STRESS, 
PSI 
100% 
LIMIT LOAD 
:NDURANCE 
LIMIT 
I- STRAIN, lN/,N 
NAVWEPS 00-8OT-80 
OPERATING STRENGTH LIMITATIONS 
- 
i\ 
ULTIMATE 
STRENGTH 
LIMIT LOAD 
Figure 5.6. Typical Stress Strain Diagram for a High Strength Aluminum Alloy 
the “endurance limit.” If the operating cyclic 
stresses never exceed this “endurance limit” an 
infinite (,or in some cases “near infinite”) num- 
ber of cycles can be withstood without fatigue 
failure. No significant fatigue damage accrues 
from stresses below the endurance limit but 
the value of this endurance limit is approxi- 
mately 30 to 50 percent of the yield strength 
for the light alloys used ia airkraft construc- 
tion. The rate of fatigue damage caused by 
stresses only &g/&y above the endurance limit 
is insignificant. Even stresses near the limit 
load do not cause a significant accumulation of 
fatigue damage if the frequency of applicatibn 
is reasonable and within the intended mission 
requirement. However, stresses above the 
limit load-and especially stresses well above 
the limit load-create a very rapid rate of 
fatigue damage. 
inherent high yield strength and low ductility 
of typical aircraft metals. These same over- 
stresses cause high rate of fatigue damage and 
create premature failure of parts in service. 
The effect of accumulated overstress is rhe 
formation and propagation of fatigue cracks. 
While it is sure that fatigue crack always will 
be formed before final failure of a part, accumu- 
lated overstress is most severe and fatigue 
provoking at the inevitable stress coticentra- 
tions. Hence, disassembly and detailed’inspec- 
tion is both costly and time-consuming. To 
prevent in-service failures of a basically sound 
structure, the part must be properly maintained 
and operated within the design “envelope.” 
Examples of in-service fatigue failures are 
shown in figure 5.7. 
A puzzling situation then exists. “Over- 
stress” is difficult to recognize because of the 
The operation of any aircraft and powerplant 
must be conducted withm the operating limita- 
tions prescribed in the flight handbook. No 
hearsay or rumors can be substituted for chc 
345