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COMSOL, structural 3D, shell elements, livelink for Matlab, mphmatrix, constrained mass and stiffness matrices

Posted Aug 13, 2020, 7:34 a.m. EDT Structural & Acoustics, LiveLink for MATLAB, Structural Mechanics Version 5.5 1 Reply

Lucas Silveira
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Hello,

I wonder how to obtain the mass and stiffness matrices of the problem I am working with. The structural problem is modelled using 3D shell elements, with a total of 624 nodes of which 12 nodes are constrained, i.e. with 612*6=3672 degrees of freedom. My intention is to obtain the mass and stiffness matrices of the constrained structure, i.e. the matrices with size (3672x3672) and the description of the degrees of freedom. The livelink for matlab function mphmatrix gives me either:

Unconstrained mass and stiffness matrices E and K with size (14832x14832)

Constrained mass and stiffness matrices Ec and Kc with size (12240x12240)

I don't understand why the constrained matrices have more degrees of freedom than: number of degrees of freedom per node x number of nodes

Kind regards,

Lucas

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1 Reply Last Post Oct 10, 2020, 7:54 p.m. EDT
COMSOL Moderator

Hello Lucas Silveira

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Theodore Bantounas
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Posted: 4 years ago Oct 10, 2020, 7:54 p.m. EDT

Hej Lucas,

After checking with Livelink the nodes coordinates the number for different nodes is 2472. If you multiply with the number of DoFs you get your full matrix size. An eliminated matrix has removed the DoFs related to boundary conditions. A way to verify the matrices is eig(A,B) and remembering that the eigenfrequencies that you ask from Comsol should be calculated from the mass and stiffness matrix.

https://www.comsol.com/multiphysics/eigenfrequency-analysis

Hej då, Theodore

Hej Lucas, After checking with Livelink the nodes coordinates the number for different nodes is 2472. If you multiply with the number of DoFs you get your full matrix size. An eliminated matrix has removed the DoFs related to boundary conditions. A way to verify the matrices is eig(A,B) and remembering that the eigenfrequencies that you ask from Comsol should be calculated from the mass and stiffness matrix. https://www.comsol.com/multiphysics/eigenfrequency-analysis Hej då, Theodore

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