Failure analysis of rotating shaft in main undercarriage

A rotating shaft was found to have some cracks at a hole edge and the hole inwall during overhauling after the main undercarriage had completed 3 times the target life in fatigue testing. The rotating shaft in the main undercarriage completed 4 times the target life in fatigue testing altogether. The material of the rotating shaft is 30CrMnSiNi2A super-strength steel. In order to find out the failure mode and cause, a number of testing was performed, including appearance inspection, residual stress testing, fracture surface observation, energy spectrum analysis, microstructure examination, hardness testing and chemical composition analysis. The relationship between fatigue crack propagation rate and crack length was given by the fatigue fractography quantitative analysis. The results indicate that the fracture of the rotating shaft was caused by high cycle fatigue cracking, and the cracks initialized before 2 times the target life and 5 loading spectrums. The initiation of the cracks was mainly related to the large load spectrum stress, damage of the source region side and residual stress. On the basis of the analysis above, the assembly process and surface treatment were improved. As a result, the rotating shaft is now able to stand security life testing (with 7 times of target life).