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National Advisory Committee for Aeronautics, Technical Notes - Application of Statistical Methods to Study of Gas Turbine Blade Failures

An investigation was conducted to determine the applicability
of statistical methods as an approach to the evaluation of materials
for use in gas turbines by determining the frequency distribution
of time until failure of gas-turbine blades. Gas-turbine blades
of a cast cobalt-base heat-resistant alloy were run in gas-turbine

wheels of approximately 12%;inch total diameter. The wheels were
operated at accelerated-life conditions: a rotor speed of
22,500 rpm and an indicated turbine-inlet-gas temperature of 18500 F.

Two wheels, each incorporating 142 inserted blades of this
alloy were tested. In one wheel the entire complement of blades
failed. In the other wheel 64 of the blades were failed. The
complete frequency distribution of time until blade failure is
presented for the first wheel and cumulative frequency curves are
presented for both wheels. The blades in the first wheel had
an average life of 25.52 hours with a standard deviation of
11.49 hours.

The need for representative perfomance-life figures as sup-
plied by statistical methods of analysis for use in comparative
evaluation and for correlation of perfomance data with laboratory
properties, especially during research and development, is indicated.
Use of time for initial failure to evaluate material performance
will probably yield results that differ from those obtained by the '
use of a representative average value.

Gas-turbine cycle efficiency and power output are dependent
to a large extent upon the gas temperature used. The maximum
allowable temperature is limited by the capacity of the materials,
particularly that of highly stressed parts such as turbine blades
to perform satisfactorily for a sufficient period of time to result
in a practical over-all economy. Life expectancies of turbine parts
currently are short even at relatively low operating temperatures.
new materials have been developed and older ones have been improved
in an attempt to increase life expectancy. As such materials
become available, a comparative indication of their potential per—
formance must be obtained.