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Linear Buckling no eigenvalue found error

Posted 2020年10月29日 GMT+1 05:38Structural & AcousticsVersion 5.44 Replies

Chanho Jeong
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I've tried a simple buckling test. But an error occured

-Feature: Eigenvalue Sover 1 (sol1/e1) No eigenvalues found.

The simulation is a really simple one. - Just a cylinder(radius=0.03m , Height=1m) - Structural steel material - Fixed bottom boundary and -10N (z direction) boundary load.

I'm trying the 3D model, Solid Mechanics, Linear Buckling. Can everyone tell me what the problem is? Or is it a Comsol version issue?

Thank you

4 Replies Last Post 2020年11月5日 GMT+1 06:12
Henrik Sönnerlind COMSOL Employee

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Posted: 4 years ago 2020年10月29日 GMT+1 18:41

The underlying reason is the symmetry of the structure, which leads to that there is a double eigenvalue with an eigenmode which can bend in an arbitrary direction in the xy-plane.

There are several possible solutions. Three of them are:

  • Select to solve for 2 (or more) eigenvalues instead of 1.
  • Set a manual value (>2) for the size of the Krylov space in theEigenvalue Solvernode.
  • Relax the relative tolerance to 1e-4 in theEigenvalue Solvernode.

The behavior is a bit odd; the solver should be able to handle this case. We will have to look into it.

-------------------
Henrik Sönnerlind
COMSOL
The underlying reason is the symmetry of the structure, which leads to that there is a double eigenvalue with an eigenmode which can bend in an arbitrary direction in the xy-plane. There are several possible solutions. Three of them are: * Select to solve for 2 (or more) eigenvalues instead of 1. * Set a manual value (>2) for the size of the Krylov space in the *Eigenvalue Solver* node. * Relax the relative tolerance to 1e-4 in the *Eigenvalue Solver* node. The behavior is a bit odd; the solver should be able to handle this case. We will have to look into it.
Chanho Jeong
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Posted: 4 years ago 2020年10月30日 GMT+1 07:05

Thaks a lot. By simply solving with 2 eigenvalues, the error dissapeared.

However there is one more issue about the result I can not understand. When I adjust the deformation scale factor to 1 the result is a lot weird and unrealistic. It looks more like 100000 times of the scale factor. It doesn't matter how much small load I apply (-0.000001 N/m^2 for example).

Is there any method i can solve this problem? Thank you.

Thaks a lot. By simply solving with 2 eigenvalues, the error dissapeared. However there is one more issue about the result I can not understand. When I adjust the deformation scale factor to 1 the result is a lot weird and unrealistic. It looks more like 100000 times of the scale factor. It doesn't matter how much small load I apply (-0.000001 N/m^2 for example). Is there any method i can solve this problem? Thank you.
Henrik Sönnerlind COMSOL Employee

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Posted: 4 years ago 2020年10月30日 GMT+1 14:43

The amplitude of an eigenmode is per definition arbitrary and has no physical meaning.

You can control how the scaling is done by modifyingScaling of eigenvectorsin the settings forEigenvalue Solver.

-------------------
Henrik Sönnerlind
COMSOL
The amplitude of an eigenmode is per definition arbitrary and has no physical meaning. You can control how the scaling is done by modifying *Scaling of eigenvectors* in the settings for *Eigenvalue Solver*.
Chanho Jeong
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Posted: 4 years ago 2020年11月5日 GMT+1 06:12

Thanks a lot. I really appreaciate your help.

So it sounds like I can not expect the exact deformed geometry visually. Anyway, there is still some phenomena I can't understand when performing buckling simulation.

The simulation is still simple. - Version 5.4, 3D model, Solid Mechanics, Linear Buckling. - Just a cylinder(radius=0.03m , Height=1m) - Structural steel material - Fixed bottom boundary - Top boundaty has prescribed displacement -0.1m z direction (x,y are fixed to 0) .

There isn't an error but the result is not acceptable First, checking the z direction deformation (w-Displacement field, Z component), the top boundary hasn't moved at all. Second, maybe becasue of the first reason, total displacement (solid.disp-Total displacement-m) also says that there isn't displacement for the top boundary.

Could you also tell me why this happen or any solution for this problem? Thank you

Thanks a lot. I really appreaciate your help. So it sounds like I can not expect the exact deformed geometry visually. Anyway, there is still some phenomena I can't understand when performing buckling simulation. The simulation is still simple. - Version 5.4, 3D model, Solid Mechanics, Linear Buckling. - Just a cylinder(radius=0.03m , Height=1m) - Structural steel material - Fixed bottom boundary - Top boundaty has prescribed displacement -0.1m z direction (x,y are fixed to 0) . There isn't an error but the result is not acceptable First, checking the z direction deformation (w-Displacement field, Z component), the top boundary hasn't moved at all. Second, maybe becasue of the first reason, total displacement (solid.disp-Total displacement-m) also says that there isn't displacement for the top boundary. Could you also tell me why this happen or any solution for this problem? Thank you

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