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Overview
Thermal fatigue is a specific form of fatigue driven by varying metal temperature gradients and ensuing differential thermal expansion. In general, fatigue is a complex metallurgical process that is cycle-dependent and in which failure of a component occurs due to repeated or cyclic loading, which creates cyclic stresses. Furthermore, cyclic stresses can result from mechanical loading (applied loading or vibration) or thermal loading (metal temperature gradients). The application of these cyclic stresses produces slip lines in the crystals of a metal that develop into small cracks. These small cracks then propagate, coalesce, and result in an eventual fracture, which usually involves little-to-no gross plastic deformation.
Consequently, at one time, fatigue failures were mistakenly thought to show that the metal had lost its ductility. To this end, fatigue failure involves progressive, localized, permanent structural change due to fluctuating stresses and strains. To accurately estimate the fatigue life (or cycles to failure) in a component, the fatigue driving force and resistance parameters need to be well-understood. As such, geometry, material properties, and accurate characterization of the applied loading (fatigue driving force) are required. Furthermore, fatigue test data that describes material behavior as a function of stress or strain magnitude and the number of accumulated cycles (fatigue resistance parameters) is also required to carry out meaningful remaining life calculations.
This short eBook is designed to offer practical guidance for, and real examples of, in-service degradation or failures attributed to thermal fatigue. We will begin with a brief historical overview, followed by a discussion on the detection of thermal fatigue, including typical damage locations, morphology, and common inspection techniques. This primer will also highlight how thermal fatigue can be characterized and evaluated using engineering calculations, fitness-for-service methods, or advanced analysis. Finally, we will address what steps can be taken to mitigate or remediate thermal fatigue damage on affected equipment, including design/fabrication recommendations and process/operating practices.
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