Reconstruction of fatigue crack growth in aluminum lithium and aluminum copper fastened lap joints under marker band loading

A comparative study on the fatigue crack growth behavior in a typical third generation Al/Li alloy (AA2198-T8) and traditional aerospace aluminum alloy (AA2024-T3) was conducted. Fatigue crack growth history was reconstructed using marker band loading sequences (i.e., applying two different R-ratios). Effect of alloying and loading direction (relative to the rolling direction) was discerned from the fatigue fracture surface morphologies, which were markedly different in the two alloys. The marker band imprints aided in identifying suspected fatigue crack initiation region and estimating crack growth rate. A data reduction scheme was used to estimate crack initiation cycle.