NAVWEPS 00-8OT-80  OPERATING STRENGTH LIMITATIONS  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.  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  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.  345  STRESS,  PSI  100%  LIMIT LOAD  :NDURANCE  LIMIT  I-  STRAIN,  lN/,N  -  i\  ULTIMATE  STRENGTH  LIMIT  LOAD 