What does "fatigue" refer to in the context of materials?

In the context of materials, "fatigue" specifically refers to the weakening that occurs in a material as a result of repeated or cyclic loading over time. Under such conditions, a material may not fail immediately but can develop microcracks that grow progressively with each load cycle, ultimately leading to fracture. This phenomenon is critical in engineering and design as materials may appear intact under normal conditions but can fail unexpectedly due to fatigue if not properly accounted for in stress analysis.

The understanding of fatigue is essential for ensuring that structures or components can withstand the operational stresses they encounter throughout their lifespan without experiencing catastrophic failure. This is particularly relevant in applications where materials are cyclically loaded, such as in bridges, aircraft, and machinery.

The other choices, while related to properties of materials, do not accurately define fatigue. The breaking point, loss of shape under heat, and impact resistance are distinct concepts that address different types of material failure or deformation.