Gas Turbine Handbook : Principles and Practices

194 Gas Turbine Handbook: Principles and Practices
Figure 12-6. Turbine blade material loss.
increase the natural frequency. Conversely, damage occurring at or
near the blade root weakens the blade structurally and decreases the
blades natural frequency. Blade natural frequencies are above the
normal running speed of a gas turbine. However, a reduction in the
blades natural frequency could move it closer to the engines normal
running speed.
Critical decisions, as discussed above, should be made only with
the help of an engineer experienced with gas turbine failure signatures.
Major manufacturers constantly test their designs and often
run components (blade, nozzle, etc.) to failure. Throughout my years
in this industry, I have witnessed numerous component failures—
usually after the fact. However, on one occasion I inspected a first
stage turbine blade failure in progress. After an increase in vibration
was observed, the engine was boroscoped. A single turbine blade had
failed at the blade mid-span. Normally this would be sufficient cause
for immediate overhaul. However, as this was an experimental engine
the decision was made to continue running until vibration reached 4
times acceptable running levels. The gas turbine was run for an additional
8 hours before overhauling the unit. Vibration had peaked
and stabilized at 8 mils. The damage found in overhaul was no more
severe than had been observed in my boroscope inspection.