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- S.N. curve is basically plot of cyclic stress verses number of cycles, it shows all possible variations of stresses with respect to cycle or time.
- Fatigue is failure phenomenon in which an object is applied by cyclic load causing cyclic stress generation which is very less than yield or ultimate stress values and due to generation of micro cracking and its propagation the failure of material occurs.
Both ferrous and non-ferrous materials show different behaviour in fatigue mechanisms.

The frequency of fatigue defines the time require to fail and the amplitude of cyclic stress applied defines failure intensity.

- Ferrous materials have a distinct feature in which upto a certain critical value fatigue becomes max, while of non-ferrous metals with increasing number of cycles the fatigue increases and leads to failure.
- The metal has a unique property which we call it as endurance limit or fatigue limit, it is the maximum stress that can be applied to an object for infinite number of cycles without fatigue failure.
- Ferrous alloys and titanium alloys have a distinct limit, amplitude below which there appears to be no number of cycles that will cause failure. Other structural metals such as aluminium and copper do not have a distinct limit and will eventually fail even from small stress amplitudes. In these cases, a number of cycles (usually 107) is chosen to represent the fatigue life of the material.

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