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In the present article an in-service fatigue failure of a large propellershaft is analyzed and discussed. Observed crack growth path and fatigue life arecompared with fracture mechanics calculations for various loading modes to determinethe most likely fatigue damage mechanism. It was found that the fatigue crack leadingto failure of the shaft emanated from a flaw on the surface. The initial flaw had a depthin the range of 0.5-1.0 mm. The subsequent crack shapes were revealed from crackfront beach marks. Larger cracks had approximately a semi-elliptical shape with anaverage aspect ratio near 0.8. Although the design stresses in the shaft were fluctuatingshear stresses due to torsion, unforeseen rotating bending stresses may have occurreddue to misalignment of the shaft bearings. Based on the observation of the crack frontshapes, it was shown that the fatigue failure of the shaft was driven by a multiaxialstress situation dominated by rotating bending stresses at an early stage.