SERVICE LIFE

The various components of the aircraft and powerplant structure must be capable of oper­ating without failure or excessive deformation throughout the intended service life. The repetition of various service loads can produce fatigue damage in the structure and special attention must be given to prevent fatigue failure within the service life. Also, the sus­taining of various service loads can produce creep damage and special attention must be given to prevent excessive deformation or creep failure within the service life. This is a particular feature of components which are subjected to operation at high temperatures.

FATIGUE CONSIDERATIONS. The fa­tigue strength requirement is the considera­tion given the cumulative effect of repeated or cyclic loads during service. While there is a vague relationship with the static strength, repeated cyclic loads produce a completely separate effect. If a cyclic, tensile stress is 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 occurs. In this fashion, a metal can be failed at stresses much lower than the static ultimate strength.

Of course, the time necessary to produce fatigue failure is related to the magnitude of the cyclic stress. This relationship is typified by the graph of figure 5-1- The fatigue strength of a material can be demonstrated by a plot of cyclic stress versus cycles of stress required to produce fatigue failure. As might be expected, a very high stress level requires relatively few cycles to produce fatigue failure. Moderate stress levels require a fairly large number of cycles to produce failure and a very low stress may require nearly an infinite num­ber of cycles to produce failure. The very certain implication is that the aircraft must be capable of withstanding the gamut of service loads without producing fatigue failure of the primary structure.

For each mission type of aircraft there is a probable spectrum of loads which the air­craft will encounter. That is, various loads will be encountered with a frequency particular to the mission profile. The fighter or attack type of aircraft usually experiences a pre­dominance of maneuver loads while the trans­port or patrol type usually encounters a pre­dominance of gust loads. Since fatigue damage

is cumulative 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 through the anticipated service life without the occurrence of fatigue failure. To prove this capability of the structure, various major components mast be subjected to an accelerated fatigue test to verify the resistance to repeated loads.

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 oper­ating strength limits—recurring overstress causes a high rate of fatigue damage.

There are many examples of the detrimental effect of repeated over stress on service life. One major automobile manufacturer adver­tised his product as “guaranteed to provide

100,0 miles of normal driving without me­chanical 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 hot­rod 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 100,000 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 relationships for aircraft and powerplant structures.

CREEP CONSIDERATIONS. By definition, creep is the structural deformation which oc­curs 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 high operating temperatures of gas tur­bine components furnish a critical environment for creep conditions. The normal operating temperatures and stresses of gas turbine com­ponents 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 com­ponents. Gas turbines require high operating temperatures to achieve high performance and efficiency and short periods of excessive tem­peratures can incur highly damaging creep rates. •

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 limita­tions due to creep conditions,