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This study investigated the effects of multiaxial cyclic loading on the initiation and propagation of fatigue cracks in an industrial-grade aluminum alloy. Four different loading cases, including 90° out-of-phase and asynchronous loading, were used under the control of strain. The study used cellulose acetate film replication and SEM fractography techniques to increase the understanding of the evolution of fatigue damage on specimen surfaces. The material exhibited a shear damage mechanism with a significant amount of small cracks coalescence. It was explained why, despite a high agreement with the assumptions of the critical plane approach, the predicted fatigue life was overestimated at high loading levels. A correlation between the crack initiation period and overall fatigue life, regardless of loading case, was presented, which has a potential for practical applications.
Seyda et al. (Fri,) studied this question.