Uniaxial tension tests on uniform cross section specimens of Ti-6Al-4V
and aluminum 7475-T651 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 paint. Above this stress, the xray
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 exceeding 2-3 grain
or domain diameters. Further work is required to refine the model for
real systems and to assess the importance of material effects acting
in conjunction with the continuum effect.
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