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## Simulation Mechanical and Multiphysics

New Member
Posts: 2
Registered: ‎12-09-2012

# Large Deformations?

215 Views, 2 Replies
12-09-2012 08:06 PM

I'm trying to run a simple FEA analysis on a solid object. See image below (mm units).

The object is made of Aluminium, and opposing forces (approx 500N) are to be applied on the top and bottom to squeeze together the ends.

It seems simple enough, however whenever I apply a pressure force, even at 0.000000001N/mm^2, the body deforms excessively and the two ends crossover (it baffles me why the two surfaces superimpose on each other!?) The below image shows the simulation result for a pressure of 0.0000000001N/mm^2 applied to the upper surface, and the lower surface is constrained as 'fixed'.

I've spent many hours trying to figure out what is wrong with this, but it's to no avail. Can someone assist?

Distinguished Contributor
Posts: 141
Registered: ‎06-13-2012

# Re: Large Deformations?

12-09-2012 11:40 PM in reply to: nathan.c.hill

Hi Nathan,

If it were me, I would first do a Linear Static Stress analysis and display the displacement results in non-exaggerated view which is done by setting the Displaced Model Options>>Scale Factor to an absolute value of 1. This is to determine whether the two surfaces come into contact or not. An alternative to this is to judge the contact based on the displacement of the contact points and the size of the gap. If the two surfaces still overlap each other, I would perform a MES analysis and apply surface contact between the inner surfaces. Otherwise, the software will treat the surface as free/no contact and may freely pass through each other.

Regards

Ilyas

*Expert Elite*
Posts: 570
Registered: ‎08-30-2012

# Re: Large Deformations?

12-10-2012 12:37 PM in reply to: nathan.c.hill

Hi Nathan,

1. What type of analysis are you performing? Linear? MES? something else?
2. What is the displacement magnitude? Your image does not show the legend scale, so I'm not sure if you are referring to the results being too large (larger than 3 mm) or simply the image of the arms apparently overlapping.

As Ilyas explained, you can exaggerate the displacements as much or as little as you want. In Linear stress, you almost have to exaggerate the displacements. Otherwise, a displacement like 0.001 mm in your model would never be visible. But when exaggerated by a factor of 10000, it would look as if the arms were passing through each other. "Results Contours > Displacement > Show Displaced > Displaced Options".

PS. If the arms do come into contact due to the full load, you probably need to run an MES analysis and use surface to surface contact. It would be hard to do the simulation in linear because you would need to hand-draw the gap elements between the two arms, which requires the nodes on the two surfaces to match, which cannot be done with a CAD model. (But easily done with a hand-built model.)

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

Simulation Mechanical user since Dec 1997