Thermal fatigue is a kind of fatigue failure characterized by macroscopic fractures caused by cyclic thermal loads and strains caused by temperature fluctuations, spatial temperature gradients, and high temperatures in the presence of limited thermal deformation. Thermal fatigue can develop in the absence of mechanical stresses. Heat-treated materials such as stainless steel, carbon steel, and copper alloys can suffer from thermal fatigue damage if they are exposed to heat above their recrystallization temperature for a long period of time. Heat-treated metals that are used for equipment that is exposed to high temperatures, such as gas turbines, contain crystalline lattice defects that act as nucleation sites for stress corrosion cracking (SSC). SSC can develop even without external agents being present if the material is exposed to excessive internal temperatures for an extended period of time. SSC can cause considerable damage to metal parts that are used in critical engine systems because it can lead to loss of aircraft control or destruction of the engine itself. SSC can also cause serious problems for people who work with heat-treated metals because they may not be aware of any cracks that have not yet developed into full-scale failures. Thermal fatigue occurs when heat causes small cracks to grow under load, resulting in large-scale fracture of the material. The rate at which this happens depends on several factors including the heat treatment of the metal, its thickness, and how long it is exposed to high temperatures.