NAVWEPS 00-8OT-80 
OPERATING STRENGTH LIMITATIONS 
is cti~n&zti~e during cyclic stressing, the useful 
service life of the aircraft must be anticipated 
to predict the gross effect of service loads. 
Then, the primary structure is required to 
sustain the typical load spectrum rhrough the 
anticipated service life without the occurrence 
of fatigue failure. To prove this capability 
of the structure, various major components 
must be subjected to an accelerated fatigue 
test to verify the resistance to repeated loads. 
The design of a highly stressed or long life 
structure emphasizes the problems of fatigue. 
Great care must be taken during design and 
manufacture to minimize stress concentrations 
which enhance fatigue. When the aircraft 
enters service operation, care must be taken in 
the maintenance of components to insure proper 
adjustment, torquing, inspection, etc., as proper 
maintenance is a necessity for achieving full 
service life. Also, the structure must not be 
subjected to a load spectrum more severe than 
was considered in design or fatigue failures 
may occur within the anticipated service life. 
With this additional factor in mind, any pilot 
should have all the more respect for the oper- 
ating strength limits-recurring overstress 
causes a high rate of fatigue damage. 
There are many examples of the detrimental 
effect of repeated overstress on service life. 
One major automobile manufacturer adver- 
tised his product as “guaranteed to provide 
100,000 miles of normal driving without me- 
chanical failure.” The little old lady from 
Pasadena-the original owner of ALL used cars 
-will probably best the guaranteed mileage 
by many times. On the other hand, the hot- 
rod artist and freeway Grand Prix contender 
do not qualify for the guarantee since their 
manner of operation could not be considered 
normal. The typical modern automobile may 
be capable of 60,000 to l~,OOO miles of normal 
operation before an overhaul is necessary. 
However, this same automobile may encounter 
catastrophic failures in a few hundred miles if 
operated continually at maximum torque in 
low drive range. Obviously, there are similar 
relationships for aircraft and powerplant 
structures. 
CREEP CONSIDERATIONS. By definition, 
creep is the structural deformation which oc- 
curs as a function of time. If a part is subjected 
to a constant stress of sufficient magnitude, the 
part will continue to develop plastic strain and 
deform with time. Eventually, failure can 
occur from the accumulation of creep damage. 
Creep conditions are most critical at high 
stress and high temperature since both factors 
increase the rate of creep damage. Of course, 
any structure subject to creep conditions should 
not encounter excessive deformation or failure 
within the anticipated service life. 
The high operating temperatures of gas tur- 
bine components furnish a critical environment 
for creep conditions. The normal operating 
temperatures and stresses of gas turbine com- 
ponents create considerable problems in design 
for service life. Thus, operating limitations 
deserve very serious respect since excessive 
engine speed or excessive turbine temperatures 
will cause a large increase in the rate of creep 
damage and lead to premature failure of com- 
ponents. Gas turbines require high operating 
temperatures to achieve high performance and 
efficiency and short periods of excessive tem- 
peratures can incur highly damaging creep 
rates. 
Airplane structures can be subject to high 
temperatures due to aerodynamic heating at 
high Mach numbers. Thus, very high speed 
airplanes can be subject to operating limita- 
tions due to creep conditions. 
AFROELASTIC EFFECTS 
The requirement for structural stiffness and 
rigidity is the consideration given to the inter- 
action of aerodynamic forces and deflections of 
the structure. The aircraft and its components 
must have sufficient stiffness to prevent or