During the performance testing of a CFM56-3C1 engine (engine s/n 725274) in an engine test cell, 7 July 2004, a severe shift in the engine exhaust gas temperature was observed when the engine was operated at take-off power. Subsequent borescope inspection revealed that sections of two, adjacent, high-pressure turbine (HPT) blade airfoils had broken away. Examination of the blades revealed that blade s/n GSH81 fractured through the blade airfoil section as a result of fatigue crack growth. Fatigue cracking initiated in the fourth internal rib from a planar defect created by intergranular oxidation. The loss of material from the leading edge of the adjacent blade, s/n 331R5, was a secondary event.
HPT blade fracture control depends on the prevention of intergranular oxidation that creates defects that allow fatigue crack propagation to occur under the thermal and alternating stress conditions imposed on a blade.
Variability in nature of defects created by intergranular oxidation may be related to variations in the grain structure of other blades of the same design and the effectiveness of oxygen diffusion barriers at the surface of the internal ribs in the blades.