Abstract: An in-flight shutdown engine is disassembled and key components are taken to a lab for investigation. It is found by disassembling investigation that a cluster of low pressure turbine stage 4 (LPT4) vanes causes the trouble. The cluster of vanes fracture at the hook and cock backwards so as to break all LPT4 blades and part of LPT5 and almost all LPT6 vanes and blades. Macro and micro observation results show that all vanes fracture because of high cycle fatigue caused mainly by small transition radii of the hook. Calculation by means of the finite element method indicates that the small transition radii significantly decrease the ability of the blades to resist vibration stress. In addition, the size of the transition radii is below the technical requirements of the manufacturer. Metallurgical observation reveals that the vanes have been welded and reprocessed for repair, which results in the small transition radii condition.

Key words: fatigue, fracture, failure analysis, in-flight shutdown, turbine blade