CRITICAL MECHANISMS IN THE DEVELOPMENT OF FATIGUE CRACKS IN 2024-T4 ALUMINUM.
Summary rept. 1 Apr 67-31 Mar 68,
MIDWEST RESEARCH INST KANSAS CITY MO
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Notched samples of 2024-T4 aluminum were partially fatigued and examined by means of optical and electron microscopy to identify the critical mechanisms for crack nucleation. Under fully reversed axial loading which led to fatigue lives of 10 to the 5th power - 16 to the 6th power cycles, it was found that deformation in the notch began with the formation of fine slip followed by intensification of slip around inclusions. This intensified slip was accompanied by the production of small dislocation loops and dipoles. Fatigue cracks nucleated at large 1 micron impurity inclusion clusters, and no correlation could be found between the nucleation event and the formation of fine slip, concentrated slip, or dislocation dipoles and loops. Surface diffusion layers of Zn and Ag, designed to harden the surface layer and prevent local plastic deformation, were not effective in extending fatigue life. All of the evidence favors the view that fatigue cracks nucleated at the interface between surface inclusions and matrix by a mechanism which is relatively insensitive to local plastic deformation, but whose probability increases with inclusion size. Author
- Metallurgy and Metallography