Fractographic Analysis of Fatigue Crack Growth under Two-Blocks Loading on 2024-T351 Sheet Specimens

摘要

Fatigue crack growth in 2024-T351 sheet material (6.35 mm) under load sequenceswith periodic overload blocks are analyzed nearly cycle by cycle in the electron microscope. The purpose of the investigation is to study the interaction effects between the load cycles with different amplitudes. The crack increment of the first overload cycle in an overload block is significantly larger (giant striation) than that of the direct following overload cycles and it increases systematically with the number of the small amplitude cycle applied before the overload block. It is probably a result of more primary plastic deformation around the crack tip. The crack growth situation after the first overload becomes approximately the same as in CA tests. The crack growths in the small-amplitude blocks are retarded immediately after applying the overloads. The crack growth rates decrease subsequently (delayed retardation) to a minimum value following a similar trend.