CONSTRAINT INDUCED STRESSES AND THE HARD SURFACE SOFT SURFACE QUESTION IN FATIGUE

Cu, 80-20 alpha-brass and 70-30 alpha-brass single crystals were cycle d 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 shea r stress on the slip system. When half the gage length was electrolyti cally polished in specimens unloaded from tension to zero load, bendin g occurred in a manner indicating that the residual sigma(zz) stress w as higher at the interior than at the surface. Unloading from compress ion to zero load indicated a reversal in residual sigma(zz) stress dis tribution. Finite element method (FEM) analysis, taking into account t he slip behavior of a single crystal and the effect of end constraints occurring in the test, confirmed that there was little difference bet ween the resolved shear stress on the slip system at the surface and t he interior during deformation. The FEM calculations also indicated th at the residual sigma(zz) stress was nonuniform, in agreement with the bending experiments. These results, therefore, indicate that a differ ence in flow stress between surface and interior is not necessary for bending to take place when half the gage length is electropolished.