EFFECT OF OVERSTRESS ON SERVICE. LIFE

Accumulated periods of overstress can create a very detrimental effect on the useful service life of any structural component. This fact is certain and irreversible. Thus, the opera­tion of the airplane, powerplant, and various systems must be limited to design values to prevent failure or excessive maintenance costs early in the anticipated service life. The operating limitations presented in the hand­book must be adhered to in a very strict fashion.

In many cases of modern aircraft structures it is very difficult to appreciate the effect of a moderate overstress. This feature is due in great part to the inherent strength of the materials used in modern aircraft construction. As a general airframe static strength require­ment, the primary structure must not expe­rience objectionable permanent deformation at limit load or failure at 150 percent of limit load (ultimate load is 1.5 times limit load). To satisfy each part of the requirement, limit load must not exceed the yield stress and ulti­mate load must not exceed the ultimate stress capability of the parts.

Many of the high strength materials used in aircraft construction have stress-strain dia­grams 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 of limit load corresponds to ultimate stress of the material, 100 percent of limit load corre­sponds 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 pro­duce damage easily visible to the naked eye. This lack of immediate visible damage with “overstress” makes it quite difficult to recog­nize or appreciate the long range effect.

A reference point provided on the stress strain diagram of figure 5.6 is a stress termed

the ‘ ‘endurance limit. ” If the operating cyclic stresses never exceed this “endurance limit" an infinite (or in some cases “near infinite”) num­ber of cycles can be withstood without fatigue failure. No significant fatigue damage accrues from stresses below the endurance limit but the value of this endurance limit is approxi­mately 30 to 50 percent of the yield strength for the light alloys used in aircraft construc­tion. The rate of fatigue damage caused by stresses only slightly above the endurance limit is insignificant. Even stresses near the limit load do not cause a significant accumulation of fatigue damage if the frequency of application is reasonable and within the intended mission requirement. However, stresses above the limit load—and especially stresses well above the limit load—create a very rapid rate of fatigue damage.

A puzzling situation then exists. “Over­stress” is difficult to recognize because of the

inherent high yield strength and low ductility of typical aircraft metals. These same over­stresses cause high rate of fatigue damage and create premature failure of parts in service. The effect of accumulated overstress is the formation and propagation of fatigue cracks. While it is sure that fatigue crack always will be formed before final failure of a part, accumu­lated overstress is most severe and fatigue provoking at the inevitable stress concentra­tions. Hence, disassembly and detailed inspec­tion is both costly and time-consuming. To prevent Іп-service failures of a basically sound structure, the part must be properly maintained and operated within the design “envelope.” Examples of in-service fatigue failures are shown in figure 5.7.

The operation of any aircraft and powerplant must be conducted within the operating limita­tions prescribed in the flight handbook. No hearsay or rumors can be substituted for the

accepted data presented in the aircraft hand­book. All of the various static strength, service life, and aeroelastic effects must be given proper respect. An airplane can be over­stressed with the possibility that no immediate damage is apparent. A powerplant may be operated past the specified time, speed, or temperature limits without immediate appar­ent damage. In each case, the cumulative effect will tell at some later time when in­service failures occur and maintenance costs increase.