Near-threshold fatigue crack growth in 8090 Al-Li alloy |
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Authors: | X J Wu W Wallace A K Koul M D Raizenne |
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Affiliation: | (1) Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council of Canada, K1A 0R6 Ottawa, ON, Canada |
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Abstract: | Near-threshold fatigue crack growth was studied in 8090-T8771 Al-Li alloy tested in moist laboratory air. The testing was
conducted using (1) the ASTM E-647 load-shedding procedure, (2) a power-law load-shedding procedure, and (3) a constant-amplitude
(CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs)
with different crack closure levels under identical testing parameters, the conventional ΔK
eff (=K
max —K
op) fails to correlate the test data and the modified ΔK
eff (=K
max
-
χKop, where χ is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel
slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains
the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF)
in terms of a microstructure-environment synergy. The transition is related to the material’s short-transverse grain size. |
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Keywords: | |
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