Fatigue Testing
A rotating beam fatigue-testing machine
The fatigue test is dynamic type of test, which determines the relative behavior of material when subjected to repeated or fluctuating loads. It determines the resistance of a metal to repeated or alternate loads.
Suitable bases for design involving static load is the yield strength of the material. But if it is a fatigue loading, in which load is repeatly applied and removed, the design criterion is the endurance strength.
The endurance strength is the stress (S) that will cause the metal to fail, after the large no [N] of application of that stress.
Fig (a) show fatigue testing machine, a machine consist of
- An electric motor capable of running at 10000 RPM
- A large bearing whose purpose is to release the motor of the large bending movement, which is applied, to the specimen.
- Collet to hold the specimen
- A rotating lever arm subjected to downward force in order to place the specimen in a state of bending.
The upper surface of the specimen remains in tension while it lowers surface experiences compression (fig). While the specimen is rotating, any given position on the surface of the specimen alternates between the state of maximum tensile stress and a state of maximum compressive stress.
The maximum value of the [tensile or compressive] stress is usually lower than the yield strength of the material. The test is continued i.e. the cycles of stress are applied until the specimen fails or until the limiting number of cycles has been reached.
A no. of specimen of the same material are fatigue tested under different stress level and results are plotted on a semi logarithmic scale with the stress(S) on the y-axis and number of cycles [N] to cause failure of the specimen on x-axis
Fig
The result it is an S-N [fatigue] curve. S-N curve gives an idea of endurance limit or fatigue limit.
Fig (14.9b) shows two different type of S-N curve of one showing a definite leveling of the curve, which indicates that the material has a definite fatigue limit. Commonly ferrous metals shows distingue fatigue limit while non-ferrous metals do not.
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