In this paper I broaden the common text about stress theory for cases when the system of curvilinear coordinates used for the solution of a problem is not orthogonal. In this case, as it is possible to associate four different matrices to the stress dyadic of a point, I physically interpret the elements of all these matrices. That implies generalizing the classical principle of reciprocity of the tangential stress in orthogonal planes. In the remaining part of this paper I derive the main classical results related to eigenvalues and eigenvectors of the stress dyadic.
Stress
