Fatigue in material science refers to:

Fatigue in material science specifically refers to the weakening of a material that occurs when it is subjected to repeated loading and unloading cycles over time. During these cycles, even if the load is below the material's ultimate tensile strength, microstructural changes can accumulate, leading to the development of cracks. These cracks may eventually grow to a point where they cause the material to fail, often without any visible indication of the deterioration prior to fracture.

This phenomenon is significant in engineering because components that experience cyclic loads, such as those in vehicles, bridges, and machinery, must be designed to withstand fatigue. Understanding fatigue helps engineers choose appropriate materials and design geometries that minimize stress concentrations and improve the longevity and reliability of structures and components.

The other options describe different aspects of material behavior but do not accurately capture the essence of fatigue. Instantaneous failure refers to a sudden fracture, and consistent deformation under load describes creep, while an increase in strength after processing relates to phenomena like work hardening or heat treatment rather than fatigue.