The aim of the paper is to present a fatigue crack growth analysis thru a fractography study of different crack paths obtained in different operating conditions of the pressure vessel P355NL1 steel. All experiments were performed on CTS (Compact Tension Shear) specimens under the mixed mode (I+II) loading conditions. These stress states aim to reflect simple biaxiality stress states in a pressurized cylinder. In the experiments, different operating conditions were considered. Two different kinetic fatigue fracture diagrams were proposed for mixed mode fatigue crack growth rate estimation. The obtained results suggested a fact that energy approach describes synonymously the kinetics of fatigue crack growth under mixed mode loading conditions with respect to the mean stress effect – R-ratio. Finally, the fractography analysis sugessted that the fatigue fracture surface topography changes with the mixity level.

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