Fatigue of Aluminum


Concepts Shown:

cyclic loading, fatigue, endurance limit


One 1100 aluminum rod approximately 30 inches long and 5/16 of an inch in diameter.


Ask a student to bend and straighten the rod until it breaks. It is not necessary for the rod to straighten completely. Also, count the number of cycles until the rod breaks.


The rod underwent cyclic loading known as fatigue. Each time it was loaded (bent/straightened), its strength decreased. Fatigue cracks originated at the bend, the point of highest stress. The rod broke when its strength fell below the endurance limit, the maximum stress allowable for unlimited cyclic loading. S-N curve (4140 steel): Strength decreases as the number of cycles increases. At the endurance limit, the number of cycles becomes indeterminately large. Make an S-N curve for the rod by utilizing its ultimate strength, 75 MPa, and the number of cycles until failure. Use the 4140 steel curve as an example. Other examples of fatigue include (a) tension-compression cycles of an axle of a train, (b) the rotor of a generator accelerating and deceleration during starting and stopping, and (c) tension stresses from pressurization and depressurization of an airplane fuselage.


Dave Goodman

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