Analytical study of unexpected failure of a landing craft’s intermediate propeller shaft via fracture cross-section examination

This study investigates the unexpected premature failure of the left intermediate propeller shaft of a dual-propulsion landing craft. A minor surface defect of unknown origin, most likely introduced by a machining error during repair, significantly reduced the shaft’s fatigue life. To elucidate the failure mechanism, the defect is modeled as a small semi-elliptical surface crack using finite element method (FEM) and linear elastic fracture mechanics (LEFM). The evolution of the crack shape is simulated via the fatigue crack growth rate equation, where the equivalent stress intensity factor serves as the primary variable. Several definitions of this factor are examined to evaluate their capability in accurately predicting crack growth behavior and fatigue life. Torsional vibration–induced shear stresses are experimentally measured on a similar propeller shaft operating within the barred speed range. By comparing the simulation results with the geometric characteristics of fracture cross-section obtained through the TS09 Total Station, the capability of fracture mechanics to simulate crack evolution is confirmed. The results reveal that the combined effect of unforeseen bending-induced normal stress amplitudes and torsional vibration shear stresses is the primary driver of the catastrophic propeller shaft failure. Finally, the findings of this research are incorporated into the propeller shaft maintenance program as practical guidelines to prevent similar failures in the future.