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Oscillations in my model

Nina_Boursier

Description: Oscillations and energy errors in my Radioss model

Product/Topic Name : Radioss 2025

Hi everyone,

I'm trying to model a knee prothesis in Altair Radioss

I modelled it as represented below :

The tibial implant is not represented
A rotation movement is applied to the femoral implant, around the Flexion extension axis
An antero-posterior force is applied to the Tibial insert, along X2 axix
A vertical force is applied on the tibial insert along z axis (this force always tend to keep contact between the femoral implant and the tibial insert)
A rotation torque is applied around the Z axis

Forces and displacements are applied on remote point, linked to the solids using rigid bodies.

The contact between the Femoral implant and the Tibial insert is a Type 7 contact

After running mu calculation, I got a 60% error. The Dt noda is quite low, but tif I increase it, I the have an important DM/M which is not acceptable either.

Does anyone have an idea from wher my problems could come from ?

Many thanks for your help !

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NolanMcP

Hi Nina,

Thank you for including a diagram of your model and the energy plots. That makes it much easier to understand what is going on.
When you say "60% error", are you referring to the energy error reported in the 1.out file?
Next, the contact energy is definitely too high. This can mean any of the below things:
- There are large penetrations in your model that are not resolved
- The contact is too stiff
- The contact is too soft

To address the contact energy, first check the model and make sure that there are no large initial penetrations.
If there are no major initial penetrations and if you are modelling with solid elements, I suggest using /INTER/TYPE24 instead of /INTER/TYPE7.
If you are modelling using shell elements, I recommend first trying to use /INTER/TYPE25 instead of Type7. Type25 automatically created a symmetric interface in the background.
If you prefer to continue using Type7, then you should create a symmetric interface automatically. Simply duplicate the contact and then swap the secondary and main segments.

Please refer to the Altair Best practice document linked below when defining your interfaces. Usually updating a contact definition is enough to reduce the contact energy back to normal levels:

https://community.altair.com/discussion/39772/radioss-modeling-best-practices

Does this help?

Nolan
Altair Support

Nina_Boursier

Many thanks for your answer. I'll try to check about it.

When I say "60% error", I'm indeed referring to the energy error of the 1.out file.

Okay, I'm actually modelling with solid emlements, so I'll try with the /INTER/TYPE24

I'll come back to you after I got some results.

NolanMcP

Hi Nina,

Having a temporary large error % is normal when imposed displacement, velocity, or acceleration is used or any time external force is added to the model. In a stable model, the energy error will approach zero as the simulation proceeds.

Additionally, the energy error reported in the 1.out file only accounts for Kinetic Energy, Internal Energy, and External Work. Contact energy and hourglass energy are not considered. The full explanation for how energy error is calculated can be found in the link below:

https://help.altair.com/hwsolvers/rad/topics/solvers/rad/faq_rad_results_checking_r.htm

Let me know if the behavior improves after updating the contact definitions!

Nolan