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Dirk76

Validation of 1D and 3D simple model

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Hi all,

 

I was running optistruct 13.0 in order to make a correlation between a 1D and 3D model. First of all I simply created a fixed beam at one end, and the other end with a force of 1 kN. The outputs were very simple to obtain, indeed I could actually have pretty similar results in terms of stresses, displacements and constraining reactions. For the 3D the force and the constraint were applied using RBE2 elements, considering a node as independent and the ones on the cross-section of the beam as dependents, as you can see in the figures below I attached. Now the thing is, when I tried to make a correlation between a slightly different model than the one mentioned above, I couldn't obtain similar results, instead they were really different. I made a comparison between stresses, displacements and constraining reactions. The force applied was 1 kN as well, on the free end of the "T-structure". The section used for both 1D and 3D was an IPE360. For the beam of 1 m (the one where I applied the force) I used a freeze contact with the one of 2 m (fixed at both ends), in order to replicate the same condition of the 1D. The displacements were really different, almost 2 order of magnitudes of difference... the stresses obtained in the 1D was 4 times bigger than the one in the 3D and the constraining reactions (MRy) obtained didn't equal the real flexural moment generated by the force applied at the free end (it should equal 250000 N*mm as in the 1D, for a fixed-fixed condition). I attached all the comparisons and the .hm file if you wanna see the pre-process of the analysis. Could someone give me an explanation on this? Thanks in advance for any replies!

constraint.PNG

force.PNG

displ.PNG

MRx.PNG

MRz.PNG

Ry.PNG

von mises.PNG

displ.PNG

MRx.PNG

MRz.PNG

Ry.PNG

von mises.PNG

model_1D.hm

model_3D.hm

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Hi Prakash, thanks for the reply.

 

Yes, you'll find them attached to the post as well as the comparison of the results in terms of stress, displacements and constraining reactions. As you can see from first sight, the two models are practically correlated, the slightly differences are acceptable since we're comparing a 1D model with a 3D one. I think that the value of 1.020e+06, in the 3D, for MRx is due to the fact that the independent nodes have an offset of 10 mm from the central node on the neutral axis, for both the applied force and the constraint. In the 1D you have, instead, the correct value since you apply directly the force and the constraint on the nodes of the beam. If you try to make a simple calculation with pencil and paper, you'll find these values for the two models, meaning they work pretty well.

1D.PNG

3D.PNG

displ_1D.PNG

displ_3D.PNG

MRx_1D.PNG

MRx_3D.PNG

Ry_1D.PNG

Ry_3D.PNG

von mises_1D.PNG

von mises_3D.PNG

model_1D.fem

_odel.3D

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