The current methods of fatigue life estimation do not model the interaction of stress with microstructural discontinuities. An experimental study of fatigue crack nucleation in 7075-T6 and 7079-T6 aluminum alloy sheet was performed to obtain more information on the role of discontinuities in the microstructure and to determine the relative contributions of each type of discontinuity. Some specimens were machined from recently manufactured material without any coating (cladding or anodizing). Some were fabricated from old clad-anodized material from an aircraft fuselage and old anodized material from a stock of unused wing panels. In the uncoated materials, large constituent particles and the surface roughness were found to be important factors in fatigue crack nucleation. In the coated materials, discontinuities related to the cladding and anodizing appeared to be the controlling factors in crack nucleation and particles did not play any role. The coated specimens showed a low scatter and a relatively low fatigue life. Multiple nucleation sites were observed in the coated materials. In the uncoated 7075-T6 materials in this study, large, iron-bearing, constituent particles were a factor in 25% of the failures, but marks due to material manufacture on the wide (LT) faces of the specimens appear to have been the predominant source of failure. In the coated 7075-T6511 and 7079-T6 specimens, all nucleation sites were located on a wide face, and the main controlling factor for crack nucleation appears to have been the coating: anodizing, shot-peening and anodizing, or cladding and anodizing. Particles played no role in fatigue crack nucleation in the coated specimens. The fatigue test data also indicated that the coatings were detrimental in fatigue. Another difference observed in the coated (clad and/or anodized) specimens was the presence of multiple nucleation sites on some of the fracture surfaces.
Determination of fatigue related discontinuity state of 7000 series of aerospace aluminum alloys
Bestimmung des ermüdungsbezogenen Diskontinuitätszustandes der 7000er Reihe von Raumfahrt-Aluminiumlegierungen
Engineering Failure Analysis ; 14 , 4 ; 673-685
2007
13 Seiten, 11 Bilder, 2 Tabellen, 22 Quellen
Article (Journal)
English
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