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Plastic castings of fatigue cracks were produced by a process of vacuum infiltration and subsequently examined in a scanning electron microscope. Study of the castings revealed that the crack tip geometry in the 2024-T3 and 7075-T6 alloys was essentially the same. Crack advance in air is initially “brittle” in both alloys, with the crack tip subsequently blunting to an elliptical geometry at maximum load. Brittle crack extension presumably results from tensile rupture due to the presence of water vapor. At minimum load the crack tip has been resharpened but remains generally elliptical and singly notched.
In contrast, crack advance in vacuum appears to result solely from slip activity in the absence of any active species to promote failure by tensile rupture. Crack branching is believed to be a significant means of accommodating plastic flow near the maximum of the load cycle. The crack front itself appears to be highly disorganized due to crack propagation at different levels and on different slip systems in adjacent grains of the polycrystalline materials.
Castings of air fatigued specimens also shed some new light on plasticity just behind the crack tip and on contact between the two fracture surfaces during crack closure at a larger distance behind the crack tip.
aluminum alloys, striation formation, fatigue crack growth, environment influence, crack tip geometry
Associate Professor of Mechanical Engineering, University of Missouri, Independence, Mo.
Professor, Delft University of Technology, Delft,