text: the “endurance limit.” 
  If the operating cyclic 
  an 
  stresses never exceed this “endurance limit” 
  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 
  load-and especially stresses well above 
  limit 
  the 
  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 
  Examples of 
  shown in figure 5.7. 
  
  the design “envelope.” 
  
  in-service fatigue  failures are 
  
  limit 
  
  A puzzling situation 
  
  “Over- 
  to recognize because of the 
  
  then exists. 
  
  stress” is difficult 
  
  345 