What is fatigue failure in materials?

Fatigue failure in materials refers to the progressive and localized structural damage that occurs when a material is subjected to repeated cycles of loading and unloading. This phenomenon typically happens at stress levels that are significantly lower than the material's ultimate tensile strength. Over time, even small fluctuations can lead to the development of microscopic cracks that grow and propagate until they finally result in a complete fracture of the material.

Understanding fatigue failure is crucial, especially in applications where components experience fluctuating loads, such as in bridges, aircraft, and machinery. It is not a single event but rather a process that unfolds over time, emphasizing the importance of considering fatigue in material selection and design processes. The other options describe different types of failure or wear mechanisms that do not capture the unique characteristics of fatigue.