RELATIONSHIP OF SURFACE EFFECTS TO THE MECHANICAL BEHAVIOR OF METALS.
Technical summary rept., 1 Feb-30 Jul 65,
MARTIN CO BALTIMORE MD
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The role of the surface is discussed in terms of its effect on the mechanical behavior of metals. There is adequate evidence which demonstrates that a region of high dislocation concentration exists in the region at the surface of a deformed metal. This dislocation rich layer impedes the movement of mobile dislocations. It was shown in aluminum by means of strain rate cycling tests that for a given applied stress, the average velocity of the dislocations is affected by the surface. It appears possible to explain the surface effects in terms of stress field associated with the surface layer. For aluminum crystals additional internal dislocation barriers were not formed in Stage I and it was possible to show that Stage I ends when the difference between the applied stress and the surface stress on the secondary slip system equals the critical resolved shear stress. The effect of the surface was found to be greater on polycrystalline specimens than on single crystals. For Armco iron, the surface stress was almost twice as great as that due to internal obstacles. It is also shown that the yield point in high purity metals is associated with the surface layer. Author
- Properties of Metals and Alloys