This Demonstration plots the tension variation at the surface of a cylindrical specimen in a rotating beam fatigue test.

The fatigue life of a material is commonly tested by applying a cyclical load to it, alternating between tension and compression. For cylindrically shaped specimens, the rotating beam fatigue test can be used. In this test, the specimen is subject to a sinusoidal bending moment while it is being rotated at a steady speed. The center line of the specimen forms a neutral axis. The amplitude of the bending moment applied is proportional to the distance from that neutral axis. It is highest at the surface and zero at the neutral axis itself. The number of cycles until failure is a good measure for the fatigue life of the material.

In the elastic domain, tension is proportional to deformation, which can be expressed by , where is Young's modulus and is the elongation of the material. For a cylindrical specimen of radius , bent over a radius at rpm in the rotating beam test, this means (for the outer fibers at the surface) that .

A smaller bending sector is used to accommodate specimens with larger radius. The angular speed in the Demonstration is only indicative.