Open Collections

Fatigue crack propagation in Al-Li alloy 8090 : environmental effects

University of British Columbia

Fatigue crack propagation behaviour in Al-Li alloy 8090 plate were studied in four main testing environments: a relatively inert environment - desiccated air; and three freely corroding aqueous environments consisting of distilled water, 0.6 M NaCl and 1 M AlCl₃. It was found that the major role of the aqueous environments in the T-L orientation crack growth behaviour of the Al-Li alloy plate was to promote S-L splitting (delamination) at grain boundaries, with a subsequent effect on the stress state at the crack tip. The splitting preceded the main crack advance, and helped to keep the crack in the same macroscopic crack plane and restricted the fracture surface roughness, and consequently reduced crack closure effects. In contrast, the absence of the splitting effects in desiccated air, combined with the planar deformation features and the strong crystallographic texture of the Al-Li alloy plate, led to out-of-plane cracking and ridge formation in the mid-thickness of the specimen. This resulted in severe crack growth retardation and a crack growth rate plateau at ΔK values above ∼ 3 MPa.m[sub ½]. Analyses of the observations led to the conclusion that the S-L splitting phenomenon is associated with both localized anodic dissolution processes and hydrogen embrittlement effects. The effect of changing the loading frequency was not obvious, except in the dry air environment where decreasing the frequency from ∼80 to ∼0.5 Hz led to the disappearance of the crack growth rate plateau. The fatigue cracking resistance of the Al-Li alloy plate is superior in the dry air environment and slightly better or equivalent in the aqueous environments when compared to a conventional crack-tolerant Al-alloy plate, 2024-T35 1. The effects of a re-aging treatment on fatigue cracking in the S-L orientation of the Al-Li plate were also studied. The presence of aqueous environments also accelerated crack propagation in the S-L orientation, and crack propagation is faster in the S-L orientation than in the T-L orientation. The re-aging treatment was effective in increasing the short-transverse fracture toughness and also improved the intergranular corrosion resistance of the material. However, the re-aging treatment did not improve the corrosion fatigue crack propagation resistance.

Thesis/Dissertation

application/pdf

2009-06-04

For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.

http://hdl.handle.net/2429/8772

Materials Engineering

University of British Columbia

UBCV

DSpace