Extrusion/intrusion structures as quantitative indicators of accumulated fatigue damage

Two approaches to aircraft fatigue monitoring by the computer-aided analysis of surface structures are described: (a) application of fatigue indicators attached to the aircraft unit; (b) the direct observation of the alclad aluminium alloys surface. The evolution of aluminium surface state has been monitored at various fatigue loading regimes. Some parameters have been used for the quantitative analysis of surface structures: (a) density of slip lines for single-crystals; (b) intensity and fractal dimensions of the deformation relief for polycrystalline aluminium. The possibility of a fatigue process prediction both at crack initiation stage and fatigue crack propagation stage is shown.