Significant fatigue crack branching is observed for Al–Cu–Mg alloy under both spectrum and constant loading.

Branched cracks are classified into two types based on the branching processes at macroscopic scale.

There is a relation between the appearance of the novel crack branching and load interval.

The grain shapes and their interaction with the plastic zone size contribute to the appearance of this crack branching.

The surface secondary crack is located near the plastic zone boundaries of the main crack.