NAVWEPS oo-8oT-80 
OPERATING STRENGTH LIMITATIONS 
of stress the plot of stress and strain is essen- 
tially a straight line, i.e., the material in this 
range is elastic. A stress applied in this range 
incurs no permanent deformation and the ma- 
terial returns to the original unstressed shape 
when the stress is released. At higher values 
of stress the plot of stress versus strain develops 
a distinct curvature in the strain direction and 
the material incurs disproportionate strains. 
High levels of stress applied co the part and 
then released produce a permanent deforma- 
tion. Upon release of some high stress, the 
metal snaps back-but not all the way. The 
stress defining the limit of tolerable permanent 
strain is the “yield stress” and stresses applied 
above this point produce objectionable per- 
manent deformation. The very highest stress 
the material can withstand is the “ultimate 
stress.” Noticeable permanent deformation 
usually occurs in this range, but the material 
does have the capability for withstanding one 
application of the ultimate stress. 
The relationship between the stress-strain 
diagram and operating strength limits should 
be obvious. If the aircraft is subjected to a 
load greater than the limit, the yield stress 
may be exceeded and objectionable permanent 
deformation may result. If the aircraft is 
subject to a load greater than the ultimate, 
failure is imminent. 
SERVICE LIFE 
The various components of the aircraft and 
powerplant structure must be capable of oper- 
ating without failure or excessive deformation 
throughout the intended service life. The 
repetition of various service loads can produce 
fatigue damage in the structure and special 
attention must be given to prevent fatigue 
failure within the service life, Also, the sus- 
taining of various service loads can produce 
creep damage and special attention must ‘be 
given to prevent excessive deformation or 
creep failure within the service life, This is 
a particular feature of components which are 
subjected to operation at high temperatures. 
FATIGUE CONSIDERATIONS. The fa- 
tigue strength requirement is the considera- 
tion given the cumulative effect of repeated 
or cyclic !oads during service. While there is 
a vague relationship with the static strength, 
repeated cyclic loads produce a completely 
separate effect. If a cyclic, tensile stress is 
applied to a metal sample, the part is subject 
to a “fatigue” type loading. After a period 
of time, the cyclic stressing will produce a 
minute crack at some critical location in the 
sample. With continued application of the 
varying stress, the crack will enlarge and 
propagate into the cross section. When the 
crack has progressed sufficiently, the remaining 
cross section is incapable of withstanding the 
imposed stress and a sudden, final rupture 
occurs. In this fashion, a metal can be failed 
at stresses much lower than the static ultimate 
strength. 
Of course, the time necessary to produce 
fatigue failure is related to the magnitude of 
the cyclic stress. This relationship is typified 
by the graph of figure 5.1. The fatigue 
strength of a material can be demonstrated by 
a plot of cyclic stress versus cycles of stress 
required to produce fatigue failure. As might 
be expected, a very high stress level requires 
relatively few cycles to produce fatigue failure. 
Moderate stress levels require a fairly large 
number of cycles to produce failure and a very 
low stress may require nearly an infinite num- 
ber of cycles to produce failure. The very 
certain implication is that the aircraft must 
be capable of withstanding the gamut of 
service loads without producing fatigue failure 
of the primary structure. 
For each mission type of aircraft there is 
a probable spectrum of loads which the air- 
craft will encounter. That is, various loads 
will be encountered with a frequency particular 
to the mission profile. The fighter or attack