Design and Stress Analysis of Extraterrestrial ... - The Black Vault

at which the blade obtained the initial
crack) has a characteristic
flat polished surface.
In the cross section where the blade was
broken and the crack has not yet developed, the fracture surface
is
crystalline.
The breakdown of one blade usually leads to the failure of the
remaining blades and the turbine as a whole.
To prevent vibration breakage and control it
successfully when
it does appear, it is necessary to correctly evaluate the possible
sources of the onset of the defect. For this purp% 4t is necessary
to determine the natural frequencies of blade vibration; to determine
the disturbing forces and to produce their wave analysis; to construct
a frequency diagram and reveal the possible resonant frequenciez
over the range of operating revolutions of the motor and find which are
the most dangerous. The various vibrational modes follow: flexural,
torsional, flexural-torsional, and edge vibration (when almost tne
entire surface of the blade is fixed and only individual poiits
on the edge, sometimes the closed areas of the blade, vibrate).
Edge vibrations have very high-frequencies and appear in fineý
blades.
Flexural vibrations are the most dangerous because their
frequency can be within the range of the working rpm. The other
wave forms o! turbine blades have a high frequency and do not always
fall within the range of working rpm's.
Edge vibrations have a frequency on the order of 10,000 vibrations
per second and can be excited with a certain combination of
nozzle and rotor blade quantities during vibrational combustion in
the combustion chambers. These vibrations as yet have not been
generalized.
blades.
Figure 3.22 shows the types of vibration cracks in
turbine
Let us examine only the flexural vibrations of blades.
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