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50 Hz frequency range and 1 % was above 51.5 Hz. Thus the resonant vibration was highly probable
and could lead to high vibratory stress. In addition to resonant vibration, the fluctuations from
under-frequency to over-frequency operation and frequent interruption of the normal operation
obviously would have caused excessive vibration which the defective and corroded braze joints were
unable to withstand leading to situations conducive to crack initiation and ultimately to failure.
From our simulated HCF test it was concluded that damage accumulation at a lower load can lead
to crack initiation rapidly even though the mean stress is increased slightly. If thc braze joint had
been strong enough, the short term effect of resonance or frequency fluctuation demanded from the
system of power supply could have been overcome and the blades would have survived. Thus, faulty
brazing and improper operating conditions had a combined effect leading to the present failure. It
appears that the degradation of the braze joint was the primary cause and the vibrations causing
high cycle fatigue was the ultimate cause of failure of the turbine blades.

5. CONCLUSIONS
The work has shown that the LP blades of a steam turbine, whose material conditions were found
to be good, failed due to high cycle fatigue. The fatigue loading conditions were brought about
primarily by the failure of braze joints which had been used to join the lacing rods to the blades to
impart rigidity.
Due to failure of the brazed joints, the natural frequencies of the blades were affected. Fretting
between the lacing rod and blade interface was operative and led to the formation of weak points
favourable for fatigue crack initiation. The failure of the brazed joint was due to a poor brazing
operation carried out during the earlier overhauling period, worsened by corrosion during service.
The crack growth was aided by excessive vibration due to frequent fluctuation from under-frequency
to over-frequency operation. The effect of resonant vibration, which also seemed to occur, would
lead to an increase in AK value and mean stress of the cyclic loading and favour the situation of
crack initiation and growth.

Acknowledgements-The authors would like to thank Drs R. N. Ghosh, S. Tarafder, V. Ranganath, N. Parida and G.
Sridhar for many stimulating discussions. The authors are grateful to Prof. I?. Ramachandra Rao. Director, National
Metallurgical Laboratory, for encouragement and permission to publish this work.

REFERENCES
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