What does the term 'fatigue' refer to in materials science?

In materials science, the term 'fatigue' refers to the phenomenon where materials experience progressive and localized structural damage due to repeated or cyclic loading. This process often occurs at stress levels that are lower than the material's ultimate tensile strength. Over time, these repetitive loads can lead to the initiation and growth of cracks, ultimately resulting in failure.

Understanding fatigue is crucial for designing components that are subjected to fluctuating stresses, such as in bridges, aircraft, and machinery. The fatigue process is characterized by several stages: initial crack formation, crack propagation, and eventual final failure. Unlike immediate failure, which can occur due to excessive stress, fatigue failure happens over a period of time and can be difficult to predict, making it a critical consideration in materials engineering and safety assessments. The other options do not accurately capture the essence of fatigue as it specifically relates to the gradual damage incurred from cyclic stress.