EFFECT OF SURFACE ON THE MECHANICAL PROPERTIES OF METALS.
Summary rept. 1 Feb-31 Oct 65,
MARTIN CO BALTIMORE MD
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Measurements were made on single crystal and polycrystalline specimens of high purity aluminum and Armco iron to determine the net shear stress acting on mobile dislocations, t the stress due to internal obstacles opposing the motion of the mobile dislocations, ti and the stress associated with the dislocation rich surface layer, ts. For aluminum single crystals, ti is 0 in Stage I and increases rapidly in Stages II and III. For the polycrystalline aluminum, ti increases at the onset of deformation. Compared to the plastic stress, tp, the stress ratios ttp, titp and tstp are the same for single and polycrystalline aluminum at the same strain in Stages II and III. The surface stress for iron and molybdenum is much larger than that for aluminum. A preliminary investigation on molybdenum indicates that as the ratio of the surface to the plastic stresses in a direction normal to the cross section of the specimen approaches 1, the specimen becomes brittle. It also appears that the fatigue mechanism may be studied by the change in yield point due to the surface layer. Measurements before and after cyclic stressing show that the surface stress and the effect of internal obstacles are not influenced by the cycling phase. Author
- Metallurgy and Metallography