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[STRUCT] Acceleration not working for Axisymmetric
 
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andrew.kelly
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Joined: 06 Mar 2012
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Location: Columbus, OH

PostPosted: Fri Jul 17, 2015 6:33 pm  Reply with quote

Using Mechanical V16.1 for an axisymmetric model involving contact, large deflection, and plasticity.

I want to get the "free state" of my model after deformation,. The last LS has a -Y acceleration with the bottom part fixed. So the reaction force of the bottom should be the self-weight of the parts.

I can't get this LS to converge. It's like the acceleration isn't working.

I originally solved it with Weak Springs On. It converged, but the graphics showed the parts separated by a large distance. They yo-yoed past their initial position, which makes absolutely no sense to me. I can't share the graphics with a customer, because it looks dumb (and I'd have to write a dissertation explaining it).

A few weeks ago, I made a weak body-to-ground spring with a small pre-tension to pull the parts together, but that gives me the heebie-jeebies. It's right up there with Bond-o, duct tape, and JB Weld. Acceleration seemed like the more elegant (and proper) way to obtain the final free state.

Yes, my materials have density.

The geometry shows no volume / no mass. That's probably the correct interpretation within the geometry branch because they are just surfaces at that point. It's only during the solution phase that the axisymmetric idealization is considered, so the parts should "acquire" their mass during the solve.

Is acceleration un-supported for axisymmetric? Any other clever ideas?
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Andrew C. Kelly, P.E.
Honeywell Sensing & Productivity Solutions
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andrew.kelly
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PostPosted: Fri Jul 17, 2015 7:17 pm  Reply with quote

I just discovered "Standard Earth Gravity".

When using "Standard Earth Gravity", the free state reaction force is the self-weight, and the graphics look correct.

Why didn't a manual acceleration in the -Y direction work the same way?

Bonus points: For the free state, why does weak springs sling the parts past their initial positions? Isn't the rigid body equilibrium position the same as the initial position (accounting for the residual plastic deformation, of course).
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Andrew C. Kelly, P.E.
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liam.mealey1
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Joined: 08 Dec 2014
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PostPosted: Mon Jul 20, 2015 9:26 am  Reply with quote

Did you apply the acceleration in the correct orientation for your problem (I'm not entirely sure I've fully understood your setup)?

See the following two statements from the Acceleration page of the Mechanical User's Guide:

If desired, acceleration can be used to simulate gravity (by using inertial effects) by accelerating a structure in the direction opposite of gravity (the natural phenomenon of). That is, accelerating a structure vertically upwards (+Y) at 9.80665 m/s2 (in metric units), applies a force on the structure in the opposite direction (-Y) inducing gravity (pushing the structure back towards earth). Units are length/time2.

Alternatively, you can use the Standard Earth Gravity load to produce the effect of gravity. Gravity and Acceleration are essentially the same type of load except they have opposite sign conventions and gravity has a fixed magnitude. For applied gravity, a body tends to move in the direction of gravity and for applied acceleration, a body tends to move in the direction opposite of the acceleration.

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aaron.caba
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PostPosted: Mon Jul 20, 2015 11:28 am  Reply with quote

Quote:
If you're used to the sign convention, you can move past this. For the rest of us, the sign convention just creates confusion and pain.


I second that statement. I don't use either enough to remember the sign conventions. I usually end up starting with a very small dummy model and apply my accelerations or gravity to that before I move to the real big models.

Aaron
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andrew.kelly
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PostPosted: Mon Jul 20, 2015 12:15 pm  Reply with quote

Probably that stupid sign convention thing. It's consistent with bodies passing their initial position with weak springs on, and a failure to converge with weak springs off.

It would be helpful if "standard gravity" and acceleration followed the same sign convention. For "standard gravity" in the -y direction, bodies move in the -y direction. For acceleration in the -y direction, bodies move in the opposite direction. Inconsistent.

If you're used to the sign convention, you can move past this. For the rest of us, the sign convention just creates confusion and pain.
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