Sequence effects of small amplitude cycles on fatigue crack initiation and propagation in 2024-T351 aluminum

Damage contributions from small amplitude cycles for 2024-T351 aluminum are studied experimentally in both low- and high-cyclic fatigue (LCF and HCF) regimes. The small amplitude cycles correspond to 100% and 50% of the endurance limit. Two sequences (constant amplitude to block loads, or vice versa) with varying cutoff points during the sequencing are studied. Results indicate that small amplitude cycles could contribute significantly to the accumulated damage. The damage contribution is analyzed in terms of crack initiation and propagation lives. In general, block-to-constant sequencing affects mostly the initiation, while constant-to-block sequencing affects both initiation and propagation lives.