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It is well known that variable amplitude loading produces progressionmarks on fatigue crack surfaces that are related to the loading sequence. These marksare generally a local change in the crack path. In this paper, a number of simpleunderload loading sequences were used to investigate the influence that underloadshave on a crack path and to develop a better understanding of the formation offatigue striations. The material chosen was 2024-T3 and results were compared topreviously investigated 7050-T7451. These two alloys and heat treatments are twovery common high strength aluminium alloys and heat treatments used in aircraftdesign. They represent the underaged and overaged conditions in aluminium alloys.However, AA2024-T3 and AA7050-T7451 aluminium alloys are known to possesdifferent chemical composition, mechanical properties and micro-structures, it wasshown that both materials shear essentially similar fracture features corresponding tocrack propagation at cycle-by-cycle level. It also appears that despite existingdifferences, similar failure mechanisms might take place. The exact mechanism ofcrack path change is still uncertain at the moment; however, it is believed that crackpath changes are formed as a consequence of the slip bands formation ahead of crack path changes are formed as a consequence of the slip bands formation ahead of cracktip (loading part of the cycle) followed by crack tip collapse (unloading part of thecycle).