Near-Threshold Fatigue Crack Growth in 8090 Al-Li Alloy

摘要

Near-threshold fatigue crack growth was studied in 8G9G-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 DELTA K_(eff) (=K_(max) - K_(op)) fails to correlate the test data and the modified DELTA K_(eff) (=K_(max) - K_chi(op)), where x is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rapture 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.