×
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!
  • Students Click Here

*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

Jobs

what BC's should I apply?

what BC's should I apply?

what BC's should I apply?

(OP)
I have a spherical thin film supported by a ring at film's
circumference. When I cool the system to an uniform temperature, intuitively it seems to me that both the ring & film should shrink together if CTE of film is same as that of the ring. In other words, it would be a sphere of smaller radius after cooling. My question is how do I model the boundary conditions for this problem without any rigid body motion?

I would really appreciate your thoughts on this.

Thank you.

RE: what BC's should I apply?

Use symmetry, so that you only model a quarter of the hemi-sphere. The model is restrained normal to each plane of symmetry and also at the ring's circumference. This prevents rigid body motion without affecting any results.

RE: what BC's should I apply?

If you can't use John's suggestion of symmetry then appropriate placing of weak springs in the structure will also provide you with an unconstrained structure but without rigid body motion.


------------
See FAQ569-1083 for details on how to make best use of Eng-Tips.com

RE: what BC's should I apply?

If you do not apply any loads that would make the structure want to embark on a rigid body motion, then any statically determinate set of restraints will serve to remove the "mathematical possibility" of such a motion.  These restraints can be weak springs if you want, but in theory they can be rigid.

It is only if your restaints are not statically determinate that you NEED to use weak springs.  (There are circumstances under which it is not possible to use a statically determinate set of restraints.)

Either way, at the end of the analysis the very first thing you should check is that you have zero forces in all components of your restraining system.

RE: what BC's should I apply?

> Either way, at the end of the analysis the very first thing you should check is that you have zero forces in all components of your restraining system.

Although any restraint with give you a reaction force, however small. The trick however is to reduce this reaction to as small a value as possible, and obviously one in which the response of the system becomes unaffected.


------------
See FAQ569-1083 for details on how to make best use of Eng-Tips.com

RE: what BC's should I apply?

Drej, using a statically determinate set of restraints with a balanced load, the reactions should be only round off errors and thus effectively zero.

RE: what BC's should I apply?

That's would I said in my post (ref. "however small").


------------
See FAQ569-1083 for details on how to make best use of Eng-Tips.com

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!


Resources