ABSTRACT
Consider a two-dimensional (2D) case of a plane stress, as described in Appendix A. Stress at a point is a tensor, which means that three components are needed to dene it. Having those three components, σx, σy, and τ in some coordinate system is not always meaningful. For this reason, one often seeks the extreme values or three stress invariants, σmin, σmax, and τm. Having these three claries the state of stress, but does not answer the essential question: Will the material suffer permanent deformation or damage because of being stressed to that level?
