Metal fatigue is a term for structural damage suffered by metal materials when they are repeatedly subjected to stress. Manufacturers often use a technique known as cyclical loading to test the strength of metals; the process involves placing a load onto the metal, removing it and placing the same load back onto the metal repeatedly. Even though the load stays the same and even though the load is within the limits of the metal, metal fatigue is the name for the structural damage that results from repeated cyclical loading.
When a load is placed on a piece of metal, the metal expands slightly until the load is released. The expansion and contraction of the metal as loads are added and removed causes microscopic cracks to appear in the surface of the metal. As the metal is subject to more and more cyclical loading, the cracks get larger and larger, and eventually reach a critical size at which they tear the fabric of the metal. Under these conditions, the metal can completely disintegrate, again even though the load is well within the mass threshold of the metal.
De Havilland Comet Airliner
One of the most famous instances of metal fatigue occurred in 1952. The de Havilland Comet Airliner was the first commercial airliner to use jet engine technology. Barely a year into its service as a commercial airliner, many of the jets suffered accidents that later were understood to be the result of metal fatigue. Designers failed to take metal fatigue into account and the repeated stress of the jet's cabin depressurizing and repressurizing caused metal fatigue so severe that gashes would rip through the side of the plane in mid-flight.
Scientists use a figure known as ultimate tensile strength (UTS) to aid in their determination of the amount of metal fatigue a material can endure. Steel, for instance, has a UTS value of around 160,000 pounds per square inch (psi), depending on the slickness of the metal surface. The "endurance limit" -- the amount of metal fatigue steel can withstand without breaking down, is around 45 to 50 percent of UTS, or around 80,000 psi. Keep in mind that these values are highly speculative and depend on numerous factors, including the slickness of the surface of the steel and the type of steel.