NAVWEPS 00-8OT-80  OPERATING STRENGTH LIMITATIONS  relationships  for aircraft and powerplant  structures.  is cti~n&zti~e  during cyclic stressing, the useful  service life of the aircraft must be anticipated  to predict the gross effect of service loads.  Then, the primary structure is required to  sustain the typical load spectrum rhrough the  anticipated service life without the occurrence  of fatigue failure.  To prove this capability  of the structure, various major components  must be subjected to an accelerated fatigue  test to verify the resistance to repeated loads.  CREEP CONSIDERATIONS. By definition,  creep is the structural deformation which oc-  curs as a function of time. If a part is subjected  to a constant stress  of sufficient magnitude, the  part will continue to develop plastic strain and  deform with time.  Eventually, failure can  occur from the accumulation of creep damage.  Creep conditions are most critical at high  stress  and high temperature since both factors  increase the rate of creep damage. Of course,  any structure subject to creep conditions should  not encounter excessive  deformation or failure  within the anticipated service life.  The design of a highly stressed  or long life  structure emphasizes the problems of fatigue.  Great care must be taken during design and  manufacture to minimize stress concentrations  which enhance fatigue.  When the aircraft  enters service operation, care must be taken in  the maintenance of components to insure proper  adjustment, torquing, inspection, etc., as proper  maintenance is a necessity for achieving full  service life.  Also, the structure must not be  subjected to a load spectrum more severe than  was considered in design or fatigue failures  may occur within the anticipated service life.  With this additional factor in mind, any pilot  should have all the more respect for the oper-  ating  strength limits-recurring  overstress  causes  a high rate of fatigue damage.  The high operating temperatures  of gas tur-  bine components furnish a critical environment  for creep conditions.  The normal operating  temperatures and stresses  of gas turbine com-  ponents create considerable problems in design  for service life.  Thus, operating limitations  deserve very serious respect since excessive  engine speed or excessive  turbine temperatures  will cause a large increase in the rate of creep  damage and lead to premature failure of com-  ponents. Gas turbines require high operating  temperatures  to achieve high performance and  efficiency and short periods of excessive  tem-  peratures can incur highly  damaging creep  rates.  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  operation before an overhaul is necessary.  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 range,  Aileron reversal, divergence,  flutter, and vibration should not occur in the  range of flight speeds which will be normal  operation for the aircraft.  AFROELASTIC EFFECTS  330 