Learning Forum

[javat33489]

Hello everyone. I am calculating the compression of a rubber sample in LS-Dyna.
I am doing two tests, the first time I set the force on the pressing body to 40 tons. The second time I set the displacement on the pressing body to 50 mm. All other settings are the same.

1. When using force, the compression body does not behave physically, compression occurs from above and the lower part as if it has not yet had time to react:

At 40 tons, compression occurred by 35 mm.

2. When using displacement, the body behaves physically and is compressed from both sides, from the side of the compression body and from the side of the thrust body. I install the force reaction sensor on the compression body and see that when moving and compressing the rubber by 50 mm, the load is only 1.5 tons:

Why is this happening and how to fix it? Is the force reaction lying? Or how to set up calculation when using force?
Thanks.

[Chris Quan]

Are you doing the simulation using Explicit or Implicit Analysis of LS-DYNA? Are you trying to simulate a static or quasi-static compression of rubber?

If you are using the LS-DYNA Explicit Analysis to simulate a static or quasi-static event, it is true that displacement-controled boundary condition is usually showing more accurate results than force-controlled boundary condition.

The simulation time is quite short: 1E-3 seconds. If a constant Force BC is used, the rubber acts like having a huge impact force from another body and thus its deformation is mostly in the local impact area. 

When Displacement BC is used, the reaction force is increased gradually as shown in your picture. So the entire rubber material has more time to react to the external loading and spread its deformation globally rather than locally. 

[javat33489]

Yes sir, I doubled the calculation time and everything worked.

[valerym.m5]

it would be helpful if you gave access to your model?

What model do you use for rubber?
How did you calculate the constants for this model?
and so on..