Abstract
A premature failure of alternator assembly of an aircraft was reported. After stripping of the assembly, one of the pole piece was found fractured. Material of the pole piece was low carbon steel (AISI C1006). Hardness of the pole piece was slightly higher than the standard values. Detailed investigations revealed that the pole piece was fabricated by cold forging, which induced fine surface cracks. Subsequently, during service the fatigue cracks initiated and propagated under high cyclic fatigue loadings from pre-existing fine surface cracks.
A modified design of the pole piece was suggested after determining the endurance limit using “four point correlation method” and stress levels during service using “finite element modeling”. The modified design helped to reduce the stresses well below the endurance limit from 27 MPa to <10 MPa; for previous and modified designs, respectively, thereby extending the serviceable life.
A premature failure of alternator assembly of an aircraft was reported
•Fatigue failure occurred due to surface cracks, which were formed during cold forging•A modified design was suggested after theoretical investigations of the endurance limit and stress levels during service.•The modified design helped to reduce the stresses well below the endurance limit from27 MPa to <10 MPa.•A delay in fatigue crack initiation occurred; extending the serviceable life.