Abstract
The effects of microstructure and specimen thickness on the crack closure and subsequently on the fatigue crack propagation in an Al-Li 8090 alloy have been investigated. The δ′ structure exhibiting planar slip characteristic shows a higher fatigue crack growth resistance than the (S′ + δ′) structure having homogeneous slip behavior especially at the lower ΔK region, due to the substantial extrinsic closure effects. It has been, however, observed that the effect of specimen thickness on the fatigue crack closure and crack growth rate varies with the microstructure. In the δ′ structure, a higher crack growth rate is observed in the thick specimen than in the thin specimen due to the larger amount of crack closure than in the former. It is also considered that the relative contribution of RIC to the overall closure would be large in the δ′ structure. On the other hand, in the (S′+δ′) structure, a faster crack advance is observed in the thin specimen at lower ΔK region presumably due to a larger plastic zone size through the thickness direction than in the thick specimen.
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