Constraint induced stresses and the hard surface-soft surface question in fatigue

Cu, 80-20 α-brass and 70-30 α-brass single crystals were cycled in a strain controlled tension-compression mode. Chemical polishing over the entire gage length of specimens in the grips at zero load and zero strain and reloading indicated no detectable difference in flow stress, or equivalently no significant difference in the resolved shear stress on the slip system. When half the gage length was electrolytically polished in specimens unloaded from tension to zero load, bending occurred in a manner indicating that the residual σ_zz stress was higher at the interior than at the surface. Unloading from compression to zero load indicated a reversal in residual σ_zz stress distribution. Finite element method (FEM) analysis, taking into account the slip behavior of a single crystal and the effect of end constraints occurring in the test, confirmed that there was little difference between the resolved shear stress on the slip system at the surface and the interior during deformation. The FEM calculations also indicated that the residual σ_zz stress was nonuniform, in agreement with the bending experiments. These results, therefore, indicate that a difference in flow stress between surface and interior is not necessary for bending to take place when half the gage length is electropolished.