text: Many of the high strength materials used in 
  aircraft construction have stress-strain dia- 
  grams typical of figure 5.6. One feature of 
  these materials is that the yield point is at 
  some stress much greater than two-thirds of 
  the ultimate stress. Thus, the critical design 
  condition is the ultimate load. 
  If 150 percent 
  load corresponds to ultimate stress of 
  of limit 
  the material, 100 percent of limit 
  load corre- 
  sponds to a stress much lower than the yield 
  stress. Because of the inherent properties of 
  the high strength material and the ultimate 
  factor of safety of 1.5, the limit load condition 
  is rarely the critical design point and usually 
  possesses a large positive margin of static 
  strength. This fact alone implies that 
  the 
  structure must be grossly overstressed to pro- 
  duce damage easily vidble to the naked eye. 
  This lack of immediate visible damage with 
  “overstress” makes it quite diflicult to recog- 
  nize or appreciate the long range effect. 
  
  Accumulated periods of overstress can create 
  a very detrimental effect on the useful service  A reference point provided on the stress 
  strain diagram of figure 5.6 is a stress termed 