Many applications exist where a material is subjected to a repeated cyclic stress. These materials are found to fail after a large number of applications of the stress even though the cyclic stress that is applied is below their yield strength. This mode of failure is termed fatigue.
The reason that these materials fail is due to progressive stepwise crack growth through the sample. Cracks create stress concentrations that raise the level of stress to a level above the yield strength of the material in the vicinity of the crack. Repeated cyclic stresses propagate the crack until failure.
The most common method to measure the fatigue resistance of a material is the rotating cantilever beam test.
A cylindrical specimen is mounted on a motor driven chuck with a weight suspended from one end.
As the specimen rotates each side is periodically stressed in tension and compression under the force of gravity.
As the specimen rotates the stress at any given point goes through a complete cycle from maximum tension to maximum compression.
The maximum stress, σ, that acts on the specimen, in either tension or compression, is given by:
Where l is the length of the specimen, F is the load, and d is the diameter of the specimen.