Failure analysis of the end of a shaft of an engine

The aim of this paper is to analyse the root causes of the premature failure of the end of the shaft of an engine, which was theoretically manufactured using a C45 steel. This shaft presented a keyway which was damaged and repaired by welding, machining a new one in another zone of its periphery. Neither the carbon content nor the mechanical properties of the steel agreed with those specified in the purchase order, having strength significantly lower than the envisaged one. Examination of the fracture surface revealed that the origin of the failure was sited in one corner y of the first keyway which was incorrectly repaired. From this origin, cracking progressed by fatigue until the area which remained joined was not enough for bearing the applied loading, inducing the failure. The marked damage suffered by the fracture surface precluded evaluating the existing ratio between fatigue crack growth and final fracture and determining if the stress level was high. Observation of the metallographic samples confirmed the deficient union between the key used for repairing the previous damage and the shaft promoting a faster cracking. Consequently, the failure of the shaft was attributed to the sum of various factors (lack of strength, incorrect design and execution of the repair) which have promoted the failure.