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Why does OptiStruct result differ greatly from those of ANSYS?

Altair Forum User

Hi all,

I want to compare the result of shell and solid element for a thin plate bending analysis. So I select a thin plate with dimension 100*100*2mm, the four edge of the plate are all clamped, and a uniform pressure is applied on the top surface.

However, I got a different result from OptiStruct and Ansys with the same solid model. Both the material and the mesh are the same. The in-plane element size is 1.25*1.25mm, and there are two elements through its thickness. The maximum stress in X direction from OptiStruct and Ansys is 74.4MPa and 158.7MPa. It's so different. What's more, according to the theory, the maximum stress may be 154MPa, which makes me suspect the optistruct result.

<?xml version="1.0" encoding="UTF-8"?><?xml version="1.0" encoding="UTF-8"?>

The maximum stress theory for four clamped thin plate under uniform pressure loading is given in following link

http://www.roymech.co.uk/Useful_Tables/Mechanics/Plates.html

What's the reason for this strange result? I have attached my HM file.

In addition, is there any document which explain the different options meaning of stress result type and average method? See figure below

2017-09-14_195741.png.54828a59e3a5717fbd08cd3573849c10.png

Thank you

Roy

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QuyNguyenDai

If the same FE mesh, same material and the same BC, you have to get 'the same' results.

So please check:

Mesh
MAT
BC

Altair Forum User

If the same FE mesh, same material and the same BC, you have to get 'the same' results.

So please check:

Mesh

MAT

BC

Agree..

Also, unit consistency, Did you scale the model by any chance?

Altair Forum User

Did you validate the result?

First, we need to investigate from which solver is the error is coming from.

WIth just results values, it is difficult to come to a conclusion.

The results of ANSYS are correct. I am solving same model with optistruct to get same results.

Altair Forum User

If the same FE mesh, same material and the same BC, you have to get 'the same' results.

So please check:

Mesh

MAT

BC

Material and BC are all fine. Somewhat mesh is different. Because ANSYS meshes the model automatically whereas in hypermesh I did it manually. But I think it wont make such a big difference.

Altair Forum User

Agree..

Also, unit consistency, Did you scale the model by any chance?

Yes, I have cross verified the total mass of the model. Also checked units of Density, E and Poissons ratio. All are fine

QuyNguyenDai

That's IMPOSSIBLE !!!

I don't 'believe' to you until you show/share us two your models.

Altair Forum User

@Rohit Ch

I am sorry, with limited information, it is very difficult to judge.

gopal_rathore

Sorry I cant share the result file as it is confidential. But i can share values of natural frequencies.

in Optistuct first natural frequency is 13.87 Hz

in ANSYS wb first natural frequency is 594 Hz

So can u plz tell me what are the possibilities of wrong input data??

The difference shown above is huge. Definitely modelling difference is expected in terms of fe steps. To conclude atleast share images of both cases showing constraint , system masses, frequency results etc.