FEM, Matlab and the modes of an element

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SUMMARY

The forum discussion centers on the eigenvalue analysis of an 8x8 stiffness matrix for a 4-noded isotropic element in a plane strain problem using Matlab. The user reports obtaining five non-zero eigenvalue modes and three zero-eigenvalue modes, which they believe should represent two pure translations and one rotation. The main challenge is to modify the Matlab code to visualize these zero eigenvalue modes correctly, as the current output appears to be combinations of these modes rather than distinct translations and rotations. The discussion also touches on the implications of using the stiffness matrix without a mass matrix in the context of finite element analysis (FEM).

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  • #31
Trying2Learn said:
Matlab reports these eigenvectors and I do not like the way they look. If I am a first time FE coder and I experiment to see the modes, and get those shapes, NOTHING in them informs me that three of the eigenvectors would have been MORE INFORMATIVE had I seen them as pure translations and one pure rotation. How would I have known this in advance.

Well, the zero-valued eigenvalues is a big give-away... They never occur for fully constraint problems.
 
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  • #32
Arjan82 said:
Well, the zero-valued eigenvalues is a big give-away... They never occur for fully constraint problems.
And, oddly, THAT was what I wanted to hear. Those last two sentences, were what I was looking for. And, now that you said them, it is obvious. I should have known that.

Thank you for patience (I know I can be stubborn -- sorry if it came across that way).
 
  • #33
Well, we took some detour, but we got there in the end :) All is ok!
 

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