Influence of foreign object impact mode on fatigue performance of 2198-T8 Al-Li alloy thin sheets for fuselage

The 2198-T8 is the advanced aircraft aluminium-lithium alloy used for airframe construction. Based on the digital image correlation technique, the postimpact fatigue performance of 2198-T8 aluminium-lithium alloy sheets was analysed. The foreign object impact mode includes a single impact, continuous twice-impacts at the same position and impacts at two parallel positions. For each mode, the collapse behaviours with different impact energies and insert dimensions were investigated. The results could show that residual fatigue life is sensitive to the impact mode, and through digital image correlation (DIC) nephogram analysis, on the surface of dented specimens, there exists the redistribution of strain field. For three types of impact modes, the concept of average damage volume is introduced to fit the effect of dent depth and width on residual fatigue life; the goodness of fit for the whole average damage volume is 89.5%. The crack initiation and propagation of the postimpact fatigue specimen will be affected by the stress concentration and microstructure, and the local multiaxial strain status around the dent will accelerate the crack propagation.