Page 286 - Aircraft Stuctures for Engineering Student

8.7 Fatigue 267

or, substituting for r(ue) from Eq. (8.51)
3.23 105~;5.’6
E(ue) = 1000/~0
Equation (8.54) then becomes
(
D, = - 1; ( g)2 Su,e 7 SA:-) ’ ( -3.23 x 5.26 x 105u;5.’6
Sqrn 10001~0 ) dtle
Substituting for and Skqm we have

or

D, = 16.99 x lo2 ( - Jm (u;: 2u;5.26 + u;6.26) due
:)2
2/10 ur u2 Uf
from which

or, in terms of the aircraft speed Ve

(8.59)

It can be seen from Eq. (8.59) that gust damage increases in proportion to V:/e5.26 so
that increasing forward speed has a dramatic effect on gust damage.
The total fatigue damage suffered by an aircraft per flight is the sum of the damage
caused by the ground-air-ground cycle, the damage produced by gusts and the
damage due to other causes such as pilot induced manoeuvres, ground turning and
braking, and landing and take-off load fluctuations. The damage produced by
these other causes can be determined from load exceedance data. Thus, if this extra
damage per fight is De,,, the total fractional fatigue damage per flight is
Dtotal = DGAG + DgRav + Dextra
or

Dtotal = 4.5/NG + DgRav -k Dextra (8.60)
and the life of the aircraft in terms of flights is
Nflighr = l/l)total (8.61)

8.7.5 Crack propagation

We have seen that the concept of fail-safe structures in aircraft construction relies on
a damaged structure being able to retain sufficient of its load-carrying capacity to