text: There are many examples of the detrimental 
  effect of repeated overstress on service life. 
  One major automobile manufacturer adver- 
  tised his product as “guaranteed to provide 
  100,000 miles of normal driving without me- 
  chanical failure.” 
  The little old lady from 
  Pasadena-the original owner of ALL used cars 
  -will 
  probably best the guaranteed mileage 
  by many times. On the other hand, the hot- 
  rod artist and freeway Grand Prix contender 
  do not qualify for the guarantee since their 
  manner of operation could not be considered 
  normal. The typical modern automobile may 
  be capable of 60,000 to l~,OOO miles of normal 
  is necessary. 
  operation before an overhaul 
  However, this same automobile may encounter 
  catastrophic failures in a few hundred miles if 
  operated continually at maximum torque in 
  low drive range. Obviously, there are similar 
  
  Airplane structures can be subject to high 
  temperatures due to aerodynamic heating at 
  high Mach numbers. Thus, very high speed 
  airplanes can be subject to operating limita- 
  tions due to creep conditions. 
  
  The requirement for structural stiffness and 
  rigidity is the consideration given to the inter- 
  action of aerodynamic forces and deflections of 
  the structure. The aircraft and its components 
  must have sufficient stiffness to prevent or 
  minimize aeroelastic influences in the normal 
  flight 
  reversal, divergence, 
  flutter, and vibration should not occur in the 
  range of flight speeds which will be normal 
  operation for the aircraft. 