This is simply because stresses that cause yielding result in rapidly increasing deformation due to the contribution of plastic strain. As discussed in Section 1.2 and illustrated by Fig. 1.2, any strain in excess of the elastic strain o/ E is plastic strain and is not recovered on unloading. Hence, plastic strains result in permanent deformation. Such deformation in an engineering member changes its dimensions and/or shape, which is almost always undesirable. Thus, the first step in engineering design is usually to assure that stresses are sufficiently small that yielding does not occur, except perhaps in very small regions of a component.