Metal fatigue is the gradual weakening of metal after extensive use. Such weakening is caused by the repeated application of tension, pressure, or other forms of stress. Stress cycles often alter the molecular structure of the materials so that they crack. Metal fatigue usually begins at the surface of a metal piece where small defects, or even minute tool marks, serve as a concentration point for stress. The crack spreads through the piece, eventually making it too weak to carry its normal load. Engineers allow for metal fatigue in planning airplanes, bridges, and machinery.
