×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

What would be the right way to model rigid body motion in FEM?

What would be the right way to model rigid body motion in FEM?

What would be the right way to model rigid body motion in FEM?

(OP)
Dear all,

I'm just starting off in the field of FEM. Please bear with me if this is a bad question. I'm curious how do we model rigid body motion in FEM.

The first way I can think of is to discard the stiffness term totally (set stiffness matrix to 0). That way, the equation of motion becomes F = Ma.

Another way is to set young's modulus to an infinite value, although that is not realistic.

However, setting the stiffness matrix to 0 and letting young's modulus increase to infinite seems to be controversial to each other. Increasing stiffness would increase the oscillating frequency. But dropping the stiffness term would not increase the oscillation of the structure. I'm not sure if I've missed something on that problem.

Thanks!

RE: What would be the right way to model rigid body motion in FEM?

Setting stiffness to 0 would prevent forces being transmitted between any nodes. So only the nodes that you apply a load to would move.

You could use a high stiffness so that vibration frequencies are too high to appear with whatever solution method you're using.

A more elegant way would be using constraint equations to connect the DOFs without using stiffness. That'll allow you to eliminate redundant DOFs the system of equations to leave only the rigid body DOFs. I think this is what RBE2 in Nastran does.

RE: What would be the right way to model rigid body motion in FEM?

(OP)
Thank you very much whitwas! Your explanation is very clear and helpful.

RE: What would be the right way to model rigid body motion in FEM?

I think you need to use a mechanism simulation software, like Adams. I think the problem will be in writing equations that correctly describe the motion of the different parts.

another day in paradise, or is paradise one day closer ?

RE: What would be the right way to model rigid body motion in FEM?

Rigid bodies have infinite stiffness terms for the degrees of freedom which are attached to the rigid body and move with it and zero stiffness terms between the degrees of freedom moving with the body and the stationary degrees of freedom (fixed to the ground).

Best regards,

George Papazafeiropoulos

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

eBook - The Future of Product Development is Here
Looking to make the design and manufacturing of your products more agile? For engineering and manufacturing organizations, the need for digital transformation of product development processes just became more urgent than ever so we wanted to share an eBook that will help you build a practical roadmap for your journey. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close