A fatigue test is a type of materials test in which the goal is to determine the point at which the material will fail. Fatigue is a problem for all materials, caused by wear and tear as they are used. In a fatigue test, fatigue is deliberately induced to find the stress limits of the material. Materials for certain applications may need to meet specific requirements in terms of fatigue. For example, metals used in a bridge need to be able to endure very high stress loads.
Fatigue is caused by things like motion, oscillations, vibration, pressure, wear, and loading. To borrow the example of a bridge again, bridges are subjected to stress as they move in the wind, stress during seismic events, and the daily vibration and load of thousands of cars. Over time, fatigue leads to cracking in the material, and will eventually cause failure; either the material will shear off, or the cracks will cause a fracture of structural components, leading to collapse.
Whether it's a bedframe or a bridge, fatigue is an issue. In a fatigue test, the materials used are subjected to vibration and other kinds of stress and carefully monitored. As soon as signs of fatigue start to appear, they are noted and tracked, charting how long it takes for fatigue and eventual failure to occur. The point at which fatigue starts to happen is the lower stress limit. Fatigue testing is sometimes referred to dynamic testing, because it involves a variety of techniques to pummel and essentially assault the material to find its stress limits.
With something like a piece of furniture, the furniture itself may be put through a fatigue test. With a chair, for example, a machine might simulate someone flopping into the chair over and over, or might create the twisting stresses which happen when the chair is dragged across a floor, with or without an occupant. In the case of large equipment, individual machine components are put through fatigue testing, since the entire piece of equipment may not fit in the testing area.
The results of fatigue testing are kept on file, and may need to be filed with a government agency, in some cases. For example, people working on a high rise building need to show proof that a fatigue test was done on components and that the components passed. This information can be useful if a failure occurs, creating a paper trail which can be used to figure out when things started to go wrong.