Challenge

A high-strength alloy-steel shaft had failed prematurely in service. The objective was to determine the failure mechanism, identify the root cause of crack initiation and establish appropriate measures to prevent recurrence.

Engineering Approach

A forensic investigation of the failed shaft was undertaken to identify the fatigue mechanism and determine the location and cause of crack initiation. Examination of the fracture surface and shaft microstructure identified corrosion pitting and micro-cracking associated with the initiation region. A hydraulically actuated cyclic fatigue test rig was subsequently designed and constructed to reproduce the loading conditions and investigate the failure mechanism experimentally. The combined forensic analysis, stress assessment and physical testing were used to establish the root cause of the premature failure.

Outcome

The investigation determined that the shaft had failed by fatigue under relatively low bending stress. Crack initiation was traced to microscopic corrosion damage associated with corrosion pitting, which provided initiation sites for subsequent fatigue cracking. The susceptibility was associated with the particular high-strength alloy steel and its operating environment. Recommendations were developed covering alternative material selection and corrosion-protection methods that would provide the required environmental protection without adversely affecting the surface fatigue performance of the shaft.