Fatigue Fracture of a Locomotive Diesel Engine Cardan-Shaft

A locomotive diesel engine cardan-shaft fractured when trial test for 360 h. Fractographic investigation reveals that the failure mechanism of cardan-shaft is multiple origins fatigue fracture. The crack origins are situated on the external circle surface of the transition fillet root of the splined portion, which acts as a stress concentrator. The macro-structure segregation and corresponding inhomogeneous microstructure are presented in cardan-shaft material. And the presence of more ferrite and abnormal Widmannstätten structure ferrite in core microstructure led to the low core hardness (below the minimum value required). Therefore, the estimated fatigue endurance limit of the cardan-shaft material is lower than the expected value by approximately 20%. Along with the stress concentrator in transition fillet, the fatigue crack initiated and propagated under the torsion load and the axial tensile load, ultimately leading to premature fatigue fracture of cardan-shaft. The defective forging and the heat treatment processes should be responsible for the occurrence of defective microstructure and the consequence of low core hardness.