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The fracture surfaces of fracture toughness specimens comprise three zones; the fatigue pre-crack, the ductile portion, and a stretched zone between those two. The ASTM Subcommittee on Fractography is working on means to correlate features of the stretched zone with fracture toughness parameters. A thorough analysis of the stretched zone of an aluminum alloy fracture toughness specimen was undertaken. Quantitative measurements of the zone width and qualitative evaluation of the zone profile on stereo electron microfractographs gave the following results: The average width of the stretched zone was 10 ± 1 μm. The range of zone widths was 0 to 23 ¼m. The profile of the stretched zone was quite varied, but, statistically, the stretched zones of the two fracture halves appear to form a V, the apex of which is the start of the ductile (dimpled) fracture.
fractography, fractures (materials), ductility, toughness, electron microscopy, stereoscopes, aluminum alloys, fatigue (materials), cracking (fracturing), plastic properties, breaking, strains
Metallurgist, Nucleonics Laboratory, General Electric Company, Vallecitos Nuclear Center, Pleasanton, Calif.