A YIELD SURFACE APPROACH TO THE ESTIMATION OF NOTCH STRAINS FOR PROPORTIONAL AND NONPROPORTIONAL CYCLIC LOADING

An analytical method is developed to estimate notch root strains in a notched bar of elastic-plastic, isotropic material subjected to propor tional and nonproportional multiaxial nominal loading. The method uses anisotropic plasticity theory to define a structural yield surface in nominal stress space that incorporates both the isotropic material pr operties and the anisotropic geometry factors of the notch. Notch root plastic strain increments and anisotropic work-hardening effects are then related to this yield surface using standard methods of plasticit y. Comparisons of the proposed method with previously published strain estimates using the finite element method for a notched shaft under p roportional nominal bending and torsion, and with strain gage measurem ents of a circumferentially notched solid steel shaft subjected to a s eries of box-shaped nonproportional loading paths in tension-torsion n ominal stress space are presented. The strain calculations agree well both qualitatively and quantitatively using an appropriate nominal loa d-notch plastic strain relationship, and are suitable for strain-life fatigue calculations.