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The fatigue phenomenon can be described as a sequence of four phases: (1) crack nucleation, (2) microcrack propagation, (3) macrocrack propagation, and (4) final failure. The last phase is not discussed. Technical problems related to the other phases are indicated. The continuum mechanics approach is discussed, including the application of the stress intensity factor to microcracks and macrocracks. Several factors are mentioned that cannot be accounted for by continuum mechanics. Questions regarding the microstructural aspects of crack growth are analyzed, and a tentative model for crack growth is outlined. The discussion is illustrated by results of recent NLR investigations regarding the growth of microcracks and the influence of several factors on the propagation of macrocracks, including mean stress, variable amplitude loading, sheet thickness, sheet width, heat treatment, manufacture, batch, rolling direction, environment, and frequency. All data apply to aluminum alloys and mainly to 2024-T3. The main results are summarized in a number of conclusions, and problems for further research are listed.
fatigue (materials), crack nucleation, crack propagation, microcracks, macrocracks, continuum mechanics, stress intensity factor, microstructure, dislocations, inclusions, environment
National Aerospace Laboratory NLR, Amsterdam,