What is the primary characteristic of fatigue in materials?

The primary characteristic of fatigue in materials lies in the way cracks develop as a result of repeated loading and unloading cycles. Fatigue is a process that occurs over time, typically under fluctuating stresses, which can lead to the initiation and growth of cracks within a material. These cracks usually start at points of stress concentration and can grow gradually until they reach a critical size, at which point the material may suddenly fail, despite being subjected to loads much lower than its ultimate tensile strength.

This phenomenon is particularly critical in engineering contexts where materials are subjected to cyclical loads, such as in bridges, aircraft components, and mechanical parts. Understanding fatigue is essential for predicting the lifespan of components and ensuring safe operation under varying load conditions.

The other characteristics listed do not accurately describe fatigue. Immediate failure under stress pertains to brittle fracture; permanent deformation under tension is related to plastic deformation; and being easily fractured by impact aligns more with materials' brittleness rather than fatigue behavior. Therefore, the concept of crack development through repeated loads is the defining attribute of material fatigue.