Fatigue

Most solids fatigue under cyclic loading. Fatigue is actually a crack forming process. Small micro-fractures will develop and grow under cyclic tension loading. As the cracks grow longer, the strength of the material is compromised to the extent that failure will occur if the cycling continues.

While fatigue is a "random" process, it is characterized by well defined and repeatable results on bulk material. On a small scale it is less repeatable and indeed the impact of fatigue may be reduced by the use of small scale elements such as filaments in carbon-fiber and fiber-glass.

Measuring fatigue

By measuring the magnitude and number of stress cycles, the remaining safe life of a machine or structure can be estimated. The method, known as "rainflow analysis" is not without its drawbacks. If the component being monitored has stress in different directions, or the structural material has not been characterized for fatigue, then it may be necessary to undertake destructive tests to calibrate the procedure.

In some instances, using the more direct method of acoustic emissions may be more appropriate, although calibration is still necessary.