When a material is subjected to varying load, it fails at a stress below the yield point. such type of failure of the component is known as fatigue. Also, the varying loads are known as fatigue loads and the variable stresses induced in the component are known as Fluctuating stresses.

It has been observed that, when the mechanical component is subjected to fluctuating loads, it fails at a stress considerably below the ultimate strength and quite frequently even below the yield strength. Such type of failure is known as fatigue failure. The most distinguish characteristic of the fatigue. The most distinguishing characteristics of the fatigue failure is that the stresses are repeated a very large number of times. It has been found that the magnitude of the stress at which fatigue failure occurs decreases as the number of stress cycle increases.

A fatigue failure begins with a small crack. The initial crack is 50 minute that it cannot be detected by a naked eye and sometimes, it is even difficult to detect in X-ray inspection.

The crack initiates at a point of discontinuity in the component, such as change in cross-section, key-way or hole, less obvious points at which crack may initiate are inspection or stamp marks, internal cracks or even irregularities caused by machining . Once a crack is initiated, the stress-concentration effect becomes greater and the crack progresses more rapidly.

In case of static failure, there is always sufficient plastic deformation prior to failure which gives visible warning in advance. But fatigue failure gives no warning. It is sudden, total and hence, dangerous. In fatigue failure, the cracks are not visible till they reach the surface and by that time the fatigue has already occurred.