CBAR and RBAR

[nvhuser]

Hello everyone,

For a given hinge model of a car door, somebody replaced a CBAR by a RBAR connection. The frequency response results in the door outer panel were considerably changed.
The CBAR is referenced to an isotropic material with the properties of the Steel. Both approaches allow the hinge to rotate in the z-direction (C=12345).
Could please anybody explain it? Thank you

 

[crostolo]

Hi nvhuser,
a RBAR is a "a rigid bar with six degrees-of-freedom at each end." So it is normal the results changed a lot, because a rigid element has a infinite stiffness.
Take a look at the Quick Reference Guide for more info

 

[nvhuser]

Ok, but both models are free to rotate in Z-direction.
I mean, RBAR is defined as: RBAR,EID,GA,GB,12345

and CBAR as: CBAR,EID,PID,GA,GB
The effect of torsional stiffness in CBAR is neglected by nastran. So, since CBAR has the same young modulus as steel (210 GPa), I don't see any reason for this difference in results.

 

[Naffoussi]

Hi nvhuser,

The frequency response analysis is based on normal modes which depends a lot of stiffness of the structure.
As crostolo told you a RBAR is much more stiffer than a CBAR. That is why you've obtained a results changes.

Finally to understand how to model a hinge using both CBAR and RBAR look at the attached pic. you can find more details in the Element Library of Nastran page 206.

Regards,

Seif Eddine Naffoussi, Stress Engineer

http://www.Innovamech.com

33650 Martillac û France

 

[nvhuser]

Hi, good to know, I thought 210GPa was stiff enough to be considered as perfectly rigid.
Thank you for your help