boundary conditions

[akshparm]

Hi
Just wanted to ask i want to simulate a thin plate and determine the buckling load from the eigenvalue. The eigenvalue i have determined is very high to the power of 9. Wanted to ask its a simply supported plate, what should be the boundary conditions.
Thank You

 

[TGS4]

If it's simply-supported, then there would be no moment restraint.

Can you describe your problem in a little more detail, please? An eigenvalue for a buckling problem with a result order-of-magnitude 10^9 is very bizarre.

 

[akshparm]

Thank you for the response. Basically its a thin aluminium plate of 400x400x10 mm which is being simualted to determine its buckling characterisitics. So what it is a simply supported plate and im having trouble with the boundary conditions as then i want my result to be compared to the theoretical solution. and it will be compressed at the two end. So the inital results was indeed very high so i think the problem lies with the boundary conditions. I am using abaqus, and a appliyng a step input for buckle. i then apply the boundary conditions and apply a shell load of magnitude 1, is this also correct to do?

 

[TGS4]

Simply-supported means just that - no moment restrain.

Tell me more about this "shell load" that you are applying. That may have something to do with this.

 

[akshparm]

The Shell edge load is of magintude 1 and is simlar to a moment. Do you think i should make the magintude higher.
The initial boundary conditions which i inputted to get the high value for the eigenvalue were:
A) U3 ticked on all side
B) U2 ticked on two parallel sides
C) U1 ticked on one of the parallel sides
D) then shell edge load on the opposite side of side in C

Are these boundary conditions applied correct?

 

[TGS4]

The Shell edge load is of magintude 1 and is simlar to a moment.

A force/load and a moment are generally mutually exclusive.

A picture would really help. I can infer the orientation of your plate (sort of), but I can't say for certain.

 

[akshparm]

Basically ive now changed the magnitude of the shell load to 1000000. And applying at both ends to represent a compressive strength is that correct to do? U3 = 0 on all edges. while U1 = 0 on the top and bottom edges, U2=0 on the left and right edges. Picture illustrates the boundary conditions. Upon the results the eigen value is 2.09 so the Critcial load for that will be 2.09x10^6. but my theoretical reults it is 1.58x10^6. can you tell me what i am doing wrong please?

 

[TGS4]

It's unfortunate that it needs to be said, but here it is... A picture describing the orientation of a model and the boundary conditions, etc is only really useful when it has a COORDINATE TRIAD included.

Nevertheless, you calculated a value using FEA of 2.09 compared to a theoretical value of 1.58. Is you result independent of the mesh (have you performed a mesh-convergence study)?

 

[akshparm]

Thank you. I have performed mesh convergence and this is the result it converges to. Any other suggestions which might help. are the boundary conditions simulted for a simply supported plate with compression correct from the picture. The % error from the reults is very high and i want to get it to possibly <5%. Thank you for the continued response.

 

[TGS4]

What is the membrane stress (compared to yield) at the theoretical buckling load?

 

[akshparm]

I havent studied that.

 

[akshparm]

Any other suggestions as i am having great difficulty with the boundary conditions?

 

[TGS4]

What is the membrane stress (compared to yield) at the theoretical buckling load?

Perhaps you should start by finding out the answer to this question. (Hint: not all buckling is necessarily linear and therefore able to be determined by an eigenvalue solution. I'm not entirely convinced that your issue is boundary condition related.)

 

[akshparm]

yes it seems that the boundary conditions are correct, however could there just be a internal error due to the eigenvalue? From the eigenvalue how do i determine the critical load? I have added a shell edge load of magnitude 1000000 from this i then multiply it by the eigenvalue is it correct in saying this?