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We can rotate the local coordinates of the element so that the stress tensor becomes diagonal. The new coordinate system would be the principal stress axes of which are in fact the eignevectors of the stress tensor.

Once we have the eigenvectors ( which are generally orthogonal), we can find the rotation matrix to rotate the coordinates. We usually find the rotation matrix using direction cosine or Euler angels and a bit of work is required.

But I have found that ( I'm not sure yet) in Cartesian coordinates, the eignevectors can be readily used to construct the rotation matrix. Just set up a 3x3 matrix whose rows are the eigenvectors!

I have used the method in my code and the results "seems" right but I need someone to confirm this. I am also worried for instances that the eigenvectors are not orthogonal: would this lead to wrong transformation ( i.e with the transformation the stress tensor wont be diagonal)?

Thanks

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# Rotating the coordinates to coincide the principal axes

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