs are not suppose to "change" the results other than to create a statically stable model. If you make the springs too stiff, then part of the load is transmitted to the ground through the springs, and that makes the model inaccurate compared to reality.

 

"the coefficients of friction do not have any effect at all". What motion is the friction supposed to affect?

 

I think we will not know for sure what the results are showing until you provide an archive of the model (amazing that no one suggested an archive before now, see Create, Post, or Provide an Archive of your model) or some results images that show the legend.

 

Thanks Ilyas. You found the thread regarding "rotation results" that I could not find.

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Contributor
yasmind
Posts: 23
Registered: ‎02-04-2013

Re: surface contact

02-08-2013 09:27 AM in reply to: ilyas

Hi,

Again thank you very much for your replies. John, yes, it was a displacement graph, just to give you an idea how the things were going.

I have different loading schemes to simulate, one of which is an out of plane load, applied on one face of the top horizontal element. In this case I changed the option in the 3d springs, assigned to the model for surface contacts to work, from translation to rotation, but once again, I experience my initial problem (pins go out of their places, please see the attached figure). I tried to increase the stiffness values even excessively, but the results do not change.

Thanks.

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Contributor
yasmind
Posts: 23
Registered: ‎02-04-2013

Re: surface contact

02-08-2013 09:32 AM in reply to: AstroJohnPE

By the way, while saying that the value of friction coefficient does not effect the results, I meant that I change the friction between different parts in my model, i.e. pins and other elements, etc, but I always get the same results. The coefficient of friction should make a difference, as the frame displaces, i.e. as the pins rotate in the mortices, and different parts rub against each other. Right?

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Valued Mentor
AstroJohnPE
Posts: 501
Registered: ‎08-30-2012

Re: surface contact

02-08-2013 02:49 PM in reply to: yasmind

It could be that the friction force is not large enough to prevent motion. Since the analysis type is static, and because their is no sliding coefficient of friction (at least not in linear static stress), the friction force goes to 0 if the friction force is not large enough to prevent motion.

 

By the way, did you specify a "shrink fit" type of contact for the mortice joints? Without that, would there be any loads that create a normal force needed to create a friction force?

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Valued Mentor
AstroJohnPE
Posts: 501
Registered: ‎08-30-2012

Re: surface contact

02-08-2013 02:53 PM in reply to: yasmind

Rotational constraints have no effect on brick elements (or tetrahedron elements). So essentially your pins had no constraints, and you were back to the original problem.

 

Three points (not in a straight line) held in X, Y, and Z translation (with weak springs in this case) are sufficient to prevent rigid body motion. Hopefully, this will give better results.

 

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Contributor
yasmind
Posts: 23
Registered: ‎02-04-2013

Re: surface contact

02-19-2013 09:07 AM in reply to: AstroJohnPE

Hi,

Sorry for my late reply.

Again for my frame under out-of-plane loading:

I changed my weak springs so as to have translational constraints rather than radial ones as you said, and I changed all my contact options into shrink fit with an automatic interference. 

However, if I leave the stiffness values as s I used in in plane lateral loading case, the pins do not move together with the rest of the frame, which gets displaced in the out of plane direction (see the last attached figure). And if I increase the stiffness to prevent this, then the whole model becomes extremely and unrealistically rigid.

Another problem is the interference value. A very small change in interference change drastically all the results.

Do I have to carry out a nonlinear MES analyses in order to make sure that sliding works correctly?

 

 

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