×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• 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.

# 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.

### 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.

#### Resources

Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now
The Great Project Profitability Debate
A/E firms have a great opportunity to lead the world into the future, but the industryâ€™s greatest assetâ€”real-time dataâ€”is sitting wasted in clunky, archaic ERP platforms. Learn how real-time, fully interactive dashboards in a modern ERP allow you to unlock data that will shape the future of the world. 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:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!