Question: Explain fatigue and how Fatigue limit is tested?

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- Fatigue is a failure phenomenon in which an object fails by cyclic stress which is below yield stress or ultimate stress, and occurs due to formation and propagations of micro cracking due to cyclic loadings.
- This kind of failure occurs when a load is periodically applied over an object for N number of cycles.
- Fatigue limit or Endurance limit is limiting value of stress at which object never fail for infinite number of cycles of loading.
- The basic apparatus of fatigue testing equipment is: Motor, Support bearings, Load bearings, Specimen, Loadings
Following is testing apparatus:

The material is fitted with load bearings and that setup is mounted over main bearings.

- The shaft is coupled to motor for cycling action for load.
- The load is added on load bearings for creating cyclic loading, and the deformation in specimen is observed.
- The deformation of specimen gives the bending stress for a particular load. And the graph is plotted for various values of loadings and according to a graph we get a critical value which is known as Fatigue limit or endurance limit.
- Below which all values of stress are safe for fatigue loading for infinite number of cycles.

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