Analysis of a J69-T-25 engine turbine blade fracture

The fracture of a turbojet engine turbine blade was investigated. Visual and surface examination showed that the turbine blade had initially cracked by a fatigue mechanism over a period of time and then failed by overload at the last moment. The fatigue crack initiated on a surface damaged by rubbing. Also the cracked turbine blade was severly damaged by hot gas flow and discolored. This heat damage made the gamma prime phase in the matrix (Ni-base superalloy) coarsen and lowered the fatigue strength of the base material assisting to the premature fatigue fracture. The decrease of the strength of the material due to degradation of gamma prime phase was verified by hardness measurements. The possible relevance of other parts attached to the engine shaft to the fracture was reviewed. From this review it is inferred that the root cause of cracking and excessive heat damage might be attributed to eccentricity of the shaft resulting from various reasons—shaft misalignment, uneven wear of bearing elements and mismatch in clearance, etc. However this is an assumption that should be verified by positive supporting evidence from condition monitoring of engines.