The 8 Mode Shapes of a plane finite element

AI Thread Summary
The discussion centers on the eight mode shapes and eigenvalues of a single plane stress or strain finite element, specifically a first-order quadrilateral element with eight degrees of freedom. The original poster recalls having previously written a FORTRAN code to obtain these mode shapes and is now attempting to replicate this in MATLAB but is encountering confusing results. The eight mode shapes include various stretches and shear modes, along with three zero eigenvalue modes representing free translations and rotation. The poster seeks resources specifically focused on the mode shapes of a single element, rather than modal analysis on a mesh. The conversation confirms the correct identification of the mode shapes, prompting the poster to troubleshoot their MATLAB code.
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What are the eight mode shapes
I recall, about 30 years ago, seeing the eight mode shapes (and eigenvalues) of a single plane stress (or strain) finite element.

Also, years ago, I wrote a FORTRAN code to obtain them (using IMSL libraries)

I know there are eight (because the plane quad element had 8 degrees of freedom)

I also saw them in a book somewhere (but I do not remember the book).

There were eight mode shapes: stretch in 1, stretch in 2, a horizontal shear, a vertical shear and one other. THen there were three zero eigenvalue modes (free translation in two directions and a free rotation)

I am trying to write a MATLAB code to produce them, but I get strange results taht make no sense and am confusing myself (I do not know what Matlab is doing -- or maybe I am coding it wrong).

Can anyone point me to a site that has the eight shapes?

I am NOT looking for a eigenvalue analysis on a mesh (all my google searches bring up modal analysis on a mesh--I do not want that). I am looking for the analysis on ONE element
 
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So, you are looking for mode shapes of a first-order quad element with no boundary conditions applied? Here's what I got in Abaqus (outline of the underformed shape is shown for each mode for reference):
mode shapes.png
 
EXACTLY! That was what I remembered. I KNEW there is something wrong with my Matlab code. Now I must find the error.

THANK YOU VERY MUCH!
 
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