Load sequence effects on the fatigue crack growth in a cylinder subjected to combined rotary bending moment and axial force loads

• Crack growth in a hollow cylinder subjected to combination of rotary bending and tension is studied.
• An important result of the crack geometry evolution during loading cycles is presented.
• The crack geometry during growth process depends on the life fraction (N/NR).
• For the variable loading, it depends only on the accumulated life fraction of the already applied cycles.
• This conclusion explains the damage accumulation laws in fatigue based on life fraction.

This study used a fatigue crack growth model of a semi-elliptical external surface crack in a hollow cylinder subjected to a combination of rotary bending moment and axial force. In the literature, several authors have explored the problem of crack growth in a hollow cylinder but only for each loading case separately: cyclic tension, rotary bending and cyclic bending. Furthermore, no one has studied the effect of load sequence for this case of structure and loading. In this paper, the study shows the evolution of crack geometry during loading cycles seemingly depends only on accumulated life fraction of the already applied cycles (life fraction is the ratio of the number of load cycles N   and the crack growth life NRNR corresponding to the constant cyclic loading). Therefore, the fatigue crack growth is independent of the load level and the load sequence. This remark can be used to explain the damage accumulation models in fatigue based on life fraction.