Crack growth behavior under biaxial fatigue with phase difference

• Studied fatigue crack growth behavior under biaxial fatigue with a phase difference.
• A crack splits into two symmetric cracks under biaxial fatigue with phase difference.
• Analytical and finite element analyses confirms experimental splitting observations.
• Total strain energy release rate of split cracks is equal to that of initial crack.
• As far the authors are aware, this is the first study in this respect.

Crack growth behavior of aluminum alloy 7075-T6 was characterized under in-plane biaxial tension–tension fatigue with phase differences of 90° or 180° between the two applied orthogonal cyclic loads. The initial single crack, created under the biaxial fatigue without any phase difference, splits into two symmetric cracks under the biaxial fatigue with the phase difference. The split cracks grow without any further branching. Directions of split cracks deviate sharply from the direction of the initial single crack. Under both phase differences of 90° and 180°, lengths of both split cracks are almost the same at a certain number of cycle. Strain energy release rate versus crack growth rate relationships of the split cracks are almost equal to each other. Further, sum of strain energy release rates at a given crack growth rate of both split cracks is equal to that of a single crack under the biaxial fatigue without phase difference. Analytical and finite element analyses are presented to explain the splitting of a crack due to the phase difference between the applied biaxial cyclic loads.