During the operation of shaft under repeated stresses, it came to know that shaft fails at a value below yield point of the material of the shaft. This type of failure is called fatigue. In this type of failure, crack generates at microscopic level and it grows due to the application of shaft. Such type of failure takes place without giving any indication.
There are some factors which affect the fatigue of the material. These factors are size of component, number of load reversals and relative magnitude of static and fluctuating load.
Consider a standard polished specimen on which we apply rotating mirror beam method.
In this method, we load our specimen into fatigue testing machine. As our specimen start rotating, it completely undergoes completely reversed stress cycle. This behavior is shown in the figure (b).
After that we record numbers of cycles which are necessary to cause failure in the shaft at particular value of stress. Then we make a graph by using these values. If we further observe the behavior of material, we came to know that our material does not fail, if we apply stress below dotted line. Due to this reason, we call dotted line as fatigue or endurance limit (σe) of the material and is defined as the value of stress which a polished specimen can withstand for infinite number of cycles, usually we take 107 times as infinite number of cycles.
It has been noted that if a shaft is experiencing only reversed bending stress then we use endurance limit, otherwise we use endurance strength of the material. It is a type of stress when we apply on the material gives us safe operation of the shaft.
If we look at the figure (b), we came to know that maximum value of stress in tension is equal to maximum value of stress in compression. But in case of fluctuating stresses, we see the combination of steady stresses and a component of completely reversed stress component σv.