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

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ii
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Fig. 3.20. Compensation of gas- M , '
dynamic bending moments by
moments from centrifuqal. forces.' "
Total stresses and safety factors of blades
In each cross section of a blade the total stresstis defined'
as the sum of tensile and bending stresses:
4 UP= + (T.u- (3.17)
The safety factor is determined from expression
a , (3.18)
t
where at is the rupture strength of the blade material taking
into account temperature and time.
Obviously the safety factor of the blade along the radius will
not remain constant; it will change depending upon the variation and
stresses a along the radius and the quantity at 3 ' due tT the
variable temperature of the blade (Fig. 3.21). Usually the mininium
safety factor is n = 1,2-1.5 and corresponds to the unsafe sections
of the blade.
For turbine blades the unsafe section, . counting. from
the root, is approximately 20-30% of the length' of the blade.
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