Aircraft Reciprocating-Engine Failure - Pilot und Flugzeug

Figure 7.8:
Cause-and-effect diagram for the creation of a stress
concentrating feature in the little-end housing during
operation
7.5.3 Connecting rod big-end fatigue fracture control
Fatigue fracture control is based on ensuring that the magnitude of alternating
stresses created during engine operation does not exceed the endurance strength of
the connecting rod big-end. Because the big-end housing is a bolted assembly, and
because it is larger than the little-end housing, fracture control is more complex. In
addition to stresses developed through gas pressure loads and inertia, stresses
arising from housing flexure need to be considered. A fracture control plan also has
to be developed for the bolts used to assemble the housing. The control of bolt
fatigue fracture is based on the establishment of a preload in the bolt, of sufficient
magnitude, to prevent the alternating stresses in the bolt exceeding the endurance
strength of the bolt.
Connecting rod big-end fatigue fracture may occur if: any one, or combination, of
the following conditions occurs:
• the engine operating limitations are exceeded (the tension stress developed in
the connecting rod is at a maximum when the engine is operated at maximum
speed under conditions of low engine load, the compression stress developed in
the connecting rod is at a maximum when the engine is operated under
conditions that create maximum combustion gas pressures);
• the material fatigue endurance strength is lower than the specified value;
• the flexure of the housing is increased while operating within specified engine
limits;
• stress concentrating features are created in the housing during manufacture,
operation or maintenance.
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