First of all, thanks for all the helpful comments to my previous posts. I'm trying to get a grasp of stress tensors and have been doing some studying. In the literature i've been looking at, it says something about the eigenvalues of stress tensors and the principle stresses. This is where I'm stuck. Is there a relationship btwn the eigenvalues of the stress tensors and the principle stresses? If so how would I derive that relationship?