Residual Stress Changes in Fatigue. Volume 2. A Continuum Surface Layer Effect in Polycrystalline

Uniaxial tension tests on uniform cross section specimens of Ti-6A1-4V and aluminum 7475-T65l show that the stress in the direction of load application determined by x-ray diffraction and the nominal applied stress display the expected linear correspondence up to a maximum stress somewhat below the bulk yield point. Above this stress, the x-ray stress is noticeably less than expected. Upon unloading, there is a substantial acquired compressive residual stress. Because the x-ray diffraction measurements provided average stress values in a surface layer of only a few grain diameters, these results suggest that the surface layer of the metal is yielding at a lower stress than the bulk. This anomalous behavior is duplicated qualitatively with a continuum model of an aggregate of soft and hard square grains obeying the Von Mises yield criterion. The effect is purely mechanical. No material effects, such as lower yield point or dislocation density for the surface grains, are invoked. The continuum effect decreases rapidly with depth, becoming negligible for depths in 2-3 rain or domain diameters. MM