text: 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, 
  loads produce a completely 
  repeated cyclic 
  If a cyclic, tensile stress is 
  separate effect. 
  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 
  In this fashion, a metal can be failed 
  occurs. 
  at stresses much lower than the static ultimate 
  strength. 
  
  FATIGUE CONSIDERATIONS. The 
  
  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 
  is the “ultimate 
  the material can withstand 
  stress.” Noticeable permanent deformation 
  usually occurs in this range, but the material 
  does have the capability for withstanding one 
  application of the ultimate stress. 