NAVWEPS oo-8oT-80  OPERATING STRENGTH LIMITATIONS  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 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.  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.  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.  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  type of aircraft usually experiences a pre-  dominance of maneuver loads while the trans-  port or patrol type usually encounters a pre-  dominance of gust loads. Since fatigue damage  SERVICE LIFE  328 