It has been found that metals give way in some cases under smaller loads than they could originally carry.

This is called fatigue, and is caused by a very great number of reversals or repetitions of load, as when a metal receives constant shocks or impacts in use. It is a popular but erroneous idea that the particles of metal change to a weakened crystalline form. Later investigations show that the loss of strength is due to the crystals of metal being so shaken that the small planes of. cleavage between them join in one continuous plane of rupture, which, when sufficiently extended, leaves the remaining sound metal unable to resist the shock brought upon it. Cases of fatigue are unusual.