A STUDY OF THE EFFECT OF SURFACE FILMS ON FATIGUE FRACTURE
Annual summary rept. for 15 Aug 1963-14 Aug 1964
MIDWEST RESEARCH INST KANSAS CITY MO
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The complete absence of a corrosive atmosphere during the deformation of aluminum leads to a postponement of the formation of slip bands in which fatigue cracks are known to initiate. This phenomenon can be explained on the basis of a lack of dislocation pinning at the surface due to oxidation processes. Surface anodic layers suppress surface slip in proportion to their thickness. Although slip may not be observed on the top of thick anodic layers, some slip can still occur at the metal-oxide interface. The amorphous nature of the anodic layer evidently prevents transmission of this step through a thick coating. Debris in the nature of dislocation loops is associated with surface slip lines, the debris being of higher density in gold than in aluminum. The difference is presumably due to a lower activation energy for climb in aluminum. The formation of the slip step results in a lattice strain surrounding the step. This strain occurs regardless of the presence of an oxide layer. This strain may be the cause of slip-step height saturation.