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RIGID BODY - Inertia inputs

Hi all,

I model a hammer, impacting a component. To reduce computation time, and as I don't need any information about the hammer's stresses and strains, I want to model it as a Rigid Body. Though, I wonder about inertia and mass input in rigid bodies, as I get the following warning and error :

WARNING ID : 542
** WARNING RIGID BODY INERTIA
ERROR ID : 274
** ERROR IN RIGID BODY DEFINITION (INERTIA)

And here my input:

/RBODY/402
HAMMER
# N_master I_sens I_skew I_spher MASS Grnod_id IKREM I_CoG Surf_id
39178 0 0 805 0
# Jxx Jyy Jzz
1.87779E+06 371038 -1.87779E+06
# Jxy Jyz Jxz
0 0 404038
# Ioptoff
0

If I delete inertia inputs, my computation works fine. However I need mass, because I work with initial velocity and mass is therefore important.

So I wonder :

- I input material (density) and mesh, so maybe I do not need inertia and mass input

- maybe I should input MASS in rigid body

- do what I did but something is still wrong ...

Thank you,

Alice

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Altair Forum User

Hi Alice,

For the rigid elements do not assign any material/property card. And no card image also for the elements. But you can assign mass to the rigid body.

Ensure no materials and properties are assigned for rigids and try running the same.

gopal_rathore

Hi,

Rbody gets intialised when you run starter. Kindly check in the starter out file for it i.e 0000.out file Are you getting some mass in it or not. If possible share the file.

As you have geometrical data of hammer, make a rigid body for this complete part and assign elastic material properties with PSOLID or PSHELL property for the hammer part. Then card edit the rbody and make ICoG: 2 with no inertia input. After this when you run the starter file, the Rbody after initialisation gets mass as well as inertia .Check this values if these are ok, you can use this methodology.

Altair Forum User

Hi George, Hi gopal_rathore,

Thank you for both of your replys.

@George P Johnson : I tried not applying any material/prop card, but then I can't define a part. this was a problem for me when I tried defining the hammer surface for interface modeling. I tried /SURF/SEG but did not succeed. Thank you for your help though.

@gopal_rathore : Thanks a lot for reminding me of the mass summary (and inertia) in the 000.out, that is indeed the best way to check, and I didn't think about it. I applied material and property as you said. However I momdeled my hammer with shell, though it is full. So mass is not right from material and geometry only. So what I did is to input a very low density in the material and add a mass in the rigid body, while leaving inertia inputs blank. This gives me the right mass, having already my center of gravity.

Thank you a lot for both of your help, I understand rigid bodies a way lot now !

Alice

Altair Forum User

Hi Alice,

You can also go through this video at https://altair-2.wistia.com/medias/v3bvgnw89l which is a webinar on modelling rigid bodies in RADIOSS.