There is a strong indication that both blades failed due to fatigue. Its nucleation was associated with the presence of corrosion on the convex side for blade no. 6 and on the concave, trailing edge for blade no. 4. The corrosion manifesting itself as pits (spherical cavities) had acted as stress raisers, which then promoted cracks. Beside that blades can failed due to accelerated by foreign material into compressor blades. Fatigue fracture covered almost the entire area of the fracture surface of blade no.6. It is typical of low stress high-cycle fatigue, i.e flate-face (plane-strain) fracture and the final fracture region was very small. On the contrary, the plane fracture surface (fatigue region) of blade no. 4 only covered about 40% of the fracture surface and showed brittle appearance. The final fracture region, which covered 60% of the fracture area, indicates the occurrence of plastic deformation. As the fatigue region is typical of low stress high-cycle one whilst the large area of final fracture seems to have been caused by a high loading stress, the failure of blade no. 4 may be assumed to be the results of fatigue and a static overloading. The latter may have been brought about by its neighbouring blades (blades no. 5 and 6) which had failed and shuttered prior to and in turn hit the blade no 4. (Tonny Sarief, Sugiarto, M. Choliq - PLN Litbang)
