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Fatigue crack branching behaviour in a dual phase steel has been investigated
using an in-situ SEM/EBSD fatigue test and a conventional da/dN test. Crack
branching results mainly from the extrusions and intrusions of slip bands
developed in the grains. The number of crack branches formed depends strongly
on the loading condition and the microstructure of the material. The in-situ
observation confirms that the formation of crack branches can significantly reduce
the crack propagation rate that leads to crack growth retardation in the main mode
I crack path. The crack branches formed are usually not ideal. They can propagate
almost transversely to the main crack direction with a mode II stress intensity
factor, SIF, and a rate that is much higher than that of the main crack.