| Fatigue damage can degrade the service lives of aging aircraft. Typical damage in aging aircraft can take the form of fatigue, corrosion, and the interactions of corrosion and fatigue. In this dissertation, the fatigue crack initiation and propagation are studied in both air and corrosive environments. While laser speckle sensor technique is employed to monitor the surface roughness changes that eventually lead to crack initiation, crack growth and final failure, moire method is used to study crack growth by obtaining the deformation patterns of cracked objects.;Since structures are pre-loaded before they are in dynamic service, the effect of pre-existing static strain on cumulative fatigue damage is discussed. Fasteners influence the strength and endurance of the whole structure; therefore, the fatigue damage to fastener holes is studied.;Corrosion and fatigue are two major factors that contribute to the aging of aircraft. These two factors can act separately, sequentially or simultaneously. Experiments are designed to investigate the fatigue life of Al 2024-T3 under three different types of environments: (1) fatigue with pre-existing corrosion, (2) initial fatigue then corrosion and then fatigue, and (3) fatigue in corrosive solution. An interesting result is that corrosion could be beneficial to fatigue process under some conditions. In the test of fatigue then corrosion and fatigue again, the total fatigue life (as a sum of the initial fatigue cycle and the additional cycle to failure) is longer than those brought to fatigue failure solely in air. On the other hand, simultaneous corrosion and fatigue reduces fatigue life drastically. As both fatigue crack nucleation and corrosion pitting are surface phenomena, their deterioration to structures is not just simply superimposed.;Catastrophic failure is the final result of the growth of cracks. To characterize crack growth, the crack tip strain model is proposed and the relationships between the crack tip strain and the crack growth rate are established. |
