Failure analysis of gas turbine rotor blades

• The damage of coating caused severe localized oxidation attack to stage I blade.
• The oxidation attack leads to oxide formation and dislodgement of surface coating.
• Fatigue cracks initiated at the pits formed due to oxidation attack.
• These cracks later propagated during service and led to fracture of stage I blade.
• The broken pieces of stage I blade in turn caused IOD to stage II and III blades.

The failure of the stage I, II and III turbine rotor blades of an aircraft engine was investigated by metallurgical analysis of the failed/damaged blades. The blades were made out of Ni-based superalloys of different grades. The aeroengine has completed about 80% of the assigned life since new and several hundred hours since the last overhaul before the failure of the blades. Two blades of I stage were found broken at the top and several blades of this stage were also found with deep cuts on one of the edges. Further, several II and III stage turbine blades have dents and nicks on their leading/trailing edges. Detailed investigation including visual examination of the blade surfaces, fractography, micro structural examination, chemical analysis and hardness measurement was carried out to identify the cause of the failure of the blades. The investigation has revealed that the damage of surface coating has caused severe localized oxidation attack to I stage blade ‘A’, leading to the formation of oxide at interface between coating and substrate and pits as well as dislodgement of surface coating at several locations on the surface of the aerofoil. Fatigue cracks have initiated at these pits and propagated during service and led to the fracture of the first stage blade. Subsequently, the broken pieces of the first stage blade has caused further damage (internal object damage) to other first stage as well as the second and the third stage turbine blades in the form of dents and nicks on leading/trailing edges by impact.