Results with SOL106

[sushi75]

Hi everybody!

I've got a question about results I obtain from SOL101 and SOL106, which are massively different, so I wonder which one is the most accurate.

The analysis is a pressurized plate, pinned at both ends.

Linear Static analysis predicts a pretty large deflection, and a high max stress.
However, the non linear one shows a small deflection and lower stresses.

So it seems that with the non linear analysis, the plate stiffness increases, but as we consider large displacement I was expecting a greater deflection.

Also, linear predicts a large bending moment, while the non linear shows high tension force on the plate (I guess what we call follower force). But I cannot really understanding physically how it works.

Thus, I'm a bit puzzled about how to decide which result is valid, and more generally how to have the flavour of the behiavour of the structure to choose between 101 and 106.

Thanks a lot for your help on that!

Cheers

 

[rb1957]

SOL101 (linear static) and SOL106 (non-linear) will give vastly different solutions for a flat plate under pressure.

The linear model is typically very inaccurate, as defelctions will quickly exceed 1/2 the thickness of the plate (invalidating the model).

SOL106 should be giving you accurate results. if you want to investigate this, load the plate with a very low pressure, and SOL101 and SOL106 should give similar answers.

another day in paradise, or is paradise one day closer ?

 

[BlasMolero]

Hello!,
Nonlinear analysis with NX NASTRAN (SOL106) is more accurate than linear static one using SOL101, of course!!. The diferences in displacement & stress results will depend of the severity of the nonlinearities present in your FE model: if you arrive to simmilar results (no difference in stress results more than 5%) then you can say that the linear static results are correct, your model is linear.

Geometric nonlinear effects should be significant if the deformed shape of the structure appears distinctive from the original geometry when you inspect it visually.

A more rigorous and quantitative definition for the large displacements can be derived from the plate theory of Kirchhoff and Love, which states that the small deflection theory is valid for a maximum deflection of less than 20% of the plate thickness or 2% of the small span length. However, this definition seems to be a little conservative for numerical analysis. Additionally, there is no distinct limit for large displacements because geometric nonlinear effects are related to the boundary conditions as well as the dimensions of the structure. If the load-deflection curve of the critical point can be estimated, the loading point should be in the nonlinear portion of the curve.

Geometric nonlinear effects in the structure involving large rotations, whether rigid body rotations or deformation induced rotations, are self-evident. Stiffening of a membrane, stiffness in a pendulum or snap-through of an arch belong to this category.

The problem you are seeing in your nonlinear SOL106 analysis is related with the stress stiffening effect: the term refers to a coupling between membrane stress & lateral displacements associated with bending. The bending stiffness of a beam, arch, plate o shell is increased by tensile membrane stress and is decreased by compressive membrane stress (buckling). And this effect is only captured by nonlinear analysis SOL106 (PARAM,LGDISP,1), the linear static analysis SOL101 simply ignores.

Stress stiffening effect (ie, geometric nonlinearity) is usually neglegible for massive bodies but may be important for thin-walled structures, then is up to you to run always a nonlinear analysis and compare with the linear solution, this way you can validate your results, OK?.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

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[jacksonfem]

Could you please post the characteristics of your model? Dimensions, materials, loads etc.
Are you sure you set the SOL 106 with right way?

 

[sushi75]

Thanks for your replies! it's much clearer now.

So what the non linear solution does, is to update the stiffness matrix during incremental steps. Is is the same approach/matrix which is used for buckling? From my understanding, buckling is using a differential stress stiffness matrix based on higher order terms of the strain-displacement relation.

I try to get the full picture...!

Thanks!